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Conservation of energy and efficiency

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Practice questions

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1.Which statement best describes the law of conservation of energy?
Easy
- AEnergy can be created but not destroyed.
- BEnergy can be destroyed but not created.
- CEnergy can neither be created nor destroyed, only transformed or transferred.
- DEnergy can be created and destroyed in equal amounts.
2.In any energy transformation, the total amount of energy before and after the transformation is the same.
Easy
True or false?
3.A light bulb converts electrical energy into light and heat. Which statement is correct?
Medium
- AThe electrical energy is completely converted into light energy.
- BThe total energy output (light + heat) equals the electrical energy input.
- CThe light energy is greater than the electrical energy input.
- DHeat energy is the useful output energy.
4.Which of the following are examples of energy transformations? (Select all that apply.)
Medium
- AA battery powering a torch (chemical to electrical to light)
- BA ball rolling down a hill (gravitational potential to kinetic)
- CA book sitting on a shelf (no transformation)
- DA microphone converting sound to electrical signals
5.Arrange the following events in the correct order for the energy transformation in a hydroelectric power station:
Medium
- Kinetic energy of spinning turbine
- Gravitational potential energy of water behind dam
- Electrical energy from generator
- Kinetic energy of falling water
6.Match each device with the main energy transformation it performs:
Medium
- Electric heater
- Solar cell
- Microphone
- Sound to electrical
- Electrical to thermal
- Light to electrical
7.Match each energy transformation to the correct example.
Medium
- Chemical → Kinetic + Thermal
- Electrical → Light + Thermal
- Gravitational Potential → Kinetic
- A ball rolling down a hill
- A burning candle
- A light bulb
8.Arrange the following steps in the correct order for the energy transformation in a hydroelectric power station.
Medium
- Kinetic energy of water turns turbine
- Gravitational potential energy of water stored behind dam
- Electrical energy from generator
- Kinetic energy of turbine turns generator

