Energy, Work & Power (Cambridge International IGCSE Physics): Exam Questions

Extract:

• Accept "light", "EM waves", "electromagnetic waves" or "infrared" for "Radiation".
• Accept "thermal", "thermal energy" or "heat" for "Heating"; do not accept "internal", which is a store, not a pathway.
• Award each pathway independently: an answer naming both light and heat scores 2 marks.

An electric current transfers energy from the battery to the filament lamp.

State the two energy transfer pathways for the energy emitted by the filament lamp.

State which store of energy in the battery is decreasing.

Extract:

An electric current transfers energy from the battery to the filament lamp.

Two energy transfer pathways for the energy emitted by the filament lamp are:

• Radiation
• Heating

The store of energy decreasing in the battery is:

• Chemical

Explain how the principle of conservation of energy applies to this circuit.

A drone is a machine that can fly. Fig. 4.1 shows a drone rising into the air, lifting a camera.

The drone obtains energy from a battery of cells.

Complete the sequence of useful energy transfers as the drone rises into the air. One part is done for you.

................................. store → kinetic store → ............................. store

Extract:

A drone is a machine that can fly. Fig. 4.1 shows a drone rising into the air, lifting a camera.

The drone obtains energy from a battery of cells.

Extended tier only

The drone can move in any direction up or down, backwards or forwards, left or right. It can also remain stationary above the ground.

Describe the motion and position of the drone when it has both a large quantity of potential energy and a small quantity of kinetic energy.

Extract:

A drone is a machine that can fly. Fig. 4.1 shows a drone rising into the air, lifting a camera.

The drone obtains energy from a battery of cells.

When the drone moves, it wastes some energy.

Describe what happens to this energy.

A bouncy ball is dropped from a height of 1.65 m onto a smooth wooden floor, where it bounces, reaching a height of 1.43 m.

The bouncy ball has a mass of 0.07 kg

Complete the energy transfers taking place when

(i) the ball is falling towards the floor

.................................... energy store → kinetic energy store [1]

(ii) the ball hits the floor and deforms

kinetic energy store → .................................... energy store [1]

Extract:

A bouncy ball is dropped from a height of 1.65 m onto a smooth wooden floor, where it bounces, reaching a height of 1.43 m.

The bouncy ball has a mass of 0.07 kg

Extended tier only

Calculate the energy in the gravitational potential store of the ball just before it is dropped.

energy = ....................................

Extract:

Mass, m = 0.07 kg gravitational field strength, g = 9.8 N/kg Height, h = 1.65 m E_P = 0.07 × 9.8 × 1.65 = 1.13 J

Extended tier only

Calculate the speed at which the bouncy ball hits the floor.

For the purposes of this calculation, you can assume there is no air resistance.

speed = ....................................

Extract:

A bouncy ball is dropped from a height of 1.65 m onto a smooth wooden floor, where it bounces, reaching a height of 1.43 m.

The bounce height is smaller than the height the ball was dropped from.

Explain why this is the case.

Identify the unit for energy.

Tick one box.

square N

square K

square J

Describe the link between work done is and energy transferred.

Fork-lift trucks are used to move crates in warehouses. Fig. 1.1 shows a crate being lifted to high shelf.

Fig. 1.1

The fork-lift truck lifts a crate of 450 N to a shelf that is 3.9 m above the ground.

Calculate the work done to lift the crate.

work done = ....................................

Extract:

The fork-lift truck lifts a crate of 450 N to a shelf that is 3.9 m above the ground. Calculate the work done to lift the crate.

Use the equation for work done:

• W = F d = 450 × 3.9
• W = 1755 N m

Extended tier only

The total energy input for the fork-lift truck to lift the crate is 2500 J

Calculate the efficiency of the fork-lift truck.

efficiency = ....................................

Extract:

Gravitational field strength g = 9.8 N/kg. Expected answer 953 J, rounded to 950 J. Accept 972 J (or 970 J) for full marks if g = 10 N/kg is used. Accept the unit N m as well as J.

A group of students want to measure their power output when climbing a ladder.

