4.2.1.1Standard circuit diagram symbols
Circuit diagrams use standard symbols. You should be able to draw and interpret circuit diagrams.
The symbols are a language, and drawing them sloppily costs marks. Two rules cover most of it: wires are drawn as straight lines meeting at right angles, and a component sits in a gap in the line rather than beside it.
Common mistake
An ammeter goes in the loop; a voltmeter goes across the component. Putting a voltmeter in series is the single most common circuit-diagram error, and it would stop the circuit working.
4.2.1.2Electrical charge and current
For electrical charge to flow through a closed circuit, the circuit must include a source of potential difference.
Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:
charge flow = current × time Q = I t
charge flow, Q, in coulombs, C; current, I, in amperes, A (amp is acceptable for ampere); time, t, in seconds, s.
You should be able to recall and apply this equation.
A current has the same value at any point in a single closed loop. Charge is not used up as it goes round — it is the energy that is transferred, not the charge.
Worked example
A current of 0.25 A flows for 4.0 minutes. Calculate the charge that flows.
- Convert the time to seconds: 4.0 × 60 = 240 s.
- Write the equation: Q = I t.
- Substitute: Q = 0.25 × 240.
- Give the unit: coulombs.
Answer: 60 C
Common mistake
Time must be in seconds. Leaving it in minutes makes the charge 60 times too small — this is the most frequent slip in the whole of electricity.
- Electric current
- a flow of electrical charge; its size is the rate of flow of charge, given by charge flow = current × time
- Potential difference
- the driving force that pushes charge round a circuit, measured in volts
4.2.1.3Current, resistance and potential difference
The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component. The greater the resistance of the component, the smaller the current for a given potential difference across the component.
Current, potential difference or resistance can be calculated using the equation:
potential difference = current × resistance V = I R
potential difference, V, in volts, V; current, I, in amperes, A; resistance, R, in ohms, Ω.
You should be able to recall and apply this equation.
Questions will be set using the term potential difference, but you will gain credit for the correct use of either potential difference or voltage.
Worked example
A potential difference of 6.0 V is applied across a resistor and a current of 0.40 A flows. Calculate the resistance.
- Write the equation: V = I R.
- Rearrange for resistance: R = V ÷ I.
- Substitute: R = 6.0 ÷ 0.40.
- Give the unit: ohms.
Answer: 15 Ω
Required practical 3
Investigate the factors affecting the resistance of electrical circuits
- Draw the circuit first: the test wire or resistors in series with an ammeter and a power supply, and a voltmeter connected across the part being tested.
- For the wire: tape a length of resistance wire beside a metre rule, and connect a flying lead so the length in the circuit can be varied.
- Set a length, close the switch, record the current and the potential difference, then open the switch again straight away so the wire does not warm up.
- Repeat for a range of lengths, calculating R = V ÷ I each time.
- For the resistors: measure the current and potential difference for a single resistor, then for two in series, then for the same two in parallel.
- Repeat each reading and take a mean.
- Resistance
- the opposition to current in a component, measured in ohms; the greater the resistance, the smaller the current for a given potential difference
4.2.1.4Resistors
You should be able to explain that, for some resistors, the value of R remains constant, but that in others it can change as the current changes.
The current through an ohmic conductor at a constant temperature is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.
The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.
The applications are examinable. A thermistor in a thermostat: as the room warms, the thermistor's resistance falls, the circuit detects the change and switches the heater off. An LDR switching lights on when it gets dark: as the light fades, the LDR's resistance rises, and the circuit switches the lamp on.
You should be able to explain the design and use of a circuit to measure the resistance of a component, by measuring the current through and the potential difference across the component, and to draw an appropriate circuit diagram using correct circuit symbols. You should also be able to use graphs to explore whether circuit elements are linear or non-linear, and relate the curves produced to their function and properties.
Required practical 4
Investigate the I–V characteristics of a variety of circuit elements
- Connect the component in series with an ammeter and a variable resistor, with a voltmeter across the component.
- Use the variable resistor to set a series of potential differences, recording the current at each.
- Reverse the connections to the power supply and repeat, so the graph covers negative values as well.
- Plot current on the vertical axis against potential difference on the horizontal axis.
- For the diode, add a protective resistor in series, because a forward-biased diode has almost no resistance of its own.
- For the resistor, take readings quickly and switch off between them, so its temperature stays constant.
Common mistake
A curved I–V graph does not mean the component is broken or the readings are wrong. It means the resistance changes — which is exactly what a filament lamp is expected to do.
- Ohmic conductor
- a conductor in which the current at constant temperature is directly proportional to the potential difference, so its resistance remains constant as the current changes
Before you practise
- An ammeter goes in series; a voltmeter goes in parallel across the component.
- Charge flows only if the circuit includes a source of potential difference.
- Current is the rate of flow of electrical charge: Q = I t, with time in seconds. Recall this equation.
- Current has the same value at any point in a single closed loop.
- V = I R must be recalled; the greater the resistance, the smaller the current for a given potential difference.
- An ohmic conductor at constant temperature gives a straight line through the origin — resistance stays constant.
- A filament lamp's resistance increases as it heats; a diode conducts one way only; a thermistor's resistance falls as temperature rises; an LDR's falls as light intensity rises.
- Required practical 3: resistance against wire length, and resistors in series and parallel.
- Required practical 4: I–V characteristics of a filament lamp, a diode and a resistor at constant temperature.