A battery raises the potential energy of a charge by an amount equal to q?V. ?V is the potential difference of the battery.

1
A battery raises the potential energy of a charge
by an amount equal to q?V. ?V is the potential
difference of the battery.
2
The chemical energy of the battery decreases by
the same value q ?V. As the charge moves
through the circuit the energy of the particle
drops by the same value q ?V.
3
So a battery raises the potential energy of a
charge and a circuit lowers the PE by the same
amount. The voltage rise in a battery is equal to
the voltage drop in the circuit.
4
In any circuit, the total of all voltage rises is
equal to the total of all voltage drops. A
voltage rise is an increase in PE per charge a
voltage drop is a decrease in PE per charge.
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Power is work/time or ?PE/time. Electric power
is calculated from this equation P I ?V.
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The SI unit of power is the watt, W.1 W 1 J/ 1
s.
7
P IV and V IR can be combined to formP
I2R and P V2/R.
8
An electric space heater is connected across a
120 V outlet. It dissipates 1320 W of power.
Calculate the resistance of the heater.
9
Electric companies charge by units of energy, but
they do not use joules. The unit used is
kilowatthours.
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A kilowatthour is the energy delivered in 1 hr
at a constant rate of 1 kW.
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1 kWh 3,600,000 J, or 3.6 X 106 J, or 3.6 MJ.
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How much does it cost to operate a 100.0 W light
bulb for 24 h if electrical energy costs 0.080
per kWh?
13
Electrical wires have an energy loss calculated
by P I2R. This is called an I2R loss.
14
Electric companies want to decrease this I2R loss
when delivering electricity. This can obviously
be done by decreasing I or R.
15
Decreasing I is more effective since it is
squared. P IV so P stays the same if I is high
and V is low or if I is low and V is high.
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Since decreasing I is preferable due to I2R loss,
electricity is transferred at high V and low I.
17
Power plants transport electricity at potential
differences of up to 765,000 V.
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A transformer reduces this voltage to about 4000
V. At your home it is reduced by another
transformer to about 120 V.
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