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Page 1

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Circuit rules
+
I
R1 R2 R3



The electric current is the
same at all points around
a series circuit.
Component p.d's add up to
equal supply EMF.

Page 2

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Circuit Rules
+

V




The potential difference is
the same across all
components connected in
parallel.

Page 3

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Circuit Rules
+

I I
I1

I2

I3


Total current drawn from
the power supply equals
the sum of the individual
component currents.

Page 4

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Resistor combinations

In series:
Rtotal = R1 + R2 + R3 +...

Current is the same
through all components
connected in series

Page 5

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Resistor combinations

In parallel:
1/Rtotal = 1/R1 + 1/R2 +...

Potential difference is the
same across all
components connected in
parallel.
(Shortcut ­ if two resistors
of equal value are
connected in parallel, the
effective resistance is
HALF the value of ONE of
them)

Page 6

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Potential Divider

R1
V1
R2 V2



V2 = V1 x R2 /(R1 + R2)
R1 or R2 may be a variable
resistor, thermistor or
L.D.R

Page 7

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Electromotive Force
Electromotive Force is the
term used for the electrical
potential difference across
any device which acts as a
SOURCE of electrical
energy. This includes
batteries, solar cells,
generators etc.

Page 8

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Electromotive Force
Electromotive Force is the
energy transferred (from
other forms into electrical)
per unit charge.

= E/Q
1 Volt (V) is 1 Joule per
Coulomb.
1 V = 1 JC-1

Page 9

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Source of EMF
So the symbol used for a
battery (or rather a single
cell)...



...is really the symbol for a
source of EMF

Page 10

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Real cell
Strictly speaking, a real
cell should be drawn...



r

...as a source of EMF in
series with a resistance:
the INTERNAL
RESISTANCE of the source

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