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Welcome to our Physics lesson on Joule or kW-h?, this is the third lesson of our suite of physics lessons covering the topic of Electric Power and Efficiency, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
You may have realized in the above example that the values of energy are considerable. Only in two minutes of operation, the values of energy produced by the source were several hundreds of thousands of Joules. On the other hand, a home circuit contains a lot of electric appliances operating simultaneously and the electricity bill is paid once a month. If the electric energy consumed were calculated in Joules, the number showing the value of energy consumption would be too large. Therefore, it is better to use a more suitable unit of energy to make the work of the employees who deal with electricity bill easier but also to make the consumer to easily understand how much he has to pay. This unit is known as kilowatt - hour (kW-h). Kilowatt- hour means kilowatts multiplied by hour. Given that 1 kW = 1000 W and 1 hour = 60 ∙ 60 = 3600 s, the conversion factor between kW-h and Joule is
A normal family consumes from a few hundreds to a few thousands kWh electrical energy in a month. This makes the electric board meter easy to read and you could complete these calculations for your own home.
A family electric board meter reads 034604.5 kWh on April 1 and 035179.3 kW-h on April 30. If the price is ₤0.2/kWh, what is the energy in Joules consumed during the month of April and what is the electricity bill the owner has to pay?
First, we find the energy consumed in kW-h by subtracting the final and the initial value the electric board meter reads. We have
When converted into Joules, this value becomes
The electricity bill is calculated by considering the electricity in kWh consumed. We have
You have reached the end of Physics lesson 15.6.3 Joule or kW-h?. There are 4 lessons in this physics tutorial covering Electric Power and Efficiency, you can access all the lessons from this tutorial below.
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