ELECTRICITY 2: Electrical circuits and how to draw them, circuit symbols, parallel circuits, series circuits

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 What is an electric circuit and what is an electric current?

 How do you draw an electric circuit?

 How do you interpret a circuit diagram?

 Do you know your circuit symbols?

 What is the difference between a series circuit and a parallel circuit?

Can you interpret what happens when a circuit is switched on?

See APPENDIX 1 for a summary of all electricity equations you may need.

QUIZ on "Electrical circuits" Basic revision questions from KS3 science-physics on simple circuits, circuit symbols and components, current flow & ammeter readings, useful circuits - hazards and how they work - what have you remembered?

On this page I've referred to relative ammeter readings as a1, a2 etc., but on all other pages I1, I2 etc. will be used.

What is an electric current? What is an electrical circuit?

The diagram circuit 01 (right) is the simplest sort of electrical circuit that can do anything useful e.g. lighting a bulb (symbol ) using a single cell battery (symbol ).

The switch is closed ('on', symbol ) to complete the electrical circuit in which all the components must be connected together with an electrical conductor such as a copper wire.

This is one of the simplest circuit diagrams you can draw - so get used to them asap!

Circuit 01 is a simple closed loop and the current will be the same at any point in the circuit.

Lots more on circuit symbols in the next section and is just the wire connections!


CURRENT - An ammeter (symbol ) is included to measure the current - the rate of flow of electrical charge - usually negative electrons.

The unit of current is called the ampere, symbol A.

The flow of electric charge is usually the flow of the tiny negative particles we call electrons.

A current of electric charge can only flow round a complete circuit - as the diagram - no gaps in the wires! AND there must be source () of potential difference (p.d.) like a cell or battery to drive the electrons around.

POTENTIAL DIFFERENCE - It is the electrons (the 'charge') that transfer the electrical energy from a 'higher potential' to a 'lower potential'.

The unit of potential difference (p.d.) is the volt, symbol V e.g. a simple single torch battery might give a p.d. of 1.5 V, a car battery might deliver 12 V from six 2 V cells wired one after the other in series - more on wiring in series later.

It is the potential difference that drives the electrons round a circuit and if you increase the p.d. then you push more electrons along in a given time i.e. you increase the current.

It is the potential difference ('voltage') that 'pushes' the electrical charge (-ve electrons) around the circuit.

If the p.d. is > 0 V, current flows in one direction, if the p.d. is <0 V, the current flows in the opposite direction!, and if the p.d. = 0 V, no current flows!

The everyday term 'voltage' is strictly speaking not correct, in an exam use 'potential difference' once and then use the abbreviation 'p.d.' after that.

Circuit diagrams must be drawn with the correct symbols for the components, and normally, wires are drawn as straight lines and the switch closed ('on') to complete the circuit - so it looks as if it works!

You should be able to follow the wire from one end ('terminal') of the power supply to the other and passing through any components in the circuit.

Circuit 29 (right) is essentially the same as circuit 01 above with a resistor (symbol ).

A resistor is a two terminal component that resists the flow of electric charge - reduces the current.

It is often a thin wire relative to the width of the wire used for the rest of the circuit. This thin resistance wire can convert electrical energy into heat and light (filament bulb), heat (heating element) or just light (LED lamp).

RESISTANCE - A resistance is any component that restricts the flow of charge i.e. it opposes the current flow.

The unit of resistance is the ohm, symbol Ω.

The current flowing through a resistor depends on two factors:

(i) for a given fixed resistance, the larger the potential difference, the larger the current,

(ii) for a given fixed potential difference, the greater the resistance of a resistor, the lower the current.

For more details see 3. Ohm's Law, experimental investigations of resistance, simple graphs and calculations

where we will introduce how to wire up and use a voltmeter.

Every cell (battery) has a positive (+) and negative (-) terminal and by convention the current flows from the positive terminal round to the negative terminal (clockwise here).

Note 1: Current convention and chemistry!

This electrical current convention may be a problem in chemistry because the electrons actually flow in the opposite direction! That is, anticlockwise in circuit 29 - it is logical that negative electrons flow from negative to positive. It is important you understand this because in chemistry you study electrolysis and need to know what the electrons are doing! The reason for this clash is the current convention was adopted before scientists knew about electrons!)

Note 2: Alternating current (ac) and direct current (dc)  (for future reference)

With an alternating current (ac), the current changes direction in a cycle e.g. 5O Hz and the potential difference goes through a cycle +/- V.

With a direct current (dc) there is no reversal in current direction, it flows one way with a constant voltage (pd/V).

