Electricity podcast

JAMES: Hello and welcome to the BBC Bitesize Physics Podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I'm Ellie Hurer, a bioscience PhD researcher.

This is episode one of our eight-part series all about electricity. And today we're going to be exploring electrical charge, current, and how to calculate them.

JAMES: Let's begin.

ELLIE: When we talk about how electricity flows, there are two key C's: charge and current.

JAMES: Yeah. Let's start with charge. There are two types of electric charge: positive and negative. Charged particles, such as electrons or ions, can transfer electrical energy. In metals, these charged particles are delocalised electrons that are free to move through the whole structure. Electrons are negatively charged.

ELLIE: And now let's move on to the second C, current. Electrical current is what we call the flow of electrically charged particles, such as electrons, as they move through an electrical conductor. It's how electrical energy flows through a circuit.

JAMES: Yeah, well, we measure current in amps using an ammeter. And when we do that, we measure the number of electrons passing a point in a circuit in just one second. The current is the same at any point in a circuit, so long as it's a single closed loop.

ELLIE: There are two types of electric current.

JAMES: Yeah, we have an alternating current and a direct current. That's AC and DC.

A direct current is when the electrons just flow in one direction around a circuit. So imagine a circuit that looks like a circle, for example. In a direct current, the electric current would either flow clockwise or anti clockwise. Not both.

ELLIE: An alternating current, on the other hand, is a current where the direction of the electron flow continually reverses.

JAMES: Yeah, the best way to remember the differences between the two is actually through their names. ‘Direct’ for doesn't change direction and ‘alternating’ for alternates direction.

ELLIE: So let's talk about how those work in a circuit. For an electrical charge to flow through a closed circuit, the circuit must include a source of potential difference.

Potential difference, also known as voltage, is the difference in energy the electrons have between two different points in a circuit. In a circuit, you've got to place a voltmeter in parallel with a component in order to measure the difference in energy from one side of the component to the other.

JAMES: Yeah, and by in parallel, what we mean is the voltmeter is on a separate branch of the circuit to the component in question.

ELLIE: We'll cover the difference between series and parallel circuits in more detail in episode 3.

JAMES: So for an electrical charge to flow through a circuit, there needs to be a voltage force. Let's use a lamp for an example. For that lamp to be turned on, you need to plug it into a wall socket, because the socket is the source of the voltage. Electricity can't flow through the plug up the wire and into the light bulb unless you plug it in.

ELLIE: Right, and when the lamp is plugged in, this then means the charge can flow, so you can then have a current. So, how do you measure the size of the electric current flowing through the wire to turn the light bulb on?

JAMES: There's luckily an equation for that Ellie, so grab your pen, grab your paper, we can write this one down together, if you want to do that.

The size of the electrical current is the amount of charge that passes a point per second, and the equation for calculating that is as follows: Charge equals current multiplied by time. I'll say that again. Charge equals current multiplied by time.

ELLIE: In this equation, we use the letter Q for charge, and it is measured in coulombs.

We then use the letter I for current, which is measured in amps, with the letter A. The unit for time is seconds, which is written as T. So this equation can also be expressed as Q equals I multiplied by T.

JAMES: Let's look at a practical example, and we'll give you the chance to, of course, work out the calculation, so grab your pen and paper.

So let's imagine you've made a simple circuit in your science class. It just has a battery, a light bulb, and a switch. Super simple. How would you calculate the charge that flows through the circuit if you knew the following? It had a current of 1.5 amps, and you want to measure the charge over the course of 60 seconds. I'll let you have a think for a moment and then we'll explain.

ELLIE: The equation is charge equals current multiplied by time. So you would take the current, 1.5 amps, then multiply it by the number of seconds, which is 60 seconds, to get the answer of 90 Coulombs. You can also rearrange that equation to find the current and in that case we would write the formula down as: current equals charge divided by time.

JAMES: Yeah, let's use that same circuit as an example. How would you work out the current, if the charge in the circuit was 90 coulombs over 60 seconds? I'll give you a few seconds to pause, then we can work it out together.

So to calculate the current flowing through the circuit, you would take the charge, 90 coulombs, and divide it by the number of seconds, that was 60, to get the answer, 1.5 amps. Simple as that.

Okay, let's summarise the key points we've learned from this episode. Really good one. Number one, for electrical charge to flow through a closed circuit, the circuit must include a source of potential difference. That's also known as voltage. Potential difference is the difference in energy the electrons have between two different points in a circuit.

Secondly, electrical current. We measure that in amps, and it's the amount of charge passing a point in the circuit per second. The equation that links these three is charge equals current multiplied by time. Well, you'll see that written down as Q equals I multiplied by T.

