Hello, my name's Dr.

George.

This lesson is called "Danger from Electric Shock" and it's part of the unit Mains Electricity.

The outcome for the lesson is I can describe mains electricity in the UK and the dangers of an electric shock.

These are the key words for this lesson.

I'll explain them as we go along, but you can come back to this slide anytime to remind yourself of the meanings.

I will just read one of them now though.

Main electricity is the electricity from electric sockets and circuits in a home.

This lesson has three parts.

They're called main electricity in the UK, dangers of an electric shock and reducing dangers of shock.

In the UK, mains electricity has a potential difference of 230 volts and it supplies alternating current with a frequency of 50 hertz.

It's often described as 230 volts AC.

An alternating current or AC moves repeatedly forwards and backwards in a circuit.

The direction of flow of charge keeps changing.

50 hertz means that the current moves forwards and backwards 50 times every second.

So this is different from DC, direct current, in which the current always flows in the same direction.

Inside any cable that connects a mains appliance to the socket, there are either two or three wires, usually three.

There will always be a live wire, which is coated in brown plastic and a neutral wire, which is coated in blue plastic.

The live wire and the neutral wire are both needed to make a complete circuit, so it may look as though there's just a single wire running to your appliance.

But inside there are these two wires that are making a complete loop, and because the wires are plastic coated, they don't make a direct electrical connection between them.

We can model mains AC using a loop of string or alternatively something like a plastic hula hoop.

This hand here represents the live connection.

It pushes and pulls the string from side to side.

And it switches direction because the live voltage changes between positive and negative.

This hand represents the neutral connection, allowing the string to move easily through.

And these hands here represent the main's appliance that we're running.

These hands grip and resist the movement of the string in a similar way to how the appliance has resistance resisting the flow of current.

And in this case, or in the case of many appliances, in fact, it causes heating.

Which of these are properties of main electricity in the UK? And each time I ask a question, I'll wait for five seconds, but if you need longer, just press pause and press play when you have your answer ready.

Mains electricity supplies alternating current and the frequency is 50 hertz.

The PD isn't 240 volts, it's 230.

Now to make current go backwards and forwards 50 times each second, the voltage of the live wire changes at the same rate as that.

And that voltage changes from an average of plus 230 volts to an average of minus 230 volts.

And it's doing that changing one way and back again 50 times each second.

The average voltage of the neutral wire meanwhile remains at zero volts.

When the voltage of the live wire is plus 230 volts, there's a potential difference of 230 volts across the circuit and an electric field is set up around the circuit that pushes current around everywhere in a clockwise direction.

So conventional current flows normally from positive to negative with a battery, but it will also flow from positive to zero to neutral.

And the electrons are moving the other way.

They're attracted to the positive terminal.

When the voltage of the live wire is minus 230 volts, they're still a potential difference of 230 volts around the circuit, but in the opposite direction.

An electric field is set up around the circuit in the opposite direction from before, and the current is pushed around everywhere in the circuit in an anticlockwise direction.

So it's switched as going the other way.

Now, which of the following statements describe a neutral wire? It has blue insulation, It's coated in blue plastic and its voltage stays about the same, about zero volts.

Now can you copy and complete this main electric circuit? So complete the labels, there's a missing voltage as the wires aren't labelled and colour the wires correctly.

Press pause when you do this and press play when you're ready to check your answer.

And here's the completed circuit, The neutral wire is the blue one, which connects to zero volt terminal, and the live wire is the brown one connecting to plus or minus 230 volts.

Now let's look at the dangers of an electric shock.

As we've already seen, mains electricity in the UK is 230 volts and this is a high enough potential difference to cause an electric shock.

And the larger the current, the more harmful an electric shock could be.

So we'll look at the kind of current that you can get from an electric shock.

So you should never mess about with mains electricity.

You might occasionally hear of someone who's had an electric shock and been okay, but they got lucky.

Electric shocks can kill.

Electric current passing through a person's body can damage living tissue.

Electric current causes a heating effect and large currents can cause burns, both on the outside and the inside of the body.

And current flowing through a person's heart can stop the heart.

It stops pumping blood around the body, and in that case, they need very urgent medical help or else they will die.

If you were to accidentally touch the terminals of a nine volt battery with a wet finger, you might experience a small electric shock, a slight tingle buzzing feeling, and there'll be a current of a few milliamps through your finger.

So that's in the region of a thousandth of an amp.

But a current bigger than about five milliamps would be painful.

And if we double that up to about 10 milliamps, that's about a hundredth of an amp.

That is enough current going through you to cause muscles to contract.

If this happened because you were touching something with your hand, your hand would be forced into a hard grip and you wouldn't be able to let go.

That's because muscles are controlled by electrical impulses and the electric shock would override your own conscious control of the muscles.

A current of over 0.

1 amps, 10th of an amp, could cause serious burns or a heart attack.

So to recap, what size of current is the smallest that would cause a painful electric shock? And the answer is 0.

005 amps.

Anything lower than that, you might feel it, but it wouldn't really hurt.

A person can get a shock from main's electricity if they make a connection between a live wire and a neutral wire.

So if you somehow are in contact with both of those.

You complete an electric circuit.

The potential difference between live and neutral is across you and it pushes a current through you.

Current through the arms can cause serious burns.

A person can get shock from main's electricity in another way if they make a connection between a live wire and the ground.

So if they touch the live wire and then if they're simply standing on the ground or standing on the floor in a building which is connected to the ground, the ground has a voltage of zero volts, which is the same as the neutral wire.

So this person has a potential difference across them between one hand and their feet.

And that potential difference between live and the ground will push a current through them.

If that current passes through their heart, which it certainly can do if it's passing from one hand to their feet, then that can cause a heart attack and may stop the heart.

