In an exam you might not be told whether the
components are wired in series or parallel - you are expected to know!
In a parallel circuit, each component is
connected separately to the terminals of the power supply e.g. the +ve and
-ve terminals of a dc supply (as in the circuit diagram 35 below).
If you disconnect one component, e.g. one
of the resistors, the other components will keep working.
That is why most electrical appliances
include parallel circuit systems and enables you to switch individual things
'on and off' without affecting other devices.
Practically everything in a car is
wired in parallel and is independently controllable e.g. heater,
windscreen wiper, headlamps etc. It would be bad news if one thing
failed and the rest did the same!
In reality, most circuits are a mixture of series and
parallel wiring.
PARALLEL circuit of two
resistors
You can include a variable resistor in
the circuit to vary the p.d. and current and make a varied series of
readings to verify the pattern of resistance - which should stay constant if
they don't heat up ...
... and check that the potential
difference across each resistor is the same.
R1 and R2 are resistors
(e.g. any device/resistance) wired in parallel and again I'm assuming the circuit is
powered by a 12.0 V d.c. supply from batteries or power pack.
Reminder: Apart from the
resistors wired in parallel, if any other part of the circuit is
disconnected, then the whole circuit fails - no current can flow
Reminder: Wiring in parallel means each component is
independently connected to the positive and negative terminals of the power
supply - in this case two resistors on separate loops ...
... therefore if either
resistor 1 or 2 fails or removed-disconnected breaking that part of the
circuit, the other resistor will continue to function. This is
one reason why parallel circuits tend to be more useful than series
circuits.
It
also means you can control each component independently of the other.
For example, look at circuit 14 on
the right (ignoring the ammeters!).
This is a simple way to control two
lamps wired in parallel.
After closing switch s3 (power supply
on) you can close s1 or s2 to select the lamp to be lit b1 or b2.
Actually, the vast majority of
circuits of practical use include a mixture of series and parallel
wiring.
Ammeters: Each resistor R1
and R2 is wired in series with its own ammeter A1
and A2.
Ammeter A3 is
wired in series with everything
else.
Therefore the possible ammeter
readings are: I1, I2 and
I3
A (amps).
Ammeters are always wired in
series with any component whose current is being measured.
Voltmeters V1 and
V2
are wired in parallel with each resistor to measure the potential difference (p.d.) across
it.
Reminder: Wiring in parallel means each component is
independently connected to the positive and negative terminals of the power
supply - i.e. on separate loops ...
AND this is how you must always
wire a voltmeter in a circuit,
and ammeters are always wired
in series, at any point in any circuit.
Using some basic rules and Ohm's Law (V = IR) I'll
show you how to work out all sorts of things from just only the three pieces
of data given.
(a)
The current flowing through each resistor and the rest of the circuit
Since the resistors are wired in
parallel, they have the same maximum source p.d. across them (V1
= V2 = 12.0 V
in this case).
(You couldn't say this for the series
circuit where the total p.d. is divided up between the resistors.)
Another consequence, of wiring
components in parallel, is that the current is split between the
two or more component sections ('loops') of that part of the
circuit.
Calculations using Ohm's Law equation:
I
= V/R
I1 = Vtotal/R1
= 12.0/6.0 =
2.0
A and I2
= Vtotal/R2 = 12.0/2.0 =
6.0
A
You can add together the separate currents in
the parallel part of the circuit to obtain the total current, which must
be the same in the rest of the circuit.
I3 = I4 = I1
+ I2 = 2.0 + 6.0 =
8.0
A
Note:
(i) The current is shared
between the two branches in the circuit.
(ii) The larger the resistance,
the smaller the current:
The 2 ohm allows a flow of 6.0 A
and the 6 ohm allows 2.0 A.
(iii) Comparison with the series
circuit 34 shows that the two same resistances wired in parallel,
offer a much smaller resistance than when they are wired in series
and the current is greater.
(iv) If the two resistors R1
and R2 had
the same resistance, ammeter readings I1 and I2 would be
equal, for example if they
were two identical bulbs, they would glow with the same brightness.
parallel
circuit
series circuit