Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you can join me today for this lesson on electric cells and batteries.

It's part of the Electrolysis unit.

I'm here to work with you through this lesson and support you all the way through, especially the tricky parts.

Our learning outcome for today's lesson is, I can describe how the electric cell works and how electric cells combine to produce batteries of different voltages.

Here are our keywords for today's lesson.

Electric cell, a device that contains a store of chemicals that react to produce a voltage across two electrical contacts.

Voltage, this is a measure of the difference in energy between two parts of a circuit.

It is measured in volts.

Electrolyte, a liquid or aqueous salt solution that contains freely moving ions.

Electric battery, electric cell or several cells joined together in series that can push current around a complete circuit to transfer energy to each component.

You may wish to pause the video and note down these keywords and their meanings so that you can refer to them later on in the lesson.

In today's lesson, we have two learning cycles.

The first learning cycle is about electric cells and the second electric batteries.

So let's get started with our learning about electric cells.

An electric cell contains a limited store of chemicals that react together to produce a voltage that can push current around an electric circuit.

And that's a real key point.

Now you're gonna be familiar with these cells that you can see on the screen now, an alkaline cell or a lithium cell, and we probably in everyday language refer to them as batteries.

And they are things that we put into portable objects that need powering such as an alarm clock or a torch or scales that are used in either the bathroom or for cooking.

Electric cells are used to power portable objects and I'm sure you can come up with lots and lots of examples yourself.

And when you just have one of these, one of these batteries, as we call it in everyday language.

The scientific term that is correct is an alkaline cell or a lithium cell, also battery.

We'll learn about what a battery actually is in learning cycle two.

So what happens when an electric cell is connected into electric circuit? A chemical reaction takes place and energy is transferred as electric current flows around the circuit.

And you can see in the animation here, it's an electric charge flowing around a circuit.

The circuit's being complete and it's transferring that energy that allows the light bulb to be lit up.

And you can see the electric cell at the top.

When all the reactants have been used up, the chemical reaction stops, there's nothing left.

And so the electric current stops.

And in everyday language we often say the battery has gone flat.

What we actually mean is the chemical reaction's stopped because reactants have been used up.

So let's have a quick check for understanding.

True or false? An electric cell stops pushing current around a circuit when it has run out of energy.

True or false? Well done if you chose false.

Now what's the reason? Is it A, it stops when one of the reactants is used up, so no more energy can be transferred to the components in the circuit? Or B, it stops when the chemical store of energy is emptied? Well done if you chose A.

That is the correct answer.

As soon as one of the reactants is used up, no more energy can be transferred to the components in the circuit.

So that's what happens.

So an electric cell can be made by connecting different metals and putting them in contact with an electrolyte.

And here we can see an electric cell.

And it's similar to the setup that we've been using in electrolysis.

So we've got a beaker here with the electrolyte.

Electrolyte just being a liquid or a solution that contains ions that can move.

So for example, here we've got an aqueous solution of ammonium chloride.

We've got two different metals, we've got magnesium metal and we've got copper metal and we can see that they're connected together and the vault metre has a reading.

The electrodes react with the ions of the electrolyte and that's what produces the voltage across the cell.

The reactions at each electrode creates a charge difference between them.

And when the electrodes of different metals are connected, charge will flow.

And this is really important.

So if we just put the magnesium metal into the electrolyte and didn't connect it to the copper metal, we wouldn't get that charge flowing.

We need to think about what happens when reactions take place.

What is happening here is a metal is being oxidised and you'll know from previous learning that when something is oxidised, it loses electrons.

And now we need to think of those electrons as being electrical charge.

The electrons that are lost gain energy.

So as those electrons are pulled away or they are lost from the metal, they are increasing in energy.

The energy per unit charge is measured in volts and this is what creates the potential difference between the metals.

So a voltage is measured across the cell when the two metals are connected.

This next point is really important.

The metal with the higher reactivity pushes the electrons towards the other metal.

And where it's got a higher reactivity, it means that the electrons are being lost that are being, when it's oxidised, have more energy.

And so they are pushed towards the other metal.

Metals found higher up the reactivity series push the electrons more than those lower down the series.

So once again, we need to refer to that reactivity series of metals to be able to predict what the voltage will be when we make up a cell of different metals.

So really important to go back to that previous learning.

In this cell, the electrons will flow from magnesium to copper because magnesium is much more reactive than copper.

So it will be oxidised and it'll push its electrons towards the copper metal.

And you will see that by these arrows that have been added to our diagram.

Again, it's important to understand at which direction the electron flow is going.

It's from the more reactive metal to the less reactive metal.

