OxfordAQA International GCSE Physics: Charge, current, potential difference and circuit symbols (9203) – Revision Notes

These notes condense section 3.5.1 Electrical circuits, statements a to g, of the OxfordAQA International GCSE Physics (9203) specification, for exams from May/June 2018 onwards. For full explanations, read the study guide. Then try the practice questions. See also the OxfordAQA International GCSE Physics hub, the printable checklist and the free 10-minute diagnostics.

Key definitions

Term	Meaning (3.5.1)
Free charges	In a metal, many electrons are free to move through the lattice (a)
Static electricity	Unbalanced charge that has nowhere to flow because no conducting route exists (b)
Discharge	Charge flowing when a conducting route is provided (b)
Electric current	The rate of flow of electric charge, in amperes (c)
Voltage of a source	The energy the source gives to charges as it drives them round the whole circuit, in volts (d)
Potential difference	The energy transferred by charges in a component, in volts (e)

Questions use the term potential difference. The specification says correct use of either potential difference or voltage gains credit.

Equations

Equation	Quantities and units	Rearrangements
I = Q / t	current I in A, charge Q in C, time t in s	Q = I t, t = Q / I
V = E / Q	V potential difference (V), E energy (J), Q charge (C)	E = Q V, Q = E / V

One ampere is one coulomb per second. One volt is one joule per coulomb.

Two-step link: charge from the current first, then energy from the charge.

E = Q V = (I t) V

Unit prefixes to recall: 1 mA = 0.001 A, 1 ms = 0.001 s, 1 kJ = 1000 J, 1 minute = 60 s and 1 hour = 3600 s.

Method in steps

1. Write the quantity you want and list the data.
2. Convert time to seconds and milliamperes to amperes.
3. Rearrange the equation, then substitute.
4. Finish with the unit, and round sensibly.
5. If there is a second part, check whether you can use your first answer.

Free charges and static electricity

• Metals conduct because they contain many free electrons. Insulators have almost none.
• A current needs a source and a complete conducting loop.
• Conventional current leaves the positive terminal. The electrons in the metal move the opposite way.
• Rubbing two insulators transfers electrons from one to the other. The surface that gains electrons is negative and the surface that loses them is positive.
• The charge stays put because it has no conducting route. A conductor held in the hand loses its charge because charge flows away through you.

Lightning in four steps

1. Collisions inside a storm cloud separate charge.
2. The ground becomes charged with the opposite sign to the base of the cloud.
3. The potential difference between cloud and ground becomes very large.
4. The air stops insulating, a conducting path forms and charge flows in a flash. This is the discharge.

Worked reminders

Charge from current. A current of 1.8 A flows for 2.5 minutes.

t = 2.5 × 60 = 150 s
Q = I t = 1.8 × 150 = 270 C

Current from charge. 45 C passes in 0.30 ms during a spark.

I = Q / t = 45 / 0.00030 = 150 000 A = 1.5 × 10⁵ A

Energy from charge. 22 C is driven round a circuit by a 9.0 V battery.

E = Q V = 22 × 9.0 = 198 J

Potential difference from energy. A motor transfers 330 J from 55 C.

V = E / Q = 330 / 55 = 6.0 V

Energy shared round a circuit. A 6.0 V battery drives 40 C round a circuit with a lamp and a buzzer. The lamp has 2.5 V across it.

Energy supplied = 40 × 6.0 = 240 J
Lamp transfers  = 40 × 2.5 = 100 J
Buzzer transfers = 240 - 100 = 140 J, so V = 140 / 40 = 3.5 V

Reading thermistor data. A thermistor on a fixed supply carries 8.0 mA at 25 °C and 26 mA at 55 °C. The current rises, so the resistance falls. In 5.0 minutes at 55 °C, Q = 0.026 × 300 = 7.8 C.

