Explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes.
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GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Resistors
Current, potential difference and resistance
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the scientific explanation of resistors to a relevant example.
Analyse a new situation involving resistors and explain the scientific reasoning.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
Power Sources and Switches
- A cell or battery provides electrical power to a circuit.
- A switch controls the flow of electricity by being either closed (allowing current to flow) or open (breaking the circuit).
Lamps, Fuses, and Diodes
- A filament lamp is a small bulb that produces light when current flows through it.
- A fuse is a safety component that breaks (melts) if too much current flows through the circuit, protecting other components.
- A diode only allows current to flow in one direction, blocking it in the reverse direction.
- A light-emitting diode (LED) emits light when current flows through it in the forward direction, and is commonly found in alarm clocks, traffic lights, and modern light bulbs.
Measuring Components: Ammeters and Voltmeters
- An ammeter measures current in a circuit and must always be connected in series.
- A voltmeter measures potential difference (voltage) across a component and must always be connected in parallel.
Resistors: Fixed and Variable
- A fixed resistor provides a set, unchanging amount of resistance (measured in ohms, Ω) in a circuit.
- A variable resistor allows the resistance to be manually adjusted, giving control over the current flowing through the circuit.
Light-Dependent Resistors (LDRs)
- An LDR is a resistor whose resistance decreases as the intensity of light increases, and increases in darkness.
- LDRs are used in practical applications such as automatic night lights and burglar alarms, where changes in light levels trigger a response.
Thermistors
- A thermistor is a resistor whose resistance decreases as temperature increases, and increases as temperature falls.
- Thermistors are used as temperature sensors in devices such as car engines and electronic thermostats.
I-V Graph for a Fixed Resistor
- The I-V graph for a fixed resistor is a straight line passing through the origin, showing a directly proportional relationship between V and I.
- Reversing the connections on the power supply produces negative values of both potential difference and current, extending the straight line into the negative region.
- The graph is symmetrical about the origin, with the same gradient in both the positive and negative directions.
- What is a Filament Lamp?
- A filament lamp works by passing a current through a thin metal filament, which heats up to a very high temperature and emits light.
- As the current increases, the filament's temperature rises, which causes the resistance of the lamp to increase.
- Unlike a fixed resistor, the filament lamp is a non-ohmic conductor because its resistance changes with temperature.
I-V Graph for a Filament Lamp
- The I-V graph for a filament lamp is a curve, not a straight line, because resistance increases as temperature increases.
- At low values of potential difference, the graph is approximately proportional (nearly straight), but it curves and flattens at higher values.
- When reversed, the graph produces a mirror-image curve in the negative region, giving an overall S-shaped curve.
- The flattening of the curve at higher potential differences shows that current increases by a smaller and smaller amount as resistance grows.
- What is a Diode?
- A diode is a component that only allows current to flow in one direction, known as the forward direction.
- In the reverse direction, the diode has a very large resistance, which effectively prevents any current from flowing.
- Diodes are used in circuits where it is important to control the direction of current flow.
I-V Graph for a Diode
- In the negative (reverse) region of the I-V graph, the current is zero for all values of negative potential difference.
- In the positive (forward) direction, current only begins to flow at approximately 0.6–1 V, known as the threshold voltage.
- Once the threshold voltage is exceeded, the current rises steeply as potential difference increases.
- The I-V graph for a diode is highly asymmetric, with a flat line at zero in the negative region and a steep curve in the positive region.
Comparing the Three I-V Graphs
- The fixed resistor produces a straight line through the origin, indicating constant resistance and ohmic behaviour.
- The filament lamp produces an S-shaped curve, indicating increasing resistance at higher temperatures.
- The diode produces a graph with zero current in the reverse direction and a steep rise in the forward direction above the threshold voltage.
- Being able to recognise and sketch all three I-V graphs is an essential skill for GCSE Physics.
Ohmic vs Non-Ohmic Conductors
- An ohmic conductor obeys Ohm's Law, meaning current is directly proportional to potential difference at a constant temperature.
- A non-ohmic conductor does not obey Ohm's Law because its resistance changes under different conditions, such as temperature or direction of current.
- The fixed resistor is the only ohmic conductor of the three components studied; the filament lamp and diode are both non-ohmic.
Measuring I-V Characteristics in the Lab
- A voltmeter is connected in parallel with the component to measure potential difference across it.
- An ammeter is connected in series with the component to measure the current flowing through it.
- By reversing the connections on the power supply, negative values of potential difference and current can be recorded, allowing the full I-V graph to be plotted.
- Readings are taken at several values of potential difference to build up a table of results from which the I-V graph can be sketched.