Section 1 of 9Basic components
You should know the circuit symbols for: cell and battery (power sources), switch, fixed resistor, variable resistor, filament lamp, ammeter (circle with A), voltmeter (circle with V), fuse, diode and LED, thermistor and LDR. Circuit diagrams use straight lines drawn with a ruler, and components are placed in simple loops with no gaps.
Remember that a cell is a single unit and is drawn with one long and one short line, with the longer line being the positive terminal. A battery is drawn with several pairs of lines. Components must be connected in the correct way round when they have polarity, such as diodes and LEDs and electrolytic capacitors.
Section 2 of 9Variable resistor
A variable resistor has a slider that changes the length of resistance wire in the circuit, thereby changing the resistance and the current. It can be used as a dimmer for a lamp or a volume control. When used as a potential divider, all three terminals are connected, so the output voltage varies from zero up to the supply voltage.
A variable resistor can also be used as a rheostat in series with a lamp.
- which is useful for controlling sensors or for the variable supply in an I–V experiment.
Section 3 of 9Thermistor (NTC)
The resistance of a negative-temperature-coefficient thermistor decreases as temperature rises.
A thermistor is used in a potential divider with a fixed resistor, so the output voltage changes with temperature. A transistor or a relay can then use that voltage as a switch to turn on a heater or fan. This idea of a sensor and a switch is the basis of automatic control in heating systems and fire alarms.
Section 4 of 9Light-dependent resistor (LDR)
The resistance of an LDR decreases as the light intensity increases.
- while darkness gives a large voltage across it,
- which can switch on a lamp.
Section 5 of 9Diode and LED
A diode allows current in one direction only (forward direction, the direction of the arrow in its symbol).
To calculate the series resistor for an LED, subtract the LED's forward voltage from the supply voltage and divide by the required current.
- so if it is connected the wrong way round, it will not light and no current will flow.
Section 6 of 9Choosing the right component
The behaviour of these components can be found by measuring I and V and plotting graphs, as in the I–V graphs lesson, or by measuring resistance with a multimeter in different conditions, such as warming a thermistor in a water bath.
Section 7 of 9Worked examples
An LDR in a street-light circuit has a resistance of 2 MΩ in the dark and 400 Ω in daylight. Explain what happens to the current in daylight when the p.d. is constant.
I = V/R, so with a much smaller resistance in daylight the current is much larger. For 6.0 V, current in the dark is 3 µA, but in daylight is 15 mA, 5000 times larger.
A thermistor and a 100 Ω fixed resistor are in series across 6.0 V. At 20°C the thermistor is 200 Ω; at 60°C it is 50 Ω. Find the p.d. across the fixed resistor at each temperature.
At 20°C: R = 300 Ω, I = 0.020 A, V = 0.020 × 100 = 2.0 V.
At 60°C: R = 150 Ω, I = 0.040 A, V = 0.040 × 100 = 4.0 V.
The voltage across the fixed resistor increases as the temperature rises.
A red LED needs 0.020 A and drops 2.0 V. It is connected to a 5.0 V supply. What series resistor is needed?
P.d. across the resistor = 5.0 − 2.0 = 3.0 V.
R = 3.0 / 0.020 = 150 Ω.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Thermistor | Resistor whose resistance changes with temperature. |
| LDR | Light-dependent resistor, resistance falls as light increases. |
| LED | Light-emitting diode. |
| Fuse | Safety device that melts if current is too high. |
| Variable resistor | Resistor whose value can be changed by a slider. |
| Diode | Component allowing current in one direction. |