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In this lesson, we will learn how to use the formula P = E/t to calculate the rate at which energy is transferred by a device given the amount of energy it transfers in a given time.

Q1:

A 24 W fluorescent light is left on for several hours, during which time it is supplied with 432 kJ of energy. For how many hours was the light left on?

Q2:

Which of the following is the correct unit for power?

Q3:

A 250 W television and a 400 W television are both used for two hours. How much more energy does the 400 W television require to operate for this time than the 250 W television?

Q4:

A kettle is filled with water and turned on. It takes 3 minutes to boil the water, and the kettle is supplied with 3 2 4 0 0 0 J of energy in this time. What is the power of the kettle?

Q5:

A 30 mW LED in an electronic circuit is left on for 2 hours. How much energy does it transfer to its environment over this time? Assume that the LED is 1 0 0 % efficient.

Q6:

How many watts are there in 1 megawatt?

Q7:

What is 34 kJ in joules?

Q8:

Engy wants to find out how much electrical power her television needs when it is on. She watches television for a few hours and records how much energy has been supplied to it every half hour. The graph shows her results.

How much energy has been supplied to the television after 3 hours?

What is the power of her television?

Q9:

What is 6700 kilowatts in megawatts?

Q10:

Which of the following is the correct formula for the energy transferred, πΈ , in a given time, π‘ , for an electrical appliance with a given power, π ?

Q11:

What is 50 MW in watts?

Q12:

A microwave is used for 5 minutes, during which time it is supplied with 180 kJ of energy. What is the power of the microwave?

Q13:

What is 24 MW in kilowatts?

Q14:

How many joules are there in 1 kilojoule?

Q15:

What is 5 0 0 0 joules in kilojoules?

Q16:

A 60 W incandescent light bulb is left on for 30 seconds. How much energy is supplied to it over this time?

Q17:

Which of the following is the correct unit for energy?

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