Equation 3

Question 1

What is the equation for efficiency and what are all the units included in this equation

Answer 1

efficiency = useful energy transferred / total energy supplied

(Percentage efficiency=efficiency x100)

10j/100j= 1/10 = 10%

Question 2

The energy supply through a lightbulb is 200 J. A total of 28 Joules of this is usefully transferred. How how efficient is the lightbulb?

Answer 2

Efficiency= useful energy transferred / Total energy supplied =0.14
Percentage efficiency = efficiency x 100
0.14 x 100 Percentage efficiency =14%

Question 3

Two electric motors are used to lift a 2 N weight through a vertical height of 10 m
- Motor one does this in 5 seconds
- Motor two does this in 10 seconds.
What is the power of both motors?

Answer 3

W=F x d = 2 x 10 = 20 J
Work done = power x time.
P=W/t=20/5= 4w P=W/t=20/10= 2W

Question 4

Define power.

Answer 4

watts = joule / sec
power = work done / time

Question 5

What is the equation for work done and what are all the units included in this equation?

Answer 5

Work done = force x distance
W = F x d
Work done/W is measured in joules (J)
force /F is measured in Newtons (N)
distance /d same direction, measured in (m)

Question 6

Calculate the work done from the example below. Don’t forget units.

10 N force applied Box moved by 2 m

Answer 6

W = F x d
W = 10 x 2
W = 20 J

Question 7

A device has a power of 450W. It is left on for
10 seconds. How much energy is transferred?

Answer 7

power= W / t

W = p x t W=450 x 10 W=4500J

Question 8

How much time does it take for a 500W device to transfer 1500 J of energy?

Answer 8

power= w/t

t = w/p t=1500/500 t=3s

Question 9

What is the equation for Power and what are all the units included in this equation?

Answer 9

power= work done / time
power=J / t

Question 10

A motor uses a battery to turn
a)Which energy stores are involved?
b) which energy transfer is involved?

Answer 10

a) Chemical, kinetic
b) Mechanical

Question 11

A motor uses a battery to turn, the motor has a power of 12W. How much energy can it transfer in 10seconds?

Answer 11

P = E/T E = P * T
12 * 10 = 120 E = 120J

Question 12

The object needs 24 joules of energy to be lifted to the right height & uses 2 watts of power in this process. How long does this take the motor?

Answer 12

24 / 2 = 12s
E / P = T

Question 13

What is the equation for finding pressure and what are the units explain.

Answer 13

P = F / A
Pressure = Force / Area
F= N/m2
A= m2
Unit: Pascals (P)