The mains supply, plugs and static electricity
By the end of this chapter you will be able to
- Explain the difference between AC and DC, and give the mains voltage and frequency
- Name the three wires in a UK plug and say what each does
- Calculate the current from the power and pd, and choose a fuse
- Calculate the energy used in kWh and the cost of using an appliance
- Explain how friction charges an object, and why objects attract or repel
You should already know
- Power and energy in circuits (Chapter 7)
- Current and charge (Chapter 5)
Where this leads
- Electromagnetic induction and transformers (Chapter 20)
- Energy resources and the electricity supply (Chapter 4)
GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the mains supply (about 230 V, 50 Hz AC) and cell supplies (DC), Identify the live, neutral and earth wires in a UK plug, and explain the purpose of fuses, circuit breakers and double insulation, Choose a suitable fuse rating from the power and the supply voltage, Calculate the energy in kilowatt-hours and the cost of electricity, Explain static electricity: charging by friction, the movement of electrons, attraction and repulsion, and earthing.
Exercise P Do you remember?
Method. Use I = P / V.
- Divide. I = 230 / 230 = 1 A.230 / 230 = 1
- Write the answer. 1 A
Check. 1 x 230 = 230 W. ✓
Method. Multiply the numbers.
- Multiply. 3 x 2 = 6, then 6 x 1000 = 6000.6000
- Write the answer. 6000
Check. 6000 / 1000 = 6. ✓
Method. Recall which charged particles move in solids.
- Recall. Electrons are the particles that move from one object to another.electrons
- Write the answer. electrons
Check. ✓
Method. Recall the unit.
- Recall. Energy is measured in joules, J.joule (J)
- Write the answer. joule
Check. ✓
Method. Divide.
- Divide. 6 / 0.2 = 60 / 2 = 30.6 / 0.2 = 30
- Write the answer. 30
Check. 30 x 0.2 = 6. ✓
The mains supply and direct current
In direct current (DC) the charge flows in one direction all the time, as from a cell or a battery. In alternating current (AC) the direction of the current reverses many times each second. The UK mains supply is AC at about 230 V with a frequency of 50 Hz, which means the current changes direction 100 times each second.
Key ideas
- DC: the current flows in one direction, from a cell or battery.
- AC: the current changes direction; the UK mains is about 230 V at 50 Hz.
- Frequency of 50 Hz means 50 complete cycles each second.
- Use P = V x I for both AC and DC when you use the rms (average) values.
Method. Each complete cycle changes direction twice.
- Cycles. There are 50 cycles each second.50 cycles
- Changes. Each cycle has two changes of direction, so 50 x 2 = 100 changes.50 x 2 = 100
- Write the answer. 100
Check. The frequency 50 Hz gives 100 direction changes each second. ✓
Method. Use I = P / V.
- Take the numbers. P = 2300 W and V = 230 V.P = 2300, V = 230
- Divide. I = 2300 / 230 = 10 A.2300 / 230 = 10
- Write the answer. 10 A
Check. 10 x 230 = 2300 W. ✓
Method. Recall the difference between DC and AC.
- Look. A cell gives direct current, which flows in one direction.DC
- Decide. Mains gives alternating current, which changes direction.AC
- Write the answer. cell
Check. ✓
Watch out
Watch out: 50 Hz means 50 complete cycles each second, which is 100 changes of direction. Do not write 50 changes.
Exam tip
Practical safety. Mains electricity is dangerous. Never touch sockets, wires or appliances with wet hands, and never open a plug.
Exercise 8A AC, DC and the mains
Method. Recall the type of supply.
- Recall. A cell gives a steady current in one direction, which is DC.DC
- Write the answer. DC
Check. ✓
Method. Recall the type of supply.
- Recall. The mains current reverses direction, which is AC.AC
- Write the answer. AC
Check. ✓
Method. Recall the mains voltage in the UK.
- Recall. The UK mains supply is about 230 V.230 V
- Write the answer. 230 V
Check. ✓
Method. Recall the frequency.
- Recall. The UK mains frequency is 50 Hz.50 Hz
- Write the answer. 50 Hz
Check. ✓
Method. Use 50 cycles each second, with two changes per cycle.
- Double. Two changes per cycle, so 50 x 2 = 100.100
- Write the answer. 100
Check. ✓
Method. Use I = P / V.
- Divide. I = 1150 / 230 = 5 A.1150 / 230 = 5
- Write the answer. 5 A
Check. 5 x 230 = 1150 W. ✓
Method. Use I = P / V.
