Energy transfers

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Heating in Circuits

When charge flows through a component, some energy is transferred from the electrons to the component, causing its temperature to rise.
This heating effect occurs because electrons collide with the ions in the metal lattice, making the ions vibrate more.
The energy is dissipated into the surroundings by thermal conduction, convection and radiation.
The amount of heat produced depends on the current and the resistance: greater current or higher resistance produces more heat.
Reducing resistance can increase current, which may actually increase the heat produced.

Reducing Heating in Circuits

In many appliances, heat is unwanted and can be dangerous.
To reduce heating, use a lower current or use wires with lower resistance.
Copper is commonly used for wiring because it has low resistance and is relatively cheap.
Lower resistance alternatives exist but are more expensive.

Uses and Dangers of Electric Heating

The heating effect is useful in devices designed to heat things, such as kettles, ovens, toasters and domestic heaters.
For example, a kettle converts electrical work to a thermal energy store.
Unwanted heating reduces efficiency, e.g. waste heat in a light bulb or overheating in a computer.
Excessive heating can cause fires, sparks or damage to components.
Many domestic fires are caused by too much current in low-quality wiring.

Calculating Electric Energy

Work is done when charge flows through a circuit; work done equals energy transferred.
The energy transferred by electrical work depends on current, potential difference and time.
The equation is: E = P × t, where E is energy in joules (J), P is power in watts (W) and t is time in seconds (s).
Since P = IV, the equation can also be written as E = I × V × t.
The energy transferred can also be calculated using E = Q × V, where Q is charge in coulombs (C) and V is potential difference in volts (V).
When charge flows around a circuit, the energy supplied by the battery equals the energy transferred to all components.
Always convert time to seconds before using these equations.

Energy and Power

Power is defined as the energy transferred per second.
Power is measured in watts (W); 1 W = 1 J/s.
1 kilowatt (kW) = 1000 W = 1000 J/s.
Power can also be defined as work done per unit time.
The equation is P = E / t (or E = Pt), where E is energy in joules (J) and t is time in seconds (s).

Energy flow diagram: a nuclear power plant

Electrical Power

The power of an electrical device depends on its potential difference (voltage) and current.
The equation is P = V × I, where P is power in watts (W), V is potential difference in volts (V) and I is current in amperes (A).
Using V = IR, power can also be written as P = I²R or P = V² / R.
For example, a 20 Ω resistor with a current of 6 A has a power of (6)² × 20 = 720 W.
The unit of power is the watt (W), which is the same as a joule per second (J/s).

A hydropower plant: dam, powerhouse, turbine, generator, power lines and city

Energy Transfers in Appliances

Everyday appliances transfer energy electrically from the mains supply to energy stores within the appliance.
The amount of energy transferred depends on the time the appliance is switched on for and the power of the appliance.
For example, a 1 kW iron uses the same energy in 1 hour as a 2 kW iron in 30 minutes.
Energy can be transferred to the kinetic store of an electric motor (e.g. in vacuum cleaners, washing machines, refrigerators).
Energy can be transferred to the thermal store of heating devices (e.g. toasters, kettles, radiators).
Some appliances use DC cells or batteries, transferring energy from a chemical store.

Power Ratings

The power rating of an appliance tells you how much energy it transfers by electrical work every second.
Power ratings are usually given on a label, along with the required potential difference (e.g. 230 V in the UK) and frequency (e.g. 50 Hz).
The higher the power rating, the faster the energy is transferred.
For example, a 2000 W kettle transfers 2000 J of energy per second.
Plugging an appliance into a mains supply with a much higher voltage than stated can cause it to fuse, set fire or become damaged.

The National Grid

The National Grid is a system of cables and transformers linking power stations to consumers.
Electrical power is transferred from power stations to consumers using the National Grid.
Step-up transformers increase the potential difference (and reduce the current) for transmission.
Step-down transformers decrease the potential difference (and increase the current) for domestic use.
Transmitting at high potential difference reduces the current, which reduces heating in the cables and wasted energy.
This makes the National Grid an efficient way to transfer energy.

Use of Transformers

Transformers are used to increase or decrease the potential difference of an alternating current (AC).
They consist of two coils of wire (primary and secondary) around a magnetic iron core.
A step-up transformer has more turns on the secondary coil than the primary coil.
A step-down transformer has more turns on the primary coil than the secondary coil.
Step-up transformers are used between power stations and transmission cables.
Step-down transformers are used to reduce the potential difference to a much lower value for domestic use.