Oscilloscope traces comparing ac and dc current signals - showing the changing direction + <=> - oscillation of the alternating current p.d. and the constant p.d. of a direct current.

Note that some devices in the home work off a dc current - but the output from e.g. the transformer in your computer power supply, is rectified to convert it to a dc supply.


Circuit symbols and symbolism used in drawing-constructing circuit diagrams

circuit symbol for the wire in a electrical circuit.

circuit symbol for a T junction in the circuit wires.

circuit symbol for a closed switch, this completes a circuit so that it is 'on' and current flows.

circuit symbol for an open switch, this breaks a circuit so that it is 'off', and current can't flow.

circuit symbol for a two way switch, in which one route is 'open' and the other 'closed'.

, , , circuit symbols for 1, 2, 3 or many cells  wired in series (>1 cell often referred to as a 'battery'), the short stubby vertical line is the negative pole and the long thin vertical line is the positive pole.

Components in a series are wired in line with each other, end to end connecting with the +ve and -ve terminals of the power supply.

If you have two 1.5 V batteries wired in series, you add them up to get the total p.d. of 3.0 V.

You do exactly the same with resistors e.g. a 3.0 ohm and 5.5 resistor wired in series act as a total resistance of 8.5 ohms.

The 4th symbol often indicates a battery like that in a car, made up of multiple individual cells wired in series.

circuit symbol for two cells wired in parallel.

When components are wired in parallel, each one is separately connected to the +ve and -ve terminals by being connected to the main circuit at each end of the component's terminals.

If you have two cells producing the same p.d. wired in parallel, the p.d. of the circuit is just the same as one cell.

The two symbols for an electricity supply.

 Direct current (d.c. or dc) means the current only flows in one direction and the convention current flows from positive (+) to negative (-). Electrons actually flow in the opposite direction!

  Alternating current (a.c. or ac) switches direction in a continuous oscillation e.g. 50 Hz i.e. changing direction 50 times a second.

circuit symbol for a resistor, which resists the flow of an electrical current e.g. in a component, often a thinner wire than the rest of the circuit wire.

or are circuit symbols for a variable resistor.

It behaves like any other resistor, BUT, the resistance can be varied e.g. by turning a mechanical slider like in a dimmer switch for a lamp in a room.

The more of the thin resistance wire the current goes through, the greater its resistance and the smaller the current.

In the school laboratory you may come across it as a rheostat by which you can alter the resistance by moving a slider along a resistance wire.

circuit symbol for a filament single lamp bulb.

circuit symbols for two lamp bulbs wired in series.

circuit symbols for two lamp bulbs wired in parallel.

circuit symbol for a voltmeter that measures the potential difference in volts (p.d. in V).

The voltmeter is always wired in parallel across another circuit component to measure the p.d. in volts across it.

circuit symbol for an ammeter, an instrument that measures the flow of electrical current in amps (A).

This may be wired in series or parallel depending on which part of a circuit you want to know the current flow.

circuit symbol for a fuse. This melts and breaks the circuit if the current increases above a safe limit.

circuit symbol for a diode, sometimes the symbol is enclosed in a circle

A diode only allows a current to flow in one direction.

circuit symbol for a thermistor whose resistance changes with temperature i.e. the current allowed to flow varies with temperature.

circuit symbol for a light emitting diode (an LED), a semiconductor device that changes electrical energy into light energy i.e. it glows when a potential difference (voltage) is applied across it.

It is a much more efficient device than a hot filament lamp bulb.

circuit symbol for a light dependant resistor (LDR), sometimes the rectangle is enclosed in a circle

The resistance of an LDR changes depending on the intensity of light that shines on it.

The greater the light intensity, the lower the resistance and the greater the current flow.

circuit symbol for an electric motor, sometimes its just a circle with an M in it

Circuit symbols (as far as I know) NOT needed for UK GCSE physics courses

circuit symbol for capacitor, a device that stores energy in the form of electrically charged field between its plates.

circuit symbol for microphone, that converts a sound wave into an electrical signal.

circuit symbol for loudspeaker, that converts an electrical energy signal into sound energy.

circuit symbol for a transformer, which converts an a.c. current of one voltage in one input coil into an a.c. current of a different voltage in a second output coil.

circuit symbol for a bell.

circuit symbol for a buzzer.


Examples of simple circuits and their interpretation

These are circuit diagrams copied from my KS3 science-physics quizzes.

I just want you to think in 'simple' conceptual way e.g. which bulbs light up and how brightly AND compare current flow in different parts of the circuits.