- Divide. I = 100 / 230 = 0.4348... A, so 0.43 A to 2 decimal places.100 / 230 = 0.43
- Write the answer. 0.43 A
Check. 0.43 x 230 = 98.9 W, close to 100 W. ✓
Method. Recall the name.
- Recall. Steady current in one direction is direct current, DC.DC
- Write the answer. DC
Check. ✓
Method. Think about the direction of the current.
- Direction. The charge flows the same way all the time.one way
- Write the answer. Accept any sentence that says the current flows in one direction only.
Check. ✓
Method. Think about how the mains is produced.
- Source. The mains is generated by a generator that makes the current reverse.reverses
- Write the answer. Accept any sentence that says the current changes direction because the supply is generated that way.
Check. ✓
Method. Recall the mains voltage.
- Recall. The UK mains is about 230 V, which is much higher than 50 V.230 V
- Write the answer. Accept any answer that says the UK mains is about 230 V.
Check. ✓
Method. Use I = P / V.
- Divide. I = 2000 / 230 = 8.69 A, so 8.7 A to 1 decimal place.2000 / 230 = 8.7
- Write the answer. 8.7 A
Check. 8.7 x 230 = 2001 W, close to 2000 W. ✓
Method. Think about the supply in each case.
- Look. The torch uses a cell (DC); the fridge uses the mains (AC).torch
- Write the answer. torch
Check. ✓
Plugs, fuses and double insulation
The UK plug has three pins and three wires. The live wire (brown) carries the alternating pd of about 230 V. The neutral wire (blue) completes the circuit and is at about 0 V. The earth wire (green and yellow) is connected to the metal case of the appliance. If a live wire touches the case, the earth wire carries a large current, which blows the fuse and disconnects the supply.
A fuse is a thin wire that melts if the current is too big, which breaks the circuit. Its rating should be the next size above the normal current. UK plug fuses are 3 A, 5 A or 13 A. A circuit breaker does the same job by switching off, and can be reset.
Double insulation means an appliance has two layers of insulation, so the outer case cannot become live and no earth wire is needed. These appliances carry the double insulation symbol, and have only a live and a neutral wire.
| Appliance | Power | Current (P / V) | Fuse chosen |
|---|---|---|---|
| 60 W lamp | 60 W | 0.26 A | 3 A |
| 700 W toaster | 700 W | 3.04 A | 5 A |
| 2300 W kettle | 2300 W | 10 A | 13 A |
Key ideas
- Live (brown): the alternating pd of about 230 V.
- Neutral (blue): completes the circuit, about 0 V.
- Earth (green and yellow): a safety wire to the metal case.
- The fuse is in the live wire. Choose the next fuse rating above the normal current.
Method. Find the current, then choose the next fuse above it.
- Current. I = 700 / 230 = 3.04 A.3.04 A
- Fuse. The next fuse above 3.04 A is 5 A.5 A
- Write the answer. 5 A
Check. 3 A is too small; 5 A is the next size up. ✓
Method. Find the current, then choose the next fuse above it.
- Current. I = 2300 / 230 = 10 A.10 A
- Fuse. The next fuse above 10 A is 13 A.13 A
- Write the answer. 13 A
Check. A 5 A fuse would blow at once. ✓
Method. Think about the safety wire.
- Look. The wire that protects the user is connected to the case.safety
- Decide. That is the earth wire, green and yellow.green and yellow
- Write the answer. green and yellow
Check. ✓
Watch out
Watch out: the fuse goes in the live wire. A fuse rated too high (for example, 13 A for a 60 W lamp) does not protect the wiring and the appliance.
Exam tip
Exam tip. When choosing a fuse, calculate the current first, then pick the next fuse rating up. Never choose a fuse much bigger than the current.
Exercise 8B Plugs, fuses and safety
Method. Recall the colours.
- Recall. The brown wire is live.live
- Write the answer. live
Check. ✓
Method. Recall the colours.
- Recall. The blue wire is neutral.neutral
- Write the answer. neutral
Check. ✓
Method. Recall the colours.
- Recall. The green and yellow wire is earth.earth
- Write the answer. earth
Check. ✓
Method. Think about the wire that can cause a shock.
- Recall. The fuse is placed in the live wire.live
- Write the answer. live
Check. ✓
Method. Recall the standard fuse ratings.