So another quick check for understanding.

The voltage produced by an electric cell depends on A, the types of electrodes.

B, the difference in reactivity of the metal electrodes.

C, the reading on the voltmeter.

D, the type of electrolyte.

So quite a few of these answers are correct.

Well done if you've got A, the type of electrodes.

B, the reactivity in the metals of the electrodes.

And D, the type of electrolyte.

The reading on a voltmeter is just a result that is telling us what the voltage is.

So very well done if you've got all of those three correct.

So here we have our reactivity series of metals, which you should be by now very familiar with.

If a different combinations of metals are connected, the voltage changes.

So let's have a look at an example here.

So if we take magnesium metal, which is quite high up the reactivity series and we connect it to zinc metal a little bit lower down the reactivity series, we will get an electron flow between them, and the voltage in this case is 1.

61 volts.

If we were to take a different reactivity, so for example we kept our magnesium, but we replace the zinc metal with copper, then that voltage would increase.

The voltage across electric cell depends on the difference in reactivity of two metals.

So the greater the difference in reactivity, the greater the voltage.

If the reactivity is quite close together, then the voltage is going to be quite small.

So again, let's have a little check for understanding.

Which of the following combinations of metals would make an electric cell with the greatest voltage? So we're looking for the biggest difference in reactivity.

Is it A, magnesium and iron? B, magnesium and copper? C, magnesium and lead? Look at the reactivity series that we've got printed there to help.

Well done if you chose B.

If you look at your reactivity series, the biggest difference in metals of reactivity is between magnesium and copper.

So that will give the greatest voltage.

So well done if you got that right.

So we've got an example here.

In this cell, the zinc is more reactive than copper, so it will push its electrons towards copper.

And we've shown that just by an arrow going across from zinc to copper metal.

So what actually happens? Let's have a think about it for a moment.

The zinc, which is a more reactive metal, is oxidised so it loses electrons.

And again, from previous learning, we can write this as a half equation.

We can say the zinc solid goes to zinc two plus AQ, plus two electrons, which are written as E minus.

So that is what is happening at the zinc electrode.

The zinc metal is giving up two electrons and the zinc ions are going into the electrolyte and that means that the electrons are free to move around the circuit.

The copper or the less reactive metal in this case ions present in the electrolyte, for example, copper sulphate electrolyte, are reduced.

Remember those copper irons are going to be attracted towards the copper metal electrode because it's now more negative as electrons have gone around the circuit.

And we can see the new half equation here.

CU two plus aqueous plus two E minus goes to copper solid.

The ions present in the electrolyte acts as charge carriers.

The electrolyte could be different ions, they would still be attracted towards a carbon metal and act as charge carriers.

So another quick check for understanding.

True or false? In an electric cell made from magnesium and copper, the magnesium pushes the electrons around the circuit.

Is that true or false? And think about the reactivity.

Well done if you chose true.

Now let's think about the reason.

Is it A, magnesium is more reactive than copper, so it's oxidised and gives up electrons? Or B, magnesium is more reactive than copper, so the Mg two plus ions are reduced? Well done if you chose A, magnesium is oxidising.

It gives up those electrons and that pushes then the energy that's given up with it, it pushes towards the copper metal.

So very well done if you've got that right.

So that brings us to our first task.

Question one, what is an electric cell? Question two, you need to use the reactivity series of metals that we've got down the side to put the electric cells in order, starting with the lowest voltage.

And to do this, if you just write a number from one to five in each box, so the one with the lowest voltage, then write the number one in the box.

Pause the video, have a go at the questions, and then when you're ready we'll look at the answers together.

The answer to question one, electric cells contain a limited store of chemicals that react to produce a voltage that can push current around a circuit.

So if you've got that answer, very well done.

Question two, where we're using the reactivity series to try and find out, work out how reactive these cells are.

So starting with the lowest in voltage, magnesium and aluminium because if you look at that, magnesium and aluminium are just next door to each other.

Magnesium is above aluminium in the reactivity series.

So there's not going to be much difference in reactivity.

Our second one, magnesium and iron followed by magnesium and tin, magnesium and lead, and then magnesium and gold.

By the time we get to magnesium and gold, there's a massive difference in reactivity.

So we would expect that to have the highest voltage.

So very well done if you've got all of those in the correct order.

Excellent work.

So we're gonna move on to question three now.

The diagram shows an electric cell made from magnesium and silver metals.

So please label the diagram, draw an arrow to show the electron flow, and then describe how the electric current flows around the circuits.

Pause the video, have a go at the question and then when you're ready we'll have a look at the answers together.

