Understand current, potential difference and series or parallel circuits with three worked examples and 12 original questions.
Readnary original study guide
Understand current, potential difference and series or parallel circuits with three worked examples and 12 original questions.
Original AI-assisted Readnary material. Not independently academically reviewed; answers may contain errors. Check important results against your course materials.
Selected content from Physics 0625 sections 4.2 and 4.3 for 2026-2028 exams. This guide uses idealised simple resistor circuits; detailed component characteristics and laboratory wiring are not covered.
| Quantity | Meaning | Unit |
|---|---|---|
| Current | charge passing per second | ampere (A) |
| Potential difference | energy transferred per unit charge | volt (V) |
| Resistance | potential difference / current | ohm (ohm) |
Electric current is the rate of charge flow: I = Q/t, so 1 A means 1 C passes a point each second. Conventional current is drawn from the positive terminal around the external circuit to the negative terminal, although electrons in a metal move the other way. Potential difference is energy transferred per unit charge: V = E/Q. A 6 V supply transfers 6 J to each coulomb of charge across its terminals in an idealised account. Charge is not used up in a lamp.
An ammeter measures current through a branch and is placed in series with it. A voltmeter measures potential difference across a component and is placed in parallel with that component. Do not place a standard ammeter directly across a cell. For a fixed resistor at constant temperature, V = IR links potential difference, current and resistance. The ratio V/I need not stay constant for every component: a filament lamp changes temperature and resistance as it warms.
In a series loop, current is the same at every point. The supply potential difference is shared among components, so component voltages add to the supply voltage. For resistors in series, total resistance is the sum of their resistances. Increasing series resistance with the same ideal supply generally reduces the loop current. A broken component opens the only path, so the whole series circuit stops carrying current.
In a parallel circuit, each branch across the same two supply nodes has the same potential difference. The current entering a junction equals the sum of currents leaving it. Adding a branch can increase the total current drawn from an ideal fixed-voltage supply, while the current through an unchanged existing branch stays the same in the ideal model. A break in one branch need not stop current in other intact branches. State any ideal-supply assumption when applying these rules.
A charge of 24 C passes a point in 8 s. Find current.
A fixed resistor has 12 V across it and carries 0.5 A. Find its resistance.
A supply provides 2.0 A to two parallel branches. One branch carries 0.7 A. Find current in the other.
Try each question before opening its answer. Numerical answers should include your working.
I = Q/t = 18/6 = 3 A.
Q = I x t = 0.4 x 20 = 8 C.
V = E/Q = 30/5 = 6 V.
In series with the lamp, so the same branch current passes through the meter.
In parallel across the lamp's two terminals.
R = V/I = 10/2 = 5 ohm.
I = V/R = 6/20 = 0.3 A.
R-total = 3 + 7 = 10 ohm, assuming the resistors behave as stated.
Their potential differences add to 9 V, so the other is 9 - 4 = 5 V.
I-second = 1.5 - 0.6 = 0.9 A by conservation of charge.
6 V across each branch because each connects to the same two supply nodes.
Its filament heats, changing its resistance; do not treat it automatically as a fixed resistor at constant temperature.
Current is conserved at a junction; it is not consumed by a lamp. Equal voltage applies to parallel branches across the same nodes, not to arbitrary series components. The V/I ratio is not always constant for a warming filament.
Physics · IGCSE · STUDY GUIDE
Understand current, potential difference and series or parallel circuits with three worked examples and 12 original questions.
Read the original Readnary guide in PDF view or use the web lesson above. Try the questions before revealing their explained answers.
