Year 9 Physics: Changes in Energy

Introduction to Changes in Energy

Energy is the ability to do work or cause change. In physics, we often talk about different types of energy and how they change from one form to another. Understanding these changes is important because it helps us comprehend how things work in the world around us.

Types of Energy

1. Kinetic Energy: This is the energy of motion. Anything that is moving has kinetic energy. For example, a car driving down the road or a ball being thrown.
   • Formula: Kinetic Energy (KE) can be calculated using the formula: $$KE = \frac{1}{2} mv^2$$ where ( m ) is mass in kilograms and ( v ) is velocity in metres per second.
2. Potential Energy: This is stored energy. It can be gravitational (due to height) or elastic (like in a stretched spring). For example, a book on a shelf has gravitational potential energy because it can fall.
   • Formula: Gravitational Potential Energy (PE) can be calculated using: $$PE = mgh$$ where ( m ) is mass in kilograms, ( g ) is the gravitational field strength (approximately ( 9.8 , \text{m/s}^2 ) on Earth), and ( h ) is height in metres.
3. Thermal Energy: This is the energy related to the temperature of an object. When you heat something, its particles move faster, which increases its thermal energy.
4. Chemical Energy: This is the energy stored in the bonds of chemical compounds. Food and batteries are examples of chemical energy.
5. Electrical Energy: This is the energy from electric charges. For example, when you plug in a device, electrical energy powers it.

Changes in Energy

Energy can change from one form to another. This is known as energy transformation. Here are a few examples:

• A Roller Coaster: At the top of the ride, the coaster has high potential energy due to its height. As it goes down, that potential energy changes into kinetic energy as it speeds up.
• A Stretchy Rubber Band: When you stretch a rubber band, you give it elastic potential energy. When you let go, that energy changes into kinetic energy as the band snaps back.

Key Rules

1. Energy Conservation: Energy cannot be created or destroyed; it can only be transformed from one form to another. The total energy in a closed system remains constant.
2. Efficiency: Not all energy transformations are 100% efficient. Some energy is often converted into thermal energy (waste energy) during transformations.

Tips and Tricks

• Visualise: Draw diagrams of energy transformations to help remember how energy changes form.
• Real-life Examples: Think of everyday examples of energy changes, like cooking (thermal energy) or riding a bike (kinetic energy).
• Practice Problems: Work through calculations using the formulas for potential and kinetic energy to gain a better understanding.

Questions

Easy Level Questions (20)

1. What is kinetic energy?
2. What is potential energy?
3. How do you calculate kinetic energy?
4. What type of energy do batteries store?
5. What happens to potential energy when an object falls?
6. Is energy created or destroyed?
7. What is thermal energy?
8. Give an example of elastic potential energy.
9. What type of energy does food contain?
10. What happens to energy in a closed system?
11. What type of energy is used in a moving car?
12. How does a roller coaster demonstrate energy changes?
13. What is the formula for gravitational potential energy?
14. What is an example of electrical energy?
15. What happens to the energy when you heat a substance?
16. What type of energy is released when you light a candle?
17. What type of energy does a stretched spring have?
18. What is meant by energy transformation?
19. Can kinetic energy ever be negative?
20. What does it mean for an energy transformation to be efficient?

Medium Level Questions (20)

1. If a 2 kg book is on a shelf 3 m high, what is its gravitational potential energy?
2. A car with a mass of 1,000 kg is moving at 20 m/s. What is its kinetic energy?
3. Describe what happens to energy when a ball is thrown upwards.
4. How can energy be wasted in an electrical appliance?
5. Explain how energy changes when you eat food and use it to run.
6. What type of energy transformation occurs when you rub your hands together?
7. Why do we say energy is conserved?
8. How does friction affect energy transformations?
9. What is the role of gravity in potential energy?
10. How can we measure the efficiency of an energy transformation?
11. If a rubber band is stretched and then released, what type of energy transformation occurs?
12. What happens to the kinetic energy of a car when it brakes?
13. Calculate the potential energy of a 5 kg object at a height of 4 m.
14. What type of energy is involved when a chemical reaction occurs?
15. Why is it important to understand energy changes in science?
16. Describe an example of energy changing from electrical to thermal.
17. How does solar energy transform when used to power a device?
18. If a 3 kg ball is moving at a speed of 10 m/s, what is its kinetic energy?
19. How does the energy of a falling object change as it falls?
20. What factors affect the kinetic energy of an object?