Third and finally, the current has the same value at any point in a single closed circuit. Thank you for listening to Bitesize Physics. If you found this helpful, please do go back, listen again and make some notes so you can come back any time you want and revise with us.

ELLIE: In the next episode of Bitesize Physics, we're going to focus on resistance and potential difference. So be sure to listen to the next episode and the rest of the series to make sure you're ready for your GCSE exam.

BOTH: Bye!

ELLIE: Hello, and welcome to the BBC Bitesize Physics podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I’m James Stewart, I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

ELLIE: Just a reminder that we're covering lots of topics in this series, so make sure you take a look at the rest of the episodes too.

JAMES: Yeah, let's get started because today we're talking all about current, resistance and potential difference.

In the last episode of the podcast, we gave you definitions of current and charge. Feel free to go back and listen to that one, but in this episode today we're gonna be talking about how those actually work when the current flows through an electrical component, something like a wire.

ELLIE: The current through component depends on both the resistance and the potential difference, also known as the voltage across the component.

JAMES: Resistance in a circuit is provided by components. You can have a component that is called a resistor. But other components such as a bulb, motor, and even wires provide resistance.

ELLIE: Resistance reduces the current as it makes it harder for the current to flow. And there's an equation that links the amount of resistance with current and potential difference.

So grab your pen and paper. The equation is: potential difference equals current multiplied by resistance. Let me repeat that again. Potential difference equals current multiplied by resistance.

JAMES: Now remember that potential difference is also known simply as voltage and is measured in volts. Current is measured in amps, and resistance is measured in ohms. The equation on an exam paper could therefore be written as: V equals I multiplied by R.

ELLIE: I know that's a lot to take in, so maybe pause this episode for a moment and write down that equation and then let's apply it to an example.

JAMES: Imagine a circuit. It's a simple one with a lightbulb, a cell, and an ammeter. How would you calculate the potential difference if the current is 2 amps and the lightbulb has a resistance of 58 ohms?

ELLIE: Well, potential difference equals current multiplied by resistance, so 2 amps multiplied by 58 ohms. And your answer would be… 116 volts.

JAMES: If you want to try more examples just like this to prepare you for the kind of questions you might get in an exam, visit the BBC Bitesize website for quizzes and more.

ELLIE: In an electric circuit, a resistor is a component that resists current. All components in a circuit have some resistance, but there are a few specific resistors you should learn about.

JAMES: Yeah, let's look at the first type of resistor, the fixed resistor, which always has the same value for resistance. This means that if you increase the potential difference, the current must also increase, because potential difference equals current multiplied by resistance.

ELLIE: In fact, potential difference and current are directly proportional. So what that means is that if the potential difference doubles, so does the current.

JAMES: And that type of resistor is called an ohmic conductor. There are other types of resistors that aren't ohmic, which means their value for resistance changes. And I think we should get stuck into that as well. When it comes to components like lamps and thermistors, the resistance is not a constant. It actually changes as the current does.

ELLIE: And for example, let's take, say, a filament light bulb. That's the kind of light bulb that has a squiggly wire in it. In a filament lightbulb, the resistance increases as the temperature of the lightbulb increases. So once it's at its full brightness, it gets pretty hot. So the resistance will have increased.

JAMES: And that's because the particles in that filament of the lightbulb are vibrating faster because of that higher temperature, making it harder and harder for the electrons to flow through. Now, this is not a proportional relationship, as if the potential difference increases, the current does not increase at the same rate.

Let's look at another example, a diode. So in this example, the current that flows through a diode only flows in the one direction. So diodes have a very high resistance in the reverse direction.

ELLIE: In the forward direction. Diodes have a large resistance at low potential differences, but at higher potential differences, the resistance decreases a lot. So, current increases.

JAMES: Okay, let's talk about resistance in another type of component, a thermistor.

ELLIE: When the temperature of a thermistor increases, it gets hotter and the resistance decreases.

And when the temperature decreases and gets cooler, the resistance increases.

Yeah, there's one type of thermistor that you can find in many homes. A thermostat is the device people use to change the temperature of the heating around the house.

JAMES: And finally let's talk about resistance in another type of component yes, we haven't run out of components just yet, an LDR. An LDR is a light dependent resistor and it has a similar pattern to a thermostat.

Now they're the things you have on like your light sensors at home, so we use them for street lights, night lights, that kind of thing.

The resistance of an LDR decreases as light intensity increases. And the resistance increases as the light intensity decreases.

ELLIE: So, how might questions about all these different components and resistance come up in an exam?

JAMES: Great question! Uh, yeah, you might be shown or even asked to draw the correlation on a graph. So, grab your pen and your paper and we'll show you just how to do that.