So we now have our electric cell here with the labels.

So A is magnesium.

B is our electrolyte, and we don't need to actually say what type of electrolyte it is.

And C is a voltmeter.

Because magnesium is more reactive than silver, the electrons flow from magnesium around to the silver metal.

So well done if you've got those correct.

Describe how the electric current flows around the circuit.

Well your answers should include the following points, trying to put them in logical order.

And I would always use the diagram to help if I'm writing an answer to this question.

So the first thing to say is, magnesium is the most reactive metal.

So it pushes its electrons towards the silver metal.

And we may include the half equation, magnesium solid goes to magnesium two plus aqueous plus two electrons that go around the circuit.

The electrons flow through the connecting leads.

The silver ions pick up the electrons at the silver metal.

Ag plus aqueous plus E minus gives Ag solid.

And that is what actually happens as it goes around the circuit.

And that's of course assuming that our electrolyte has some silver ions in.

It may have been a different electrolyte and then the half equation would've been slightly different.

The electrolyte contains charge carriers that move through it to complete the circuit.

And that is what is really important in these questions to understand that its electrolyte contains those charge carriers.

So very, very well done if you've got that question correct and you had it in that logical format that basically takes us around the circuit.

Well done, that's excellent work.

That brings us to the end of our first learning cycle on electric cells, and we're gonna move on now to look at electric batteries.

So the electric battery is formed from an electric cell or from several cells joined together in series.

And you remember from your physics lesson that series are when they are joined in a row.

And you can see from our animation here, we have now got two of the alkaline cells next to each other and they are connected and they're forming a battery and that electrical charge is flowing around the circuit through the bulb and we now have a battery.

A battery can push current around a complete circuit to transfer energy to each component, which is why the light has lit up.

The voltage of individual electric cells are often very, very small.

And this is why batteries of several cells are used to generate higher, more useful voltages.

And you will be familiar again going back to our previous example of a torch, that you may actually put two or three alkaline cells into that torch to actually get it to light up.

So we may have come across this in your physics lessons.

We have two electric cells, they are connected in series next to each other and they form a battery.

The voltage of the battery is calculated by adding up the voltages of each cell.

So in this example, the voltage of the battery is going to be 1.

5 plus 1.

5, which equals three volts.

Okay, a quick check for understanding.

Three electric cells of voltage 1.

5, 2.

0, and 1.

0 volts are connected in series to form an electric battery.

Which statement is correct? A, the voltage of the battery is the same as the highest cell voltage, which is two volts.

B, the voltage of the battery is 1.

5 plus 2.

0 plus 1.

0 which equals 4.

5 volts.

Or C, the voltage of the battery is the average voltage of all the cells, in which case we add them up and we divide by three and our answer is 1.

5 volts.

Which answer is correct, A, B, or C? Well done if you chose B.

To find the voltage of a battery, we add up the voltages of all the individual cells and it is the total.

Excellent work.

So the voltage of the electrical battery depends on the voltage of the individual cells.

So whatever we have, we add it up and we can connect our chemical cells together.

So in this diagram here we have three electric cells connected in series to form an electrical battery.

The first one is 2.

0 volts.

Then we've got a 1.

3 volts and a 1.

5 volts.

And it's important to note that the cells are connected between each other.

The voltage of the battery is 4.

8.

We add up all the individual numbers.

Okay, so we're gonna have a look at another calculation here.

The voltage of a battery can be varied by connecting different chemical cells in series.

So to make a 12 volt battery using 1.

5 cells, what must we do? Well first of all, we connect the cells in series.

But how many? We use 12 divided by 1.

5 'cause that's the voltage and we get eight cells.

So if we are going to make a 12 volt battery, we need to connect eight 1.

5 volts cells in series and then we get that total of 12.

Okay, your turn.

This time we're going to describe how to make an 8.

0 volt battery and we're using different cells this time, 0.

8 volt cells.

So have a go.

How would you make an 8.

0 volt battery using 0.

8 volt cells? Okay, so the first thing hopefully you've said is you connect the cells in series, that's really important.

The cells were connected next to each other.

How many? Well, to work that out, we will have eight, the total voltage, divided by the voltage that we've got for each cell.

So that's 0.

8, which gives 10 cells.

So to make that eight volt battery, we will connect 10 0.

8 volt cells in series.

So very well done if you've got that correct.

Excellent work.

Some electric batteries are rechargeable and I'm sure you are used to using appliances with rechargeable batteries.

So for example, your mobile phone.

When it goes flat and stops working, you plug it into the wall and you recharge it.

You don't open it up, change the battery and throw away the old one.

