4.2CH4 Electricity and Magnetism

Electrical Quantities — Cambridge O Level Physics 5054 Revision Notes & Topical Questions

10 min read 15 parts Lesson 2 of 6

📚 Cambridge O Level Physics 5054 Hub / Topical Revision

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Part 1 of 13Electric charge and static electricity

Electric charge and static electricity

Charge

Charge a property of matter; there are two types, positive and negative, measured in coulombs (C).
Like charges repel and unlike charges attract.

Charging by friction

Charging by friction rubbing two insulators transfers electrons from one to the other.
The material that gains electrons becomes negative and the one that loses them becomes positive.
A polythene rod rubbed with a cloth gains electrons, and the cloth becomes positive.

Only electrons move

Only electrons move positive charge does not move in solids because the protons are fixed in the nuclei.

Detecting charge

Detecting charge a gold-leaf electroscope shows charge because the leaf and stem acquire the same charge and repel.

Charging by induction

Charging by induction a charged object brought near a conductor makes the near side oppositely charged,
- and earthing the far side leaves the conductor charged.

Conductors and insulators

Conductors and insulators conductors such as metals allow charge to flow, while insulators such as plastic and glass do not.

Dangers and uses

Dangers and uses static charge can cause sparks near fuels, but is used in photocopiers, paint sprays and electrostatic precipitators.

Part 2 of 13Electric fields

Electric field

Electric field a region where a charged particle experiences a force.

Direction

Direction the field direction is the direction of the force on a small positive charge.
Field lines point away from positive charges and toward negative charges.

Field patterns

Field patterns the field is radial around a point charge and uniform between oppositely charged parallel plates.

Effect on materials

Effect on materials an electric field causes the movement of charges and can polarise an insulator, which is why a charged balloon sticks to a wall.

Part 3 of 13Current, charge and potential difference

Current, charge and potential difference

Electric current

Electric current the rate of flow of electric charge, I = Q ÷ t, measured in amperes (A).
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
1 A is a charge flow of 1 coulomb per second.

Conventional current

Conventional current drawn from the positive to the negative terminal of a cell.
In metals the electrons actually move in the opposite direction.

Direct and alternating current

Direct and alternating current direct current (d.c.) flows in one direction and alternating current (a.c.) reverses direction periodically.

Measuring current

Measuring current an ammeter is connected in series and has a very low resistance.

Electromotive force (e.m.f.)

Electromotive force (e.m.f.) the electrical work done by a source in moving a unit charge around a complete circuit, measured in volts.

Potential difference (p.d.)

Potential difference (p.d.) the electrical work done per unit charge between two points, V = W ÷ Q.
1 V is 1 joule per coulomb.

Measuring p.d.

Measuring p.d. a voltmeter is connected in parallel across a component and has a very high resistance.

Quantity	Symbol	Unit	Meter
Charge	Q	coulomb (C)	-
Current	I	ampere (A)	Ammeter (series)
p.d.	V	volt (V)	Voltmeter (parallel)
e.m.f.	E	volt (V)	Voltmeter across cell
Resistance	R	ohm (Ω)	-

Part 4 of 13Resistance and Ohm's law

Resistance

Resistance the opposition to current flow in a component, R = V ÷ I.
1 ohm is the resistance when 1 V drives a current of 1 A.

Ohm's law

Ohm's law for a metallic conductor at constant temperature, the current is proportional to the p.d. across it.
A graph of I against V is a straight line through the origin.

Practical

Practical finding resistance

use an ammeter, a voltmeter and a variable resistor.

Connect the resistor in series with an ammeter, cell and variable resistor.

Connect the voltmeter in parallel across the resistor.

Record the current and p.d. for different settings of the variable resistor.

Calculate R = V ÷ I for each, or plot V against I and find the gradient.

Resistance and Ohm's law

Non-ohmic components

Non-ohmic components a filament lamp's resistance rises as it heats, so its I-V graph curves.
A diode allows current in one direction only, and a thermistor's resistance falls as temperature rises.