- Recall. The standard UK plug fuses are 3 A, 5 A and 13 A.3 A
- Write the answer. 3 A
Check. ✓
Method. Find the current, then choose the next fuse above it.
- Current. I = 60 / 230 = 0.26 A.0.26 A
- Fuse. The next fuse above 0.26 A is 3 A.3 A
- Write the answer. 3 A
Check. ✓
Method. I = 1150 / 230 = 5 A; the next fuse above 5 A is 5 A.
- Current. I = 1150 / 230 = 5 A.5 A
- Fuse. A 5 A fuse is equal to the current, so choose 5 A.5 A
- Write the answer. 5 A
Check. The fuse should be the next size up or equal, so 5 A. ✓
Method. Think about a fault where the live wire touches the case.
- Fault. If the live wire touches the case, the earth carries a large current.large current
- Effect. This blows the fuse, so the case is not left live.blows fuse
- Write the answer. Accept any sentence that says it carries a large current away and blows the fuse, so the case is not left live.
Check. ✓
Method. Think about the insulation.
- Insulation. It has two layers of insulation between the live wire and the outside.two layers
- Result. So the outside cannot become live.not live
- Write the answer. Accept any sentence that says it has two layers of insulation, so the outside cannot become live.
Check. ✓
Method. Think about how a breaker works.
- Recall. A circuit breaker switches off, and can be reset; a fuse must be replaced.reset
- Write the answer. reset
Check. ✓
Method. Compare the fuse with the current.
- Compare. A 13 A fuse is much bigger than the 2 A current, so it will not blow in a fault at 2 A.13 > 2
- Write the answer. yes
Check. ✓
Method. Use I = P / V.
- Divide. I = 1500 / 230 = 6.52 A.1500 / 230 = 6.52
- Write the answer. 6.52 A
Check. ✓
The cost of electricity
The electricity meter counts energy in kilowatt-hours. One kilowatt-hour (1 kWh) is the energy used by a 1 kW appliance running for 1 hour. So energy in kWh = power in kW x time in hours. The cost is the number of kWh times the price per kWh.
energy (kWh) = power (kW) x time (h)
cost = energy (kWh) x price per kWh
One kilowatt-hour is 3.6 million joules. The prices in this section are examples only; real prices vary.
Key ideas
- Energy in kWh = power in kW x time in hours.
- Convert watts to kilowatts by dividing by 1000.
- Convert minutes to hours by dividing by 60.
- Cost = kWh x price per kWh. Example price here: 30p per kWh.
Method. Use energy = power x time.
- Take the numbers. Power = 2 kW and time = 3 h.P = 2 kW, t = 3 h
- Multiply. Energy = 2 x 3 = 6 kWh.2 x 3 = 6
- Write the answer. 6 kWh
Check. 6 kWh / 3 h = 2 kW. ✓
Method. Multiply the energy by the price.
- Take the numbers. Energy = 6 kWh and price = 30p per kWh.6 kWh, 30p
- Multiply. Cost = 6 x 30 = 180p.6 x 30 = 180
- Write the answer. 180p
Check. 180p = £1.80. ✓
Method. Convert watts to kilowatts, then multiply by the time.
- Convert. 100 W = 0.1 kW.0.1 kW
- Multiply. Energy = 0.1 x 10 = 1 kWh.0.1 x 10 = 1
- Write the answer. 1 kWh
Check. 1 kWh = 3.6 MJ, which is 3 600 000 J. ✓
Watch out
Watch out: a kilowatt-hour is an energy unit, not a power unit. Use kW for power and kWh for energy.
Exam tip
Exam tip. Write the energy equation first. Convert the power to kW and the time to hours before you multiply.
Exercise 8C Energy in kWh and cost
Method. Use energy = power x time.
- Multiply. Energy = 1 x 2 = 2 kWh.1 x 2 = 2
- Write the answer. 2 kWh
Check. ✓
Method. Convert 30 minutes to 0.5 hours.
- Convert. 30 minutes = 0.5 h.0.5 h
- Multiply. Energy = 2 x 0.5 = 1 kWh.2 x 0.5 = 1
- Write the answer. 1 kWh
Check. ✓
Method. Convert 1500 W to 1.5 kW.
- Convert. 1500 W = 1.5 kW.1.5 kW
- Multiply. Energy = 1.5 x 4 = 6 kWh.1.5 x 4 = 6
- Write the answer. 6 kWh
Check. ✓
Method. Convert 60 W to 0.06 kW.