ELLIE: So, start off by drawing a big cross graph with two intersecting lines. Label the y axis, the vertical one, as current. And label the x axis, the horizontal one, as potential difference.

JAMES: With an ohmic conductor, resistance is a diagonal line from one corner of the graph to the other, passing through the origin, that's the intersection of the axis.

ELLIE: And when it comes to a diode, resistance increases with a diagonal line upwards once the potential difference has reached a small positive value.

JAMES: And finally when it comes to a filament lamp, resistance curves in an s shape across both the bottom left and the top right square of the graph.

ELLIE: While drawing this out might help you start to visualise it, I definitely recommend checking out the Bitesize website to see what these graphs actually look like.

Okay, so let's recap the main lessons we learned in this episode. So firstly, we learned the equation potential difference is equal to current multiplied by resistance.

Next, we learned that the current through an ohmic conductor is directly proportional to the potential difference across the resistor.

And last but not least, the resistance of components such as lamps, diodes, thermistors and LDRs is not constant.

JAMES: Thank you for listening to Bitesize Physics. In the next episode we are going to dig in to series and parallel circuits.

ELLIE: If found this helpful, go back and listen again and make some notes so you can come back to this as you revise.

BOTH: Bye!

JAMES: Hello and welcome to the BBC Bitesize Physics podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I’m James Stewart, I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: Just a quick reminder, whilst you're here in the BBC Sounds app, there's also the Bitesize Study Support podcast, which is full of tips and tricks to help you stay focused during revision, and of course, get the best out of your actual exam day.

ELLIE: Right, let's get started.

So, I want you to imagine an electric circuit, the kind of one you probably made for the first time in primary school. It's wires that connect to different electrical components, like a light bulb, a switch, and a battery.

JAMES: Yeah, there are actually two types of circuits. We have series circuits and parallel circuits.

Now in a series circuit, all the electrical components are connected by wires in one loop. So there's only one route for the current to flow. Now electrons pass through all of the components in the circuit in that one loop.

ELLIE: And on the other hand, a parallel circuit has electrical components on separate branches, so the electrons can take different routes around the circuit.

JAMES: And if you want to know exactly what that looks like, be sure to check out the BBC Bitesize website if you are someone that prefers to see rather than hear.

Series circuits and parallel circuits are both used a lot, but they have some really key differences, so let's start with series circuits.

ELLIE: Of course, so there are three key things that you need to know about series circuits. Firstly, in a series circuit, the same current flows through all of the different electrical components.

So the current at all parts of the circuit is the same. Secondly, in a series circuit, the total potential difference of the power supply is shared between all of the electrical components. And finally, the total resistance of all components in a series circuit is the sum of the resistance of each component.

JAMES: And that last point actually might come up in your exam as an equation, so we'll write that out for you. Grab a pen and paper, you can follow us through this one.

The equation to work out the total resistance of multiple components in a series circuit is: resistance total equals the sum of all the individual resistances added together.

ELLIE: For example, a circuit with three components would be R total equals R1 plus R2 plus R3. But one with two components would just be R1 plus R2.

JAMES: Yeah, so a key thing to know is that when you add a component in a series circuit, the total resistance increases because the total resistance is the sum of the resistance of each individual component.

ELLIE: Well, series circuits aren't that common in a regular house. But one great example is fairy lights, which I love.

JAMES: I'm glad we get to talk about fairy lights in the physics podcast. Um, a lot of fairy lights are designed as series circuits. There's one battery or plug, and then the lights are arranged into a circle, so the current flows in one direction.

ELLIE: You can tell that a set of fairy lights is a series circuit. If one bulb blows out, the circuit is broken, so the whole set stops working because the current flows in one loop. We know, it's so annoying, so annoying.

JAMES: But we will try and draw out a series circuit, so if you want to do that with us. Good time to grab your pen and paper.

So we want you to draw out a circuit in the shape of a square with one battery, one switch, and four light bulbs.

ELLIE: Could you say that it's like a mini set of fairy lights?

JAMES: Exactly what I would say it is.

ELLIE: Love it.

JAMES: So, from the information we've shared so far, how could you tell, in your mini fairy light circuit, that the one you're looking at is a series circuit? Have a little think for a moment.

ELLIE: So the key way to tell if a circuit is a series, is that it is one singular loop. There aren't any other branches or directions for the current to flow.

JAMES: Okay, let's move on to parallel circuits. Just like series circuits, there are three key things that you need to know about parallel circuits for your exam.

Number one, in a parallel circuit, the potential difference across each branch of the circuit is the same. In which case, the total potential difference in the battery is the same as the potential difference in each branch. But, in a parallel circuit, the current isn't the same. The total current throughout a parallel circuit is the sum of the current flowing through each of its separate branches.