I've rarely included the rectangular resistor circuit symbol here, but don't forget a lamp bulb is a resistor.

These circuit diagrams in include ammeters, switches and a simple battery power supply.

Wiring in series or parallel in the circuits is discussed.

Assume all ammeter readings e.g. a1, a2 etc. are in amperes (A).

No specific resistors or voltmeters are included at the moment and no calculations yet either!.

1. Circuit diagram 01: 1 ammeter, 1 switch, 1 cell,  and 1 bulb all wired in series in a simple single loop.

Assume bulb glows with normal brightness, so 1 cell powers 1 bulb correctly - not too dim or 'blows' the bulb!

2. Circuit diagram 02: 1 ammeter, 1 switch, 2 cells and 2 bulbs all in series.

Here we have doubled the potential difference (p.d.), but we have also doubled the resistance, the effects cancel each other out, therefore the lamp will glow with normal brightness.

3. Circuit diagram 03: 1 ammeter, 1 switch, 2 cells in series with 1 bulb all wired in series.

Here, doubling the p.d. will double the current and the bulb will glow more brightly than in circuits 01 and 02 (will probably blow the bulb!).

4. Circuit diagram 04: 1 ammeter, 1 switch, 1 cell and 2 bulbs all wired in series.

Here, doubling the resistance will halve the current and the bulbs will glow dimmer than in circuits 01 and 02.

5. Circuit diagram 05: 1 ammeter, 1 switch, 3 cells and 3 bulbs all wired in series.

Here we have tripled the p.d., but also tripled the resistance, so the bulbs will glow normally as in circuits 01 and 02.

6. Circuit diagram 06: 1 ammeter, 1 switch, 3 cells and 2 bulbs all wired in series.

Here the bulbs will glow a little more brightly than in circuits 01 and 02. Can you figure out why?

7. Circuit diagram 07: 1 ammeter, 1 switch, 3 cells and 1 bulb all wired in series.

Here the bulb will glow VERY bright for a few seconds and then burn out!

You have tripled the p.d. but kept the minimum of one resistance, too much current flowing for the bulb filament!

8. Circuit diagram 08: 1 ammeter, 1 switch, 1 cell and 3 bulbs all wired in series.

Compared to circuit 07, here the bulbs will glow very dimly, much less so than in circuits 01 and 02.

You have tripled the resistance and kept the minimum p.d.

Therefore the current flow is much lower than in circuit 07, less electrical energy to light the bulbs.

9. Circuit diagram 09: 1 ammeter, 1 switch, 1 cell and 3 bulbs all wired in series.

Here the bulbs will glow a little bit dimmer than their 'normal' brightness. Can you see why?

10. 11. Circuit diagrams 10/11:  1 ammeter, 1 switch, 2 cells in series with pairs of ammeters and bulbs wired in parallel.

Note the two slightly different styles of drawing the circuit - they both amount to the same things.

Here things are getting a bit more complicated and I'm introducing what the relative ammeter readings might be.

From now on, I'm less interested in how bright the bulbs glow, but what might the relative ammeter readings be?

Circuits 01 to 09 were simple loops and the current flow is identical at any point in the circuit.

However, here, the current is split to power each bulb individually in the parallel sections of the circuit.

The ammeter current readings a1 + a2 MUST equal ammeter reading a3 because the current flowing from the battery, even if it is split, it must be the same in total. You can't lose or gain electrons!, so a1 + a2 = a3.

Also ammeter readings a1 = a2, assuming the bulbs have the same resistance, so the same current will flow through them equally as they both experience the same p.d.

In section 3. Ohm's Law we will look at these situations in a quantitative way.

12. Circuit diagram 12: Here everything is wired in a simple loop.

The bulbs b1 and b2 will glow normally and with equal brightness, assuming they are of equal resistance.

Since everything is wired in series, all the ammeter readings will be the same, a1 = a2 = a3.

13. 14. Circuit diagrams 13/14:

Same as circuits 10/11 except nothing happens until you close the switches!

To light a bulb you must close switch s3 and either/both switches s1 and s2.

Here you can light each bulb individually, which you cannot do if they were wired in series.

15. Circuit diagram 15: Everything wired in series.

Same as circuit 12 except nothing happens until you close the switches,

and all 3 switches must be closed to light the bulbs!

16. Circuit diagram 16: The bulbs will glow very brightly and the filaments will probably burn out!

Can you see why the lamps might just light for a few seconds before going out!?

17. Circuit diagram 17: The bulbs will glow very dimly, the 4 bulbs equate to a high total resistance.

When resistances e.g. lamp bulbs are wired in series, you add them up to get the total resistance.