Hard Level Questions (20)

1. A 10 kg object is dropped from a height of 20 m. Calculate its potential energy and the kinetic energy just before it hits the ground.
2. If a car travels at 30 m/s and has a kinetic energy of 450,000 J, what is its mass?
3. Describe the energy changes that occur in a hydroelectric power station.
4. Explain the concept of energy loss in real-world applications.
5. How does the law of conservation of energy apply to a pendulum?
6. A 1.5 kg ball is thrown upwards with an initial velocity of 15 m/s. Calculate its maximum height.
7. What is the efficiency of a light bulb that converts 40 J of electrical energy into 8 J of light energy?
8. How does potential energy change when a roller coaster goes over a hill?
9. Calculate the kinetic energy of a 500 g object moving at 25 m/s.
10. Explain how energy is transformed in a battery when it powers a device.
11. What is the relationship between mass, speed, and kinetic energy?
12. Describe a real-life scenario where energy transformation is not 100% efficient.
13. How does a wind turbine convert kinetic energy into electrical energy?
14. If a 3 kg object is lifted to a height of 5 m, how much work is done against gravity?
15. How do you calculate the total mechanical energy of an object?
16. A car has a kinetic energy of 600,000 J and is brought to rest. How much work is done by the brakes?
17. How does thermal energy affect the state of matter in substances?
18. Explain the energy transformations that occur in photosynthesis.
19. If the gravitational field strength is 10 N/kg, what is the potential energy of a 2 kg object at a height of 10 m?
20. Discuss how energy conservation can help in addressing climate change.

Answers

Easy Level Answers

1. The energy of motion.
2. Stored energy.
3. $$KE = \frac{1}{2} mv^2$$
4. Chemical energy.
5. It converts to kinetic energy.
6. No, it only transforms.
7. Energy related to temperature.
8. A stretched rubber band.
9. Chemical energy.
10. Total energy stays the same.
11. Kinetic energy.
12. Potential energy changes to kinetic energy.
13. $$PE = mgh$$
14. Energy from electric charges.
15. It increases thermal energy.
16. Chemical energy is released.
17. Elastic potential energy.
18. Energy transformation refers to changing from one form to another.
19. No, kinetic energy cannot be negative.
20. It means not all energy is usable.

Medium Level Answers

1. $$PE = mgh = 2 \times 9.8 \times 3 = 58.8 , J$$
2. $$KE = \frac{1}{2} mv^2 = \frac{1}{2} \times 1000 \times 20^2 = 200,000 , J$$
3. Potential energy converts to kinetic energy as it rises and vice versa when it falls.
4. Through heat loss.
5. The body converts chemical energy into kinetic energy.
6. Friction turns kinetic energy into thermal energy.
7. Energy stays the same in total but changes form.
8. It reduces potential energy.
9. $$PE = mgh$$
10. By comparing input and output energy.
11. From elastic potential to kinetic energy.
12. The car’s kinetic energy turns to heat due to friction.
13. Wind turns the blades, generating electrical energy.
14. $$Work = mgh = 3 \times 10 \times 5 = 150 , J$$
15. Total Mechanical Energy = Potential Energy + Kinetic Energy.
16. Work done is equal to the change in kinetic energy.
17. It can change from solid to liquid to gas.
18. Plants convert solar energy into chemical energy.
19. $$PE = mgh = 2 \times 10 \times 10 = 200 , J$$
20. It reduces energy waste and promotes sustainable practices.

Hard Level Answers

1. $$PE = mgh = 10 \times 9.8 \times 20 = 1960 , J$$ ; $$KE = 1960 , J$$
2. $$KE = \frac{1}{2} mv^2 \rightarrow 450,000 = \frac{1}{2} m (30)^2 \rightarrow m = 500 , kg$$
3. Water flows, turning a turbine, generating electricity.
4. Energy is lost as heat or sound.
5. Total energy remains constant; energy changes forms.
6. $$h = \frac{v^2}{2g} = \frac{15^2}{2 \times 9.8} = 11.47 , m$$
7. Efficiency = Output energy/Input energy = $$\frac{8}{40} \times 100 = 20%$$
8. Potential energy decreases and kinetic energy increases.
9. $$KE = \frac{1}{2} mv^2 = \frac{1}{2} (0.5)(25^2) = 156.25 , J$$
10. It changes chemical energy into electrical energy.
11. Kinetic energy increases with more mass and speed.
12. Like a car engine converting fuel to motion.
13. Kinetic energy of wind turns blades into electrical energy.
14. $$Work = mgh = 3 \times 10 \times 5 = 150 , J$$
15. Total Mechanical Energy = Potential + Kinetic Energy.
16. Work done = 600,000 J.
17. Thermal energy can change solid ice to liquid water.
18. Plants use sunlight to create glucose.
19. $$Work = mgh = 2 \times 10 \times 10 = 200 , J$$
20. It promotes energy use and reduction of greenhouse gases.

This guide should help you understand the concepts of changes in energy. Keep practicing with the questions, and remember to relate these ideas to the world around you!