Part 3 The usefulness of electrical appliances. Flashcards
How do you calculate energy transfer (E=Pxt)
E is the energy transferred in kilowatt-hours, kWh
P is the power in kilowatts, kW
T is the time in hours, h.
Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.
For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.
How do you calculate the cost of mains electricity
Cost(p) = Energy (kWh) * cost per unit (P/kWh) transferred. E.g Power= 1500w =1.5kw
Time = 4 hours
Energy = 1.5 * 4 = 6kWh
cost = 6kWh * 6p / kWh = 36p
what is the energy measured in?
Joules (J). -is the amount of energy transferred by a 1w appliance in 1 second.
What is power usually measured in?
Watts (W) or kilowatts (kW). A 5kw appliance transfers 5000 J in 1 second
What to do when dealing with large amounts of electrical energy? (e.g energy used by a home in 1 week)
It’s easier to think of the time in Kilowatts and hours rather than watts and seconds, when dealing with large amounts.
What is the standard units of electrical energy measured in?
Kilowatt-hours (kWh) not joules.
What are the 2 formulas for finding the cost of electricity?
- Number of units (kWh) used = Power (kW) * Time (hours) —— Units = kW * hours.
- Cost = Number of units * price per unit ——- Cost = units * price
How do you work out an electricity meter question?
Units are usually given in kWh. You could be given 2 readings and asked to work out the total energy used over a particular time period. Just subtract the meter reading at the start of the time (the smaller one) from the reading at the end.
How do you compare two electrical appliances?
You might be given two different appliances that do the same job and asked to weigh up the pros and cons to figure out which one is more suitable for a certain situation. Think about: practical disadvantages and advantages, which is more cost effective, which uses less energy etc. Think logically.
How do you use data to compare two appliances?
You take the information provided and form a logical equation. E.g. A company is deciding whether to install a 720w low-power heater or a high-power 9kW heater. The heater they pick will be on 30 hours each week. Their electricity provider charges 7p per Kw of electricity. How much would they save if they chose the low-power heater?
Weekly electricity used by low-power = 0.720 kW * 30 h=21.6 kWh
weekly electricity used by high-power = 9 kW * 30h = 270 kWh
Total savings = (270-21.6)*7= £17.39 (to the nearest penny)
How does access to electricity effect the standard of living?
Poorer countries might not have refrigerators, which not only helps food last longer but stores important vaccines, to keep them cold. Without refrigerators it’s difficult to distribute important vaccines, can have devastating effect on countries population. Electricity also provides hospitals with x ray machines without access it could mean diagnosis and treatment is poorer, could reduce life expectancy. Communications are also effected, e.g phones, t’v, internet, etc.. Makes it harder to keep in touch.
