How do the I-V graphs of a resistor, a filament lamp and a diode differ, and what do thermistors and LDRs do?
The I-V characteristics of an ohmic resistor, a filament lamp and a diode, ohmic and non-ohmic behaviour, and how the resistance of a thermistor and an LDR varies with temperature and light.
A focused answer to OCR Gateway GCSE Physics A topic P3 on I-V characteristics, covering the graphs for an ohmic resistor, a filament lamp and a diode, ohmic and non-ohmic behaviour, and how the resistance of a thermistor and an LDR changes with temperature and light.
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What this topic is asking
OCR wants you to describe and explain the I-V characteristics of a resistor, a filament lamp and a diode, distinguish ohmic and non-ohmic behaviour, and describe how a thermistor and an LDR respond to temperature and light. This is topic P3.4 of the OCR Gateway Physics A (J249) specification, linked to the I-V characteristics practical.
I-V characteristics
In the I-V characteristics practical you vary the potential difference across a component with a variable resistor, record the current and voltage, and plot the graph (often for both directions of current).
The fixed resistor (ohmic conductor)
Because the resistance is constant, with a fixed produces the straight line. The steeper the line, the lower the resistance (more current for the same voltage).
The filament lamp
This is the practical reason a filament lamp is not an ohmic conductor: the heating effect of the current changes the resistance, so the simple straight-line relationship breaks down.
The diode
A light-emitting diode (LED) is a diode that emits light when a current flows through it in the forward direction, and is widely used as an efficient indicator and light source.
Thermistors and LDRs
A thermistor is a resistor whose resistance decreases as the temperature increases. This makes it useful as a temperature sensor in thermostats, fire alarms, fridges and car engines. A light-dependent resistor (LDR) has a resistance that decreases as the light intensity increases (lower resistance in bright light, higher in the dark), so it is used in light sensors such as automatic street lights and camera exposure controls.
Try this
Q1. State what the straight-line I-V graph of a fixed resistor at constant temperature tells you about its resistance. [1 mark]
- Cue. The resistance is constant (the current is proportional to the potential difference).
Q2. State how the resistance of an LDR changes when it is moved from a dark room into bright light. [1 mark]
- Cue. The resistance decreases.
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 20194 marksDescribe the shape of the I-V graph for a fixed resistor at constant temperature and for a filament lamp, and explain why they differ.Show worked answer →
A P3 question on I-V characteristics worth four marks. For a fixed resistor at constant temperature, the I-V graph is a straight line through the origin, showing the current is directly proportional to the potential difference, so the resistance is constant (it is an ohmic conductor) (2 marks for the straight line and the constant-resistance meaning). For a filament lamp, the graph is an S-shaped curve: as the current increases the filament gets hotter, its resistance increases, so the line curves and the current rises less steeply at higher voltages (2 marks for the curve and the temperature explanation). Markers reward the straight line for the resistor, the curve for the lamp, and heating increasing the lamp's resistance.
OCR 20213 marksDescribe how the resistance of a thermistor changes with temperature, and state one use of a thermistor that makes use of this property.Show worked answer →
A P3 question worth three marks. The resistance of a thermistor decreases as the temperature increases (and increases as it gets colder) (2 marks for the correct direction of the relationship). A use that exploits this is a temperature sensor or thermostat, for example in a fire alarm, an oven, a fridge or a car engine, where the changing resistance is used to detect or control the temperature (1 mark). Markers reward resistance falling with rising temperature and a sensible temperature-sensing application. A common error is to state the relationship the wrong way round.
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