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5.3 Emf and internal resistance. Internal resistance. * is the opposition to charge moving through a source of electricity * causes electrical energy to be dissipated inside the source as charge moves through it. Producing heat !. Electromotive force of a source.
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5.3 Emf and internal resistance
Internal resistance
* is the opposition to charge moving through a source of electricity
* causes electrical energy to be dissipated inside the source as charge
moves through it
Producing heat !
Electromotive force of a source
* is the potential (chemical) energy transferred to electrical energy when
one coulomb of charge passes through it
E = W
Q
5.3 Emf and internal resistance
E
E
V
r
r
A high resistance voltmeter measures terminal Pd.
Open circuit Pd of a cell equals the EMF in volts,
because the current through the cell is negligible.
Energy supplied
per coulomb
by cell
Energy changed
per coulomb
by external circuit
Energy wasted
per coulomb
by internal resistance
=
+
I
EMF
=
Pd across R
+
Pd across r
R
E
=
V
+
v
=
IR
+
Ir
E
=
I ( R + r)
E
5.3 Emf and internal resistance
E
E
V
r
r
Q. Find the internal resistance of a cell if its emf is 3V
and the pd across an external resistance is 2.5 volts
when 0.5 A flows.
EMF
=
Pd across R
+
Pd across r
E
=
V
+
v
“terminal pd”
“lost volts”
I
=
IR
+
Ir
E
=
+
0.5 r
2.5
3
R
=
0.5
0.5 r
r
=
1.0 ohm
5.3 Emf and internal resistance
Power
Power supplied by cell = I E
= I(IR +Ir)
= I2 R + I2r
Useful power delivered to R
Maximum power delivered to R
Power
delivered
to R
* occurs when the load resistance R
matches the internal resistance r
(R+r)I = E
I2 R = E2 R
(R+r)2
I = E
(R+r)
r
Load resistor R
5.3 Emf and internal resistance
Measurement of emf and internal resistance r
* adjust the variable resistor for
different values of current and
record the terminal pd across the cell
r
R limits the
current to
safe levels !
Terminal
Pd /V
2
Emf = 1.5V
1
Circuit current / A
0.5
1.0
5.3 Emf and internal resistance
Measurement of emf and internal resistance r
EMF
=
Pd across R
+
Pd across r
E
=
V
+
v
=
IR
+
Ir
E

Ir
=
E
IR
Terminal
Pd /V
rI

=
E
V
Compare with straight line equation:

2
=
c
mX
Y
Emf = 1.5V
m = gradient
(0, 1.5)
m = Y
m = 0.5 = 0.42
1.2
(1.2 , 1)
1
Int resistance r =
Circuit current / A
0.5
1.0