And finally, in a parallel circuit, the total resistance of two resistors is less than the resistance of the smallest individual resistor.

ELLIE: That feels like a bit of a tongue twister, so let me repeat that.

JAMES: It was.

ELLIE: In a parallel circuit, the total resistance of two resistors is less than the resistance of the smallest individual resistor.

JAMES: And a key thing to know is that when you add other resistors to a parallel circuit, the total resistance decreases because it is less than the resistance of the smallest individual resistor.

ELLIE: And this is because more current is flowing for the same potential difference, which means resistance goes down.

JAMES: And just to note, whilst we're here, when we say resistor, that doesn't just refer to a fixed resistor, for example. All components in the circuit have a resistance. And in an exam question, they might actually ask you to calculate or explain the resistance of something like a lamp.

ELLIE: Right. We know that in a parallel circuit, the current can take different routes around the circuit. So, let's dive into what they look like in a circuit diagram. Okay, we're going to stick with the same example that we used last time, fairy lights, because they're my favourite.

JAMES: Why wouldn't you, yeah?

ELLIE: Exactly. But rather than the kind of fairy lights that completely turn off whenever one bulb breaks, we're going to draw the kind of fairy lights where the set keeps on working, even if one bulb breaks.

ELLIE: So let's try drawing one. Grab your pen and paper. So, I want you to draw a long rectangle circuit with a battery and a light switch on one end. Then, draw four lines connecting the long sides of the rectangle and draw one light bulb on each row.

JAMES: Right, so how can you tell that the circuit you're looking at is a parallel circuit? Have a think for a moment.

The key way to tell if a circuit is a parallel circuit is that there are separate branches to the circuit. So if one lightbulb were to stop working, the current would still be able to flow around the circuit and light up the other lightbulbs.

ELLIE: This means that even if one bulb broke, your fairy lights would still light up.

JAMES: Phew.

ELLIE: Thank goodness.

JAMES: Okay, let's do a quick recap of the three key lessons we've learned today. Firstly, in a series circuit, the current is the same through each component. The potential difference is shared, and the total resistance for all components is the sum of the resistance of each component. Two, the equation to work out the total resistance of all components in a series circuit is: resistance total equals the sum of the resistance of each of the components.

And three, in a parallel circuit, the total current is the sum of the currents through each branch. The potential difference is the same across each branch of the circuit. And the total resistance of all resistors is less than the resistance of the smallest individual resistor.

ELLIE: Thank you, James. And sadly, guys, we're at the end of this episode about series and parallel circuits. However, in the next episode of Bitesize Physics, we're going to be talking all about the domestic uses of electricity and the three pin plug.

JAMES: Thank you for listening to Bitesize Physics.

BOTH: Bye!

ELLIE: Hello and welcome to the BBC Bitesize Physics Podcast.

JAMES: The series designed to help you tackle your GCSE in Physics and combined science. I’m James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher. Before you listen, just a reminder that you can listen to the whole series or find an episode that you want to focus on.

JAMES: Yeah, absolutely. Let's get started on today's episode, where we're going to be talking about the domestic uses of electricity and the three pin plug.

I don't know where you are right now, but there are probably, I don't know, two wall sockets in your room?

ELLIE: Yeah, there's loads in the room we're in. There's one socket that's plugged into the microphone that I'm using right now. My phone is charging on a wall socket, and there's a kettle on the other side of the room I plugged in earlier to make my tea.

JAMES: Exactly, because electricity is all around us. If you boiled a kettle this morning, or you used the computer at school, or your phone to listen to this podcast conveniently, you use electricity because it's a vital part of our everyday lives.

ELLIE: But what has this got to do with the physics exam?

JAMES: Alright, let's talk through some key facts about domestic electricity to prepare for your GCSEs.

ELLIE: Right, so most of the things we use at home are connected to the mains electricity, which is what wall sockets are connected to.

JAMES: Mains electricity is supplied using alternating current, which we usually call the AC supply. For a quick recap, an AC supply is an electric current that regularly changes its direction.

ELLIE: Whereas a direct current, or DC, only flows in one direction. But be sure to go back and listen to episode one of this series, it's brilliant, where we dig deeper into those different types of electric current.

JAMES: Here in the UK most homes and domestic buildings have an electric supply with a frequency of about 50 Hertz, that's H Zed, and a voltage of 230 volts that's measured in V if you see it. The frequency of 50 Hertz means that the current changes direction and back again 50 times per second.

A wall socket is what you put your plugs and chargers into. As you know, in the UK, most plugs have three pins, one at the top, two at the bottom. That's because most electrical appliances are connected to the mains using three core cables.

ELLIE: So if you're on your phone or on your laptop, now would be a great time to head to BBC Bitesize to see a diagram of a three pin plug.

