Pick one before you read on. It is fine to be wrong.
By the end you can
Explain why water and electricity are a dangerous mix
Explain how a fuse protects a home circuit
The electricity in a home socket is called the mains. It is very different from a torch cell. A cell gives 1.5 V. The mains gives about 110 to 240 V, depending on where you live.
Mains electricity can kill. A torch cell is safe to touch, but never touch the mains. This chapter is about staying safe and about the part that guards a home circuit, the fuse.
The current through your body depends on the voltage and on the resistance of your body. Current = voltage ÷ resistance. Dry skin has a high resistance. Wet skin has a much lower one.
Water with anything dissolved in it carries a current. Tap water, sweat and rain all have things dissolved in them. Water on your skin lowers the resistance of your body.
Worked example
Take the mains as 200 V. Where you live it may be nearer 110 V or 240 V. Dry skin has about 10 000 Ω and wet skin about 1 000 Ω. How much more current passes through wet skin?
Current = voltage ÷ resistance.
Dry skin: 200 ÷ 10 000 = 0.02 A.
Wet skin: 200 ÷ 1 000 = 0.2 A.
Wet skin against dry skin: 0.2 ÷ 0.02 = 10 times as much current.
Answer: 10 times as much current
A made up sum. A milliamp, mA, is 0.001 A, so 20 mA is 0.02 A. Real skin varies, but wet skin always lets far more current through.
A current of 0.02 A can lock your muscles, so you cannot let go. More than about 0.05 A through the body can kill. With wet skin the current in the sum is far above both. Even with dry hands the mains can kill. At mains voltage the resistance of the skin breaks down.
So never touch a switch, a plug or an appliance with wet hands. Never use electric things near water.
Now the fuse. Many things can send too much current round a circuit. Too many appliances on one socket do it. A fault does it too. Too much current makes wires hot, and hot wires can start a fire.
A fuse is a thin wire in the live wire of the circuit. It is made to melt when the current is too big. The wire melts and breaks the circuit before the other wires get hot.
The fuse is in the live wire. All the current for the heater goes through it.
Draw it first
Draw a home circuit and a blown fuse
1 working: the fuse is whole. The lamp is on.
2 blown: the fuse wire has melted. The circuit is broken, and the lamp is off.
3 switched off: the fuse is whole. The switch is open, so the lamp is off.
Appliances on one circuit are in parallel. Each takes its own current, and the fuse carries the total. Look at a socket circuit with a heater, a kettle and a lamp.
A model of a socket circuit with a 3 A fuse. Two cells stand in for the mains, and the currents are small model ones. The fuse carries the current of all three.
Worked example
A 3 A fuse guards the circuit. The heater takes 1.2 A, the kettle 1 A and the lamp 0.6 A. Ravi plugs in a fourth appliance that takes 0.6 A. What is the new total, and does the fuse blow?
In parallel, the currents add up.
Before: 1.2 + 1 + 0.6 = 2.8 A.
With the fourth: 2.8 + 0.6 = 3.4 A.
The fuse is 3 A. 3.4 A is more than 3 A, so the fuse wire melts.
Answer: 3.4 A, so the fuse blows
A circuit breaker does the same job as a fuse. It is a switch that flips off when the current is too big. You can switch it back on once the fault is fixed. A fuse wire must be replaced.
See it three ways
As a picture
In words
Keep water away from electric things. Keep your hands dry. Tell an adult about a damaged wire or a hot plug. Never touch a power line that has fallen.
In symbols
Wet skin: resistance low, current large. Fuse: current too large, wire melts, circuit breaks.
Try it
Survey the places water and electricity meet
You need: paper and a pencil, a walk round your home, looking only.
Be safe: This activity uses no electricity. Look from a safe distance and do not touch plugs, sockets, switches or wires. If you see a damaged wire, tell an adult.
Walk round your home with the paper. Do not touch any plug, switch or wire.
Look for each place where water and electricity are close. Think of the sink, the bathroom, a tap near a socket, and a cable outside in the rain.
Draw each place. Mark where the water is and where the electric thing is. Keep to a safe distance as you draw.
For each place write one habit that keeps them apart. Tell an adult about any damaged wire.
What you will see: Most homes have several places where water and electricity are close. A habit, such as drying your hands or keeping plugs off the wet floor, keeps each one safe. Your places will differ from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
It is safe to touch a switch with wet hands if the water looks clean.
