4.2.5Electricity and Magnetism
Electrical Energy AND Electrical Power — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 22 of 65 1 quiz
Lesson summary
4.2.5 Electrical Energy and Electrical Power — IGCSE PHYSICS notes: Describe energy transfers in circuits, use P = IV and E = IVt and calculate the cost of electricity in kilowatt-hours.At the supply — energy is transferred from the battery or mains to the charge.At the component…
- At the components
- Combined with V = I R
- Higher power rating
- Kilowatt-hour (kWh)
- Conversion
- High-voltage transmission
Part 1 of 8Energy transfers in circuits
Energy transfers in circuits
At the supply
- At the supply energy is transferred from the battery or mains to the charge.
At the components
- At the components energy is transferred from the charge to the components and then to the surroundings as thermal energy or light.
Resistors
- Resistors the energy becomes the internal (thermal) energy of the component through collisions of electrons with ions.
Part 2 of 8Power
Power
Power the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s.
P = I V
What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
- power (W) equals current (A) times p.d.
- (V) (Extended).
Combined with V = I R
- Combined with V = I R this gives P = I2R and P = V2/R.
Higher power rating
- Higher power rating appliances such as kettles and heaters transfer energy faster, so they cost more to run for the same time.
Part 3 of 8Energy
E = P t
- E = P t energy (J) equals power (W) times time (s).
E
I V t
What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
- energy (J) equals current (A) times p.d.
- (V) times time (s) (Extended).
Part 4 of 8The kilowatt-hour
Kilowatt-hour (kWh)
- Kilowatt-hour (kWh) the energy transferred by a 1 kW appliance in 1 hour.
Conversion
- 1 kWh = 1000 W × 3600 s = 3.6 × 106 J.
Formula
- Formula use E = P t with power in kW and time in hours to get energy in kWh.
Part 5 of 8Calculating the cost of electricity
1
Convert the power rating to kilowatts (divide W by 1000).
2
Convert the time used to hours.
3
Calculate energy in kWh = power (kW) × time (h).
4
Multiply the energy by the cost per kWh.
5
Give the answer with the correct currency unit.
Part 6 of 8Power and energy of appliances
| Appliance | Power | Comment |
|---|---|---|
| Kettle | 3 kW | High power, short time |
| Heater | 2 kW | Expensive to run for long |
| TV | 0.1 kW | Low power |
| LED lamp | 0.01 kW | Efficient |
Part 7 of 8Power transmission
1High-voltage transmission
- High-voltage transmission a high voltage gives a low current for the same power, which reduces I2R losses in the cables.
2Fuse rating
- Fuse rating it should be just above the normal working current of the appliance.
Part 8 of 8More from the specification
More from the specification
Potential difference
- Potential difference the energy transferred per unit charge between two points, measured in volts, so W = Q V.
Electrical energy and power
- Electrical energy and power energy equals current times voltage times time, and power is energy per second, P = I V.
Resistance
- Resistance the opposition to current, R = V / I, and it makes conductors transfer energy as heat.
Electrostatic charging
- Electrostatic charging rubbing a polythene rod gains electrons and becomes negative, and an electroscope's leaf rises as like charges repel.
Electric field
- Electric field the region around a charge where another charge feels a force, with field lines pointing away from positive charge.
Gold-leaf electroscope
- A device in which a charged gold leaf falls when photoelectrons discharge the plate.
Potential difference
- Potential difference the energy transferred per unit charge between two points, measured in volts, so W = Q V.
Electrical energy and power
- Electrical energy and power energy equals current times voltage times time, and power is energy per second, P = I V.
Resistance
- Resistance the opposition to current, R = V / I, and it makes conductors transfer energy as heat.
Electrostatic charging
- Electrostatic charging rubbing a polythene rod gains electrons and becomes negative, and an electroscope's leaf rises as like charges repel.
Electric field
- Electric field the region around a charge where another charge feels a force, with field lines pointing away from positive charge.
Potential difference
- Potential difference the energy transferred per unit charge between two points, measured in volts, so W = Q V.
Electrical energy and power
- Electrical energy and power energy equals current times voltage times time, and power is energy per second, P = I V.
Resistance
- Resistance the opposition to current, R = V / I, and it makes conductors transfer energy as heat.
Electrostatic charging
- Electrostatic charging rubbing a polythene rod gains electrons and becomes negative, and an electroscope's leaf rises as like charges repel.
Electric field
- Electric field the region around a charge where another charge feels a force, with field lines pointing away from positive charge.
Terminal
- Connection point of a cell, labelled + or −.
Insulator
- Material that does not allow current to flow.
Potential difference
- Work done by a unit charge passing through a component.
Electric field
- Region of space in which a charge experiences a force.
Alternating current
- Current that continually reverses direction.
Field lines
- Lines showing the direction and strength of a magnetic field.
Direct current
- Current that flows in one direction only.
Electromotive force
- Energy transferred from other forms to electrical energy per unit charge.
Resistance
- Opposition to current, R = V / I, in ohms (Ω).
Electroscope
- Instrument with a metal cap, stem and gold leaf used to detect charge.
Conductor
- Material that allows current to flow.
Key terms
| Term | Meaning |
|---|---|
| Electrical power | Rate of energy transfer, P = I V. |
| Watt | One joule per second. |
| Electrical energy | E = P t = I V t. |
| Kilowatt-hour | Energy unit equal to 3.6 × 106 J. |
| Cost | Energy in kWh multiplied by the price per kWh. |
Avoid these mistakes
1
Using time in minutes or hours in E = P t when the power is in watts — use seconds, or use kW with hours.
2
Mixing up power and energy — power is the rate of transfer in watts, energy is in joules or kWh.
3
Forgetting to convert W to kW before calculating kWh.
4
Thinking the kilowatt-hour is a unit of power — it is a unit of energy.
Marked practiceQuizzes for this lesson
Last updated:
\n