Student A has a mass of 54 kg and takes 15 s to climb the ladder which has a vertical height of 1.8 m.

Calculate the work done in raising the student's body mass as they climb the ladder.

work = ....................................

Extract:

Work done from part (a): 953 J (accept 950 J, or 972 J if g = 10 N/kg was used). Time taken 15 s. Expected answer 63.5 W, rounded to 64 W. Error carried forward: award full marks for the candidate's own part (a) value divided by 15 and correctly evaluated.

Calculate the power output of Student A in raising their body mass up the ladder.

power output = ....................................

Extract:

Efficiency 17% (0.17). Useful power output from part (b): 64 W (accept 63.5 W or 63 W). Expected answer 376 W, rounded to 380 W. Error carried forward: award full marks for the candidate's own part (b) value divided by 0.17 and correctly evaluated (e.g. 374 W from 63.5 W, 371 W from 63 W, 381 W from 64.8 W).

Extended tier only

Student A's body is only 17% efficient when climbing the ladder.

Calculate the total power input of the student to climb the ladder.

power input = ....................................

Extract:

Expected answer: chemical store (muscles) to gravitational potential store. Accept the kinetic store as an intermediate step. Accept thermal store of the surroundings as an additional destination only.

State the energy transfer taking place as student A climbs the ladder.

Extended tier only

Fig. 3.1 shows a model of a wind turbine used to demonstrate the use of wind to generate electricity. The wind is blowing towards the model, as shown.

The mass of air passing through the circular area swept out by the turbine blades each second is 7.5 kg. The energy in the kinetic store of the air that passes through this circular area each second is 240 J.

(i) Calculate the speed of the air.

speed = ........................................................ [3]

(ii) The moving air drives a generator. State the input power of the air passing through the turbine blades.

input power = ........................................................ [1]

Extract:

The mass of air passing through the circular area swept out by the turbine blades each second is 7.5 kg. The energy in the kinetic store of the air that passes through this circular area each second is 240 J.

(ii) The moving air drives a generator. State the input power of the air passing through the turbine blades.

input power = ........................................................ [1]

(ii) State the input power:

• Recall that power = the energy per second
• Therefore input power = kinetic energy
• P = 240 W; [1 mark]

Extended tier only

The output current of the generator is 2.0 A. The output potential difference (p.d.) of the generator is 11 V.

(i) Calculate the output power of the generator.

output power = ........................................................ [2]

(ii) Calculate the efficiency of the wind turbine.

efficiency = .................................................... % [2]

Extract:

The mass of air passing through the circular area swept out by the turbine blades each second is 7.5 kg.

The density of air is 1.3 kg / m³.

Calculate the volume of air passing through the circular area swept out by the turbine blades each second.

Extended tier only

A force is used to move an object from the Earth’s surface to a greater height.

Explain why the energy in the gravitational potential store of the object increases.

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Fig. 2.1 shows a train moving up towards the top of a mountain.

The train transports 80 passengers, each of average mass 65 kg, through a vertical height of 1600 m.

Calculate the increase in energy stored in the gravitational potential store of the passengers.

increase in energy = .........................................................

Extract:

The train transports 80 passengers, each of average mass 65 kg, through a vertical height of 1600 m. Calculate the increase in energy stored in the gravitational potential store of the passengers.

List the known quantities:

• Number of passengers = 80
• Mass of each passenger = 65
• Height, h = 1600 m
• Acceleration of free fall, g = 9.8 m / s^2

Calculate the total mass of the passengers, m:

• m = number of passengers × mass of each passenger
• m = 80 × 65 = 5200 kg

State the equation for gravitational potential energy:

• E_p = mgh

Calculate the increase in gravitational potential energy of the passengers:

• E_p = 5200 × 9.8 × 1600
• E_p = 81 500 000 J = 8.2 × 10^7 J

Extended tier only

The engine of the train has a power of 1500 kW. The time taken to reach the top of the mountain is 30 minutes.

