3.1 The Usefulness of Electrical Appliances

Electrical appliances use electrical energy and transfer it into a useful form of energy (and unuseful energy forms as a byproduct). The amount of energy that is used by an appliance depends on the appliances power and the amount of time that the appliance is switched on for. Power is how fast the appliance can transfer it.
Energy = Power x Time
kWh = kW x hours
Joules = W x seconds
kWh = kW x hours
Joules = W x seconds
Energy is measures in Joules (J) and 1 J is the amount of energy transferred by a 1 Watts (W) appliance in 1 second. Power is measures in Watts (W) or Kilowatts (kW). A kilowatt is 1000 wats. A 2kW appliance will transfer 2000 J in 1 second. Households tend to use a great deal of energy, so we tend to think of power in terms of kilowatts and time in hours. The standard units of electrical energy is kilowatthours (kWh).
When we know how much electricity an appliance uses, we can work out the cost of using that appliance for a certain period of time. We work out the cost using the formula below.
When we know how much electricity an appliance uses, we can work out the cost of using that appliance for a certain period of time. We work out the cost using the formula below.
Cost = number of units x price per unit
The longer that an appliance is switched on for and the greater the power of the appliance is, the more energy will be used and the greater the cost of running that item will be.
Example
Suppose that we have an 800watt washing machine that is on for 45 mins (a very quick wash). The cost of one kilowatthour is 20p. Find out the amount of electricity that is used to have this washing machine on for 45 mins and how much this would cost me.
Firstly, we need to work out how much energy the washing machine is using. We do this by using the formula below.
Suppose that we have an 800watt washing machine that is on for 45 mins (a very quick wash). The cost of one kilowatthour is 20p. Find out the amount of electricity that is used to have this washing machine on for 45 mins and how much this would cost me.
Firstly, we need to work out how much energy the washing machine is using. We do this by using the formula below.
Power x Time = Energy
0.8 kW x 0.75 hours = 0.6 kWh
0.8 kW x 0.75 hours = 0.6 kWh
We want to find the energy used in kilowatt hours. Therefore, I have converted the power into kilowatts, which is 0.8 kW and changed the time to be in hours (45 mins is 0.75 hours). Our washing machine uses 0.6 kWh of electricity for a short wash. We can now work out how much it costs me to run this washing machine by multiplying the amount of energy used by the cost of the energy. From the question we know that one kilowatthour costs 20p, so we do:
number of units x price per unit = Cost
0.6 kWh x 20p = 12p
0.6 kWh x 20p = 12p
The cost of having the washing machine on for 45 mins is 12 pence.
Mains Electricity
Houses have electricity meters that measures how much electric has been used by the household. Usually the meters are in kilowatthours (kWh). Households are asked by electricity companies to give them a meter reading at certain times of the year. The companies subtract the old meter reading from the new meter reading, to work out how much energy has been used by the household. The household will then be charged for their usage. For example, suppose that my previous meter reading (taken 3 months prior to now) for my house was 452345.21 kWh and my new reading is 452562.35 kWh. My energy company are charging me 20 pence per kWh, how much energy have I used? And how much will this cost me? Assume that I continue with the same electricity consumption, how much would my yearly bill be?
To find out how much energy I have used, we subtracted the old meter reading from the new meter reading. 452562.35 kWh  452345.21 kWh = 217.14 kWh. We then multiply this value by the amount of cost of one kilowatthour, which is 20 pence. 217.14 x £0.20 = £43.428, which rounds to £43.43.
During the 3 month, I used 217.14 kWh and this costed me £43.43.
The above figures have worked out how much electricity I have used and how much the electricity has costed me for the previous 3 months. If we wanted to work out how much electricity I will use in a year and how much this will cost me, we would multiply the energy and the cost by 4 because a year is four times greater than three month. Therefore, I would use 868.56 kWh (217.14 x 4) and this would cost me £173.71 (£43.428 x 4).
Houses have electricity meters that measures how much electric has been used by the household. Usually the meters are in kilowatthours (kWh). Households are asked by electricity companies to give them a meter reading at certain times of the year. The companies subtract the old meter reading from the new meter reading, to work out how much energy has been used by the household. The household will then be charged for their usage. For example, suppose that my previous meter reading (taken 3 months prior to now) for my house was 452345.21 kWh and my new reading is 452562.35 kWh. My energy company are charging me 20 pence per kWh, how much energy have I used? And how much will this cost me? Assume that I continue with the same electricity consumption, how much would my yearly bill be?
To find out how much energy I have used, we subtracted the old meter reading from the new meter reading. 452562.35 kWh  452345.21 kWh = 217.14 kWh. We then multiply this value by the amount of cost of one kilowatthour, which is 20 pence. 217.14 x £0.20 = £43.428, which rounds to £43.43.
During the 3 month, I used 217.14 kWh and this costed me £43.43.
The above figures have worked out how much electricity I have used and how much the electricity has costed me for the previous 3 months. If we wanted to work out how much electricity I will use in a year and how much this will cost me, we would multiply the energy and the cost by 4 because a year is four times greater than three month. Therefore, I would use 868.56 kWh (217.14 x 4) and this would cost me £173.71 (£43.428 x 4).
Comparing Appliance
In the exam, you may be asked to compare appliance/ devices. In order to do this you need to think about the advantages and disadvantages of using each appliance or device. You can also compare the cost when using the appliance or devices. See the quiz at the end of the section for product comparisons.
In the exam, you may be asked to compare appliance/ devices. In order to do this you need to think about the advantages and disadvantages of using each appliance or device. You can also compare the cost when using the appliance or devices. See the quiz at the end of the section for product comparisons.
Electricity Not Being Available
When electricity is not available we can use batteries or elastic potential energy to power devices. Our homes have batteries in them. An example of a device that has elastic potential energy is a windup radio. A user winds up the radio and then the elastic potential energy slowly releases and this powers the radio.
A lot of developing nation do not have access to reliable electricity and this has a great effect on the standard of living in these areas. For example, if there is no electricity, hospitals may not be able to use equipment that requires electricity such as Xrays, MRI scanner and may not be able to operate due to poor lighting. Also, we use electricity to keep our food fresh and safe and if there is unreliable electricity there is a greater chance of food going off and becoming inedible. Furthermore, communication can be effected by a lack of electricity.
When electricity is not available we can use batteries or elastic potential energy to power devices. Our homes have batteries in them. An example of a device that has elastic potential energy is a windup radio. A user winds up the radio and then the elastic potential energy slowly releases and this powers the radio.
A lot of developing nation do not have access to reliable electricity and this has a great effect on the standard of living in these areas. For example, if there is no electricity, hospitals may not be able to use equipment that requires electricity such as Xrays, MRI scanner and may not be able to operate due to poor lighting. Also, we use electricity to keep our food fresh and safe and if there is unreliable electricity there is a greater chance of food going off and becoming inedible. Furthermore, communication can be effected by a lack of electricity.