A step-up transformer: fewer turns on the primary coil than the secondary coil, wound on a soft iron core.

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Xem trước miễn phí — 8 trên 63 câu hỏi. Đăng ký để xem tất cả.

1.What is the unit Watts (W) equivalent to?
Easy
- AJ/A
- BJ/s²
- CJ/s
- DJ/V
2.
Table 1 shows the power rating for some domestic electrical appliances.
Electrical Appliance Power (W)
Kettle 2400
Clock 10
Lamp 50
Iron 1250
Which appliance transfers stored energy to other types of energy at the fastest rate?
Medium
- AKettle
- BClock
- CLamp
- DIron
3.
Table 1 shows the power rating for some domestic electrical appliances.
Electrical Appliance Power (W)
Kettle 2400
Clock 10
Lamp 50
Iron 1250
Which appliance transfers stored energy to other types of energy at the slowest rate?
Medium
- AKettle
- BClock
- CLamp
- DIron
4.There is a current of 2.3 A in the radio when the radio is working correctly. Which of these should the technician choose to protect the radio circuit?
Medium
- AA 2 A fuse
- BA 5 A fuse
- CA 10 A fuse
- DA 13 A fuse
5.Which of the following is the correct equation for the power of a component in terms of current and resistance?
Medium
- AP = I²R
- BP = IR
- CP = I/R
- DP = R/I
6.Which of the following statements about the National Grid are correct? (select all that apply)
Medium
- AStep-up transformers increase the voltage before transmission.
- BStep-down transformers decrease the voltage for domestic use.
- CTransformers change the power so that less energy is wasted.
- DHigh voltage transmission reduces the current in the cables.
- EThe National Grid uses direct current (DC) from power stations.
7.Power is the same as energy.
Easy
True or false?
8.A step-up transformer has more turns on the secondary coil than on the primary coil.
Easy
True or false?

