POWER AND ENERGYLesson 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).

FORMULASPower 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 WTherefore, 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 ΩP = ?

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 WV = 120 VI = ?

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 WΔ 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) C1A 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) C1A 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) C1A 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) C110 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