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
sciencephysics 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 I_{1}, I_{2}
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 drawingconstructing 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.
TOP OF PAGE
Examples of simple circuits and their interpretation
These are circuit
diagrams copied from my
KS3 sciencephysics 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 twoway 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 = I^{2}R
(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
TOP OF PAGE
Electricity and
magnetism revision
notes index
1.
Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I^{2}R,
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, IV 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 lefthand rule, F = BIL gcse physics
12.
Generator effect, applications e.g. generators
generating electricity and microphone
gcse
physics revision
IGCSE revision
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