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POWER AND ENERGY
Lesson 9
ELECTRICAL SAFETY
Electricity can be hazardous; it must be treated with
caution.
 The table below shows human sensitivity to electric
current.

Current (A, AC)
Current (A, DC)
Reaction
4 x10-4
4 x10-4
Slight sensation
9 x10-3
0.05
Shock
0.02
0.07
Muscles paralyzed
0.1
0.5
Fibrillation and
death
Overloaded Circuits are dangerous as they
exceed the circuit’s safe limit.
 This can lead to a short circuit or arc.
 This leads to a circuit with little or no resistance;
this causes a huge increase in current which can
lead to overheating.


To prevent this from happening, a fuse will blow
or a circuit breaker will trip.
GROUNDS


To ensure that electrical products are made to
standard, electrical devices must pass safety
standards. Products sold in Canada must be met
the safety standards of CSA or UL.
Insurance companies will not cover fires started
by products that have not met these standards.
ELECTRICAL POWER AND ENERGY
Power is described as the rate of transforming
energy, essentially the rate at which work is
done.
 Power can be measured in joules per second (J/s),
or watts (W).

FORMULAS
Power
Energy
Time
ΔE is the energy transformed in joules (J)
Δt is time interval in seconds (s)

Power can also be found in other electrical
systems when voltage and current is given
Power
Potential Difference Current
FORMULAS FROM THE RESISTANCE
LESSON
Resistance
Voltage
Current
POWER

If
And

Than,
, Than

POWER

If
And

Than,
, Than

Also

One watt is a small amount of power, so electrical
power is often stated in kilowatts (1 kW = 103) or
megawatts (1MW = 106)
EXAMPLE 1: HOUSEHOLD POWER

What is the maximum power that can be drawn
from a standard 120 V household circuit that has
a circuit breaker (or fuse) of 15 A?
V = 120 V
 I = 15 A
P = ?

P = IV
 P = (15 A) x (120 V)
 = 1800 W


Therefore, the maximum power on any 15
amp breakered circuit is 1800 W.
EXAMPLE 2: POWER TO THE AMP
Calculate the power rating of a stereo amplifier
(not the speaker power) if it is plugged into a
standard 120 V outlet and has a resistance of 120
Ω.
 V = 120 V
 R = 120 Ω
P = ?

=
120 W
Therefore, the power rating of
the stereo is 120 W.

We could have also found I using the formula,
and then used the formula
EXAMPLE 3: POWER IN THE CURRENT

How much power is dissipated in a circuit with a
15 Ω resistor that draws a current of 10.0 A
I
= 10.0 A
 R = 15 Ω
P = ?
P
= 1500 W
 Therefore,
the power dissipated in
the circuit is 1500W.
EXAMPLE 4: CURRENT IN A CIRCUIT

Calculate the current in a small colour TV
connected to a 120 V circuit and uses 180 W
P
= 180 W
 V = 120 V
I = ?
I

= 1. 5 A
Therefore, the TV draws a current of 1.5 A
EXAMPLE 5: POWER USAGE

A 1.2 KW hair dryer is used for 5.0 min.
Calculate the energy consumed in joules and
megajoules.
P
= 1.2 KW, 1.2 x 103 W
 Δ t = 5.0 min = 5.0 min x 60 s
 =3.0 x 102 s
 ΔE = ?
Δ
E = 3.6 x 105 J
 Therefore, the energy consumed is 3.6 x
105 J
QUESTIONS
Calculate the energy consumed for the following
questions. T (2) C1
 A 75 W stereo operated for 16 h.
 An air conditioner rated at 6.5 x 102 W is
operated for 8.2 hours.
 For each of the following, calculate the time the
device was used for. T (2) C1
 A 800 W drill uses 2400 Js, how long was the
drill used for?
 A Coffeemaker 1200 W coffee maker uses 2.16
x 106 J.

For each of the following questions calculate the
power of each device. T (2) C1
 A hair dryer uses 4.5 x 105 J in 5.0 min.
 A commercial sized heater uses 8.64 x 106 J in
1.0 hours.
 Calculate the power produced by a 12 V battery
charger if it delivers T (2) C1
 10 A in the fast charge mode.
 2 A in the trickle charge mode
 A portable heater, rated at 1 KW, is plugged into
a 120 V outlet. How many amperes will it draw?
T (1) C1