JAMES: So a plug has three main copper wires, and you can identify them by the colour of the plastic insulation they are covered in.

ELLIE: The brown wire is the live wire. It's the wire that electric current travels through. The live wire carries the alternating potential difference from the electric supply.

JAMES: Yeah, and that live wire is at 230 volts, so it can be really dangerous.

ELLIE: The wire with green and yellow stripes is called the earth wire.

JAMES: Which you can remember by thinking about how so much of nature on earth is green.

ELLIE: Also, the earth wire is a safety wire to stop the appliance becoming live. The earth wire is at zero volts as it only carries a current if there is a fault.

JAMES: And finally, the blue wire is called a neutral wire.

ELLIE: Which you can remember by thinking about how a lot of blue things like the ocean and the sea are calming colours.

JAMES: That’s right. The neutral wire connects the cable in the wall and completes the circuit. The neutral wire is at or pretty close to earth potential at zero volts.

ELLIE: So, just to help you remember, earth is green and yellow, like so much of nature. Blue is neutral, like the sea and sky, and the live wire is brown.

JAMES: All the wires in a plug play a key role, but the earth wire is extra important because it keeps us safe. The earth wire provides a path for current to flow from the case of the device to the ground if there is a fault.

JAMES: Let's say we were looking at the plug for an electric hob. If the live wire were to become loose, it would be really dangerous if it were to touch the casing of the hob, as anyone touching the appliance would be electrocuted.

ELLIE: So what the earth wire does is direct the electric current to the ground instead of to the person touching the appliance.

JAMES: Exactly, so always be careful when it comes to electrical appliances and if you ever see or feel a spark, stop using that appliance and tell a responsible person, not me or Ellie, that there's a fault because no one wants to get an electric shock.

ELLIE: Alright guys, it's time to recap the key things that we've learnt. So in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230 volts. A three pin plug includes a live wire, an earth wire and a neutral wire.

JAMES: And finally that earth wire is essential to making an appliance safe as it earths the electric current that flows through an appliance if there's a fault.

Thank you for listening to Bitesize Physics. If you found this helpful please go back and listen again and make some notes along the way and come back here whenever you want to revise. There's loads more resources available as well on the BBC Bitesize website, so be sure to check those out too.

ELLIE: And in the next episode of Bitesize Physics, we're going to dig into energy transfers in everyday appliances, so be sure to have a listen.

BOTH: Bye!

JAMES: Hello and welcome to the BBC Bitesize Physics Podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I’m James Stewart, I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: And today we're going to be talking about energy transfers in everyday appliances.

ELLIE: From the plug you use to charge your phone, to the batteries you use to power a remote, we're going to talk all about the electricity you use at home. So, let's begin!

JAMES: Every electrical appliance in your house is designed to transfer energy from one store to another. For example, in a TV remote, the chemical energy stored in the battery is transferred into the electrical energy store in the remote that's used to turn it on and let you change channels.

ELLIE: Or in a laptop, the electrical energy stored in your mains electricity is used to charge your laptop by transferring into the chemical energy store in the battery.

JAMES: In the UK, all electrical devices in a home should have labels on them that show the power rating of the device.

ELLIE: Power is how much energy is transferred per second. Which means something has a higher power rating if it transfers more energy in a given time. The unit of power is watts. And the symbol for that is a capital W.

JAMES: So if you're near a microwave or a kettle, look out for a sticker that shows how many watts that device operates on.

ELLIE: Okay, so let's get to our first equation of the day. Whoop whoop! Grab your pen and paper because we're about to hear how to calculate energy transferred.

Alright guys, so energy transferred equals power multiplied by time. Energy transferred is measured in Joules, J. Power is measured in watts, W, and time is measured in seconds, S. Sometimes energy transferred might be called ‘work done’.

JAMES: Shall we look at a practical example for this? I always find that it helps me try and figure this out. So imagine you went swimming at the weekend and you left with really wet hair. You'd probably reach for the hair dryer straight away, right? But how much energy would you use? Let's work it out.

ELLIE: So, if your hairdryer has the power of 1,800 watts and you use it for five minutes, you would multiply 1,800 watts by 300 seconds to calculate that the energy transferred would be 540,000 joules.

JAMES: So how does energy transfer actually work? A good way to look at energy transfer is through the appliances that keep us hot or cool us down.

ELLIE: Let's say it's a hot summer day and your room is scorching. You might buy a small handheld fan to cool you down. When you pop the batteries into the fan and turn it on, there's a variety of energy transfers going on.

JAMES: Work is done when the chemical energy store in the battery, is transferred into the electrical energy store, which is then transferred into the kinetic energy store of the electric motor that makes the fan spin around and cool you down.