- Convert. 60 W = 0.06 kW.0.06 kW
- Multiply. Energy = 0.06 x 10 = 0.6 kWh.0.06 x 10 = 0.6
- Write the answer. 0.6 kWh
Check. ✓
Method. Multiply the energy by the price.
- Multiply. Cost = 4 x 30 = 120p.4 x 30 = 120
- Write the answer. 120p
Check. 120p = £1.20. ✓
Method. Find the energy first, then the cost.
- Energy. Energy = 2 x 5 = 10 kWh.10 kWh
- Cost. Cost = 10 x 30 = 300p.10 x 30 = 300
- Write the answer. 300p
Check. 300p = £3.00. ✓
Method. The lamp uses 1 kWh.
- Energy. Energy = 0.1 x 10 = 1 kWh.1 kWh
- Cost. Cost = 1 x 30 = 30p.30
- Write the answer. 30p
Check. ✓
Method. Compare the energy in 1 hour.
- Kettle. Energy = 2 kW x 1 h = 2 kWh.2 kWh
- Lamp. Energy = 0.1 kW x 1 h = 0.1 kWh.0.1 kWh
- Write the answer. kettle
Check. ✓
Method. One kWh is 1 kW for 1 hour.
- Convert. 1 kW = 1000 W, and 1 hour = 3600 s, so 1000 x 3600 = 3600000 J.1000 x 3600 = 3600000
- Write the answer. 3600000 J
Check. 3.6 million joules. ✓
Method. Convert 2 h 30 min to 2.5 h.
- Convert. 2 h 30 min = 2.5 h.2.5 h
- Multiply. Energy = 2.5 x 2.5 = 6.25 kWh.2.5 x 2.5 = 6.25
- Write the answer. 6.25 kWh
Check. ✓
Method. Look at the unit.
- Unit. The kW is power and the h is time.kW x h
- Result. Power multiplied by time gives energy.energy
- Write the answer. Accept any sentence that says it is power multiplied by time, so it measures energy.
Check. ✓
Method. Multiply by 7 days.
- Multiply. Energy = 12 x 7 = 84 kWh.12 x 7 = 84
- Write the answer. 84 kWh
Check. 84 / 7 = 12. ✓
Method. Power is proportional to cost for the same time.
- Ratio. 2 kW / 1 kW = 2.2 / 1 = 2
- Write the answer. 2
Check. ✓
Method. Think about the equation.
- Equation. Energy = power x time, so less power or less time gives less energy.less energy
- Cost. Less energy means a lower cost.lower cost
- Write the answer. Accept any sentence that says use appliances with lower power, or use them for less time.
Check. ✓
Static electricity
When two different insulating materials are rubbed together, electrons can move from one to the other. The material that gains electrons becomes negatively charged; the material that loses electrons becomes positively charged. This is static electricity: the charge stays on the object rather than flowing in a circuit.
Like charges repel (push apart), and opposite charges attract (pull together). A charged object also attracts an uncharged one, because it induces an opposite charge in it.
A charged object can be earthed: the charge flows to the Earth through a conductor, so the object becomes neutral. A large build-up of charge can cause a spark.
| Material rubbed | Charge on the material | Charge on the other material |
|---|---|---|
| a balloon rubbed on wool or hair | negative (gains electrons) | positive (loses electrons) |
| a glass rod rubbed with silk | positive (loses electrons) | negative (gains electrons) |
| a plastic comb through hair | negative (gains electrons) | positive (loses electrons) |
Key ideas
- Friction moves electrons from one material to the other.
- A material that gains electrons is negative; one that loses electrons is positive.
- Like charges repel; opposite charges attract.
- Earthing lets the charge flow away, so the object becomes neutral.
Method. Think about which particles move in friction.
- Look. Electrons are the particles that move from the wool to the balloon.electrons
- Decide. The balloon gained electrons, so it is negative.electrons
- Write the answer. electrons
Check. Protons are fixed in the nucleus and do not move. ✓
Method. Think about like charges.
- Look. Both rods have the same charge: negative.like charges
- Decide. Like charges repel.repel
- Write the answer. repel
Check. Opposite charges would attract. ✓
Method. Think about the paper.
- Induce. The balloon’s negative charge pulls positive charge towards the near side of the paper.induced positive
- Attract. The induced positive side is attracted to the balloon.attract
- Write the answer. Accept any sentence that says the balloon induces a positive charge in the paper, so they attract.
Check. ✓
Watch out
Watch out: only electrons move in static charging. Protons do not move, so do not say that protons are gained or lost.
