P2 - Conservation of Energy and Power

Question 1

What is the conservation of energy principle?

Answer 1

Energy can be transferred usefully, stored or dissipated but can never be created or destroyed.

Question 2

Is all energy transferred between stores? Explain.

Answer 2

Not all energy is transferred usefully. Some energy is dissipated usually as thermal energy

Question 3

Why is dissipated energy called wasted energy?

Answer 3

Because the energy is stored in ways that are not useful and can not be accessed and eventually ends up in thermal stores

Question 4

Describe the energy transfer for a closed system.

Answer 4

Neither matter nor energy can enter or leave there for the net change in the total energy of a closed system is always zero.

Question 5

What is the formula for energy transferred?

Answer 5

Energy transferred (J) = power (W) * time (s) 
E = W * t

Question 6

A motor transfers 4.8 KJ of energy in two minutes. Calculate it’s power in watts. (3 marks)

Answer 6

E=W*t
4800j = x * 120s
4800j/120s=x
x=40

Question 7

A clothes dryer has the power of 4 kW. It takes 45 minutes to dry one load of clothes. Electricity cost 14p per kWh. What is the cost of drying one load. (4 marks)

Answer 7

14 kWh = 1 hour 
45 minutes = 0.75
E=W*t
x = 4 * 0.75
x = 3 kWh
14*3= 42p