Energy, Work & Power (Cambridge O Level Physics)

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.

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

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.

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.

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 = .........................................................

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.

Fig. 3.1 shows a girl throwing a heavy ball.

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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 = ...........................................................

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.

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.

Calculate the efficiency of the kettle.

Fig.4.1 represents a hydroelectric system for generating electrical energy.

Describe how the power station uses the water in the reservoir to generate electricity.

State three advantages of generating electricity using a hydroelectric system compared with using a coal-fired power station.

An electric drill is shown in Fig.4.2. 

Complete the sentences about the electric drill.
Use words from the box. Each word may be used once, more than once, or not at all.

Fig. 5.1 shows a wind turbine.

Describe how the wind turbine produces electricity.

Wind turbines are used in many countries to replace coal-fired power stations.

A tidal barrage (dam) produces electricity using tides. Fig. 5.1 shows a diagram of a tidal barrage (simplified).

The water behind the barrage (dam) has most of its energy in one of its energy stores. State the name of this energy store.

Explain how the tidal barrage (dam) produces electricity.

Fig. 5.1 shows a geothermal power station. It generates electricity.

In a geothermal power station, the process of generating electricity includes seven stages.

Four of the stages are shown below.

The flow chart in Fig. 5.2 shows the seven stages, but it is incomplete. Complete the flow chart by adding the letters P, Q, R and S in the correct sequence.

The cost of electricity obtained from a geothermal power station is similar to the cost of electricity obtained from wind turbines.

 
Describe one advantage and one disadvantage of using a geothermal power station to generate electricity compared with using wind turbines.

Nuclear fission is used in nuclear power stations to release energy.

Describe how the energy is used to generate electricity.

Describe two environmental problems that arise from using nuclear power stations.

Fig. 2.1 shows a black car going up a hill on a sunny day.

Fig. 2.1

State:
 

. [1]

       [1]

The car accelerates up the hill. 

In addition to an increase in thermal energy, there are other energy transfers taking place. 

Describe the other energy transfers.

At one point in the motion, the kinetic energy of the car is 90 kJ. 

The mass of the car is 800 kg.

Calculate the speed of the car.

speed ...................................................  m/s 

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.

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

The 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.

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

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

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

Extended

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.

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.

The salmon swim further upstream and encounter a waterfall that is much higher than the previous one.

The salmon attempts the jump, 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.

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.

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

Fig. 1.1

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 and the time taken was measured to be 1.2 s.

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

Calculate the total energy he used.

A solar panel receives energy from the Sun at a rate of 5.0 kW.

Thermal energy is transferred from the solar panel to water with an efficiency of 20%.

Cold water of mass 15 kg enters the solar panel every hour.

The specific heat capacity of water is 4200 J / (kg°C).

Calculate the temperature increase of the water.

temperature increase = .....................................................°C 

State and explain one advantage and one disadvantage of heating the water in a solar panel compared with heating the water in a coal-burning boiler.

Energy sources used to generate electricity are shown in the box.

Which energy sources are non-renewable?

Draw a ring around each energy source that is non-renewable.

The diagram shows a geothermal power station.

Describe how the geothermal power station generates electricity.

In America, it is estimated that tidal power costs $150 per MWh, whereas coal costs $60 per MWh. This is partly due to construction costs of the turbines for tidal power. 

Suggest why it is expensive to construct tidal turbines. 

Fusion is occurring in the centre of a star. The region in which a certain type of fusion is occurring has a diameter of 150 000 km.

Calculate the surface area of this sphere. 

The weight of the star around the core exerts a large force on the core. 

The core experiences a pressure of 26.5 × 10¹⁵ Pa.

Compare the advantages and disadvantages of nuclear fusion and nuclear fission.

Calculate the number of joules in 1 GWh.

Tidal power electrically transfers energy in the kinetic store of moving water. 

When the tide is moving, a portion of sea water passes through the turbine. The turbine has a cylindrical shape as the turbine spins in a circular path. A turbine is shown in Fig. 1.1.

The cylinder is 24 m long and has a radius of 5.0 m. The tide is travelling at 6.0 m/s.

Some ocean environmental groups oppose the use of tidal power, stating that it can physically harm aquatic life, as well as impacting their ability to communicate. 

Suggest why these statements could be correct.

A coal power station has an average output of 400 MW. Explain how, in a power station, burning a fuel causes a generator to produce an alternating current.

The Sun has a total power output of 3.8 × 10²⁶ W across its whole surface. The surface of the Earth receives 5.0 × 10⁻⁸ % of this power. 

Calculate the total power the surface of the Earth receives from the Sun.

The power that the Earth receives from the Sun is actually twice the percentage given in part (b).