Exam tip
Practical safety. Use a balloon and small paper pieces, with a grown-up or teacher. Do not use mains or any charged object near eyes or electrical equipment.
Exercise 8D Static charge, attraction and earthing
Method. Recall the particles.
- Recall. Electrons move, from one material to the other.electrons
- Write the answer. electrons
Check. ✓
Method. Electrons are negative.
- Recall. Gaining electrons gives the material a negative charge.negative
- Write the answer. negative
Check. ✓
Method. Losing electrons leaves more protons than electrons.
- Recall. Losing electrons gives the material a positive charge.positive
- Write the answer. positive
Check. ✓
Method. Like charges repel.
- Recall. Two positive charges repel each other.repel
- Write the answer. repel
Check. ✓
Method. Opposite charges attract.
- Recall. A positive and a negative charge attract each other.attract
- Write the answer. attract
Check. ✓
Method. Recall the word.
- Recall. Earthing lets the charge flow to the Earth, so the object becomes neutral.earthing
- Write the answer. earthing
Check. ✓
Method. Gaining electrons makes an object negative.
- Recall. The balloon gained electrons, so it is negative.negative
- Write the answer. negative
Check. ✓
Method. Losing electrons makes an object positive.
- Recall. The rod lost electrons, so it is positive.positive
- Write the answer. positive
Check. ✓
Method. Think about the wall.
- Induce. The charged balloon induces an opposite charge in the wall surface.induced
- Attract. Opposite charges attract, so the balloon sticks.attract
- Write the answer. Accept any sentence that says the balloon induces an opposite charge in the wall, so they attract.
Check. ✓
Method. Think about a large build-up of charge.
- Build-up. Charge builds up on the object.build-up
- Spark. When the charge is large enough it jumps across the gap as a spark.spark
- Write the answer. Accept any answer that says the charge builds up until it jumps across the gap as a spark.
Check. ✓
Method. Think about the induced charge.
- Induce. The negative rod pulls the positive charges in the paper closer to it.positive side
- Attract. The induced positive side attracts the rod more than the negative side repels it.attracts
- Write the answer. Accept any answer that says the rod induces a positive charge on the near side of the paper.
Check. ✓
Method. Opposite charges attract.
- Predict. A positive and a negative rod have opposite charges, so they attract.attract
- Write the answer. attract
Check. ✓
Method. Think about where the protons are.
- Protons. Protons are fixed in the nucleus of the atom.fixed
- Electrons. Only electrons move in static charging.electrons move
- Write the answer. Accept any sentence that says the protons are fixed in the nucleus and electrons are the particles that move.
Check. ✓
Chapter summary
| Skill | Method |
|---|---|
| Identify the mains | About 230 V, 50 Hz, alternating; a cell gives DC |
| Plug wires | Live brown (fuse), neutral blue, earth green and yellow |
| Choose a fuse | I = P / V; the next fuse above: 3 A, 5 A or 13 A |
| Energy and cost | kWh = kW x h; cost = kWh x price |
| Static electricity | Friction moves electrons; like charges repel, opposite charges attract |
Can you…?
- I can name the three wires in a UK plug and say what each does
- I can choose a fuse from the power and voltage
- I can calculate energy in kWh and the cost
- I can explain how friction charges an object
- I can explain attraction and repulsion of charged objects
Key terms
- live wire
- the wire at about 230 V, usually brown, which contains the fuse
- neutral wire
- the wire at about 0 V that completes the circuit, usually blue
- earth wire
- a safety wire, green and yellow, connected to the metal case
- fuse
- a thin wire that melts if the current is too large
- kilowatt-hour (kWh)
- the energy used by a 1 kW appliance in 1 hour
- static electricity
- a charge that builds up on an object and does not flow in a circuit
- earthing
- letting a charge flow to the Earth so the object becomes neutral
Exercise 8Z Check-up
Method. Recall.
- Recall. The UK mains is about 230 V.230 V
- Write the answer. 230 V
Check. ✓
Method. Recall.
- Recall. The fuse is in the live wire.live
- Write the answer. live
Check. ✓
Method. Use I = P / V.
- Divide. I = 920 / 230 = 4 A.920 / 230 = 4
- Write the answer. 4 A
Check. ✓
Method. Choose the next fuse above the current.
- Choose. The next fuse above 4 A is 5 A.5 A
- Write the answer. 5 A
Check. ✓
Method. Use energy = power x time.
