10.2Standard 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.
10.3Series and parallel
There are two ways of joining electrical components, in series and in parallel. Some circuits include both series and parallel parts.
For components connected in series:
• there is the same current through each component • the total potential difference of the power supply is shared between the components • the total resistance of two components is the sum of the resistance of each component:
Rtotal = R1 + R2 resistance, R, in ohms, Ω
For components connected in parallel:
• the potential difference across each component is the same • the total current through the whole circuit is the sum of the currents through the separate components • the total resistance of two resistors is less than the resistance of the smallest individual resistor.
You should be able to describe the difference between series and parallel circuits, and use circuit diagrams to construct and check series and parallel circuits that include a variety of common circuit components.
- Series circuit
- a circuit in which components are joined one after another, so the same current passes through each, the supply potential difference is shared between them, and the total resistance is the sum of the separate resistances
- Parallel circuit
- a circuit in which components are joined across the same two points, so each has the same potential difference across it and the total current is the sum of the currents through the branches
10.7–10.11Electrical 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
10.4–10.6, 10.12–10.13Current, 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
Before you practise
- Learn the standard symbols — a diagram drawn with the wrong symbol says the wrong thing.
- A series circuit has one path; a parallel circuit has more than one, so each branch gets the full potential difference.
- An ammeter goes in series with the component; a voltmeter goes in parallel across it.
- charge = current × time, in coulombs.
- Current is the rate of flow of charge, and in a metal it is a flow of electrons.
- Current is the same everywhere in a series circuit, and is conserved at a junction in a parallel one.
- Potential difference is the energy transferred per unit charge: energy transferred = charge × potential difference.
- A current only flows if the circuit is complete and there is a source of potential difference.