1.7 Energy, Work, & Power - Exam Ques

Question 1

Define efficiency as

Answer 1

Define efficiency as:

(a)

(%) efficiency =
(useful energy output)
────────────── (× 100%)
(total energy input)

(b)

(%) efficiency =
(useful power output)
────────────── (× 100%)
(total power input)

recall and use these equations (USEFUL/TOTAL)

Question 2

The efficiency of the solar panel is 70%.

Calculate the power of the solar radiation incident on the panel.

Meaning to calculate what?

THERMAL ENERGY / TIME
OVER INPUT = percentage, in decimals

BC POWER OUTPUT = ENERGY/TIME

BC POWER = WORK DONE/TIME

Answer 2

Power input using

power input / power output x 100 formula

The incident power of the solar radiation is calculated as follows;

Efficiency = (power) output/(power) input (× 100)
OR (4100 / 5) 100 70
power input
× = OR (4100 100)
power input
× OR rearranged C1
Power input = 1200W
Thus, the incident power of the solar radiation is 1,185.6 W.

Question 3

Specific heat capacity - define

Answer 3

Energy required to increase temperature of 1kg of a substance by 1 degree Celsius

Question 4

draw a graph of extension against load for a spring which obeys Hooke’s law.

Extension - y-axis
Load - x-axis

Answer 4

Straight line through origin

Question 5

Forms of energy that don’t derive energy from the Sun?

Answer 5

GEOTHERMAL AND NUCLEAR FISSION – does not derive energy from the sun

Question 6

Which nuclear reaction takes place here – fusion/fission for nuclear power station and the Sun?

Answer 6

fission - nuclear power station.

FUSION - the sun

Question 7

COMPRESSION OF SPRINGS IN CAR SEAT? Which energy?

Answer 7

Strain - the energy due to the compression of springs in a car seat

Question 8

Which energy resource - obtain electricity WITHOUT producing heat to boil water?

Answer 8

hydroelectric – energy resource OBTAIN ELECTRICITY WITHOUTTT PRODUCING HEAT TO BOIL WATER

Question 9

Energy from uranium is transferred to electrical energy in a nuclear power station.

Correct order of stages of process?

Answer 9

Reactor
Boiler
Turbine
Generator

Question 10

Airplane lands. As it descends, speed reduces. Energy changes during descent?

Answer 10

Kinetic + gravitational –> thermal (heat)

Question 11

Same units? Which?

Answer 11

Energy and work,

but NOT power.

Question 12

Fig. 3.2 shows a model train, travelling at speed v, approaching a buffer.

The train, of mass 2.5 kg, is stopped by compressing a spring in the buffer. After the train has stopped, the energy stored in the spring is 0.48 J.

Calculate the initial speed v of the train.

Answer 12

Use of 1/ 2mv2

0.5 × 2.5 × v2 = 0.48

2v = 0.48/(0.5 × 2.5) OR v2 = 0.384

v = 0.62m/ s

Question 13

Fig. 2.1 shows a conveyor belt transporting a package to a raised platform. The belt is driven by a motor.

(a) The mass of the package is 36kg. Calculate the increase in the gravitational potential energy (g.p.e.) of the package when it is raised through a vertical height of 2.4m.

increase in g.p.e. = ?

Answer 13

mgh OR 36 × 10 × 2.4

864 J OR Nm

Question 14

The package is raised through the vertical height of 2.4m in 4.4s.

Calculate the power needed to raise the package.

power =

[WORK DONE/TIME]

Answer 14

P = E/t

196 W

Question 15

(c) The electrical power supplied to the motor is much greater than the answer to (b).
Explain how the principle of conservation of energy applies to this system.

Answer 15

total energy is constant OR initial energy = final energy [1]

some energy is dissipated into the surroundings [1]

Question 16

d) Assume that the power available to raise packages is constant. A package of mass greater than 36kg is raised through the same height.

Suggest and explain the effect of this increase in mass on the operation of the conveyer belt.

Answer 16

Work done/energy used (to raise mass) is greater [1]

P = E/t AND power is constant [1]

speed reduced / time taken is longer [1]

Question 17

An athlete of mass 64 kg is bouncing up and down on a trampoline.

At one moment, the athlete is stationary on the stretched surface of the trampoline. Fig. 3.1 shows the athlete at this moment.