Resistance of a wire

Resistance of a wire it increases with greater length and decreases with a larger cross-sectional area.
Doubling the length doubles the resistance, and doubling the area halves it.

Temperature effect

Temperature effect the resistance of a metal increases as it gets hotter because the ions vibrate more and collide with electrons more often.

Part 5 of 13Electrical energy and power

Electrical power

Electrical power the rate of energy transfer, P = IV, in watts.
What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Electrical energy

E = IVt, the energy transferred in time t.
Using V = IR, the power is also P = I²R or P = V² ÷ R.

kWh

kWh the energy supplied by a 1 kW appliance in one hour; cost = kWh × price per kWh.
A 3 kW heater used for 2 h uses 6 kWh.

E.m.f. and energy

E.m.f. and energy a cell with an e.m.f. of 1.5 V gives 1.5 J to each coulomb that passes through it.

Heating effect

Heating effect current produces thermal energy in a resistance, used in kettles, toasters and fuses.

Part 6 of 13Practical uses and sources

Cells and batteries

Cells and batteries a cell converts chemical energy to electrical energy, and a battery is two or more cells in series.
Cells in series add their e.m.f. values.

Mains and generators

Mains and generators a generator converts kinetic energy to electrical energy, and the mains supply is a.c.

Charge in a circuit

Charge in a circuit the charge passing a point in a given time is Q = It.
A current of 2.5 A for 20 s moves 50 C.

Conservation of charge

Conservation of charge charge is neither created nor destroyed, so the current into a junction equals the current out.

Part 7 of 13Series and parallel links and units

1Linking the equations

Linking the equations charge, current, p.d., resistance and power are connected by Q = It, V = IR and P = IV.
A quick path is to find the current first, then use it to find the other quantities.

2Units in order

1 A = 1 C/s, 1 V = 1 J/C, 1 Ω = 1 V/A and 1 W = 1 J/s.

3Metals and electrons

Metals and electrons in a metal the free electrons drift slowly through the lattice of positive ions,
- but the effect of the e.m.f. travels very quickly.
This is why a lamp lights almost instantly when a switch is closed.

4Insulators in static electricity

Insulators in static electricity charge stays where it is put on an insulator, so rubbed plastic can attract small pieces of paper.

5Earthing

Earthing connecting a conductor to the ground lets charge flow away, so it is used to prevent dangerous build-up on fuel tankers.

6Graph of I against V for a lamp

Graph of I against V for a lamp it curves because the resistance rises as the filament gets hotter.
For a fixed resistor the graph is a straight line through the origin.

Part 8 of 13Experiments on charging and conduction

Experiments on charging and conduction

Charging by friction

Charging by friction rub a polythene rod with a dry cloth, then hold it near small pieces of paper or a suspended charged rod.
The rod picks up the paper, and two rods rubbed the same way repel, which shows two kinds of charge.

What moves

What moves only negative charge (electrons) transfers when solids are rubbed.
The material that gains electrons becomes negative, and the one that loses electrons becomes positive by the same amount.

Experiment to find conductors and insulators

Experiment to find conductors and insulators connect a test circuit.

Build a series circuit with a cell, a lamp and two gaps at the ends of crocodile clips.

Touch the clips on each material in turn, such as copper, steel, plastic, wood and glass.

If the lamp lights, the material is a conductor.

If the lamp stays off, the material is an insulator.

Electron model

Electron model in a conductor such as a metal there are free electrons that can drift through the structure when a p.d. is applied.
In an insulator the electrons are held tightly to their atoms, so no current flows.

Examples

Examples copper, aluminium and graphite are conductors, and plastic, rubber, glass and dry air are insulators.

Part 9 of 13Electric field patterns

1Direction

Direction the direction of an electric field line at a point is the direction of the force on a positive charge there.

2Point charge

Point charge the field lines are straight and radial, pointing outward for a positive charge and inward for a negative charge.

3Charged conducting sphere

Charged conducting sphere the lines are radial and leave or enter the surface at right angles, like those of a point charge,
- and there is no field inside the sphere.