ELLIE: And we can see energy transfer in the appliances that warm us up too. Imagine it's a cold, snowy winter's night. Let's say you have one of those stand-alone electric heaters. When you plug it into the AC mains of your wall socket, it transfers energy from the electrical energy store into the thermal energy store of the heater to warm up your room.

JAMES: Energy transfer is linked to the potential difference a charge carries. Because when the charge transfers some energy to an appliance, the potential drops in the charge.

ELLIE: And to calculate the amount of energy transferred, you'll need another equation, so grab that pen and paper again so you can jot this down.

JAMES: Okay, here we go. Energy transferred equals charge flow multiplied by the potential difference. And here are the units of measurement. Energy transferred, or work done, is measured in joules, J. Charge flow is measured in coulombs, C. And potential difference is volts, V.

ELLIE: So let's look at a real life example. Imagine you're throwing a party and put fairy lights up around the room to make it feel a little bit magical. I do love my fairy lights, but how would you calculate the energy transferred by the fairy lights?

JAMES: Well, if the charge flow in the lights was 10 Coulombs and the potential difference was 20 volts, you would take 10 Coulombs and multiply it by 20 volts, which would give you the answer 200 Joules.

And finally, let's talk about the link between power, current and voltage. And you'll want to grab your pen and paper one last time for this one. Power equals potential difference multiplied by current. Power is measured in watts, W. Potential difference is measured in volts, V. And current is measured in amps, A.

ELLIE: Or, you could use another equation to calculate the power. So power equals current squared multiplied by resistance. Power is measured in watts W, current is measured in amps A, and resistance is measured in ohms.

JAMES: Okay, let's recap the three key lessons we learnt here. Firstly, every electrical appliance in your home is designed to transfer energy from one store to another. Secondly, the amount of energy an appliance transfers depends on how long the appliance is switched on and the power of the appliance. The equation to calculate that is energy transferred equals power multiplied by time, or energy transferred equals charge flow multiplied by the potential difference.

And finally, power equals potential difference multiplied by current, or, you can use an alternative equation, get ready for this one, power equals current squared multiplied by resistance.

ELLIE: Okay, and one last note. We've given you a lot of equations in this episode.

JAMES: So many. So many.

ELLIE: So many. So if you're a little bit overwhelmed, which is understandable, be sure to head to the BBC Bitesize website to read through all of this and study at your own pace.

JAMES: Yeah, that's good advice. We also wanted to give you some practical advice for your exam. So when you see a question about these topics, follow these three steps, okay? Number one, first and foremost, write down the values that you've been given.

Secondly, write down the thing you are being asked to find out or calculate. And thirdly, check through your list of equations to decide which one is the correct use. It's like having superpowers and picking the right superpower for the problem.

ELLIE: Also, in a question worth more marks, like 5 or 6, you might need to use more than one equation.

And we're at the end of the episode, so thank you so much for listening to BBC Bitesize Physics. If you found this helpful, go back and listen again and make some notes so you can come back to this as you revise.

JAMES: In the next episode of Bitesize Physics, we're going to be learning about something called the National Grid. Spoiler alert, it's not a big game of 4-in-a-row.

ELLIE: Oh, man. Bye!

ELLIE: Hello and welcome to the BBC Bitesize Physics podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I’m James Stewart and I’m a climate science expert and TV presenter.

ELLIE: And I'm Ellie Hurer, a bioscience PhD researcher.

JAMES: Let's get stuck in.

Let's start with pylons. They carry electricity cables high above the ground and they belong to something called the National Grid.

ELLIE: The National Grid is a system of cables and transformers that link power stations where electricity is generated to buildings, homes and consumers across the country.

JAMES: The electricity we use in our daily lives is generated at power stations around the UK. But it's transferred in different ways using transformers.

ELLIE: Not the type of transformer that turns from a car into a crime fighting robot though. It’s an electric transformer. A transformer is a device that changes the potential difference or voltage of an electrical supply. And there are two types to know about.

JAMES: The first one is a step-up transformer and it's used to increase or ‘step up’ the voltage or potential difference of an electric current. As we increase the voltage, the current decreases. Whereas a step-down transformer is used to decrease, or ‘step down’, the voltage or potential difference of an electric current. As we decrease the voltage, the current increases.

ELLIE: So what’s that got to do with the National Grid?

JAMES: Well, electric current is transferred straight from a power station to your home in one smooth go. Before electrical power leaves a power station, it goes through a step-up transformer to transfer the power at a very high voltage, we're talking about 400,000-ish volts. This makes the current much lower.

ELLIE: The current travels through the wires until it reaches a local area and goes through another kind of transformer, a step-down transformer. There, the voltage is decreased to around 230 volts for domestic use. This means the current increases.

