Section 1 of 7Components that supply and control
Cell and battery
- Cell and battery A cell provides the voltage (potential difference) that drives current round a circuit.
- A battery is two or more cells joined together.
Switch
- Switch A closed switch completes the circuit so current flows;
- an open switch leaves a gap and the current stops.
Fixed resistor
- Fixed resistor Limits the current. Its resistance cannot be changed.
Variable resistor
- Variable resistor Has a slider that changes its resistance, so the current can be adjusted.
- Dimmer switches and volume controls use one.
Section 2 of 7Sensors and one-way components
| Component | What it does | Example use |
|---|---|---|
| Thermistor | Resistance falls as temperature rises and rises as it gets colder | Thermostats in radiators, kettles and heaters |
| Light-dependent resistor (LDR) | Resistance falls as light intensity rises and rises as it gets darker | Street lamps that switch on automatically at dusk |
| Diode | Allows current in one direction only, the direction the triangle points; it can convert a.c. to d.c. | Rectifying circuits |
| Light-emitting diode (LED) | A diode that gives out light when current passes through it | Lighting, TV displays, road signs |
| Lamp | Transfers electrical energy as light and heat | Torches, indicators |
Section 3 of 7Meters
Measuring in circuits
Ammeter
- Ammeter Measures current in amps.
- It is always connected in series, so that the same current passes through it as through the component.
Voltmeter
- Voltmeter Measures potential difference (voltage) in volts.
- It is always connected in parallel across the component being measured.
Resistance of meters
- Resistance of meters In exam questions, an ideal ammeter has negligible resistance and an ideal voltmeter draws negligible current,
- so they do not change the readings.
Section 4 of 7Drawing a circuit diagram
1
Draw a power source first: a cell, battery or power supply with the correct symbol.
2
Add a switch so the circuit can be opened and closed, and check it is drawn closed when current is meant to flow.
3
Add the components in the order given, using only the standard symbol for each.
4
Place the ammeter in series in the main loop, then connect the voltmeter in parallel across the component being measured.
5
Draw the wires with a ruler as straight lines that meet exactly at the symbols, leaving no gaps.
6
Mark conventional current as flowing from the positive terminal round to the negative terminal.
Section 5 of 7Checking a circuit is correct
Needs an energy source
- Needs an energy source Without a cell or power supply there is no potential difference to push charge round.
Needs a complete loop
- Needs a complete loop Electrons can only flow if there is an unbroken path.
- A gap or an open switch stops the current.
Diodes must face the right way
- Diodes must face the right way Current only flows if the diode's triangle points in the direction of conventional current.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Potential difference | The energy transferred per unit charge between two points in a circuit, measured in volts. |
| Series | Connected in a single loop, one component after another, so the same current flows through each. |
| Parallel | Connected across a component on a separate branch. |
| Thermistor | A resistor whose resistance decreases when its temperature increases. |
| LDR | A light-dependent resistor, whose resistance decreases when light intensity increases. |
| Diode | A component that lets current pass in one direction only. |
Section 7 of 7Exam technique
1
Connecting a voltmeter in series with a component: it must be in parallel across it.
2
Reversing the sensor behavioursthermistors and LDRs both have resistance that decreases as temperature or light increases.
3
Drawing a diode facing against the current direction and still expecting current to flow.
4
Leaving gaps in wires or drawing a switch open in a circuit that should be working.
5
Using non-standard or sketched symbols, which may lose marks if the symbol is ambiguous.