More questions

9.Which of the following are consequences of the law of conservation of energy? (Select all that apply.)
Medium
- AEnergy can be created from nothing in nuclear reactions.
- BPerpetual motion machines of the first kind are impossible.
- CThe total energy in an isolated system is constant.
- DEnergy can be transformed from one form to another.
10.A coal-fired power station has an efficiency of 35%. This means that for every 100 MJ of chemical energy in the coal, the useful electrical energy output is:
Medium
- A35 MJ
- B65 MJ
- C100 MJ
- D135 MJ
11.A pendulum swings from its highest point to its lowest point. Ignoring air resistance, which statement is correct about the energy changes?
Hard
- AKinetic energy increases and gravitational potential energy decreases, total energy constant.
- BKinetic energy decreases and gravitational potential energy increases, total energy constant.
- CBoth kinetic and potential energy increase, total energy increases.
- DBoth kinetic and potential energy decrease, total energy decreases.
12.A student drops a ball from a height of 2.0 m. The ball bounces back to a height of 1.5 m. Which statement best explains why the ball does not return to its original height?
Medium
- AThe law of conservation of energy does not apply to bouncing balls.
- BSome of the kinetic energy is converted to heat and sound energy during the collision with the ground.
- CThe ball loses mass during the bounce.
- DGravitational potential energy is not conserved in any system.
13.Which of the following are examples of energy dissipation? (Select all that apply.)
Medium
- AA car engine converting fuel energy into heat and sound.
- BA light bulb converting electrical energy into light and heat.
- CA battery storing chemical energy.
- DA pendulum swinging in air gradually slowing down.
14.In any energy transformation, the total amount of useful energy always remains the same.
Easy
True or false?
15.Match each energy transfer or transformation to the correct description.
Medium
- A battery powering a torch
- A ball dropped from a height
- A car braking to a stop
- Chemical → electrical → light (and heat)
- Gravitational potential → kinetic
- Kinetic → heat (and sound)
16.Arrange the following energy transformations in the correct order for a wind turbine generating electricity.
Medium
- Kinetic energy of wind
- Electrical energy
- Kinetic energy of turbine blades
- Kinetic energy of generator rotor
17.A solar panel has an efficiency of 18%. It receives 1200 J of light energy. How much electrical energy does it produce?
Medium
- A216 J
- B66.7 J
- C1200 J
- D984 J
18.A perpetual motion machine of the first kind is impossible because it would violate the law of conservation of energy.
Easy
True or false?
19.In an experiment to determine the efficiency of a solar panel, a student measures the light energy incident on the panel and the electrical energy output. Which of the following would cause the calculated efficiency to be lower than the true value?
Medium
- AThe light meter overestimates the incident light energy.
- BThe electrical output is measured accurately but the light energy is underestimated.
- CThe panel is partially shaded during the experiment.
- DThe voltmeter used to measure output has a high resistance.
20.Which of the following are valid methods to measure the useful output energy of a device? (Select all that apply.)
Medium
- AMeasuring the temperature rise of the device's surroundings.
- BMeasuring the work done by the device against a known force.
- CMeasuring the electrical power input and multiplying by time.
- DMeasuring the kinetic energy of a mass accelerated by the device.
- EMeasuring the heat dissipated by the device.
21.In an experiment to measure the efficiency of a light bulb, it is sufficient to measure only the electrical power input and the light output in lumens.
Easy
True or false?
22.Arrange the following steps in the correct order for a student to determine the efficiency of a solar panel:
Medium
- Measure the electrical energy output using a joulemeter.
- Calculate efficiency = (useful output / total input) × 100%.
- Measure the area of the solar panel.
- Measure the light energy incident per square metre using a light meter.
- Calculate the total light energy incident on the panel.
23.Match each type of energy loss in a motor with the correct method to reduce it:
Medium
- Friction in bearings
- Resistance in wires
- Air resistance on moving parts
- Use lubricants
- Use thicker wires
- Streamline the shape
24.A student uses a joulemeter to measure the electrical energy supplied to a motor and the work done by the motor in lifting a weight. The student finds that the electrical energy input is 50 J and the work done lifting the weight is 35 J. Which of the following is the best estimate of the energy dissipated as heat in the motor?
Hard
- A15 J
- B35 J
- C50 J
- D85 J
25.The law of conservation of energy states that energy can be created or destroyed.
Easy
True or false?
26.What is the term for energy that is not usefully transformed and is often dissipated to the surroundings?
Easy
- AWasted energy
- BUseful energy
- CKinetic energy
- DPotential energy
27.Match each device with the main energy transformation it performs.
Medium
- Electric heater
- Solar panel
- Loudspeaker
- Electrical → heat
- Light → electrical
- Electrical → sound
28.Match each type of energy loss in a motor to the correct method to reduce it.
Medium
- Friction in bearings
- Electrical resistance in coils
- Air resistance on rotating parts
- Use lubricants
- Use thicker wires
- Streamline the shape
29.Arrange the following energy transformations in the correct order for a coal-fired power station generating electricity:
Medium
- Chemical energy in coal
- Thermal energy in steam
- Kinetic energy of turbine
- Electrical energy from generator
30.Which of the following are examples of useful energy output? (Select all that apply.)
Medium
- ALight from a lamp
- BHeat from a computer processor
- CSound from a speaker
- DFriction in brakes
31.Which of the following are valid methods to reduce energy loss in a motor? (Select all that apply.)
Medium
- AUse lubricants to reduce friction
