Electrical Power — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

Section 1 of 9Power

Power

Power the rate at which energy is transferred by an appliance.

P

E / t

What the symbols mean: P = power (W = J s⁻¹) · W or E = work done / energy transferred (J) · t = time (s) · F = force (N) · v = velocity (m s⁻¹)

• P is power (W), E is energy (J) and t is time (s).

Work done

• Work done power can also be written P = W / t, since work done equals energy transferred.

Unit

• Unit the watt (W) is one joule per second.
• Larger units are the kilowatt (1 kW = 1000 W) and megawatt (1 MW = 10⁶ W).

Section 2 of 9Electrical power equations

P = I V

• P = I V the power dissipated by a component equals current times p.d.

Using V = I R

• Using V = I R this gives P = I²R and P = V² / R.

Same current

• Same current use P = I²R, so a thick low-resistance cable stays cool.

Same p.d.

• Same p.d. use P = V² / R, as for appliances in parallel on the mains.

Section 3 of 9Power ratings

Meaning

• Meaning a label of 230 V, 2000 W means the device transfers 2000 J each second at 230 V.

Normal current

• I = P / V = 2000 / 230 = 8.7 A.

Lower voltage

• Lower voltage the device would draw less current and have less power.

Comparing

• Comparing a 100 W lamp transfers energy ten times as fast as a 10 W lamp.

Power and energy

• Power and energy a higher power means more energy each second, but total energy also depends on how long the device is on.

Section 4 of 9Choosing a fuse

1

Find the power rating of the appliance and the mains p.d.

2

Calculate the normal current with I = P / V.

3

List the standard fuse ratings: 3 A, 5 A and 13 A.

4

Choose the rating just above the normal current.

Too low

• Too low the fuse blows during normal use.

Too high

• Too high it does not protect the flex from overheating.

Section 5 of 9Comparing appliances on the mains

Same p.d.

• Same p.d. appliances on the mains have 230 V across them, so their currents are proportional to their power ratings.
• A 1 kW toaster draws about 4.3 A and a 2 kW kettle about 8.7 A.

Overloading

• Overloading plugging many high-power devices into one socket adds their currents and can overload the cable and fuse.

Series resistors

• Series resistors the common current means the larger resistor dissipates more power.

Parallel resistors

• Parallel resistors the common p.d. means the smaller resistor dissipates more power.

Section 6 of 9Power loss in transmission

Loss

• Loss the power lost in cables is I²R.

Reducing loss

• Reducing loss transmit at a high voltage and low current, because P = I V lets a given power be sent with a small current if V is large.

Halving the current

• Halving the current it cuts the heating loss to a quarter.

Section 7 of 9More from the specification

More from the specification

Coulombic repulsion between charges

• Coulombic repulsion between charges identical charges repel each other with electrostatic force, dissipating electrical power when moved through resistive circuits.

Electrostatic repulsion and power dissipation

• Electrostatic repulsion and power dissipation driving charges against repulsive forces requires electrical energy, dissipating electric power across resistive heating elements.

Static Electricity and Charging by Friction

• When insulating materials like a polythene rod are rubbed with a cloth, friction transfers negative electrons, leaving the polythene negatively charged and the cloth with an equal positive charge.

Electric Charge (Q) and Current (I)

• Electric current is the rate of flow of charge carriers around a closed circuit, defined by the formula I = ΔQ / Δt and measured in amperes (A),
• where 1 coulomb equals 1 ampere-second.

Potential Difference and Voltage Measurement

• Potential difference (voltage) is the electrical energy transferred per unit charge between two points in a circuit (V = W / Q), measured in volts using a high-resistance voltmeter in parallel.

Electrical Power in Circuits

• The rate at which electrical energy is transformed in a circuit component is electrical power P, calculated using P = IV = I²R = V² / R,
• and measured in watts (W) or joules per second.

Electromotive force

• Energy transferred from other forms to electrical energy per unit charge.

Gold-leaf electroscope

• A device in which a charged gold leaf falls when photoelectrons discharge the plate.

Mains supply

• Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

Charge

• Quantity of electricity in coulombs, Q = It.
  What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

Electroscope

• Instrument with a metal cap, stem and gold leaf used to detect charge.

Conductor

• Material that allows current to flow.

Terminal

• Connection point of a cell, labelled + or −.

Insulator

• Material that does not allow current to flow.

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.

Section 8 of 9Key terms

Term	Meaning
Power	Rate of energy transfer, P = E / t.
Watt	One joule per second.
Power rating	Power and voltage marked on an appliance.
Fuse	Thin wire that melts when the current exceeds its rating.
I2R loss	Power lost as heat in a conductor.

Section 9 of 9Exam technique

1

Mixing power and energy — a high-power device does not use more energy if it is on only briefly.

2

Using the e.m.f. instead of the p.d. across the component in P = V²/R.

3

Choosing a fuse far above the working current — it never protects the circuit.

4

Forgetting to change kW to W in calculations with joules.