Tell the electric company how
much electrical energy is used
Measure kW hrs (energy units
like Joules)
In most homes and factories outlets
vary in voltage (110 V, 220 V, etc)
To determine energy use per outlet
 V=IR, I = q/t, W = Vq
Meters measure voltage and current
instantaneously and calculate power
and energy
Power = energy = Vq
Since I = q
Power, P = VI
Units: (A)(V) = C J = J = W
s C s
**Electrical meters measure kW hrs → (P)(t)
Devices (TV, Refrigerator, etc) are rated for maximum
power/voltage/etc. (They stop working if exceeded)
Devices can be overpowered by accident by
 overloading outlets – draws too much current
plugging too many things into the same outlet
Ground Fault (short circuit) – contact is made
between live and ground conductors
Example: Working hair dryer in water – plumbing is
connected to ground
Preventing Accidental Overload
Circuit breakers
small filaments (like light bulbs)
Burn out when too much current is drawn
bimetallic strip heats up when too much current is drawn
bends when heated which opens switch to prevent
current flow
GFI outlets and breakers
Complex circuitry that senses too much current almost
 Opens circuit at outlet or breaker
0.00l A and higher – feel shock
0.01A and higher – unable to release
0.02 A and higher – paralyzes respiratory muscles
(you can’t breath)
0.1 A and higher – ventricular fibrillation (erratic
1.0 A and higher – “cooked”
Skin has very high resistance
– approximately 100,000 Ω
So, V = IR
10 V = I (100,000 Ω)
I = 0.0001 A
Remember! You can’t have a current without a
To calculate the power dissipated in a resistor:
P = IV
If I is unknown then because V = IR, P = V2/R
If V is unknown then because V = IR, P = I2R
To calculate the total power dissipated by the circuit use
the total current and total voltage
Total power dissipated in the circuit = sum of the power
dissipated in each resistor
Ptotal = P1 + P2
Figure 3

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