Electricity

1. 5.2 - Internal Resistance, Electrical Power,
Resistors in Series & Parallel
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2. Internal Resistance
All cells are made of materials that have
resistance, this is called Internal
Resistance (r)
If the cell is connected to an External
Resistor (R) then some of the energy is
converted from electrical energy to heat
energy inside the cell.
The PD available for the circuit is EMF (E)
therefore less than the EMF of the
cell.
This picture shows that some of
the EMF is lost before the charges
even leave the battery. PD (V) 2

3. Internal Resistance (cont)
I
Using Ohms Law:
E= Ir + IR
I
For the main Resistance
in the circuit:
V=IR
Missing volts:
Ir
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4. More Internal Resistance
If you connected a wire directly from one end of the I
battery to the other then the Resistance (R) would
be very small.
The Current (I) that flows would be enormous.
I
This would cause loads of heat to be wasted by the
current as it moves through the Internal Resistance
of the battery.
The battery would get very hot and run out of
energy very fast. This is called a short circuit.
Try it with the PhET Circuit Construction Kit:
http://phet.colorado.edu/en/simulation/circuit-
construction-kit-dc or click on the picture
OR connect some steel wool across a battery. 4

5. Electrical Power (P)
Power means the energy per second (Watts or Js-1)
Remember some definitions:
By substitution:
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6. More Electrical Power Equations (P)
Using: and Ohms
Law:
We can find the Power DISSIPATED (converted to heat) by
a resistor.
By substituting from Ohms Law: V=RI OR I=V/R
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7. Combining Resistors
1. Series Circuits
Putting two resistors together is like putting two separate flights of
stairs one after the other. The stairs are harder to climb
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8. Combining Resistors
2. Parallel Circuits
Because there are more ways of getting round the circuit this is like
making the stairs wider. The Resistance is less.
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9. See how combinations of Resistors work:
http://www.walter-fendt.de/ph14e/combrlc.htm or click on the picture
Add Resistors and then highlight areas of the circuit to see the
total resistance. Impedance is another word that can be used for Resistance
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