1. Power and Energy

2. Power Electric power - the rate at which electric energy is transformed into another form of energy such as light or heat. P=IV P = power (J/s = watt = W) I = electric current (A) V = potential difference (V) The unit of electric power is the watt (1W = 1 J/s)

3. Two Alternate Formulas for Power By substituting V/R for I, P = V 2 R By substituting IR for V. P = I 2 R

4. Power (Joules/sec)– Scale Electric range >12000 W Electric clothes dryer  5000 W Electric shaver  15 W Clock  5 W

5. Travelling Overseas? In the US and Canada the standard outlet works on 120 V. In Europe the standard outlet works at 240 V. This is why an adaptor is needed to use you hairdryer in any hotel room.

6. Light Bulbs The Watts marked on a light bulb indicates how fast it will convert electrical energy into heat and light. Incandescent bulbs – about 2% of energy is converted to light. Fluorescent bulbs – about 9.5% of energy is converted to light (4 X more efficient).

7. Sample Problem Calculate the resistance of a 60 W bulb designed for 12 V.

8. Give it a go What is the maximum power consumption of a 6.0 V tape player that draws a maximum of 450 mA of current? Answer: 2.7 W An 800 W hair dryer has a resistance of 18. What is the current through the hair dryer? Answer: 6.7 A

9. Electrical Energy With electric bills, you pay for energy not power. to calculate the electric energy used by a device multiply power consumption by the time the device is on. E = Pt E = energy (J) P = power (W) t = time (s) Since power is measured in J/s and time is measured in s, energy is measured in J. We could also write the units of energy as Ws.

10. Kilowatt –hour (kWh) The electric company uses a larger unit, the kilowatt-hour (kWh). 1 kWh = 1000 W x 3600 s = 3.6 x 10 6 J Any formula for P can be substituted into the formula Example P = I 2 R so…. E = I 2 Rt

11. Examples An electric heater draws 15.0 A on a 120 V line. How much does it cost to operate the heater for 30.0 days if the heater is used for 3.00 h per day and the electric company charges 10.5 cents per kWh? ($17.00) A small electric furnace operating on 100 V expends 2.0 kW of power. a) What current is in the circuit? (20A) b) What is the resistance of the furnace? (5 Ω) c) What is the cost of operation for 24 h at $0.05/kWh? ($2.40)