- Multiply. Energy = 3 x 2 = 6 kWh.3 x 2 = 6
- Write the answer. 6 kWh
Check. ✓
Method. Multiply by the price.
- Multiply. Cost = 6 x 30 = 180p.6 x 30 = 180
- Write the answer. 180p
Check. ✓
Method. Recall.
- Recall. The UK mains frequency is 50 Hz.50 Hz
- Write the answer. 50 Hz
Check. ✓
Method. Recall.
- Recall. Electrons are the particles that move.electrons
- Write the answer. electrons
Check. ✓
Method. Like charges repel.
- Recall. Two negative charges repel.repel
- Write the answer. repel
Check. ✓
Method. Think about the insulation.
- Insulation. Two layers of insulation keep the case from becoming live.two layers
- Write the answer. Accept any sentence that says it has two layers of insulation so the outside cannot become live.
Check. ✓
Total 31 marks. Allow about 31 minutes.
Exercise 8X Exam-style questions
Method. Recall the safety wire.
- Recall. The earth wire is green and yellow and is connected to the case.green and yellow
- Write the answer. earth, green and yellow
Check. ✓
Method. Use energy = power x time.
- Multiply. Energy = 1.2 x 3 = 3.6 kWh.1.2 x 3 = 3.6
- Write the answer. 3.6 kWh
Check. ✓
Method. Use I = P / V, then choose the next fuse above the current.
- Current. I = 1150 / 230 = 5 A.5 A
- Fuse. The fuse must be at least 5 A, so the 5 A fuse is chosen.5 A fuse
- Write the answer. 5 A
Check. ✓
Method. Find the daily energy, then multiply by 7.
- Daily. 15 minutes = 0.25 h, so energy per day = 2 x 0.25 = 0.5 kWh.0.5 kWh
- Week. Energy in a week = 0.5 x 7 = 3.5 kWh.0.5 x 7 = 3.5
- Write the answer. 3.5 kWh
Check. ✓
Method. Describe the charging and the attraction.
- Charging. Friction moves electrons from the jumper to the balloon.electrons move
- Charge. The balloon gains electrons, so it is negative.negative
- Induce. Near the wall, the balloon induces a positive charge in the surface.induced
- Attract. Opposite charges attract, so the balloon sticks to the wall.attract
- Write the answer. Accept any answer that says electrons move from the wall or jumper to the balloon, so the balloon is negative and induces an opposite charge in the wall, so they attract.
Check. ✓
Method. Convert each to kW, then add the energies.
- Heater. Energy = 3 x 1 = 3 kWh.3 kWh
- Fan and lamp. Fan = 1 kWh and lamp = 0.1 kWh.1 + 0.1
- Total. Total = 3 + 1 + 0.1 = 4.1 kWh.3 + 1 + 0.1 = 4.1
- Write the answer. 4.1 kWh
Check. ✓
Method. Think about what a fuse must do.
- Purpose. A fuse protects the wiring and appliance from too much current.protects
- Too big. A fuse that is too big allows a larger current before it melts.too big
- Risk. So the wiring could overheat before the fuse melts.overheat
- Write the answer. Accept any answer that says a fuse that is too big does not melt quickly enough, so the wires or appliance could overheat before it protects them.
Check. ✓
Method. Work out both charges.
- Charges. Rubbing glass with silk makes the glass lose electrons, so it is positive.positive
- Result. Two positive charges repel.repel
- Write the answer. Accept any answer that says both rods are positive, so they repel.
Check. ✓
Method. Convert minutes to hours for kWh, then multiply by 3600 for joules.
- Hours. 3 minutes = 0.05 h.0.05 h
- kWh. Energy = 2.4 x 0.05 = 0.12 kWh.0.12 kWh
- Joules. Energy in J = 2400 W x 180 s = 432000 J.432000 J
- Write the answer. 0.12, 432000
Check. 0.12 kWh is 432000 J, since 1 kWh = 3600000 J. ✓
Method. Follow the fault in order.
- Fault. The live wire comes loose and touches the metal case.live touches case
- Earth. The earth wire is connected to the case and gives a low-resistance path.earth path
- Current. A large current flows through the earth wire and the live wire.large current
- Fuse. The fuse melts and breaks the circuit, cutting off the supply.fuse blows
- Safe. The case is no longer live, so the user is protected.safe
- Write the answer. Accept any answer that says the live wire touches the case, the earth wire carries a large current, and the fuse blows to cut off the supply.
Check. ✓