JAMES: So can you explain why a potential difference is increased if it's then going to be decreased later?

ELLIE: It's to make the transfer more efficient. When the electric current in a cable is higher, more of the energy is lost as it's transferred into heat, so high currents waste energy and money.

JAMES: By using step-up and step-down transformers, the National Grid is able to transport the same amount of electric power at a higher voltage and lower current to save money and save energy.

ELLIE: But the National Grid does even more than that. At what time of the day do you think that people in the UK use the most electricity?

JAMES: Like 6, 7 o'clock when people get home from work, or having dinner, watching telly.

ELLIE: Yeah, so in the UK we usually use the most electricity between 7am to 11am in the morning, when lots of people are getting ready for work and school. And then again, from 5pm to 9pm, when some people are making dinner, relaxing and getting ready for bed.

JAMES: And we use more electricity during the winter months too. When there's a big event happening like the Football World Cup or during the school holidays when people are at home a little bit more.

ELLIE: Yeah, the National Grid is designed to manage the demand for electricity during those busy hours. The people who work there predict when more electricity will be needed and produce it based on this. So usually, power stations run well below their maximum capacity to enable them to ramp up power output when needed.

JAMES: So the next time you see a pylon in the middle of the countryside, remember it's connected to the National Grid and it's actually responsible for getting electricity into your home so you can switch your plug on in your bedroom.

ELLIE: Right, I think it's time to recap some of the lessons we've learned in this episode. Firstly, the National Grid is a system of cables and transformers linking power stations to consumers. Secondly, the National Grid uses transformers to make energy transfer more efficient. And thirdly, the National Grid uses step-up transformers to increase the voltage, or potential difference, decreasing the current as it leaves the power station. Then it uses step-down transformers to decrease the voltage, or potential difference, which in turn increases the current before it can be used in homes, factories and buildings.

JAMES: Thank you for listening to Bitesize Physics. If you found this helpful, go back and listen again, and make some notes on your way. Feel free to come back here any time to help you revise.

ELLIE: In the next episode of Bitesize Physics, we're going to be talking all about static, so be sure to listen to the next episode. So I'll see you then.

BOTH: Bye!

JAMES: Hello and welcome to the BBC Bitesize Physics Podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I’m James Stewart, I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: And today we're going to be talking about static charge and how different objects repel and attract.

ELLIE: Well, I'm ecstatic to hear that. Let's begin.

JAMES: Have you ever taken a jumper out of the tumble dryer and felt a little electric shock when you've done it? Or maybe you've done that thing where you rub a balloon against your head to make your hair stand up in the air. If so, you've experienced something called static.

ELLIE: So static charge is an electric charge that accumulates on an insulated object, for example, because of friction. And just so you remember, something is insulated if it doesn't easily conduct electrical charge.

JAMES: Static charge occurs because the electrons are not free to move around in an insulator. So when they are transferred, they build up in one place. That's why we call it static charge. With static meaning stationary or simply can't move.

ELLIE: When certain insulating materials are rubbed against each other, for example, hair on a balloon, they become electrically charged.

JAMES: Secondly, negatively charged electrons are rubbed off one material and on to another. The material that gains electrons becomes negatively charged, and the material that loses electrons is left with an equal positive charge.

ELLIE: So in the hair and balloon example, the balloon would gain electrons and become negatively charged, whereas your hair would lose electrons and become positively charged.

JAMES: Exactly, so when you move the balloon away from your hair, you'll find that the two insulating materials, the balloon and the hair, would actually attract each other, making your hair stand up as it reaches towards the balloon. And the two objects exert equal and opposite forces on each other, even though they're no longer touching.

ELLIE: You can see a similar effect when it comes to clothes that have come out of a tumble dryer. Sometimes you'll notice a spark between different jumpers, or feel a little shock maybe as static electricity is produced between them through friction.

JAMES: Yeah, let's talk about the science behind that, shall we? So as static charge builds up, the potential difference between an object and the Earth, or something connected to the Earth, gets bigger. Now, in this example, it's actually you who's connected to the Earth.

ELLIE: If this difference gets big enough, then the charge can jump across the gap and cause a spark. Though, this is usually quite small and just felt as a static shock.

JAMES: Another good example is when you jump on a trampoline.

When you jump up and down on the trampoline, charge builds up as you rub your feet on the bottom of the trampoline with each jump.

ELLIE: But then, when you reach out to help someone else onto the trampoline, the charge jumps from you to them, and it causes a static shock.

JAMES: In the case of static charge, opposites attract. This is why the negative and positively charged objects would attract each other.

ELLIE: But if you were to bring together two objects with a negative charge, they would repel each other. This type of attraction between oppositely charged objects is called electrostatic attraction.

