Welcome to the World of Energy!
Have you ever wondered why you need to eat breakfast to run around, or why a toy car stops moving after a few seconds? The answer to almost every "why" in Physics is Energy. In this chapter, we are going to explore what energy is, how it is stored, how it moves from one thing to another, and how we measure and pay for it at home.
1. What is Energy?
Energy is a bit like money in a bank account. You can’t necessarily see it, but you know it’s there because it allows you to "buy" actions. In science, energy is a calculable property of a system that is conserved across changes.
Important Rule: Energy is measured in Joules (J). For larger amounts, we use Kilojoules (kJ).
\(1 \text{ kJ} = 1000 \text{ J}\)
Energy in Food
Food nutrition labels show the amount of chemical energy stored in food, usually given in kilojoules (kJ). A healthy daily diet requires a balanced intake of energy to meet our daily needs for growth, repair, and movement.
Energy Stores
Energy can be stored in different ways within a system:
- Chemical: Found in food, batteries, and fuels (like wood, coal, or petrol).
- Kinetic: Found in moving objects.
- Gravitational Potential: Found in objects raised above the ground.
- Elastic: Found in objects that are stretched or squashed (like a spring).
- Thermal (Internal): Stored in materials due to the motion and spacing of their particles.
- Magnetic: In magnetic fields between attracting or repelling poles.
- Electrostatic: In electric fields between interacting charges.
- Nuclear: Stored in the nucleus of atoms.
2. Energy Transfers and Changes in Systems
When changes happen in a system, energy shifts between stores via physical processes:
- Mechanically: By forces acting over a distance (like lifting a weight or stretching a spring).
- Electrically: When an electric current completes a circuit.
- By Heating: When energy transfers from a hotter region to a cooler region by conduction or radiation.
- By Radiation: Transfer via waves, such as light or sound.
Heating, Conduction, and Thermal Equilibrium
Energy naturally transfers from a hotter object to a cooler object, reducing the temperature difference until both reach the same temperature. This state is called thermal equilibrium.
- Conduction: Heat transfer through materials (especially good conductors like metals).
- Insulators: Materials (like foam, wood, or wool) that conduct heat very poorly, slowing down thermal energy loss.
- Radiation: Infrared radiation transfers energy through transparent media or empty space without needing particles.
Simple Machines
Simple machines (such as levers, pulleys, and ramps) can provide a bigger force, but only at the expense of a smaller movement. Because work done = force × displacement, the product of force and displacement remains unchanged (ignoring friction).
3. The Law of Conservation of Energy
This is a fundamental rule in physics:
Energy cannot be created or destroyed. It can only be transferred from one store to another.
The total energy before any change in a closed system is always equal to the total energy after the change. For example, when dropping an object, gravitational potential energy transfers to kinetic energy, and on impact transfers to thermal energy and sound in the surroundings.
4. Power and Domestic Energy Calculations
Power is the rate at which energy is transferred or work is done. It tells us how many joules of energy are transferred per second.
The Formula:
\(P = \frac{E}{t}\)
Where:
P = Power, measured in Watts (W) or kilowatts (kW) (\(1 \text{ kW} = 1000 \text{ W}\))
E = Energy transferred, measured in Joules (J)
t = Time, measured in Seconds (s)
Calculating Domestic Fuel and Electricity Bills
Electricity suppliers measure large amounts of domestic energy use in kilowatt-hours (kW h) rather than joules.
A kilowatt-hour is the amount of energy transferred by a \(1 \text{ kW}\) appliance running for \(1 \text{ hour}\):
\(\text{Energy (kW h)} = \text{Power (kW)} \times \text{Time (hours)}\)
To calculate the cost of electricity on domestic fuel bills:
\(\text{Cost} = \text{Energy used (kW h)} \times \text{Cost per kW h}\)
Example: A \(2 \text{ kW}\) electric heater is left on for \(3 \text{ hours}\). If electricity costs \(30\text{p}\) per \(\text{kW h}\):
Energy used = \(2 \text{ kW} \times 3 \text{ h} = 6 \text{ kW h}\)
Cost = \(6 \text{ kW h} \times 30\text{p} = 180\text{p}\) (or £1.80).
5. Energy Resources
We rely on different energy resources to generate electricity and supply fuel for heating and transport:
Non-Renewable Resources
Resources that are being used up faster than they can be replaced. They will eventually run out.
- Fossil Fuels: Coal, oil, and natural gas. Burning them releases carbon dioxide, contributing to climate change.
- Nuclear Fuel: Materials such as uranium that release energy from nuclear reactions without emitting greenhouse gases, but produce radioactive waste.
Renewable Resources
Resources that are naturally replenished at a rate equal to or faster than they are consumed.
- Solar: Generating electricity directly via photovoltaic cells or collecting heat.
- Wind: Wind turbines driven by air currents.
- Hydroelectric: Turbines driven by falling water.
- Geothermal: Utilizing heat from within the Earth.
- Biomass: Fuel produced from recently living plant or animal material.
Final Quick Tips for Success
- Remember the units: energy is in J or kJ (or kW h for household bills); power is in W or kW.
- When calculating electricity cost, ensure appliance power is in kW and time is in hours.
- Thermal energy always transfers from hotter to cooler until thermal equilibrium is reached.
- Simple machines can multiply force, but the total work (force × displacement) does not increase.