Energy stores and systems

These revision notes cover the section on Energy Stores and Systems assessed as part of the AQA GCSE Physics and Combined Sciences: Trilogy qualifications.

Welcome to the fascinating world of energy stores and systems! If you’ve ever wondered how a moving car stops or why boiling water in an electric kettle eventually gets hot, you’re about to find out.

What is a System?

Imagine you’re holding a basketball. Alone, it’s just a ball. But once you throw it, the ball, your arm, and even gravity become part of something bigger—a system. A system is an object or a group of objects that interact with each other. The magic begins when there are changes within this system, leading to energy transformations.

basketball player throwing a ball, demonstrating an energy system

Energy Stores

Energy can be stored in various ways within a system. Here are the primary stores you should know about:

  1. Kinetic Energy: The energy of motion. The faster something moves, the more kinetic energy it has.
  2. Gravitational Potential Energy: Energy stored due to an object’s position in a gravitational field. Higher up means more energy!
  3. Elastic Potential Energy: Stored when materials stretch or compress.
  4. Chemical Energy: Found in food, fuel, and batteries, this energy is released during chemical reactions.
  5. Thermal Energy: The total energy of all the particles in an object. The hotter it is, the more thermal energy it has.

Changes in Energy Stores

When a system changes, so does the way energy is stored. Let’s explore some common situations:

1. An Object Projected Upwards

Imagine throwing a basketball upwards. As it leaves your hand (kinetic energy), it slows down, converting kinetic energy into gravitational potential energy. At its highest point, the movement halts momentarily, meaning all the kinetic energy has been converted into gravitational potential energy. As it falls, this process reverses.

2. A Moving Object Hitting an Obstacle

When a car crashes into a barrier, its kinetic energy is rapidly converted into other forms, such as thermal energy (heat) and sound energy. The deformation of the car and barrier also indicates that some energy is stored as elastic potential energy.

Energy exchange when a car hits a barrier. Crumpled vehicle from impact.

3. An Object Accelerated by a Constant Force

When you push a skateboard, work is done (force applied over a distance), transferring chemical energy (from your muscles) to kinetic energy in the skateboard. The faster the skateboard moves, the more kinetic energy it accumulates.

skateboarder demonstrating chemical energy transferring into kinetic

4. A Vehicle Slowing Down

Brakes on a vehicle convert its kinetic energy into thermal energy through friction. This is why brakes get hot after being used for a while.

5. Bringing Water to a Boil in an Electric Kettle

The electric kettle converts electrical energy into thermal energy, heating the water. The water’s increase in temperature indicates a rise in its thermal energy.

kettle boiling water turning electrical energy into thermal (heat) energy

Calculating Changes in Energy

Understanding energy transformations involves some calculations. Here are formulas for the main energy stores:

  • Kinetic Energy (KE): KE = 1/2 mv^2 where m is mass in kilograms and v is velocity in meters per second.
  • Gravitational Potential Energy (GPE): GPE = mgh where m is mass, g is the acceleration due to gravity (9.8m/s^2 on Earth), and h is height in meters.
  • Work Done by Forces: Work = force × distance where force is in newtons and distance in meters.

Let’s apply these to our examples:

  • A basketball thrown upwards: Use the kinetic energy formula at the start and gravitational potential energy at the highest point.
  • A car crashing: The kinetic energy before the crash can be calculated, showing how much energy is available to be converted.
  • Heating water in a kettle: The work done by the electrical current can be related to the increase in water’s thermal energy.

Can you work out which types of energy increase and decrease at different points during a rollercoaster ride? For example, the cart will have the greatest GPE (Gravitational Potential Energy) at the highest point of the track.

rollercoaster demonstrating how energy converts from one form to another throughout the ride

Summary and Tips for Understanding

Understanding energy in systems boils down to recognizing the types of energy involved and how they transform when a system changes. Here are some tips:

  • Always identify the system and the energy stores involved.
  • Use the right formulas to quantify changes in energy.
  • Practice with real-life examples to see these principles in action.

Video

 

Quiz

Write out your answers on a piece of paper, then check them against the solutions below.

Questions

Multiple Choice Questions

  1. In physics, a system refers to:
    • A) A collection of scientific methods.
    • B) The total energy within a closed space.
    • C) An object or group of objects where energy changes occur.
    • D) A mathematical model for energy conservation.
  2. Which of the following is NOT a form of energy storage?
    • A) Elastic
    • B) Nuclear
    • C) Sonic
    • D) Chemical
  3. The energy stored due to an object’s position above the ground is called:
    • A) Kinetic energy
    • B) Elastic potential energy
    • C) Chemical energy
    • D) Gravitational potential energy
  4. The formula “KE = 1/2 mv^2” is used to calculate:
    • A) Gravitational potential energy
    • B) Elastic potential energy
    • C) Kinetic energy
    • D) Thermal energy
  5. When boiling water in an electric kettle, electrical energy is mainly converted into:
    • A) Mechanical energy
    • B) Thermal energy
    • C) Chemical energy
    • D) Kinetic energy

True or False

  1. Energy in a system can be destroyed. (True/False)
  2. A vehicle slowing down converts kinetic energy into thermal energy due to friction. (True/False)
  3. Chemical energy is the energy stored in the bonds between atoms in molecules. (True/False)

Short Answer

  1. Describe the energy transformation in an electric kettle when water is boiled.
  2. Explain the energy changes when a ball is thrown upwards and then falls back to the ground.

Calculation Problems

  1. What is the gravitational potential energy of a 5 kg object placed 15 meters above the ground? (Use g = 9.8 m/s^2)
  2. Find the kinetic energy of a 1200 kg car moving at 15 m/s.

End of questions.

Answers

  1. C
  2. C
  3. D
  4. C
  5. B
  6. False
  7. True
  8. True
  9. When water is boiled in an electric kettle, electrical energy is converted into thermal energy. This increases the water’s temperature until it reaches its boiling point, converting some of the water into steam.
  10. As the ball is thrown upwards, kinetic energy is converted into gravitational potential energy. At the peak of its trajectory, the ball has maximum gravitational potential energy and minimal kinetic energy. As it falls, the gravitational potential energy is converted back into kinetic energy until it hits the ground.
  11. GPE = mgh = 5 kg * 9.8 m/s^2 * 15 m = 735 Joules.
  12. KE = 1/2 mv^2 = 1/2 * 1200 kg * (15 m/s)^2 = 135,000 Joules.