Why it feels right: Clean water looks harmless, and the water does not change the switch.
The test that breaks it: Do not test it with electricity. Use the sum instead. With dry skin, 0.02 A passes. With wet skin, 0.2 A passes, which is 10 times as much. Dry hands are not safe on the mains.
What is true: Water on your skin lowers its resistance. Even clean-looking water has things dissolved in it. Dry your hands before you touch any electric thing.
Key words
mains
the electricity supply to a home. It can kill.
fuse
a thin wire that melts when too much current flows.
circuit breaker
a switch that flips off when too much current flows. It can be reset.
Back to your guess
The answer is C: Water with things in it carries current, and wet skin lets far more current through.
Water with anything dissolved in it carries a current. Wet skin lets far more current through the body. The mains is not stronger, but the body lets more current pass.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Why is it dangerous to touch a switch with wet hands?
See the answer
Wet skin lets much more current pass through the body.
Water on the skin lowers its resistance. With the same voltage, a lower resistance means a bigger current.
If you chose B: The voltage of the mains stays the same. It is the resistance of your body that falls when your skin is wet. Go to S3.10.4.
If you chose C: Pure water is a poor conductor. The water we use has things dissolved in it, and they carry the current. Go to S3.10.8.
What does a fuse do when too much current flows?
See the answer
Its thin wire melts and breaks the circuit.
The fuse wire is thin, so it gets hot first and melts. The circuit is broken before the other wires get dangerously hot.
If you chose A: A fuse cannot make the current smaller. Its wire melts and stops the current. Go to S3.10.8.
If you chose C: A melted fuse wire stays broken. It must be replaced. A circuit breaker is the part you can switch back on. Go to S3.10.8.
A socket circuit has a 5 A fuse. A heater takes 3 A and a kettle takes 4 A. Both are plugged in. What current flows in all, and what happens to the fuse?
See the answer
7 A, so the fuse blows
The currents add up: 3 + 4 = 7 A. That is more than 5 A, so the fuse melts.
If you chose A: You took the larger current only. In parallel the currents add, so both appliances count. Go to S3.10.3.
If you chose B: You multiplied the currents. In parallel the currents add up. Add them. Go to S3.10.3.
Mei is flying a kite. The string tangles in an overhead power line. What should she do?
See the answer
Let go, move far away and tell an adult.
A power line is at a very high voltage. Never touch it, the string, or anything that touches it. Keep away and tell an adult.
If you chose B: The string may carry electricity from the line. Never touch it. Let go and move away. Go to S3.10.8.
If you chose C: Never go near a power line. The current can jump and can kill. Leave the kite and tell an adult. Go to S3.10.8.
Joseph's 5 A fuse keeps blowing when he runs the heater and the kettle on one socket. He wants to put a thick wire where the fuse goes, so it never blows. Is that a good idea?
See the answer
No. Use one appliance at a time. Ask an adult to fit a new fuse of the same size.
The fuse blows because the heater and kettle together take too much current. One appliance at a time cuts the current. A new fuse must be the same size, so it still melts before the wires get hot. A thick wire would let the wires overheat.
If you chose A: The fuse is thin on purpose. With a thick wire nothing melts. The wires in the walls can get hot and start a fire. Go to S3.10.8.
If you chose C: The other wires are made for a smaller current than the heater and kettle take together. Without the fuse they overheat. Go to S3.10.8.
From an earlier chapter
Review: a 8 V supply is joined to a resistor of 20 Ω. What is the current?
See the answer
0.4 A
Current = voltage ÷ resistance = 8 ÷ 20 = 0.4 A.
If you chose A: You multiplied the voltage by the resistance. Divide the voltage by the resistance. Go to S3.10.4.
If you chose B: You added the voltage and the resistance. Divide the voltage by the resistance. Go to S3.10.4.
A heater takes 1 A and a kettle takes 5 A. They are on one circuit in parallel. What total current flows through the fuse?
See the answer
6 A
The currents add up in parallel. 1 + 5 = 6 A.
Keep it
Copy this into your exercise book.
Mains can kill. Wet skin lets far more current through, so keep hands dry. A fuse is a thin wire that melts when the current is too big. A circuit breaker can be reset.
Draw: A home circuit with a supply, a fuse, a switch and a lamp. Mark the fuse wire.
