Textbook Solution: Energy | IGCSE Cambridge Science for Year 6 - Class 6 PDF Download

Energy at Work

Q1. In Activity 6.2, which of the devices you investigated transferred energy from a battery or power supply?

Ans: The Electric lamp transfers energy from a battery or power supply.

Q2. What was the energy source for each of the other devices you tried out?

Ans: For the Hole punch, the energy source is the mechanical energy provided by the push from the hand.

Q3. Wind turbines generate electricity. What is the source of that energy?

Ans: The source of the energy that wind turbines convert into electricity is kinetic energy from the wind.

Q4. A candle flame can be used to produce light. Describe the source of that energy.

Ans: The source of the energy in a candle flame is the chemical energy stored in the wax, which is released as heat and light when the wax burns.

Q5. Mia holds a ball at head height. She then drops the ball and it falls to the ground.

a) What is the source of the energy that makes the ball move?

Ans: The source of the energy that makes the ball move is gravitational potential energy, which is converted to kinetic energy as the ball falls.

b) What has happened to that energy at the point just before the ball hits the ground?

Ans: Just before the ball hits the ground, most of the gravitational potential energy has been converted into kinetic energy.

Q6. Draw energy transfer diagrams like the one in figure 6.5 to describe the energy changes when:

a) a car accelerates

Ans: Chemical energy in fuel -> Kinetic energy of the car.

b) a girl climbs a rope ladder

Ans: Chemical energy in muscles -> Potential energy as the girl gains height.

c) water in a river goes down a waterfall

Ans: Potential energy due to height -> Kinetic energy as water falls.

d) a pan of water is heated over a flame

Ans: Chemical energy in the flame -> Thermal energy in the water.

Q7. How does a battery transfer energy into an electric lamp?

Ans: A battery transfers chemical energy to electrical energy, which the lamp then converts into light and heat.

Q8. How does a ringing bell transfer energy to your ears?

Ans: A ringing bell vibrates to produce sound waves, which transfer kinetic energy through the air to your ears.

Q9. Name two machines or other objects that work by receiving a transfer of energy from the Sun. Describe how each machine or object transfers the energy it receives, and what the energy is transferred to.

Ans: Solar panels and plants. Solar panels convert solar energy into electrical energy, while plants convert solar energy into chemical energy through photosynthesis.

Q10. Look at figure 6.7. Think about the energy changes that happen when a pole-vaulter runs with a long pole, places one end of the pole in the ground and makes a running jump upwards, still holding onto the end of the pole.

a) What energy does the vaulter have when running?

Ans: The vaulter has kinetic energy when running.

b) How is energy transferred to the pole when the athlete places one end of the pole into the ground and makes a running jump upwards, still holding onto the end of the pole?

Ans: Kinetic energy is transferred into elastic potential energy stored in the bent pole.

c) How does the pole then return energy back to the vaulter?

Ans: The pole returns energy back to the vaulter by converting the elastic potential energy back into kinetic energy, propelling the vaulter upwards.

Energy Dissipation

Q1. For each of the following devices, describe the useful energy transfers and what they make happen:

For example:

a washing machine heating water and doing work to move clothes and water

a) an oven

b) a torch

c) a loudspeaker

d) a drill

e) a battery

Ans:

• An oven: Converts electrical energy into thermal energy to heat and cook food.
• A torch: Converts electrical energy from batteries into light and heat energy to illuminate dark areas.
• A loudspeaker: Converts electrical energy into sound energy to produce audible sound.
• A drill: Converts electrical energy into mechanical energy to rotate a drill bit and create holes.
• A battery: Stores chemical energy and converts it into electrical energy to power devices.

Q2. For each of the devices in question 1, identify at least one wasted energy transfer.

Ans:

• An oven: Some of the thermal energy is lost to the surrounding environment instead of being used to cook food.
• A torch: Some of the electrical energy is converted into unwanted heat energy.
• A loudspeaker: Some of the electrical energy is lost as heat instead of being fully converted to sound.
• A drill: Some mechanical energy is lost as heat due to friction.
• A battery: Some chemical energy is lost as heat during energy conversion and discharge.

Q3. When a car engine burns fuel, only about 20% of energy stored in the fuel is usefully transferred into motion of the vehicle. Describe in what ways energy is dissipated in the engine.

Ans: When a car engine operates, the majority of the energy is lost primarily as heat. This heat is dissipated through the exhaust system and the cooling system (radiator). Some energy is also lost as sound energy through the noise of the engine and exhaust system, and further losses occur due to friction within the engine and drivetrain components.

End of chapter review

Q1. Name the way that energy is transferred when:

(a) You hear music

(b) You see pictures on a television screen 

(c) Your bicycle rolls downhill and gathers speed

(d) Your food is cooked in an oven. 

Ans: 

a) You hear music - Sound energy

b) You see pictures on a television screen - Light and electrical energy

c) Your bicycle rolls downhill and gathers speed - Kinetic energy

d) Your food is cooked in an oven - Thermal energy

Q2. Give the term for energy that is stored in:

(a) A moving object

(b) A hot object.