(b) The stretched surface of the trampoline begins to contract. The athlete is pushed vertically
upwards and she accelerates. At time t, when her upwards velocity is 6.0 m / s, she loses
contact with the surface.

(i) Calculate her kinetic energy at time t.

Answer 17

i) (KE =) ½ mv2 in any form C1
1200J

Question 18

(ii) Calculate the maximum possible distance she can travel upwards after time t

Answer 18

(G)PE (gained) = KE (lost) in any form

(G)PE = mgh

1.8 m

Question 19

iii) In practice, she travels upwards through a slightly smaller distance than the distance calculated in (ii).

Suggest why this is so.

Answer 19

air resistance

Question 20

(c) The trampoline springs are tested. An extension-load graph is plotted for one spring. Fig. 3.2
is the graph.

i) State the name of the point X.

Answer 20

limit of proportionality

Question 21

ii) State the name of the law that the spring obeys between the origin of the graph and
point X.

Answer 21

Hooke’s law

Question 22

(b) The train travels a distance of 4.0 km along a straight, horizontal track.

(i) Calculate the work done on the train during this part of the journey.

work done =

Answer 22

(work done =) F × x

1.4 x10^9 J

Question 23

4 An electric train is initially at rest at a railway station. The motor causes a constant force of 360 000 N to act on the train and the train begins to move.

(a) State the form of energy gained by the train as it begins to move.

Answer 23

Kinetic energy

Question 24

(ii) The mass of the train is 450,000 kg.

Calculate the maximum possible speed of the train at the end of the first 4.0 km of the journey.

maximum possible speed =

Answer 24

work done = kinetic energy OR ½mv2

(v2=)2WD÷ m OR 2×1.4(4)×109 ÷4.5×105
OR 6400

80m/ s

Question 25

(c) After travelling 4.0 km, the train reaches its maximum speed. It continues at this constant speed on the next section of the track where the track follows a curve which is part of a circle.

State the direction of the resultant force on the train as it follows the curved path.

Answer 25

perpendicular (to curved path) OR centripetal OR towards centre (of circle)

Question 26

(b) Coal-fired power stations are polluting.
State an advantage of using coal as a source of energy

Answer 26

(relatively) cheap [1]

OR widely/always available available

Question 27

A coal-fired power station generates electricity at night when it is not needed.
Some of this energy is stored by pumping water up to a mountain lake. When there is high demand for electricity, the water is allowed to flow back through turbines to generate electricity.

On one occasion, 2.05 × 10^8 kg of water is pumped up through a vertical height of 500 m.

(i) Calculate the weight of the water

Answer 27

2.05×10^9N

Question 28

(ii) Calculate the gravitational potential energy gained by the water.

energy gained =

Answer 28

use of mgh OR weight × h

1.03×1012 J

Question 29

iii) The electrical energy used to pump the water up to the mountain lake is 1.2 × 1012 J.
Only 6.2 × 1011 J of electrical energy is generated when the water is released.

Calculate the efficiency of this energy storage scheme.

efficiency =

Answer 29

output energy÷ input energy OR 6.2×1011 ÷ 1.2×1012

0.52 OR 52%

Question 30

Fig. 3.1 shows a skier taking part in a downhill race.

(a) The mass of the skier, including his equipment, is 75 kg. In the ski race, the total vertical change in height is 880 m.

Calculate the decrease in the gravitational potential energy (g.p.e.) of the skier

Answer 30

(g.p.e.=) mgh OR 75 × 10 × 880

= 6.6 × 10^5 J/Nm

Question 31

b) The skier starts from rest. The total distance travelled by the skier during the descent is
2800 m. The average resistive force on the skier is 220 N.
Calculate

(i) the work done against the resistive force,
work done =

Answer 31

(work =) Fs /Fd OR 220 × 2800 C1
= 6.2 × 105 J /Nm OR 620kJ / kNm

Question 32

b) ii) the kinetic energy of the skier as he crosses the finishing line at the end of the race.

kinetic energy =

[2]

Answer 32

(ii) answer to (a) – answer to (b)(i)

e.g. (k.e.=) 6.6 × 105
– 6.2 × 105
= 4.0 × 104 J OR 44 kJ

OR 6.6 × 105
– 6.16 × 105
= 4.0 × 104 J

Question 33

c) Suggest why the skier bends his body as shown in Fig. 3.1.

Answer 33

increase stability [1]

(to go faster by) reduced air resistance [1]