Calculate the efficiency of the engine in raising the 80 passengers 1600 m to the top of the mountain.

efficiency = .........................................................

State what is meant by the principle of conservation of energy.

Extended tier only

Fig. 3.1 shows a girl throwing a heavy ball.

(i) State the energy transfers that take place from when the girl begins to exert a force on the ball until the ball hits the ground and stops moving.

(ii) The mass of the ball is 4.0 kg. The girl exerts a force on the ball for 0.60 s. The speed of the ball increases from 0 m/ s to 12 m/s before it leaves the girl’s hand.

Calculate:

1. the momentum of the ball on leaving the girl’s hand

momentum = ...........................................................[2]

1. the average resultant force exerted on the ball.

average resultant force = ...........................................................[2]

Extended tier only

Fig. 3.1 shows an aircraft on the deck of an aircraft carrier.

The aircraft accelerates from rest along the deck. At take-off, the aircraft has a speed of 75 m/s.

The mass of the aircraft is 9500 kg.

Calculate the energy in the kinetic store of the aircraft at take-off.

energy = ...........................................................

Extract:

The aircraft accelerates from rest along the deck. At take-off, the aircraft has a speed of 75 m/s. The mass of the aircraft is 9500 kg. Calculate the energy in the kinetic store of the aircraft at take-off.

energy = 2.7 × 10^7 J

The catapult provides a constant force for a distance of 150 m along the deck.

On an aircraft carrier, a catapult provides an accelerating force on the aircraft. The catapult provides a constant force for a distance of 150 m along the deck.

Calculate the resultant force on the aircraft as it accelerates. Assume that all of the kinetic energy at take-off is from the work done on the aircraft by the catapult.

force = ...........................................................

An electric kettle contains water at a temperature of 19 °C. The kettle has a power rating of 3.0 kW and is switched on for 3.5 minutes.

Calculate the energy supplied to the kettle by the electricity supply.

energy = ........................................................

Extract:

At 3.5 minutes, the temperature of the water reaches 100 °C. The volume of the water in the kettle is 1700 cm^3 and its density is 1.0 g/cm^3. The specific heat capacity of water is 4200 J/(kg °C). Initial temperature = 19 °C. Final temperature = 100 °C. Volume of water, V = 1700 cm^3. Density of water, ρ = 1.0 g/cm^3. Specific heat capacity of water, c = 4200 J/kg °C. The equation for the change in energy associated with a temperature change: ΔE = m × c × ΔT. The equation relating mass, volume and density: ρ = m / V. Rearranged: m = ρ × V.

Extended tier only

At 3.5 minutes, the temperature of the water reaches 100 °C. The volume of the water in the kettle is 1700 cm³ and its density is 1.0 g/cm³. The specific heat capacity of water is 4200 J/(kg °C).

Calculate the energy gained by the water.

energy = ........................................................

Extract:

Energy supplied to the kettle by the electricity supply = 630 000 J Energy gained by the water = 580 000 J

Extended tier only

Calculate the efficiency of the kettle.

efficiency = ........................................................

Fig. 4.1 shows an electric motor and pulley wheel being used to raise a load M. The electric motor uses a belt to turn the pulley wheel

When the electric motor lifts the load, it transfers energy. Fig. 4.2 shows the energy transfers.

Write on Fig. 4.2 to complete the label in each box. The first label is done for you.

Fig. 2.1 shows water stored in a reservoir behind a hydroelectric dam.

State the form of the energy stored in the water in the reservoir that is used to generate electricity

Extended Tier Only

Fig. 8.1 shows two vertical, cylindrical tubes and a cylindrical magnet all held in a vacuum

One tube is made of plastic and the other tube is made of copper. The two cylindrical tubes have identical dimensions.

The magnetic field of the small, cylindrical magnet is extremely strong.

Initially, the magnet is at rest at the top of the plastic tube.

The magnet is released and it falls through the plastic tube without experiencing a resistive force. The magnet takes 0.67 s to fall to the lower end of the plastic tube.

The mass of the magnet is 0.012 kg.