4Between two parallel charged plates

Between two parallel charged plates the lines are straight, parallel and equally spaced from the positive to the negative plate,
- so the field is uniform.
Lines near the edges bulge outward, but the end effects are not examined.

Part 10 of 13Current direction, electron flow and meters

1Direction of flow

Direction of flow conventional current runs from positive to negative around a circuit,
- while the free electrons actually drift from negative to positive.

2Metals

Metals a p.d. makes free electrons move through the metal lattice, which is the electrical current.

3Ammeters

Ammeters analogue and digital ammeters have several ranges, such as 0 to 1 A and 0 to 10 A.
Start on the largest range and move to a smaller one for a more precise reading.

4Voltmeters

Voltmeters they also have ranges, and the correct reading is taken from the scale that matches the range chosen.

Part 11 of 13Combining cells

1Cells in series

Cells in series the total e.m.f. is the sum of the e.m.f. values of the cells.
Three 1.5 V cells in series give 4.5 V.

2Identical cells in parallel

Identical cells in parallel the e.m.f. is the same as that of one cell.
Parallel cells do not give a larger voltage, but they last longer because each supplies a share of the current.

3Volt

Volt it is defined by the joule per coulomb, so 1 V = 1 J/C.

4Amp

Amp it is a coulomb per second, so 1 A = 1 C/s.

Part 12 of 13Current-voltage graphs

1Fixed resistor

Fixed resistor the graph is a straight line through the origin, so the resistance is constant and the resistor obeys Ohm's law.

2Filament lamp

Filament lamp the line curves and flattens as the p.d. increases.
The filament gets hotter, its resistance increases, so the current rises less quickly for each extra volt.

3Diode

Diode there is almost no current in the reverse direction,
and in the forward direction there is little current until about 0.6 V, after which the current rises steeply.

4Why heating raises resistance

Why heating raises resistance the metal ions vibrate more and collide with the electrons more often, which opposes the flow of charge.

Part 13 of 13Worked examples

A current of 0.80 A flows through a lamp for 5.0 minutes. Calculate the charge that flows. [3]3 marks
- t = 5.0 × 60 = 300 s.
- Q = It = 0.80 × 300.
- Q = 240 C.
A resistor has a p.d. of 12 V across it and a current of 0.40 A. Find its resistance and power. [3]3 marks
A 2.0 kW kettle is used for 4.0 minutes. Calculate the energy in joules and the cost at 15 cents per kWh. [4]4 marks
A wire has a resistance of 6.0 Ω. Find the resistance of a wire of the same material with twice the length and the same area. [2]2 marks
A cell of e.m.f. 9.0 V drives a charge of 15 C around a circuit. How much energy is transferred? [2]2 marks
A diode, a resistor and a filament lamp are tested and their I-V graphs plotted. Match each description to a component: (a) straight line through the origin, (b) curves, gradient falls, (c) no current for negative voltage. [3]3 marks

Key terms

Term	Meaning
Charge	A property of matter measured in coulombs, either positive or negative.
Current	The rate of flow of charge, I = Q ÷ t, in amperes.
Potential difference	Energy transferred per unit charge between two points, in volts.
E.m.f.	The energy supplied by a source per unit charge passing through it, in volts.
Resistance	The opposition to current, R = V ÷ I, in ohms.
Ohm's law	Current in a metallic conductor is proportional to p.d. at constant temperature.
Power	The rate of energy transfer, P = IV, in watts.
Electric field	A region in which a charge experiences a force.

Avoid these mistakes

Connecting a voltmeter in series — a voltmeter goes in parallel, an ammeter in series.

Saying a battery stores charge — it provides energy to charge that is already in the circuit.

Saying current is used up — the same current flows around a series circuit; it is energy that is transferred.

Forgetting to convert minutes to seconds in Q = It — time must be in seconds.

Saying all conductors obey Ohm's law — only at constant temperature and for metals.

Last updated: 19 October 2026