Electrical Power, Efficiency, and Consumption 3.1

Quick Formula Recap: Current (I) = amount of charge moving past a certain point, in a specific amount of time  I = q/t = Amps Potential Difference (Voltage) = energy gained or lost by each coulomb  V = E/q (Volts) Resistance = ratio of potential difference across a load to the current through the load  R = V/I (Ohms)

One More Ratio to Consider: We know how much energy each coulomb contains (voltage) But… how fast does each coulomb use that energy? POWER!!

Energy and Power Energy is the ability to do work Power is the rate that energy is produced (or consumed) in a given amount of time Power = Energy/time P = J/s = Watts (W) Pt E

Power Problems: How much energy is used up by a 40 W bulb left on for 45 seconds? G:P = 40W t = 45 s R: E = ? A: P = E/t E = Pt S: E = (40 W)(45 s) E = 1800 J S: 1800 J of energy are used in a 40 W bulb, in 45 seconds

Power Problems: A motor uses up 4000 J of energy in 20 seconds. What is the motor’s power? G:E = 4000 J t = 20 s R: P = ? A: P = E/t S: P = 4000 J/ 20 s P = 200 J/s = 200 W S: The motor’s uses 200 Joules every second, or the motor’s power is 200 W

Total Energy? So…  Energy is Joules  Power is how much energy/ second (J/s) = W What about the total amount of energy used in a certain amount of time? An analogy can be useful…

An Electrical Analogy: TERMANALOGYANALOGY UNIT SCIENTIFIC UNIT EnergyMoney$Joules (J) PowerRate of pay Rate you earn (or spend) your money $/hJ/s = Watt (W) 1 Watt = 1J/s Total Energy Rate of pay for a specific time period $/h time worked W h “Watt-hour”

Units of Energy Wh is very tiny. Too tiny to be useful  Like being paid in cents/hour Households use kWh  1 kWh = 1000 Wh  1 Wh = kWh 1 kW = 1000 W 1 MW = W 1 GW = W

Units of Energy One kilowatt-hour is the amount of energy transmitted by one thousand watts of power over a period of one hour In Ontario, the current cost of electricity is 5.6 cents for the first 1000 kWh during the winter and the first 600 kWh in summer  After that it is 6.5 cents/kWh

Cost of Electricity The cost of electricity varies throughout the day and the throughout the year Canadians pay for the amount of energy used (kWh)  kWh x $/kWh  50 kW h X $0.056/kWh = $ 2.80 Cost to operate = power used x time x cost of electricity

Phantom Power Power being used by a device even when it is turned off  VCRs, TVs, stereos, computers and kitchen appliances The solution?  A power bar!  Use the switch on the power bar to cut all power to the appliance

Cost of Electricity A laptop uses a 75 W adapter when it is plugged in Calculate how much it would cost to operate the laptop if it is used for 3 hours every day, for a year.  Calculate total number of hours used 365 days x 3 hrs/day = 1095 hrs  Convert power to kW (1 kW = 1000 W) 75 W = kW  How much energy in total? kW x 1095 hrs = kWh  Cost: Winter: kWh x kWh = $4.85