Thêm câu hỏi

9.Match each term to its correct description.
Medium
- Power
- Energy transferred
- Charge flow
- Rate of energy transfer
- Work done when charge flows
- Measured in coulombs
10.Place the stages of electricity transmission in the correct order.
Medium
- Power station generates electricity
- Step-up transformer increases voltage
- Transmission cables carry electricity
- Step-down transformer decreases voltage
- Electricity reaches homes
11.Why is electrical energy transmitted at high voltage in the National Grid?
Hard
- ATo increase the current and reduce heating.
- BTo reduce the current and reduce energy wasted as heat.
- CTo increase the power so more energy is transferred.
- DTo make the electricity safer for consumers.
12.The heating effect of current is always wasted energy.
Easy
True or false?
13.Which of the following is a disadvantage of the heating effect of current?
Medium
- AIt can be used in electric heaters.
- BIt can cause a fire hazard.
- CIt can be used in toasters.
- DIt can be used in kettles.
14.Which equation links energy transferred, charge flow and potential difference?
Medium
- AE = QV
- BE = Q/V
- CE = V/Q
- DE = Q + V
15.Match each appliance to the main energy transfer it produces.
Medium
- Kettle
- Vacuum cleaner
- Toaster
- Thermal energy to water
- Kinetic energy to suction
- Thermal energy to bread
16.Order the steps to show how energy is transferred in a circuit.
Medium
- Energy transferred to electrons at power supply
- Electrons flow through circuit
- Electrons transfer energy to components
- Components transfer energy to surroundings
17.Transformers change the power of the electricity they transmit.
Easy
True or false?
18.Which of the following appliances transfers energy mainly to the kinetic energy store of a motor?
Medium
- AToaster
- BKettle
- CWashing machine
- DRadiator
19.Which of the following statements about power are correct? (select all that apply)
Medium
- APower is the rate of energy transfer.
- BPower is measured in joules.
- CPower is the energy transferred per second.
- D1 W is equal to 1 J/s.
- EPower is the same as energy transferred.
20.Which of the following are advantages or disadvantages of the heating effect of current? (select all that apply)
Medium
- AIt is useful in an electric kettle.
- BIt can cause a fire hazard.
- CIt always increases the efficiency of a device.
- DIt is useful in an electric toaster.
- EIt never causes any energy loss.
21.Match each term with its correct description.
Medium
- Step-up transformer
- Step-down transformer
- National Grid
- Increases voltage and decreases current
- Decreases voltage and increases current
- System of cables and transformers linking power stations to consumers
22.Match each appliance with the main energy store it transfers energy to.
Medium
- Electric kettle
- Electric motor
- Electric heater
- Thermal energy store
- Kinetic energy store
23.Place the following stages of electricity transmission in the correct order.
Medium
- Step-up transformer increases voltage
- Electricity transmitted through cables
- Step-down transformer decreases voltage
- Electricity used in homes
24.In a step-down transformer, the secondary coil has fewer turns than the primary coil.
Easy
True or false?
25.The heating effect of current can be reduced by decreasing the current in the wires.
Easy
True or false?
26.Which equation links power, current and resistance?
Medium
- AP = I²R
- BP = IR
- CP = I/R
- DP = V²/R
27.A 12 V car battery supplies a current of 4.8 A to a headlamp. What is the power supplied to the headlamp?
Medium
- A57.6 W
- B2.5 W
- C0.4 W
- D16.8 W
28.An electric heater has a power of 142 W and a resistance of 0.68 Ω. What is the current through the heater?
Medium
- A14.4 A
- B208.8 A
- C0.0048 A
- D9.6 A
29.Which of the following is the correct equation for energy transferred by charge flow?
Medium
- AE = Q × V
- BE = Q / V
- CE = V / Q
- DE = Q + V
30.A 2000 W iron is used for 5 minutes. How much energy is transferred?
Medium
- A600 000 J
- B10 000 J
- C400 J
- D120 000 J
31.The graph of current against potential difference for a component curves so that the current increases less steeply at higher potential differences. Which component shows this variation?
Medium
- AA filament lamp
- BA low value resistor at constant temperature
- CA high value resistor at constant temperature
- DA diode
32.A student records the current in a lamp for several potential differences: 0.06 V gives 0.05 A, 0.12 V gives 0.08 A, 0.18 V gives 0.10 A, 0.24 V gives 0.12 A, 0.30 V gives 0.13 A and 0.36 V gives 0.13 A. The student concludes that the current is directly proportional to the potential difference. Which statement best comments on this conclusion?
Medium
- AIt is not valid because the current does not increase in proportion to the potential difference
- BIt is valid because the current always increases as the potential difference increases
- CIt is not valid because the current stays constant at the highest potential differences
- DIt is valid because doubling the potential difference doubles the current
33.A technician connects four devices to a 12 V car battery, each with its own switch and fuse. The currents in the devices are 4.8 A, 0.6 A, 2.3 A and 1.5 A. What is the current in the wires to the battery when all four devices are switched on?
Medium
- A9.2 A
- B3.7 A
- C1.2 A
- D27.6 A
34.Four devices in a car are connected in parallel to the battery, each with its own switch. What happens to the overall resistance of the circuit when one of the devices is switched off?
Medium
- AThe overall resistance increases
- BThe overall resistance decreases
- CThe overall resistance stays the same
- DThe overall resistance becomes zero
35.A 2 kW electric heater and a 100 W desk lamp are both connected to the 230 V mains. Which statement about the energy they transfer is correct?
Medium
- AThe heater transfers 20 times as much energy per second as the lamp
