1. P OWER AND E NERGY Lesson 9

2. E LECTRICAL S AFETY 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 Slight sensation 9 x10 -3 0.05Shock 0.020.07Muscles paralyzed 0.10.5Fibrillation and death

3. 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.

4. To prevent this from happening, a fuse will blow or a circuit breaker will trip.

5. G ROUNDS

6. 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.

7. E LECTRICAL POWER AND E NERGY 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).

8. F ORMULAS PowerEnergyTime ΔE is the energy transformed in joules (J) Δt is time interval in seconds (s)

9. Power can also be found in other electrical systems when voltage and current is given PowerPotential DifferenceCurrent

10. F ORMULAS FROM THE R ESISTANCE LESSON Resistance VoltageCurrent

11. P OWER IfAnd Than,, Than

12. P OWER IfAnd Than,, Than Also

13. One watt is a small amount of power, so electrical power is often stated in kilowatts (1 kW = 10 3 ) or megawatts (1MW = 10 6 )

14. E XAMPLE 1: H OUSEHOLD P OWER 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 = ?

15. P = IV P = (15 A) x (120 V) = 1800 W Therefore, the maximum power on any 15 amp breakered circuit is 1800 W.

16. E XAMPLE 2: P OWER 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 = ?

17. = 120 W Therefore, the power rating of the stereo is 120 W.

18. We could have also found I using the formula, and then used the formula

19. E XAMPLE 3: P OWER 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 = ?

20. P = 1500 W Therefore, the power dissipated in the circuit is 1500W.

21. E XAMPLE 4: C URRENT 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 = ?

22. I = 1. 5 A Therefore, the TV draws a current of 1.5 A

23. E XAMPLE 5: P OWER 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 10 3 W Δ t = 5.0 min = 5.0 min x 60 s =3.0 x 10 2 s ΔE = ?

24. Δ E = 3.6 x 10 5 J Therefore, the energy consumed is 3.6 x 10 5 J

25. Q UESTIONS 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 10 2 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 10 6 J.

26. For each of the following questions calculate the power of each device. T (2) C1 A hair dryer uses 4.5 x 10 5 J in 5.0 min. A commercial sized heater uses 8.64 x 10 6 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