What are current, potential difference and resistance, and how do series and parallel circuits behave?
Circuit symbols, current as the flow of charge, potential difference and resistance, the equation V equals I times R, the I-V characteristics of components, and the rules for series and parallel circuits.
A focused answer to the OCR Gateway GCSE Combined Science A topic P3 on electric circuits, covering circuit symbols, current as the flow of charge, potential difference and resistance, the equation linking them, the I-V characteristics of components, and series and parallel circuits.
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What this topic is asking
OCR wants you to use circuit symbols, define current, potential difference and resistance, use the equation linking them, describe the I-V characteristics of components, and apply the rules for series and parallel circuits.
Current, potential difference and resistance
The key equation linking them is (potential difference current resistance). For an ohmic conductor (such as a fixed resistor) at constant temperature, the resistance stays constant, so the current is directly proportional to the potential difference, and its I-V graph is a straight line through the origin. In the required practical you measure the current and potential difference for a component and plot the I-V graph to see how its resistance behaves. You should also be able to recognise the standard circuit symbols for a cell, battery, switch, resistor, variable resistor, lamp, ammeter, voltmeter, diode, LED, thermistor and LDR.
Component characteristics
These behaviours come up both as graph-recognition questions and as the reason a sensor works, for example a thermistor in a thermostat or an LDR in a light-activated switch.
Series and parallel circuits
The rules for the two circuit types are a frequent source of marks:
- In a series circuit there is a single loop, so the current is the same at every point, the potential differences add up to the supply, and the total resistance is the sum of the individual resistances (adding a resistor increases the total).
- In a parallel circuit the components are on separate branches, so the potential difference is the same across each branch (equal to the supply), the branch currents add up to the total from the source, and adding a resistor in parallel decreases the total resistance (more paths for the current).
A useful check in a series circuit is that the potential differences across the components must add up to the supply voltage.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksTwo resistors of 4 ohms and 8 ohms are connected in series with a 12 V battery. Calculate the total resistance and the current in the circuit.Show worked answer →
A Physics Paper 5 series-circuit calculation. Method: in a series circuit the total resistance is the sum of the individual resistances, ohms. The current is found from rearranged to A, and in a series circuit this current is the same everywhere. Markers credit adding the resistances to get the total, and using with the total resistance to find the current. A common slip is to use only one resistor's value instead of the total when finding the current.
OCR 20214 marksDescribe how the resistance of a filament lamp changes as the current through it increases, and explain why, then describe the I-V graph you would expect.Show worked answer →
A P3 question on component characteristics. Reward: as the current increases, the filament gets hotter, so its resistance increases. This happens because the higher temperature makes the metal ions vibrate more, which makes it harder for the charge to flow. The I-V graph is therefore not a straight line: it is an S-shaped curve that is steep near the origin and flattens out at higher currents (in both directions), showing that the resistance rises as the current increases. Markers credit the resistance increasing with current, the reason (filament heating up), and the S-shaped (curved) I-V graph rather than the straight line of an ohmic resistor.
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