The same goes for a tablet or a smartwatch because these all contain rechargeable batteries.

When the battery goes flat, it's recharged by plugging into a charger that's connected to the mains electricity supply.

When this happens, the chemical reaction in a rechargeable battery is reversed.

That means it goes back to the start.

This happens when the external voltage is connected, the reaction is reversed.

It means that we can use the reaction again to push the electrons around the circuit.

An example of a rechargeable battery is a nickel-metal hydride, and we can compare it to an alkaline non-rechargeable battery.

When we compare something, we need to look at advantages and disadvantages.

So taking the two different types of batteries, for example, the alkaline battery is cheap to manufacture, but it may end up in landfill once fully discharged and it's expensive to recycle.

On the other hand, the nickel-metal hydride battery can be recharged many times.

It can then be recycled when it comes to the end of its useful life, which reduces the use of resources.

But the disadvantages, it's more expensive to manufacture and it costs more to buy.

So true or false? In a rechargeable battery, the chemical reaction is reversible.

True or false? Well done if you chose true.

The reason is when an external voltage, for example, the mains is connected across the battery, the chemical reaction is reversed.

So well done if you've got that right.

That brings us to our second task.

For the first question, we'd like you to go through and answer these different parts.

Battery consists of cells.

So part A, describe how a six volt battery can be made from 1.

5 volt cells.

B, the six volt battery is connected to a light bulb.

Describe why the brightness of the bulb dims over time.

C, after a while, the battery is recharged.

Predict the brightness of the bulb.

And D, explain why a rechargeable battery can be recharged.

So pause the video, have a go at this question and then when you're ready, we'll have a look at the answers together.

For part A, we can make a six volt battery by connecting four 1.

5 volt cells in series.

And it's important that you have connect in series.

Part B, describe what happens to the brightness over a long period of time where it's bright for most of the time, but then eventually it starts to dim before going out altogether.

Part C, the six volt battery is recharged.

What's gonna happen to the brightness of the bulb? The bulb will get brighter again.

It'll go back to its original brightness as that chemical reaction is reversed.

Part D, the chemical reaction can be reversed by connecting an external voltage across it or plugging it into a battery charger connected to the mains.

So very well done if you've got the answers to all of those questions.

Excellent work.

Let's move on to question two.

So we've got some information in this table.

It lists the voltage of different chemical cells.

So if you could work through the parts of question two.

A, why does the copper-copper cell have voltage of zero? Part B, why does the copper-zinc cell have a negative voltage? So you're gonna have to think those couple of answers through.

Part C, describe how you would make a 1.

5 battery using these metals.

And D, draw a labelled diagram to show your answer to C.

So pause the video, have a go at these questions and then when you're ready, we'll look at the answers together.

Why does the copper-copper cell have a zero voltage? Well that's because both metals or both electrodes are made of copper and so there's no difference in reactivity.

And so no electrons are pushed towards the other electrode, which means that there is no potential difference and the voltage is zero.

So well done if you've got that.

Part B, why does the copper-zinc cell have a negative voltage? We need to think this one through and try it again to get a logical order.

So let's have a go.

Zinc, which is metal two, it's more reactive than copper, which is metal one.

In the other cells where the voltage is positive, metal one is more reactive than metal two, therefore the electrons in the copper-zinc cell will flow in the opposite direction, making the voltage negative.

So well done if you've got that answer.

C, describe how you can make a 1.

5 volt battery using these metals.

Well I would connect two cells, I would connect the chromium-copper cell, which is 1.

2, to a iron-tin cell, which is 0.

3 voltage.

So together 1.

2 and 0.

3 add up to 1.

5 and I'll connect them in series.

So well done if you've got that correct.

And here is our labelled diagram.

So it's important to show both of the cells, label all the electrodes, so the copper and the chromium, and then the tin and the iron, and then connect them.

Make sure you've got the electrolytes labelled as well.

So very well done if you've got all the labels right on diagram as well.

It doesn't matter which order the cells are put in when you're doing the diagram, the key thing is make sure they are connected.

So this brings us to the end of our lesson.

So let's have a look at our key learning points from today's lesson.

An electric cell contains a store of chemicals that react to produce a voltage across two electrical contacts.

It's made of two different metals in contact with an electrolyte.

The voltage is produced across the metals.

The size of the voltage across an electric cell depends on the materials it's made from.

A battery is made of one or more electric cells, which can be connected in series to produce a larger voltage.

The chemical reaction in a rechargeable battery is reversed when an external voltage is connected across it.

I hope that you have enjoyed today's lesson and I look forward to working with you again very soon.