Topic Seven: P1.3.1 Transferring electrical energy
Electrical Devices Electrical energy is very versatile. It can be transformed into most other kinds of energy: Light bulb Electric heater Electric motor Hair dryer useful energy Light from glowing filament Thermal energy supplied to the surroundings Kinetic energy supplied to objects Kinetic and thermal energy given to the air wasted energy Thermal energy supplied to the surroundings light from the glowing heating element Thermal and sound energy supplied to the surroundings Sound energy released. Thermal energy in the dryer itself
Energy (in joules, J) is the ability to cause change: Energy and power Work (also in joules, J) is how much energy changes form. eg, chemical potential energy each transformation is work kinetic energy thermal energy Power (in watts, W) is the rate of doing work OR Power is how much energy changes form per second. sound energy
Calculating power If power (in W) is the amount of energy transformed per second......then power = energy time
If power (in watts, W) = energy (in joules, J) time (in seconds, s)...then 1W = 1J/s
Measuring power lava lamp Power = energy time = = 4,000J 100s 40W This mains joulemeter measures electrical energy flowing through three pin mains sockets It can be set for: one flash = 100J OR, one flash = 1000J vacuum cleaner Power = energy time = 120,000J 100s = 1200W
Measuring human power run up the stairs one at a time, timing how long it takes Energy used = the gain in gravitational potential energy = weight (in newtons, N) x gain in height (m) = N 500 X 40 m = J 20,000 then, Power = energy 20,000J = = 500 W time 40s Time taken to run up the stairs
walking running 110W 166W 393W 407W 145W 139W 428W 507W 183W 139W 322W 600W 149W 528W 600w
A power calculation (1) Which is more powerful a 3kW kettle or a 10,000W cooker? 3000W 10kW The 10,000W/10kW cooker is more powerful (2)There are 20 million homes in Britain. If a 3kW kettle is switched on in 1 in 10 homes, how much power would need to be supplied? 3kW = 3000W 20million/10 = 2million, so power = 2million x 3000W = 6billion W (3) A machine has a power rating of 100kW* If the machine runs for 2 minutes*, how much energy does it transfer? (higher tier) 10kW = 100,000W, and 2 minutes = 2x60s = 120s Energy = Power x Time = 100,000W x 120s = 12,000,000W = 12MW Power = energy time *traps for the unwary be careful
Paying for electrical energy The Electricity Board measures the energy it transfers to your house in "kilowatt hours" (kwh). Energy transferred = Power x time (in kwh) (in kilowatts, kw) (in hours, h) The proper unit of energy is joules!!!!!! The proper unit of power is watts!!!!!! The proper unit of time is seconds!!!!!! The Electricity Board can do this it has the power!!
attempt to convert one kwh into joules. One kilowatt is 1000W One watt = 1J/s One kw = 1,000W = 1000J in one second One hour = 60 minutes x 60s = 3,600s So, number of joules in one hour = 1,000J x 3,600s = 3,600,000J = 3.6MJ (extension: suggest a reason why the electricity board uses kwh/units instead of joules) 3,600,000J is cumbersome and intimidating. 1kWh also known as "one unit" is much more accessible.
Energy transferred = Power x time (in kwh) (in kilowatts, kw) (in hours, h) How many kwh does a 1kW heater use in 2 hours? 1kW x 2h = 2kWh How many kwh does a 120kW digger use in 30 minutes? 30 minutes = 0.5 hours so, 120kW x 0.5h = 60kWh How many kwh does a 100W bulb use in 1 day? 100W = 0.1kW, and 1 day = 24 hours so, 0.1kW x 24h = 2.4kWh
The electricity board calls each kwh a "Unit" 1kWh = 1 "Unit" To get the cost we multiply the number of units by the cost per unit. eg, if one unit costs 3p, how much would 50kWh cost? 50kWh = 50 units 50 units x 3p = 150p = 1.50
old style spinney disk electricity meter digital electricity meter
A 4000W heater is left on for 3 hours. (1) How many kilowatt hours of energy does it use? 4,000W = 4kW 4kW x 3h = 12kWh (2) How many units of electricity does it use? 12kWh = 12 units (3) How much does it cost to run if one unit costs 3p 12 units x 3p = 36p Energy (kwh) = Power (kw) x time (h)
Energy transferred = Power x time (in kwh) (in kilowatts, kw) (in hours, h) One unit = 13p 1) Calculate how 70W = 0.07kW much it costs to run refrigerator the fridge for one average power consumption = 70W year Cost = 0.07kW x (365days x 24hours) x 13p = 79.72 2) Calculate how much it costs to run the freezer for one year Power is 10x greater, so cost is 10x greater = 797.20 freezer average power consumption = 700W