- BUse thinner wires to reduce resistance
- CStreamline the shape to reduce air resistance
- DIncrease the input voltage
32.Arrange the following energy transformations in the correct order for a hydroelectric power station generating electricity.
Hard
- Kinetic energy of water turns turbine
- Gravitational potential energy of water in reservoir
- Kinetic energy of turbine turns generator
- Electrical energy from generator
33.In an experiment to determine the efficiency of a solar panel, a student measures the light energy incident on the panel and the electrical energy output. Which additional measurement would improve the accuracy of the efficiency calculation?
Medium
- AThe temperature of the panel
- BThe angle of the sunlight relative to the panel
- CThe colour of the panel
- DThe mass of the panel
34.A student investigates the efficiency of an electric motor by measuring the input electrical energy and the useful output kinetic energy. Which piece of equipment is most suitable for measuring the input electrical energy?
Medium
- AA voltmeter and an ammeter
- BA joulemeter
- CA newton meter
- DA stopwatch
35.In an experiment to measure the efficiency of a solar panel, a student places the panel at different angles to the sun. Which of the following are appropriate ways to ensure a fair test? (Select all that apply.)
Hard
- AUse the same solar panel throughout
- BMeasure the light intensity at each angle and use it to calculate input energy
- CKeep the time of exposure constant for each angle
- DChange the weather conditions between trials
- ERecord the electrical output for the same duration at each angle
36.The law of conservation of energy states that energy:
Easy
- Acan be created but not destroyed
- Bcan be destroyed but not created
- Ccannot be created or destroyed, only transformed or transferred
- Dcan be created and destroyed under certain conditions
37.The total amount of energy in an isolated system remains constant over time.
Easy
True or false?
38.A perpetual motion machine of the first kind is possible if it is perfectly designed.
Easy
True or false?
39.Which of the following is an example of energy transformation?
Medium
- AA battery storing chemical energy
- BA light bulb converting electrical energy to light and heat
- CA book sitting on a shelf having gravitational potential energy
- DA stretched rubber band having elastic potential energy
40.Which of the following are true about efficiency? (Select all that apply.)
Medium
- AEfficiency is always less than or equal to 100%.
- BEfficiency = (useful energy output / total energy input) × 100%.
- CA machine with 100% efficiency would have no wasted energy.
- DEfficiency can be greater than 100% for some machines.
41.Complete the sentence about energy.
Easy
Energy cannot be created or destroyed; it can only be ____ or ____.
42.What term describes energy that is not usefully transformed and is often dissipated to the surroundings?
Easy
43.A motor has an input energy of 500 J and produces 350 J of useful kinetic energy. How much energy is wasted?
Medium
44.A solar panel receives 800 J of light energy and converts 240 J into electrical energy. Calculate the efficiency of the solar panel as a percentage.
Hard
45.Name the three main types of energy loss in a typical motor.
Easy
____, ____, and ____
46.A hydroelectric power station has an overall efficiency of 80%. The water falling through the turbines loses 5.0 × 10⁵ J of gravitational potential energy every second. Calculate the useful electrical energy output per second (in joules).
Hard
47.Complete the sentence about energy efficiency.
Medium
The efficiency of a device is calculated using the formula: efficiency = (____ energy output) ÷ (____ energy input) × 100%.
48.Explain why it is impossible for a machine to have an efficiency greater than 100%.
Hard
49.An electric heater is 95% efficient. It wastes 50 J of energy every second. Calculate the total input energy per second (in joules).
Hard
50.A roller coaster car starts from rest at the top of a 20 m high hill. Assuming no friction, what is its speed at the bottom of the hill? (Use g = 10 m/s²)
Medium
51.Complete the sentence about energy conservation.
Easy
According to the law of conservation of energy, energy cannot be ____ or ____; it can only be ____ or ____ from one form to another.
52.What is the name given to a machine that would produce more energy than it consumes, violating the law of conservation of energy?
Medium
53.Explain why no real machine can be 100% efficient.
Hard
54.State the law of conservation of energy.
Medium
55.A student investigates the efficiency of an electric motor by measuring the input electrical energy and the useful output kinetic energy. Describe a method the student could use to measure the useful output kinetic energy. (2 marks)
Hard
56.A student investigates the efficiency of an electric motor by measuring the input electrical energy and the useful output kinetic energy. Describe a method the student could use to measure the useful output kinetic energy.
Medium
57.Explain why a perpetual motion machine of the first kind cannot exist, referring to the law of conservation of energy.
Medium
58.A motor has an input energy of 500 J and produces 350 J of useful kinetic energy. How much energy is wasted?
Medium
- A150 J
- B135 J
- C165 J
- D180 J
59.A solar panel receives 800 J of light energy and converts 240 J into electrical energy. Calculate the efficiency of the solar panel as a percentage.
Hard
- A33 %
- B27 %
- C36 %
- D30 %
60.A hydroelectric power station has an overall efficiency of 80%. The water falling through the turbines loses 5.0 × 10⁵ J of gravitational potential energy every second. Calculate the useful electrical energy output per second (in joules).
Hard
- A440000 J
- B400000 J
- C360000 J
- D480000 J
61.An electric heater is 95% efficient. It wastes 50 J of energy every second. Calculate the total input energy per second (in joules).
Hard
- A1000 J
- B1100 J
- C900 J
- D1200 J
62.A roller coaster car starts from rest at the top of a 20 m high hill. Assuming no friction, what is its speed at the bottom of the hill? (Use g = 10 m/s²)
Medium
- A18 m/s
- B24 m/s
- C20 m/s
- D22 m/s