Calculate the kinetic energy of the magnet when it reaches the lower end of the plastic tube.

kinetic energy = ........................................................

A student uses an electric motor to lift a load. Fig. 4.1 shows the arrangement.

(i) The motor exerts a force of 25 N on the load. It lifts the load a vertical distance of 2.0 m.

Calculate the work done by the motor on the load.

work done on the load = ...................................................... J [3]

(ii) State the useful energy output of the electric motor when it lifts the load 2.0 m.

useful energy output = ...................................................... J [1]

(ii) The useful energy output of the motor is less than the energy input to the motor.

Explain why the useful energy output is less than the energy input.

The student uses the motor to lift a different load. The motor does 80 J of work when it lifts this load. It takes 5.0 s to lift the load.

Calculate the power output of the electric motor.

power output = ..................................................... W

A ball is dropped from a height. Energy is transferred in the process.

(i) State the energy store that is full before the ball is dropped.

(ii) The energy from part (i) is transferred into another energy store as the ball falls.

State the name of this energy store.

State the principle of conservation of energy.*

A motor lifts a load of 550 N through a vertical height of 12 m.

(i) Calculate the work done by the motor.

(ii) The work is done in 25 seconds.

Calculate the power of the motor.

Give your answer to 2 significant figures.

A solar-powered pump is used to raise water from a well to a storage tank. The tank is $12m$ above the surface of the water in the well.

Describe the transfers of energy between stores as energy from the Sun is used to raise the water to the tank.

Show that the gravitational potential energy gained by $50\mathrm{kg}$ of water raised through a height of $12m$ is approximately $5900J$.

Extract:

Show that the gravitational potential energy gained by 50 kg of water raised through a height of 12 m is approximately 5900 J.

ΔEp = m g Δh ΔEp = 50 × 9.8 × 12 ΔEp = 5880 ≈ 5900 J

The solar panel that powers the pump has an efficiency of 18%.

Calculate the total energy input from the solar panel required to raise this water.

Suggest two advantages of using solar energy to power the water pump, compared to using a diesel generator.

Fig. 2.1 shows a fork-lift truck lifting a box.

The electric motor that drives the lifting mechanism is powered by batteries.

Name the store that energy is usefully transferred from by the batteries.

Extended tier only

The lifting mechanism raises a box of mass 32 kg through a vertical distance of 2.5 m in 5.4 s.

(i) Calculate the energy transferred to the gravitational potential energy store of the box.

energy = ...........................................................[2]

(ii) The efficiency of the lifting mechanism is 0.65 (65%).

Calculate the input power to the lifting mechanism.

input power = ...........................................................[3]

Extract:

Fig. 2.1 shows a fork-lift truck lifting a box.

The electric motor that drives the lifting mechanism is powered by batteries.

Batteries are recharged from a mains voltage supply that is generated in an oil-fired power station.

By comparison with a wind farm, state one advantage and one disadvantage of running a power station using oil.

A man is working on a platform. He uses a rope to raise a bag from the ground to the platform as shown in Fig. 4.1.

(i) Name the energy store that energy is usefully transferred to as the bag is lifted at constant speed.

(ii) The man then lifts a second bag from the ground to the platform. The first bag weighs 100 N and the second bag weighs 150 N.

On which bag of materials does the man do more work?

Explain your answer.

(iii) The man wants to determine his useful power as he lifts one of the bags. He knows the weight of the bag. State the two other quantities he needs to know.

The statements describe processes in a coal-fired power station. They are not in the correct order.

A Energy is transferred to the thermal store of the water

B Coal burns releasing energy from its chemical store

C Energy is transferred electrically via the national grid

D A turbine turns coils in a magnetic field

E Steam turns a turbine

Use the letters A, B, C, D and E to complete the flow chart explaining how the power station works.

Extended tier only

A rifle fires a bullet of mass 0.020 kg vertically upwards through the air. As it leaves the rifle, the speed of the bullet is 350 m/s.

