Spec links: 4.2.1.4 Resistors (thermistors, LDRs and I–V characteristics); 4.2.2 Series and parallel circuits · Reading time: about 4 minutes
Nobody flicks the switch
Walk down any street at dusk and watch. One by one, the streetlights flicker on. Nobody presses a switch. Later, at home, the heating clicks on because the room has cooled down. Again, nobody touched anything.
Both jobs are done by tiny, cheap components that change their resistance. Resistance tells us how hard it is for a current to flow through a component.
Light and heat sensors
A light-dependent resistor (LDR) responds to light. In bright light its resistance is low. In the dark its resistance is high. Many outdoor lights use a light sensor like this. When the resistance rises at dusk, a control circuit switches the lamp on.
A thermistor responds to temperature. As it gets warmer, its resistance falls. As it gets colder, its resistance rises. Thermistors are found in thermostats, fridges, kettles and car engines.
We can work out the current through a sensor using the equation potential difference = current × resistance, or V = I R. Imagine a thermistor in a cold room has a resistance of 4000 Ω. The potential difference across it is 6 V. Rearranging gives I = V ÷ R = 6 ÷ 4000 = 0.0015 A. If the room warms up and the resistance drops to 1000 Ω, the current rises to 6 ÷ 1000 = 0.006 A. The control circuit detects this change and turns the heating off.
Reading I–V graphs
Physicists compare components using an I–V characteristic. This is a graph of current against potential difference.
For a fixed resistor at constant temperature, the graph is a straight line through the origin. Current is directly proportional to potential difference, so the resistance stays the same. This is an ohmic conductor.
A filament lamp gives a curve. As the current increases, the filament gets hotter. The hotter it gets, the higher its resistance. So the line bends and gets flatter.
A diode lets current flow in one direction only. In the reverse direction its resistance is very high, so almost no current flows. On the graph, the line is flat on one side and climbs steeply on the other.
Series or parallel?
Sensors are usually wired in a circuit with other components. In a series circuit there is one loop. The current is the same everywhere. The potential difference from the supply is shared between the components. The total resistance is the sum of the individual resistances: Rtotal = R1 + R2.
In a parallel circuit each component has its own branch. Each branch has the same potential difference. The current splits between the branches. Adding a resistor in parallel actually reduces the total resistance. The total is always less than the smallest single resistor, because the current has an extra path to follow.
Your home lights are wired in parallel. That means one bulb can fail and the others stay on. Next time the streetlights glow at dusk, remember the small sensor quietly changing its resistance.
Key words
- Resistance: a measure of how much a component opposes the flow of current, measured in ohms (Ω).
- Light-dependent resistor (LDR): a resistor whose resistance decreases as light intensity increases.
- Thermistor: a resistor whose resistance decreases as temperature increases.
- I–V characteristic: a graph showing how the current through a component varies with the potential difference across it.
- Ohmic conductor: a conductor in which current is directly proportional to potential difference at constant temperature.
- Diode: a component that lets current flow in one direction only; it has a very high resistance in the reverse direction.
- Parallel circuit: a circuit with more than one branch; each branch has the same potential difference.
Questions
- What happens to the resistance of an LDR in the dark? [1 mark]
- State how the resistance of a thermistor changes as its temperature increases. [1 mark]
- Give two everyday uses of thermistors or LDRs mentioned in the text. [2 marks]
- Explain why the I–V graph of a filament lamp is a curve rather than a straight line. [2 marks]
- Two 20 Ω resistors are connected first in series and then in parallel.
a) State the total resistance when they are in series. [1 mark]
b) Explain why the total resistance is lower when they are in parallel. [2 marks] - An LDR is in a dark room. The potential difference across it is 12 V and the current through it is 0.0004 A. Calculate the resistance of the LDR. Give the unit. [3 marks]
- A thermistor with a resistance of 1500 Ω is connected in series with a 500 Ω fixed resistor and a 6 V battery.
a) Calculate the total resistance of the circuit. [1 mark]
b) Calculate the current in the circuit. Give the unit. [3 marks] - Describe how you could investigate the I–V characteristic of a filament lamp in the lab. Include the equipment you would use and how you would get results for both directions of current. [6 marks]
Show answers
- Resistance increases / is high (1)
- Resistance decreases (1)
- Any two from: streetlights / outdoor lights switching on at dusk (1); thermostats / heating (1); fridges (1); kettles (1); car engines (1)
- As current increases the filament gets hotter / temperature increases (1); so resistance increases (so current is not directly proportional to potential difference) (1)
- a) 40 Ω (1)
b) In parallel there is more than one path for the current / current splits between branches (1); so total current is larger for the same potential difference / total resistance is less than the smallest single resistor (1) - R = V ÷ I (1); R = 12 ÷ 0.0004 (1); R = 30 000 Ω (1)
- a) 1500 + 500 = 2000 Ω (1)
b) I = V ÷ R (1); I = 6 ÷ 2000 (1); I = 0.003 A (1) - Indicative content:
- connect the lamp in series with an ammeter and a variable resistor (or variable power supply)
- connect a voltmeter in parallel with the lamp
- adjust the variable resistor to change the potential difference
- record current and potential difference for a range of values
- reverse the connections to the lamp/supply and repeat to get negative values
- repeat readings and calculate means
- plot a graph of current (y-axis) against potential difference (x-axis)
- switch off between readings so the lamp does not overheat; take care as the lamp gets hot
- Level 3 (5–6 marks): a clear, logical method that would produce valid results, including correct placement of ammeter and voltmeter, a way of varying potential difference, reversing the current and plotting the graph; Level 2 (3–4 marks): a method with most key steps but some detail missing or not fully logical; Level 1 (1–2 marks): some relevant simple statements, e.g. use an ammeter and a voltmeter, but no coherent method
More reading: See all 30 GCSE Physics comprehension articles