Equations and efficiency

Question 1

How do you calculate GPE (gravitational potential energy)?

Answer 1

GPE = mass x gravity x
height

Gravity = 9.8

Question 2

How do you calculate kinetic energy (KE)?

Answer 2

KE= 1/2 x m x v2

M = mass
V2 = velocity squared

Question 3

How do you calculate velocity?

Answer 3

Velocity = distance
__________
Time

Question 4

How do you calculate work done?

Answer 4

W = V x I x T

Workdone x current x time

Question 5

How do you calculate Specific Heat Capacity (SHC)?

Answer 5

SHC = number of joules
           \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
  Number of   X   Number 
  Kg of mass      Of C of temperature 
                            Change

C = E
_____
M x Ø

Question 6

How do you calculate heat?

Answer 6

Heat = mass x SHC x
temperature

E = M x C x Ø

Question 7

Work done = …

Answer 7

Work done = force x
distance

               = (J) x (M)

Question 8

How do you calculate power (W)?

Answer 8

Power = Work
________
Time

W = J
____
S

Question 9

How do you calculate input energy?

Answer 9

= Energy supplied
___________________
Time taken

Question 10

How do you calculate efficiency?

Answer 10

= useful energy
transferred X 100
_________________
Energy supplied

Question 11

1) An electric motor is used to raise an object. The object transfers 150J of useful energy when the motor is supplied 250J of electrical energy.

a. What is the total energy supplied to the motor?
b. What is the useful energy that is transferred?
c. What is the efficiency?

Answer 11

a. 250J
b. 150J

c. Efficiency = 150
______ X 100
250
= 60%

Question 12

2) A filament bulb is supplied with 100J of energy. It wastes 90J as sound and heat energy.
a. What is the total energy supplied?
b. What is the useful energy transferred?
c. What is the efficiency?

Answer 12

a. 100J
b. 10J

c. Efficiency = 10
____ x 100
100
= 10%

Question 13

3) An electric heater is 90% efficient. If it is supplied with 200J of energy:

a. What is the useful energy that it transfers?
b. For every 200J of energy it is supplied with, how much does it waste?

Answer 13

a. = 200 X 90
____________ = 180
100
b. 200 - 180 = 20

Question 14

4) A fan is 50% efficient. If it transfers 400J as useful kinetic energy (KE):
a. What is the total energy supplied?
b. How much of the total energy does it waste?

Answer 14

a. = 400
______ X 100 = 800
50

b. = 800 - 400 = 400J

Question 15

5) A wind turbine has an efficiency of 55%. How much energy is required from the wind if the turbine is to generate 750J of electrical energy?

Answer 15

= energy = 55
____ = 0.55
100

Energy = 750
______ = 1363.63
0.55