18. Circuit diagram 18: 1 ammeter, 1 switch, 2 cells wired in series with 3 pairs of ammeters and bulbs wired in parallel.

If you followed the arguments for circuits 11/12 you should be able to deduce the following:

All three bulbs b1 to b3 will glow with the same brightness - all subjected to the same p.d.

Relative ammeter readings:

a1 = a2 = a3 (assuming all bulbs have the same resistance).

Total current flowing in the circuit = a4 = a1 + a2 + a3

19. Circuit diagram 19: This simple loop circuit includes a variable resistor ().

By varying the resistance, you can vary the current flow and control how brightly the bulb glows.

This is the simplest circuit to illustrate how a dimmer switch works.

The greater the resistance, the lower the current, the dimmer the bulb lights up.

21. Circuit diagram 21. Several sets of bulbs all wired in parallel.

In terms of ammeter readings and bulb brightness:

a4 = a1 + a2 + a3, but a1, a2 and a3 ammeter readings will all be different because of the different numbers of bulbs, that is each sequence of bulbs equates to a different resistance for the same potential difference.

When you have bulbs wired in series you add up the individual resistances to get the total resistance.

So, in circuit 21, for the bulbs, we have relative resistance values of 1 : 2 : 3 (left to right).

The bigger the resistance, the lower the current, so the relative ammeter readings will be a1 > a2 > a3,

and the brightness sequence for the bulbs is b1 > b2 > b3.

22. Circuit diagram 22: This is a two-way switch system e.g. for a landing light in a house.

You can switch the light on from two different locations e.g. the ground floor and first floor of a house.

25. 26. Circuit diagrams 25: When you close the switch s, only bulb b2 will light up.

The extra wire 'short circuits' and bypasses bulb b1 - virtually no current will flow through it.

The extra wire will have offer less resistance than the thin bulb filament.

In circuit 26 it is the same situation and only bulb b2 lights up AND you don't even have to close the switch.

27. Circuit diagram 27: Following on from circuits 25 and 26, when you close the switch only bulb b1 will be lit.

Virtually no current will flow through bulb b2.

APPENDIX 1: Important definitions, descriptions and units

Note: You may/may not (but don't worry!), have come across all of these terms, it depends on how far your studies have got. In your course, you might not need every formula - that's up to you to find out.

V the potential difference (p.d., commonly called 'voltage')) is the driving force that moves the electrical charge around a circuit. The p.d. across any part of a circuit is measured in volts, V.

I the current = rate of flow of electrical charge in coulombs/second (C/s), measured in amperes (amps, A).

Formula connection: Q = It,  I = Q/t,  t = Q/I, Q = electrical charge moved in coulombs (C), time t (s)

R a resistance in a circuit, measured in ohms (Ω). A resistance slows down the flow of electrical charge.

Formula connection: V = IR,   I = V/R,   R = V/I  (This is the formula for Ohm's Law)

P the power delivered by a circuit = the rate of energy transfer (J/s) and is measured in watts (W).

Formula connection: P = IV,  I = P/V,  V = P/I   also  P = I2R   (see also P = E/t below)

E = QV,  Q = E/V,  V = E/Q,   E = energy transfer in joules (J), Q = electrical charge moved (C), V = p.d. (V)

E = Pt,  P = E/t,  t = E/P,  where P = power (W), E = energy transferred (J), t = time taken (s)

Formula connection: Since E = Pt and P = IV, energy transferred E = IVt


Electricity and magnetism revision notes index

1. Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I2R, E = Pt, E=IVt

2. Electrical circuits and how to draw them, circuit symbols, parallel circuits, series circuits explained

3. Ohm's Law, experimental investigations of resistance, I-V graphs, calculations V = IR, Q = It, E = QV

4. Circuit devices and how are they used? (e.g. thermistor and LDR), relevant graphs gcse physics revision

5. More on series and parallel circuits, circuit diagrams, measurements and calculations gcse physics revision

6. The 'National Grid' power supply, environmental issues, use of transformers gcse physics revision notes

7. Comparison of methods of generating electricity gcse physics revision notes (energy 6)

8. Static electricity and electric fields, uses and dangers of static electricity gcse physics revision notes

9. Magnetism - magnetic materials - temporary (induced) and permanent magnets - uses gcse physics revision

10. Electromagnetism, solenoid coils, uses of electromagnets  gcse physics revision notes

11. Motor effect of an electric current, electric motor, loudspeaker, Fleming's left-hand rule, F = BIL gcse physics

12. Generator effect, applications e.g. generators generating electricity and microphone gcse physics revision

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