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Power and Energy

Power and Energy. 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).

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Power and Energy

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  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=IVP = 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 =V2 R • By substituting IR for V. • P = I2R

  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 = PtE = 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 106 J • Any formula for P can be substituted into the formula • Example P = I2R so…. E = I2Rt

  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)

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