JAMES: Attraction and repulsion between two charged objects are examples of a non-contact force. In case you forgot what that means, I'll quickly explain for you. A non-contact force is a type of force applied to an object by another object that's not in direct contact with it.

ELLIE: So, if you want to learn more about contact forces and non-contact forces, be sure to listen to episode one of our series on forces to find out more.

JAMES: Okay, let's recap the three key lessons we've learned here. So, firstly, when certain insulating materials are rubbed against each other, they become electrically charged. Now, that's what we call static charge.

Secondly, negatively charged electrons are rubbed off one material and on to another. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

And finally, like charged objects repel, but oppositely charged objects attract.

ELLIE: Thank you for listening to BBC Bitesize Physics. If you have found this helpful, go back and listen again and make some notes so you can come back to this as you revise.

JAMES: There’s lots more resources available on the BBC Bitesize website, so be sure to check those out. In the next, and final, sadly, episode, we're going to be learning about the essentials of electric fields, so please do join us for episode eight.

ELLIE: I can't wait.

JAMES: That's it from us.

BOTH: Bye!

JAMES: Hello, and welcome to the BBC Bitesize physics podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science. I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

This is sadly the eighth and final episode of our series on electricity. If you haven't listened to the other seven, you can go back to episode one and listen all the way through to make sure you get the most out of the series.

JAMES: Alright, let's kick off with our final episode where we are going to be talking about electric fields, how attraction works in them and how to draw one.

ELLIE: A charged object creates an electric field around itself that can influence other charged particles in the surrounding area. The closer you get to the charged object, the stronger the electric field is.

JAMES: So the further you get away from the charged object, the weaker the field is?

ELLIE: Yep, absolutely correct. And when you place another charged object inside the electric field of the first object, it experiences a force.

JAMES: And the closer the second object gets to the first object, the stronger the force it experiences is.

ELLIE: Right, and if the field is strong enough, charges can be forced through insulators such as air.

And in that case, a spark, or static discharge, might occur. This is actually what happens during a lightning strike.

JAMES: Okay, so what does an electric field actually look like? Let's draw some out together. So I want you to take your pen and your paper out, and I want you to draw two circles. So let's do one at the top of the page, and one at the bottom of the page. In the middle of that first circle, just write out the word ‘positive’ and draw a plus sign.

Then draw out a bunch of arrows starting from the circle and pointing out at the different corners of the page. And there you have a positive electric field coming from a positively charged sphere. Simple as that.

ELLIE: Alright, and let's move on to the other circle. I want you to write the word ‘negative’ and draw a minus sign in the middle of the circle. Then, draw a bunch of arrows around that circle, but this time, have the arrows pointing towards the circle, not away from it.

JAMES: And that will be an example of a negative electric field coming from a negatively charged sphere. The closer the field lines are, the stronger the forces. The further apart the field lines are, the weaker the force is.

Now the reason why we've drawn the arrows pointing out from the positive sphere and in towards the negative sphere, is because electric fields always go from positive to the negative.

ELLIE: Now, let's imagine how these two fields might interact. If the two spheres we just drew were brought closer to each other, they would attract as they have opposite charges. The closer they get, the stronger the force of attraction.

JAMES: Hopefully drawing that out helped you visualise this a bit better, but I definitely recommend checking out the BBC Bitesize website to have a look at what these fields actually look like, so then you can make your drawing more accurate.

ELLIE: Good idea, James. And finally, let's take a moment to talk about sparking.

The stronger the electric field around an object, the greater the potential difference between the charged object and Earth.

JAMES: If the strength of this field gets big enough, the air between the object and Earth can become ionized. Meaning a current flows, and that's what causes a spark.

ELLIE: Alright, so let's recap the three key lessons we've learned in today's episode.

Firstly, a charged object creates an electric field around itself. The closer to the charged object, the stronger the electric field.

Secondly, when another charged object is brought into an electric field, the force gets stronger as the distance between the objects decreases.

And finally, if an electric field is strong enough, charges can be forced through insulators such as air, and in that case, a spark or static might occur.

JAMES: And with that comes the end of our 8 part series all about electricity.

ELLIE: I hope you found it helpful, and if you didn't get the chance to listen to all of the episodes, be sure to go back so you can really dive in.

JAMES: Thank you for listening to Bitesize Physics. If you're preparing for your GCSEs, firstly, good luck, and secondly, why not also check out our Bitesize Biology podcast, or our range of Bitesize English Literature series.

ELLIE: And whilst you're in the BBC Sounds app, there's also the Bitesize Study Support podcast, which is full of tips to help you stay focused during revision and get the best out of your exam day.

BOTH: Bye!