Flashcards

State the law of conservation of energy.

Energy cannot be created or destroyed; it can only be transformed or transferred from one form to another. The total energy in an isolated system remains constant.

What is meant by 'useful energy'?

Useful energy is the energy that is transformed into the desired form for a specific purpose, such as light from a lamp or kinetic energy in a moving car.

What is 'wasted energy'?

Wasted energy is energy that is not usefully transformed and is dissipated to the surroundings, often as heat or sound.

Why can no system be 100% efficient?

In any energy transformation, some energy is always wasted, usually as heat due to friction or resistance, so the useful output is always less than the total input.

Define efficiency in terms of energy.

Efficiency = (useful output energy / total input energy) × 100%. It measures how well a device converts input energy into useful output.

A light bulb converts 100 J of electrical energy into 10 J of light and 90 J of heat. What is its efficiency?

Efficiency = (10 J / 100 J) × 100% = 10%.

What happens to the 'wasted' energy in a system?

It is transferred to the surroundings, usually as thermal energy (heat), and spreads out, making it less useful.

Give an example of energy transformation in a moving car.

Chemical energy in fuel is transformed into kinetic energy (useful) and thermal energy (wasted) due to engine friction and exhaust.

What is a perpetual motion machine of the first kind?

A hypothetical machine that would produce work indefinitely without an energy input, violating the law of conservation of energy.

Why is a perpetual motion machine of the first kind impossible?

Because energy cannot be created from nothing; the total energy output cannot exceed the energy input, so such a machine cannot exist.

How does the law of conservation of energy apply to a pendulum?

At the highest point, energy is all gravitational potential; at the lowest, it is all kinetic. Total mechanical energy remains constant if friction is ignored.

What is mass–energy equivalence?

It is the principle that mass can be converted into energy and vice versa, expressed by E = mc², where c is the speed of light.

In what context does mass–energy equivalence become significant?

In nuclear reactions, such as fission or fusion, where a small amount of mass is converted into a large amount of energy.

What is the difference between energy transfer and energy transformation?

Energy transfer is when energy moves from one place to another without changing form (e.g., heat moving from a heater to the air). Energy transformation is when energy changes from one form to another (e.g., electrical to light).

List three common forms of energy.

Kinetic energy, gravitational potential energy, thermal energy (others: chemical, electrical, light, sound, elastic, nuclear).

A roller coaster car starts at a height of 20 m. Ignoring friction, what is its speed at the bottom? (g = 10 m/s²)

Using conservation of energy: mgh = ½mv² → v = √(2gh) = √(2×10×20) = √400 = 20 m/s.

What does 'dissipated energy' mean?

Energy that is spread out to the surroundings, often as thermal energy, and is no longer available for useful work.

How can the efficiency of a device be improved?

By reducing wasted energy, e.g., using lubrication to reduce friction, or better insulation to reduce heat loss.

What is the efficiency of a device that inputs 500 J and outputs 200 J of useful energy?

Efficiency = (200 J / 500 J) × 100% = 40%.

State the first law of thermodynamics in terms of energy conservation.

The change in internal energy of a system equals the heat added to the system minus the work done by the system; energy is conserved.

What is the relationship between energy and work?

Work is the transfer of energy when a force moves an object. The amount of work done equals the energy transferred.

In an isolated system, if 50 J of chemical energy is converted to heat and sound, what is the total energy after conversion?

50 J. The total energy remains constant; it is just in different forms (heat and sound).

What is the main reason energy is 'lost' as heat in electrical circuits?

Resistance in wires causes some electrical energy to be converted into thermal energy (Joule heating).

A solar panel converts 800 J of light energy into 200 J of electrical energy. Calculate the efficiency.

Efficiency = (200 J / 800 J) × 100% = 25%.

Why is the law of conservation of energy considered a fundamental principle of physics?

It has been validated by countless experiments and is derived from the symmetry of time translation invariance (Noether's theorem).

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