- BThe heater transfers 20 times less energy per second than the lamp
- CThe heater and the lamp transfer the same energy per second
- DThe heater transfers 2000 times as much energy per second as the lamp
36.Which of the following statements about the heating effect of current are correct? (select all that apply)
Medium
- AThe greater the current, the more heat is produced
- BFor a given current, a higher resistance produces more heat
- CHeat is produced by collisions between electrons and ions in the metal lattice
- DReducing the resistance of a wire always reduces the heat produced
- EThe heating effect always makes an appliance more efficient
37.In the National Grid, transmitting electricity at a high potential difference reduces the current in the cables, which reduces energy wasted by heating.
Medium
True or false?
38.A 1 kW iron and a 2 kW iron are each used to press clothes. Which statement about the energy they transfer is correct?
Medium
- AThe 1 kW iron transfers the same energy in 1 hour as the 2 kW iron transfers in 30 minutes
- BThe 1 kW iron transfers the same energy in 1 hour as the 2 kW iron transfers in 2 hours
- CThe 1 kW iron always transfers more energy than the 2 kW iron
- DThe 2 kW iron transfers the same energy in 1 hour as the 1 kW iron transfers in 1 hour
39.Match each appliance to the main energy store that energy is transferred to.
Medium
- Electric kettle
- Electric motor in a washing machine
- Electric toaster
- Thermal energy store of the heating element
- Kinetic energy store of the rotating drum
- Thermal energy store of the heating element that toasts bread
40.Place the stages of energy transfer in a torch circuit in the correct order.
Medium
- Chemical energy store of the battery
- Charge flows through the circuit
- Energy transferred to the thermal store of the bulb filament
- Energy transferred to the surroundings by heating and radiation
41.State the equation linking power, current and potential difference.
Easy
42.An electric kettle is connected to a mains voltage of 230 V. The power of the kettle is 2860 W. Calculate the current in the kettle.
Medium
43.An electric kettle is connected to a mains voltage of 230 V. The power of the kettle is 2860 W. The kettle boils for 2.5 minutes. Calculate the energy transferred to the kettle during this time.
Medium
44.An immersion heater has a power of 142 W and a resistor with resistance 0.68 Ω. Calculate the current through the element in the immersion heater.
Medium
45.State the energy transfers in the resistor when it becomes warm.
Easy
46.State the equation linking power, energy transferred and time.
Easy
47.An electric kettle is used to boil some water. The mains supply voltage is 230 V. The power supplied to the kettle is 1.9 kW. Calculate the current in the kettle. Use the equation I = P / V.
Medium
48.A coffee machine takes 120 s to heat some water. Mains supply voltage = 230 V. Current in this coffee machine = 7.4 A. Calculate the energy transferred to the coffee machine in 120 s.
Medium
49.In a mains plug, state the name of the wire that is labelled X (the live wire).
Easy
50.In a mains plug, state the name of the wire that is labelled Y (the earth wire).
Easy
51.In a mains plug, state the name of component Z (the fuse).
Easy
52.A transformer is used to connect a laptop computer to the mains electricity supply. The input voltage to the transformer is 230 V. The output current from the transformer is 2.37 A. The transformer has an output voltage of 19.0 V. Calculate the input current to the transformer. Use the equation: input current × input voltage = output current × output voltage.
Medium
53.State the definition of power and its unit.
Easy
54.The resistor becomes warm while there is current in it. Explain, in terms of particles in the wire, why the resistor becomes warm.
Medium
55.State the dangers of overheating the resistor and explain how this can be reduced.
Hard
56.A student investigates how the current in a filament lamp changes as the potential difference across it is varied. The student replaces the filament lamp with an electric heater. The heater draws a much larger current than the filament lamp. Explain why the connecting wires used with the heater should be thicker than those used with the filament lamp.
Medium
57.Explain why the wires to the battery in a car are thicker than the wires that connect each device to its switch and its fuse.
Medium
58.An electric kettle is connected to a mains voltage of 230 V. The power of the kettle is 2860 W. Calculate the current in the kettle.
Medium
- A14.4 A
- B11.4 A
- C12.4 A
- D13.4 A
59.An electric kettle is connected to a mains voltage of 230 V. The power of the kettle is 2860 W. The kettle boils for 2.5 minutes. Calculate the energy transferred to the kettle during this time.
Medium
- A514800 J
- B429000 J
- C386100 J
- D471900 J
60.An immersion heater has a power of 142 W and a resistor with resistance 0.68 Ω. Calculate the current through the element in the immersion heater.
Medium
- A16.4 A
- B14.4 A
- C13.4 A
- D15.4 A
61.An electric kettle is used to boil some water. The mains supply voltage is 230 V. The power supplied to the kettle is 1.9 kW. Calculate the current in the kettle. Use the equation I = P / V.
Medium
- A8.26 A
- B7.26 A
- C9.26 A
- D10.26 A
62.A coffee machine takes 120 s to heat some water. Mains supply voltage = 230 V. Current in this coffee machine = 7.4 A. Calculate the energy transferred to the coffee machine in 120 s.
Medium
- A183816 J
- B245088 J
- C204240 J
- D224664 J
63.A transformer is used to connect a laptop computer to the mains electricity supply. The input voltage to the transformer is 230 V. The output current from the transformer is 2.37 A. The transformer has an output voltage of 19.0 V. Calculate the input current to the transformer. Use the equation: input current × input voltage = output current × output voltage.
Medium
- A1.196 A
- B-0.804 A
- C2.196 A
- D0.196 A