Ans: 

a) A moving object - Kinetic energy

b) A hot object - Thermal energy

Q3. Copy and complete the following sentences. Choose from the words: kinetic, dissipated, useful.

When energy is transferred, not all of the energy is _______ to us. Energy that is transferred to the surroundings by heating or sound becomes very difficult to use for doing anything else. We say the energy is                  .

Ans: When energy is transferred, not all of the energy is useful to us. Energy that is transferred to the surroundings by heating or sound becomes very difficult to use for doing anything else. We say the energy is dissipated.

Q4. Explain, using energy changes, how a rubber ball bounces.

Ans: When a rubber ball hits the ground, kinetic energy from the fall is temporarily stored as elastic potential energy when the ball deforms. This elastic potential energy is then converted back into kinetic energy as the ball returns to its original shape and bounces up.

Q5. Heating occurs in each of the following processes. For each process, describe the energy change caused by heating and state if the energy change is useful or not:

(a) cooking food in a microwave oven

(b) using an electric light.

Ans: 

a) Cooking food in a microwave oven: Electrical energy is converted to microwave radiation (a form of electromagnetic energy), which heats the food. This is useful as it cooks the food.

b) Using an electric light: Electrical energy is converted to light and thermal energy. The light is useful for illumination, while the heat is generally not useful and is considered wasted.

Q6. For the example in question 6 where the energy change was not useful, identify what useful energy transfer also occurred.

Ans: In the case of the electric light, while much of the energy is converted into unwanted heat, the useful energy transfer is the conversion of electrical energy into light energy, which provides illumination.

Q7.
(a) What energy source does an athlete use for running and jumping?

(b) There is a lot of energy dissipated when athletes use their muscles. What observation would provide evidence for this?

State how this supports the hypothesis.

A pole-vaulter can clear a much higher bar than a high-jumper. That is because the pole-vaulter can make use of the energy stored by bending a pole made of an elastic material.

(c) How does the athlete transfer energy to the pole so that energy is stored in it?

(d) Describe how bending the pole helps the pole vaulter lift themselves higher.
Ans:  

(a) The primary energy source an athlete uses for running and jumping is chemical energy stored in their muscles. This chemical energy comes from the breakdown of nutrients such as carbohydrates, fats, and proteins within the body. During physical activities like running and jumping, this chemical energy is converted into kinetic energy (movement) and thermal energy (heat). 

(b) Observation: Athletes often feel warm and sweat during and after intense physical activities such as running and jumping.

How this supports the hypothesis: The increase in body temperature and sweating provide evidence that a significant amount of energy is dissipated as heat. According to the law of conservation of energy, the chemical energy stored in muscles is not only converted into kinetic energy for movement but also into thermal energy. This supports the hypothesis that there is a lot of energy dissipation when muscles are used, as evidenced by the rise in body temperature and perspiration.

(c) The athlete transfers energy to the pole primarily through kinetic energy. Here’s how it happens:

1. Run-up Phase: The pole-vaulter runs towards the vaulting box, converting chemical energy into kinetic energy.
2. Planting the Pole: As the athlete plants the pole into the vaulting box, some of this kinetic energy is transferred to the pole.
3. Bending the Pole: The kinetic energy of the athlete is used to bend the pole, converting some of the kinetic energy into elastic potential energy stored in the pole.

(d) 
When the pole bends, it stores elastic potential energy. As the pole straightens out, this stored elastic potential energy is converted back into kinetic energy and gravitational potential energy. Here’s the process:
1. Energy Storage: As the athlete plants the pole and it bends, kinetic energy from the run-up is stored as elastic potential energy in the pole.
2. Energy Release: When the pole straightens, this stored elastic potential energy is released.
3. Vertical Lift: The release of energy propels the athlete upwards, converting the energy into gravitational potential energy, allowing the athlete to lift themselves higher than they could by jumping alone.

a) Describe the energy transfers that take place in the electric motor of the vacuum cleaner: Electrical energy is converted into mechanical energy to drive the vacuum motor, which in turn generates suction. Additionally, some of this energy is converted into heat.

b) Explain why all the energy transferred to the motor by electricity will eventually increase the thermal energy stored in the air: The inefficiencies in the motor conversion process result in some of the electrical energy being converted to heat, which is released into the room, thereby increasing the room’s temperature.

Ans:
(a) Energy Transfers in the Electric Motor of the Vacuum Cleaner

1. Electrical to Mechanical Energy: Electrical energy powers the motor, converting it into mechanical energy to drive the vacuum's fan and create suction.
2. Mechanical to Thermal Energy: Due to inefficiencies, some mechanical energy is lost as heat due to friction within the motor and fan components.
3. Additional Heat from Electrical Components: Electrical components also generate heat during operation.

(b) Explanation of Thermal Energy Increase in the Air

1. Energy Conservation: Energy is conserved and converted from one form to another.
2. Heat from Inefficiencies: Inefficiencies in the motor's conversion process generate heat.
3. Heat Dissipation: The generated heat is released into the air, increasing the room's temperature.
4. Complete Energy Conversion: All electrical energy eventually becomes thermal energy, warming the air in the closed room.