Calculate

(i) the energy in the kinetic store of the bullet as it leaves the rifle,

energy = ...........................................................[3]

(ii) the maximum possible height that the bullet can reach.

maximum height = ...........................................................[2]

Extract:

A rifle fires a bullet of mass 0.020 kg vertically upwards through the air. As it leaves the rifle, the speed of the bullet is 350 m/s.

To calculate the maximum height:

• Mass of bullet, m = 0.020 kg
• Initial speed = 350 m/s
• Recall the equation for gravitational potential energy: E_p = mgh
• Recall the link between E_k and E_p:
• All the energy will be transferred from the kinetic store to the gravitational potential store at maximum height
• E_k = E_p = mgh
• h = E_k / (mg) = 1200 / (0.020 × 9.81)
• h = 6000 m (1.s.f)

An object fired upwards will reach a maximum height where velocity will momentarily be 0. This point occurs when all the energy has been transferred from the kinetic store of the bullet to the gravitational potential store.

The actual height reached by the bullet is less than the value calculated in (a)(ii).

(i) Explain, in terms of the forces acting on the bullet, why this is so.

(ii) As the bullet rises through the air, the amount of energy in its kinetic store decreases.

State what happens to this energy.

Extended tier only

Salmon are born in freshwater rivers, spend most of their lives in the sea, but return to freshwater rivers to breed. As they swim up rivers, they often have to jump up waterfalls.

Fig. 1.1 shows a salmon attempting to jump up a waterfall with a vertical height of 0.36 m.

Fig. 1.1

The salmon has a mass of 1.84 kg.

The salmon leaves the water with a speed of 3.2 m/s.

Determine whether the salmon can make the height required for the jump.

Extract:

Fig. 1.1 shows a salmon attempting to jump up a waterfall with a vertical height of 0.36 m.

Extended tier only

Another salmon of mass 2.2 kg tries to make the jump.

Calculate the minimum speed with which the salmon would have to leave the water in order to reach the height required.

speed = ....................................

Extract:

Salmon are born in freshwater rivers, spend most of their lives in the sea, but return to freshwater rivers to breed. As they swim up rivers, they often have to jump up waterfalls.

Fig. 1.1 shows a salmon attempting to jump up a waterfall with a vertical height of 0.36 m.

The salmon has a mass of 1.84 kg.

The salmon leaves the water with a speed of 3.2 m/s.

The salmon swim far upstream and encounter a high waterfall.

The salmon attempts to jump to the top of the waterfall, but fails, and performs a somersault during the decent. The salmon straightens out before it enters the water as shown in Fig. 1.2.

Discuss whether the speed of entry into the water is greater than, less than or equal to the speed with which it leaves the water. Ignore any effects of air resistance.

Extract:

Salmon are born in freshwater rivers, spend most of their lives in the sea, but return to freshwater rivers to breed. As they swim up rivers, they often have to jump up waterfalls.

Fig. 1.1 shows a salmon attempting to jump up a waterfall with a vertical height of 0.36 m.

The salmon has a mass of 1.84 kg.

The salmon leaves the water with a speed of 3.2 m/s.

The temperature of the water at the bottom of a waterfall is greater than the temperature of the water at the top.

Suggest why this is the case.

Define power.

An amusement at a fairground uses the arrangement shown in Fig. 1.1 to measure the individual power rating of customers.

In the table below, list the quantities they must measure and the instrument used for measuring them.

Extract:

An amusement at a fairground uses the arrangement shown in Fig. 1.1 to measure the individual power rating of customers.

In the table below, list the quantities they must measure and the instrument used for measuring them.

The mass of the load on the system is 30 kg. The length of rope able to pass through the pulley is 1.3 m.

The man pulled the rope as hard as he could. The time taken was measured to be 1.2 s.

(i) Calculate the power rating of the man.

(ii) The amusement gave the man a power rating of 300 W.

Suggest a reason for the discrepancy.

Extract:

The man's body was only 19% efficient whilst pulling on the rope. Work done = 382.2 N m (from part (c))

Extended tier only

The man's body was only 19% efficient whilst pulling on the rope.

Calculate the total energy he used.

energy used = ....................................