Circuit symbols

Component	Symbol in words
Cell	The longer, thinner line is positive and the shorter, thicker line is negative
Battery	Two or more cells in a row
Switch, open or closed	Gap with a lever away from the contact, or lever resting on it
Lamp	Circle with a cross
Fuse	Rectangle with a line through its length
Resistor	Plain rectangle
Variable resistor	Rectangle with a sloping arrow through it
Diode	Triangle pointing at a bar, inside a circle
LED	Diode symbol with arrows leaving
Thermistor	Rectangle with a sloping line ending in a bar
LDR	Rectangle in a circle, arrows arriving
Ammeter	A in a circle, in the loop being measured
Voltmeter	V in a circle, across the component being measured

Thermistors and LDRs

Device	Rule	Example use
Thermistor	Resistance decreases as temperature increases	Thermostat
LDR	Resistance decreases as light intensity increases	Switching a light on when it gets dark

• With a fixed potential difference, lower resistance means a larger current. So a thermistor circuit carries more current when hot, and an LDR circuit carries more current when bright.
• Thermostat: the sensing circuit responds as the thermistor warms or cools, and the heater is switched off or on.
• Dark switch: as the light falls, the LDR’s resistance rises and the current in the sensing circuit changes. That change operates the switch for the lamp.

Model sentences for explanation questions

• Current: “Current is the rate of flow of charge, so more charge passes each second when the current is larger.”
• Potential difference: “The potential difference across the lamp is 3.0 V, so each coulomb transfers 3.0 J in the lamp.”
• Static electricity: “Electrons were transferred by rubbing, so the object has an excess of charge. It cannot flow away because the object is an insulator.”
• Discharge: “When a conducting route was provided, the charge flowed along it.”
• LDR: “In the dark the resistance of the LDR is high, so the current is small.”
• Thermistor: “As the temperature rises the resistance of the thermistor falls, so the current increases.”

Must-know distinctions

Pair	Difference
Voltage of a source and potential difference	Energy supplied per coulomb, and energy transferred per coulomb
Charge and current	Coulombs, and coulombs per second
Static and current electricity	Charge held in place, and charge flowing
Conducting and insulating	Many free charges, and almost none
Thermistor and LDR	Responds to temperature, and responds to light
Ammeter and voltmeter	Measures the current through a component, and the potential difference across it

Quick self-test

1. A current of 2.5 A flows for 4.0 minutes. Calculate the charge.
2. 72 C flows in 90 s. Calculate the current.
3. A current of 40 mA delivers 120 C. How many minutes does it take?
4. A 5.0 V supply drives 18 C round a circuit. Calculate the energy supplied.
5. A lamp transfers 120 J from 15 C of charge. Calculate the potential difference.
6. A current of 0.20 A flows for 5.0 minutes through a component with 4.5 V across it. Calculate the energy transferred.
7. Why do metals conduct but plastics do not?
8. Describe what is meant by static electricity.
9. Which has the lower resistance, a thermistor at 80 °C or at 20 °C?
10. Describe the symbol for an LDR.
11. In the symbol for a cell, which line is the positive terminal?

Answers

1. t = 240 s, so Q = 2.5 × 240 = 600 C.
2. I = 72 / 90 = 0.80 A.
3. 40 mA = 0.040 A, so t = 120 / 0.040 = 3000 s = 50 minutes.
4. E = 18 × 5.0 = 90 J.
5. V = 120 / 15 = 8.0 V.
6. t = 300 s, Q = 0.20 × 300 = 60 C, E = 60 × 4.5 = 270 J.
7. Metals contain many electrons that are free to move. Plastics have almost no free charges.
8. Unbalanced charge on an object or in a region, where the charge has no conducting path to move along.
9. The thermistor at 80 °C, because its resistance decreases as the temperature increases.
10. A rectangle inside a circle, with arrows pointing towards it.
11. The long thin line.

Where marks are usually lost

• Substituting a time in minutes or hours into Q = It.
• Forgetting to convert milliamperes to amperes.
• Writing “current is used up” instead of saying that energy is transferred.
• Writing “voltage flows” or “current across a component”. Current flows through, and potential difference is across.
• Describing lightning without saying that charge builds up and the air then conducts.
• Answering a discharge question without mentioning a conducting route.
• Stating the thermistor and LDR rules the wrong way round.
• Drawing the voltmeter in the loop or the ammeter across a component.
• Missing the unit, or writing C for current and A for charge.

Official syllabus

OxfordAQA International GCSE Physics (9203), specification version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Section 3.5.1 Electrical circuits, statements a to g.