Energy, Work, Power and Thermal Physics: notes

Forms of Energy and Conservation

• Energy is the ability to do work. It is measured in joules (J).
• Energy conversion means changing energy from one form to another, such as chemical energy in a battery becoming electrical energy in a bulb.
• Conservation of energy states that energy cannot be created or destroyed. It only changes form.
• Kinetic energy is the energy an object has because it is moving. Kinetic energy = ½ × mass × velocity².
• Potential energy is stored energy due to position or shape. Gravitational potential energy = mass × gravitational field strength × height.

Work, Power and Efficiency

• Work is done when a force moves an object through a distance. Work = force × distance.
• Power is the rate of doing work or transferring energy. Power = energy ÷ time, measured in watts (W).
• Efficiency is the useful energy output as a fraction or percentage of the total energy input.
• Worked calculation: A motor lifts a 10 kg load through 5 m in 2 s. Work done = weight × height = 10 × 10 × 5 = 500 J. Power = 500 ÷ 2 = 250 W.

Energy Resources

• Fossil fuels (coal, oil, gas) are easy to store and give high power, but they release carbon dioxide and are non-renewable.
• Hydroelectric power uses falling water, has no fuel cost, but needs dams that flood land.
• Solar power is clean and free, but it depends on sunlight and needs large storage.
• Wind power is clean, but it is unreliable and can be noisy.
• Biogas comes from decaying waste and is renewable, but it produces only small amounts of gas.
• Nuclear power gives large output with little carbon dioxide, but its waste is radioactive and dangerous.

Thermal Physics

• Kinetic theory states that all matter is made of particles that are always moving. Faster particles mean higher temperature.
• Temperature scales: Kelvin = Celsius + 273. Water freezes at 0 °C (273 K) and boils at 100 °C (373 K).
• Thermal expansion means substances get bigger when heated. Gases expand the most, then liquids, then solids.
• Heat capacity is the energy needed to raise the temperature of a whole object by 1 °C.
• Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C. Energy = mass × specific heat capacity × temperature change.
• Latent heat is the energy absorbed or released during a change of state, when the temperature stays the same.

Heat Transfer

• Conduction is heat moving through a solid by particles passing on energy. Metals are good conductors.
• Convection is heat carried by moving fluids (liquids and gases) because warm fluid rises and cool fluid sinks.
• Radiation is heat transferred by infrared waves. It needs no particles, so it works through a vacuum.
• Thermos flask: a vacuum stops conduction and convection, and a shiny silvered surface reduces radiation.
• Solar water heater: a dark, flat collector absorbs radiation, and convection moves warm water up to the storage tank.

Common Slips

• Forgetting to convert grams to kilograms or minutes to seconds before calculating.
• Mixing up work (in joules) and power (in watts).
• Using Celsius instead of Kelvin in gas law questions.
• Saying heat rises. It is the warm fluid that rises, carrying heat with it.
• Thinking a vacuum conducts heat. Radiation is the only method that crosses a vacuum.