Thẻ ghi nhớ

Define power in words and state its unit.

Power is the rate of energy transfer, or the amount of energy transferred per second. It is measured in watts (W).

What is the equation linking power, current and potential difference?

P = V × I, where P is power in watts, V is potential difference in volts, and I is current in amperes.

What is the equation linking power, current and resistance?

P = I² × R, where P is power in watts, I is current in amperes, and R is resistance in ohms.

What is the equation linking energy transferred, power and time?

E = P × t, where E is energy transferred in joules, P is power in watts, and t is time in seconds.

What is the equation linking energy transferred, charge flow and potential difference?

E = Q × V, where E is energy transferred in joules, Q is charge flow in coulombs, and V is potential difference in volts.

What is the equation linking energy transferred, current, potential difference and time?

E = I × V × t, where E is energy in joules, I is current in amperes, V is potential difference in volts, and t is time in seconds.

Why does a resistor become warm when current flows through it?

Electrons flowing through the metal collide with ions in the lattice, transferring energy to them and making them vibrate more. This increases the temperature of the resistor.

State the energy transfer that occurs in a resistor when it becomes warm.

Electrical energy is transferred to the thermal energy store of the resistor, and then dissipated to the surroundings by heating.

What two factors affect the amount of heat produced in a circuit component?

The current: greater current produces more heat. The resistance: higher resistance produces more heat for a given current.

How can the heating effect in wires be reduced?

By reducing the current in the wires, or by using wires with lower resistance. Copper is often used because it has low resistance and is relatively cheap.

State two dangers of overheating in electrical circuits.

It can cause burns if touched, and it can start fires, for example due to sparks or excessive heat in motors or low-quality wiring.

Give two useful applications of the heating effect of current.

Electric heaters, kettles, ovens, toasters, and domestic heating systems all use the heating effect.

What does the power rating of an appliance tell you?

It tells you the amount of energy the appliance transfers by electrical work every second. A higher power rating means a faster rate of energy transfer.

How does the energy transferred by an appliance depend on its power and time?

The energy transferred is directly proportional to both the power of the appliance and the time it is switched on for: E = P × t.

What is the National Grid?

The National Grid is a system of cables and transformers that links power stations to consumers, transferring electrical power across the country.

What is the role of a step-up transformer in the National Grid?

A step-up transformer increases the potential difference from the power station to the transmission cables, which reduces the current in the cables.

What is the role of a step-down transformer in the National Grid?

A step-down transformer decreases the potential difference from the transmission cables to a much lower value suitable for domestic use.

Explain why the National Grid transmits electricity at high potential difference.

For the same power, a higher potential difference means a lower current. Lower current reduces heating in the cables, so less energy is wasted, making the system more efficient.

Why are step-up transformers used before transmission cables?

They increase the voltage and decrease the current, which reduces energy wasted as heat in the cables due to resistance.

Why are step-down transformers used before electricity reaches homes?

They decrease the high transmission voltage to a much lower, safer value for domestic use.

What is the unit of power equivalent to in base units?

1 watt (W) is equal to 1 joule per second (1 J/s).

How many watts are in 1 kilowatt?

1 kilowatt (kW) = 1000 watts (W).

What is the relationship between power and energy?

Power is the rate of energy transfer, or energy transferred per second. Energy transferred = power × time.

Why are thicker wires used for appliances that draw larger currents?

Thicker wires have lower resistance, so they reduce heating and energy loss when a large current flows through them.

What energy transfers occur in a washing machine?

Electrical energy is transferred from the mains to the kinetic energy store of the motor, which rotates the drum.

What energy transfers occur in a toaster?

Electrical energy is transferred from the mains to the thermal energy store of the heating element, which then transfers energy to the bread.

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Electrical Appliance	Power (W)
Kettle	2400
Clock	10
Lamp	50
Iron	1250

Electrical Appliance	Power (W)
Kettle	2400
Clock	10
Lamp	50
Iron	1250