Download EMF and Internal Resistance

How We Load Power Supplies
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All sources have an EMF.
EMF is the open terminal voltage of the
battery.
All sources have a certain amount of internal
resistance.
Perfect batteries have 0 internal resistance.
A car battery has very low internal resistance
so is almost perfect
The car battery has a
very low internal
resistance. This means
that it can give out the
heavy current needed by
a starter motor.
You can see the heavy
wires leading to the
starter motor
E
This time we find that the terminal
voltage goes down to V.
V
It has been lost due to the internal
resistance which heats the battery
up.
V
I
Emf = Useful volts + Lost volts
V
R
In code:
E=V+v
E
r
Internal resistance
I
V
R
The cell is now a perfect battery in series with an
internal resistor, r.
o Emf = voltage across R + voltage across the internal
resistance
E =
V
+
v
o We also know from Ohm’s Law that V = IR and v =
Ir, so we can write:
E = IR + Ir
 E = I(R + r) or E = V + Ir
E
oWe adjust the variable
resistor so we can record
a range of voltages and
currents.
r
I
V
R
o We use the switch to
avoid flattening the
battery, and preventing
the variable resistor from
getting too hot.
o We plot the results on a
graph.
P.d.
(V)
Current
(A)
The graph is a straight line, of the form y = mx + c. We can make the
equation for internal resistance V = -rI + E. There are three features on the
graph that are useful:
 The intercept on the y-axis tells as the emf.
 The intercept on the x- axis tells us the maximum current the cell can
deliver when the p.d. is zero, i.e. a dead short circuit.
 The negative gradient tells us the internal resistance.