So is this true or false? Touching just a live wire can cause an electric shock.

And explain your answer.

Press pause while you're thinking about that and press play when you're ready to check your answer.

It's true.

And the reason, the live wire has a voltage of plus or minus 230 volts, and if you're touching the ground which is at zero volts, there'll be a potential difference across your body that can push current through it.

Occasionally you'll see birds perched on live electricity transmission wires high up outdoors.

But if they're only touching the live wire and not also touching something that's at zero volts, there isn't a potential difference across them and they won't get a shock.

Now can you explain why you can get an electric shock if you touch just a live wire and also why you cannot get an electric shock if you touch just a neutral wire? Press pause when you write down your answers to these questions and press play when you're ready to check them.

I'll show you some example answers that contain the points that you should try to make.

The live wire has a voltage of plus or minus 230 volts.

If you are touching the ground at zero volts, there will be a potential difference across your body that can push current through it.

Now, why you can't get an electric shock if you touch just a neutral wire? The neutral wire has a voltage of zero volts.

If you're touching the ground, that also has a voltage of zero volts and there'll be no potential difference across your body to push current through it.

Well done if you made those points in your answers.

Now for the last part of this lesson, reducing dangers of shock.

It may seem odd to you that very small currents passing through your body can do so much damage.

The current that a 1.

5 volt battery can push through a small lamp is about 0.

5 amps, half an amp, and that's much bigger than a current that can cause serious burns to a person.

Fortunately, your skin has got a very high resistance.

A potential difference of 1.

5 volts across your skin would cause a current of less than 0.

0001 amp, which is too small to notice.

The size of the current from a shock can be calculated using this equation, which is the usual equation for relating the current through something, the potential difference across it and the resistance of it.

So here I is the current measured in amps, V is the potential difference in vaults, and R is a resistance measured in ohms and you need to use the potential difference in vaults and the resistance in ohms so that the answer comes out in amps.

The resistance we'll be using here is the resistance of skin, and the PD will be the mains voltage, which is always 230 volts in these equations.

I'll show you an example calculation and then there'll be one for you to try.

A child gets an electric shock from the mains.

Their skin has a resistance of 20 kilohms. Calculate the current through their body.

So we start by writing down the quantities that we know.

We know that main's potential difference is 230 volts and we're told the resistance.

We need to convert from kilohms to ohms so that we can use this in the equation.

20 kilohms is 20,000 ohms. Write down the equation that we're going to use.

I equals V divided by R.

Substitute in the values and we get 0.

0115 amps.

We should round that to two significant figures because we only know the resistance to two significant figures.

This is big enough to be painful and to affect the muscles.

It shouldn't be life-threatening though.

Now here's one for you to try.

Another child unfortunately gets an electric shock from the mains.

Their skin has a resistance of 40 kilohms. Calculate the current through their body.

Press pause while you write out your working and press play when you have the answer ready.

And here's the solution.

Write down the PD and the resistance, which is 40,000 ohms. Use I equals V divided by R.

Write your answer to two significant figures because we have the resistance to two significant figures and we get 0.

0058 amps, which is big enough to be just painful.

The size of current caused by an electric shock depends on the resistance of a person's skin, as we just saw in those two examples, the currents were different because the resistances were different.

And the resistance of the skin depends on the resistance of both the points of contact.

Usually the wetter a person's skin, the lower its resistance.

Water is a good conductor unless it's completely pure, which it almost never is.

Tap water is not pure and water on your skin is going to have a bit of sweat mixed in with it and it will be a good conductor.

So here are some approximate resistances.

Dry skin, 50 kilohms, 50,000 ohms, sweaty skin only two kilohms, 2000 ohms, moist skin, 300 ohms and skin after soaking in a bath, just 100 ohms. So there's a really wide range of resistances here.

Getting an electric shock when your skin is wet is much more dangerous than when it's dry, and that's why there should be no mains socket in a bathroom and no normal light switches either.

If there's a light switch in the bathroom, it's worked with a pull cord so that you don't need to put your hand near a switch.

Sockets for electric razors have special anti shock protection built in.

So why are standard mains sockets dangerous in a bathroom? The answer is B, because wet skin increases the current from a shock because it has lower resistance.

Having wet skin doesn't affect the voltage.

That's 230 volts, its mains voltage.

And now a written task for you.

For each of these four situations, I'd like you to calculate the current through the body and state which of these currents are dangerous to life.

So there are four calculations to do and then four descriptions to write.

Press pause when you do this and press play when you're ready to check your answers.

And here are the answers.

With dry skin, the current is only 0.

0046 amps, painful, but not dangerous to life.

Sweaty skin, 0.

12 amps, possibly dangerous to life.

It's over a 10th of an amp.

Moist skin, we have a current of 0.

77 amps, which is definitely dangerous to life and skin after soaking in a bath, 2.

3 amps.

Again, very dangerous to life.

We've now reached the end of the lesson, so I'll give you a summary.

Mains electricity has a PD of plus or minus 230 volts, supplied by an alternating current that has a frequency of 50 hertz and moves backwards and forwards 50 times each second.

A live wire in a main circuit is brown and has a voltage of plus or minus 230 volts.

A neutral wire in a main circuit is blue and has a voltage of zero volts.

Electric current passing through a person's body can damage living tissue and possibly stop the heart pumping blood around their body.

The size of current from a shock depends on the PD across the person and the resistance of their skin.

It can be calculated using I equals V divided by R.

Current is PD divided by resistance.

Wet skin has a much lower resistance than dry skin, which means in a bathroom, an electric shock would pass bigger currents through a person.

Well done for working through this lesson.

I hope you found it interesting and it's also important to know these things so that you're safe around mains electricity.

I hope to see you again in a future lesson.

Bye for now.