AQA GCSE Physics 4.2.1 Current, Potential Difference and Resistance Exam Questions

About AQA GCSE Physics 4.2.1 Current, Potential Difference and Resistance

This is the foundation of the AQA GCSE Physics Electricity topic. You learn the standard circuit symbols, the relationships Q = I t and V = I R, and how current changes with potential difference for different components. Two required practicals sit here: investigating factors that affect resistance (such as wire length) and plotting I-V characteristics for a resistor, filament lamp and diode. It is examined on Paper 1 and leads directly into series and parallel circuits. Examiners test circuit drawing, interpreting I-V graphs and explaining the behaviour of thermistors and LDRs.

What you need to know

• Circuit symbols Know the symbols for a switch (open and closed), cell, battery, diode, resistor, variable resistor, LED, lamp, fuse, voltmeter, ammeter, thermistor and LDR, and draw circuits using them.
• Charge and current Current is the rate of flow of electrical charge. Q = I t, where Q is charge in coulombs (C), I is current in amperes (A) and t is time in s. The current is the same at any point in a single closed loop.
• Potential difference and resistance V = I R, where V is potential difference in volts (V) and R is resistance in ohms (Ω). The current through a component depends on both the resistance and the potential difference across it.
• Ohmic conductors For an ohmic conductor at constant temperature, current is directly proportional to potential difference, giving a straight line through the origin on an I-V graph.
• Non-ohmic components A filament lamp's resistance increases as temperature increases, giving a curved I-V graph. A diode lets current flow in one direction only, with very high resistance in the reverse direction.
• Thermistors and LDRs A thermistor's resistance decreases as temperature increases. An LDR's resistance decreases as light intensity increases. They are used in thermostats and automatic night lights.
• Required practicals Use an ammeter in series and a voltmeter in parallel to investigate how wire length affects resistance (resistance is proportional to length) and to plot I-V characteristics, reversing the supply to get negative values.

Common mistakes in 4.2.1 Current, Potential Difference and Resistance

• Connecting the voltmeter in series. It must be connected in parallel across the component; the ammeter goes in series.
• Saying a filament lamp's resistance increases because current increases. It increases because the filament gets hotter, so ions vibrate more and impede the flow of electrons.
• Saying current is 'used up' by components. The current is the same all the way round a series loop.
• Mixing up thermistor and LDR behaviour. Thermistor: resistance falls as temperature rises. LDR: resistance falls as light intensity rises.

Try three 4.2.1 Current, Potential Difference and Resistance exam-style questions