Starter quiz
- Which of the following are correct equations that include power?
- ✓
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- ✓
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- The potential difference across a lamp is 230 V and the current through the lamp is 0.20 A. The energy transferred by the lamp in one second is ______ J.
- '46' ✓
- Which of the following equations correctly relates current, resistance and potential difference for an electrical component?
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- ✓
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- All electrical appliances, such as laptops and vacuum cleaners, transfer energy into the surroundings by heating. This process is called …
- disintegration.
- disorganisation.
- disruption.
- dissipation. ✓
- distillation.
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- Which of the following statements about efficiency are correct?
- It can be measured in joules.
- It can be measured in watts.
- Efficiency = useful output power ÷ total input power ✓
- Efficiency = total input energy transfer ÷ useful output energy transfer
- It is the fraction of energy supplied that is usefully transferred. ✓
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- Which of the following statements describes the generator effect?
- A p.d. is induced across a conductor when the current through it changes.
- A p.d. is induced across a conductor when the magnetic field around it changes. ✓
- A magnetic field is induced around a conductor when the current in it changes.
- A magnetic field is induced around a conductor when the p.d. across it changes.
- A current-carrying conductor experiences a force when it is in a magnetic field.
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Exit quiz
- Starting with the power station, sort the following parts of the National Grid in the order in which they are connected together.
- 1⇔power station
- 2⇔step-up transformer
- 3⇔high-voltage transmission lines
- 4⇔step-down transformer(s)
- 5⇔factories and homes
- A 400 kV transmission line delivers 400 MW of power. The current through the transmission line is ______ A.
- '1000' ✓
- A transmission line has a resistance of 3.0 Ω per 100 km and a current of 1000 A through it. How much energy is dissipated by 250 km of this transmission line each second?
- 1.3 kW
- 3.0 kW
- 7.5 kW
- 3.0 MW
- 7.5 MW ✓
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- A transformer, with 100% efficiency, increases voltage from 10 kV to 40 kV. If the current through the primary coil is 60 A, the current through the secondary coil is ______ A.
- '15' ✓
- Which of the following statements about the current and voltage of transmission lines are correct?
- High power requires high voltage because power depends only on voltage.
- If transmission lines used 230 V to deliver a very high power, they could melt. ✓
- Rate of energy dissipation in electrical cables by heating depends on current. ✓
- Using a higher voltage allows more power to be delivered using less current. ✓
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- A transformer in a mobile phone charger decreases voltage from 230 V to 10 V. The current in the secondary coil is 2.0 A. If the transformer is 100% efficient, what is the current in the primary coil?
- 0.023 A
- 0.087 A ✓
- 1.6 A
- 12 A
- 46 A
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Worksheet
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Presentation
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Video
Lesson Details
Key learning points
- Power is transferred by electricity from power stations to consumers using the National Grid.
- Step-up transformers increase the transmission voltage from a power station to the transmission line.
- Step-down transformers decrease the transmission voltage from a transmission line to the consumer.
- Transmission lines dissipate some energy by heating (P = I²R).
- A very high transmission voltage reduces transmission current and the heating of transmission lines.
Common misconception
The voltage (drop) across power lines is the same as the voltage they transmit.
Spend time to clarify that the power transmitted depends on the p.d. between the live and neutral wires, and power dissipated depends on the drop in voltage from one end of a live wire to the other.
Keywords
National grid - a high voltage transmission system for getting electricity to where it is needed
Transmission line - a high voltage cable often seen between electricity pylons
Step-up transformer - a device that can change lower alternating voltages to higher ones
Step-down transformer - a device that can change higher alternating voltages to lower ones
Dissipation - the transfer of energy into the surroundings by heating
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