Energy, Work & Power

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Notes

Energy Stores & Transfers

Energy is a property stored or transferred, measured in **joules (J)**.
A **system** is an object or group of objects; changes within a system involve energy transfer.
Energy stores include: **kinetic**, **gravitational potential**, **elastic**, **magnetic**, **electrostatic**, **chemical**, **nuclear**, and **thermal**.
Energy transfer pathways: **mechanical** (force), **electrical** (charge flow), **heating** (particle conduction), and **radiation** (electromagnetic waves).
Example: a battery-powered torch transfers energy **electrically** from the **chemical store** of the battery to the **thermal store** of the bulb.
Example: a falling object transfers energy **mechanically** from its **gravitational potential store** to its **kinetic store**.

Kinetic Energy

**Kinetic energy** is the energy an object has due to its mass and speed: E_k = \frac{1}{2}mv².
Doubling mass doubles kinetic energy (directly proportional to mass).
Doubling speed quadruples kinetic energy (proportional to speed squared).
Energy is transferred to the kinetic store when an object speeds up, and away when it slows down.

Gravitational Potential Energy

**Gravitational potential energy** is the energy an object has due to its height in a gravitational field: \Delta E_p = mg\Delta h.
Lifting an object transfers energy to its gravitational potential store; lowering transfers energy away.
Work is done against the weight force to lift an object.

Conservation of Energy

**Principle of conservation of energy**: energy cannot be created or destroyed, only transferred between stores.
For a closed system, total energy remains constant: total energy in = total energy out.
Energy can be **dissipated** (spread out) to the surroundings, often as **wasted energy**.
Example: a bat hitting a ball transfers kinetic energy to the ball (useful) and to thermal stores (wasted).
**Energy flow diagrams** show stores and transfers; **Sankey diagrams** show proportions of useful and wasted energy.

Work Done

**Work done** occurs when a force moves an object over a distance: W = Fd.
Work done equals energy transferred: 1 N m = 1 J.
If no movement occurs, no work is done.
Example: a force of 500 N over 23 m does 11 500 J of work.

Power

**Power** is the rate of work done or energy transferred: P = \frac{W}{t} = \frac{\Delta E}{t}.
Unit of power is the **watt (W)**: 1 W = 1 J/s.
A more powerful machine does the same work in less time.
Example: a 2000 W iron used for 5 minutes transfers 600 000 J of energy.

Efficiency

**Efficiency** is the ratio of useful output to total input (energy or power): \text{efficiency} = \frac{\text{useful output}}{\text{total input}} \times 100\%.
High efficiency means most energy is used usefully; low efficiency means much is wasted.
Efficiency has no units; can be expressed as a decimal or percentage.
Example: a motor with 35% efficiency lifting a 7.2 kg load 5.0 m in 3 s has a power input of 336 W.

Particle arrangement in solids, liquids, and gases (states of matter).

Practice questions

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1.What is the unit of energy?
Easy
- AJoule
- BNewton
- CWatt
- DMetre
2.Which of the following is an energy store?
Easy
- AElectrical
- BKinetic
- CMechanical
- DHeating
3.The principle of conservation of energy states that energy cannot be created or destroyed. It can only be:
Easy
- Atransferred from one store to another
- Bincreased or decreased
- Cchanged into mass
- Dstored as heat
4.Work done is equal to:
Easy
- Aenergy transferred
- Bpower
- Cforce
- Ddistance
5.A car of mass 1000 kg is moving at 20 m/s. What is its kinetic energy?
Medium
- A200 000 J
- B10 000 J
- C400 000 J
- D20 000 J
6.Which of the following correctly describes the energy transfer when a ball is thrown upwards?
Medium
- AKinetic to gravitational potential
- BGravitational potential to kinetic
- CChemical to kinetic
- DNuclear to thermal
7.A force of 50 N moves an object 2 m in the direction of the force. How much work is done?
Medium
- A100 J
- B25 J
- C52 J
- D48 J
8.Which of the following is an energy transfer pathway?
Medium
- AThermal
- BMechanical
- CKinetic
- DChemical

Slides

Energy Stores & Transfers

Energy is a property stored or transferred, measured in joules (J).
A system is an object or group of objects; energy is transferred when a change happens.
Energy can be stored in kinetic, gravitational potential, elastic, magnetic, electrostatic, chemical, nuclear, and thermal stores.
Energy is transferred via mechanical, electrical, heating, or radiation pathways.

Which energy store does a moving object have?

Thermal
Kinetic
Gravitational potential
Chemical

Kinetic Energy (Extended)

Kinetic energy is the energy an object has due to its mass and speed.
Formula: Ek = ½ mv², where m is mass (kg) and v is speed (m/s).
Doubling mass doubles kinetic energy; doubling speed quadruples it.

The kinetic energy of an object is given by Ek = ____ mv².

Gravitational Potential Energy (Extended)

Gravitational potential energy is the energy an object has due to its height in a gravitational field.
Formula: ΔEp = mgΔh, where m is mass (kg), g is gravitational field strength (N/kg), and Δh is change in height (m).
Lifting an object increases its gravitational potential energy; lowering it decreases it.

What is the gravitational potential energy gained by a 10 kg mass lifted 2 m? (g = 10 N/kg)

20 J
200 J
100 J
50 J

Conservation of Energy

Energy cannot be created or destroyed, only transferred from one store to another.
For a closed system, total energy is constant.
Energy can be dissipated (spread out) to the surroundings, often as waste.

Which of these are true about conservation of energy?

Energy can be created.
Total energy in a closed system is constant.
Energy can be transferred between stores.
Wasted energy is destroyed.

Work Done

Work is done when a force moves an object in the direction of the force.
Formula: W = Fd, where F is force (N) and d is distance (m).
Work done equals energy transferred: 1 N m = 1 J.

How much work is done when a 50 N force moves an object 3 m?

53 J
150 J
15 J
500 J

Power

Power is the rate of work done or energy transferred.
Formula: P = W/t or P = ΔE/t, where P is power (W), t is time (s).
1 watt = 1 joule per second.

Power is defined as work done per ____ time.

Efficiency (Extended)

Efficiency is the ratio of useful output energy (or power) to total input energy (or power).
Formula: efficiency = (useful energy output / total energy input) × 100%.
High efficiency means little wasted energy.

A motor has a total input of 500 J and useful output of 200 J. What is its efficiency?

Energy Flow Diagrams

Energy flow diagrams show stores (boxes) and transfers (arrows).
Sankey diagrams show useful and wasted energy as proportional arrows.
Total energy in = useful energy out + wasted energy.

Which statements about Sankey diagrams are true?

Arrow width is proportional to energy amount.
Useful energy is shown by a straight arrow.
Wasted energy is shown by a straight arrow.
Total energy in equals total energy out.

Summary

Energy is stored in various stores and transferred via pathways.
Kinetic energy: Ek = ½ mv²; Gravitational potential energy: ΔEp = mgΔh.
Work done: W = Fd; Power: P = W/t; Efficiency = (useful output / total input) × 100%.
Energy is conserved; wasted energy is dissipated to surroundings.

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