Understanding how energy is stored, transferred, wasted, and conserved.
Energy is the capacity to do work. It is measured in Joules (J) and can neither be created nor destroyed.
Never say energy is "lost". It is dissipated (wasted) to the surroundings, usually as thermal energy.
Q1: A battery powers a toy car. Identify the initial store and the transfer mechanism.
Initial Store: Chemical. Transfer: Electrical Work.
Q2: When a candle burns, what are the main energy transfers happening?
Energy is transferred from the Chemical store via Heating and Radiation (light) to the surroundings.
$$ E_k = \frac{1}{2} m v^2 $$
Note: Speed ($v$) is squared, so doubling speed quadruples the energy. This is why high-speed collisions are far more dangerous than heavier vehicles at lower speeds.
$$ E_p = m g h $$
Remember: $g$ is gravity (9.8 or 10 N/kg). Height ($h$) is relative to a reference point. The reference point must be clearly stated in exam questions.
Q1: A 2kg bird flies at 10m/s. Calculate its Kinetic Energy.
$$ E_k = 0.5 \times 2 \times 10^2 = 100 \, J $$
Q2: An object of mass 5kg is lifted 2m high. Calculate the GPE gained (use $g = 9.8 \, N/kg$).
$$ E_p = 5 \times 9.8 \times 2 = 98 \, J $$
In a closed system, the total energy remains constant. It can be transferred but never created/destroyed. Example: A falling object ignoring air resistance.
Sankey Diagram: Width of the arrow represents the amount of energy.
Q1: If a motor takes in 100J and wastes 20J as heat, how much useful work is done?
80 Joules (Total - Wasted = Useful).
Q2: In a "closed system", what happens to the total amount of energy after a transfer?
It stays the same. Energy cannot be created or destroyed, only shifted between stores.
$$ W = F \times d $$
Energy is transferred when a force moves an object through a distance.
$$ P = \frac{E}{t} $$
Power is the rate of energy transfer. Measured in Watts (W).
Q1: Two athletes lift the same weight. Athlete A does it in 2s, Athlete B in 4s. Who is more powerful?
Athlete A. They transferred the same energy in less time.
Q2: Calculate the work done when a force of 50N moves a box 4 meters.
$$ W = 50 \times 4 = 200 \, J $$
$$ \text{Efficiency} = \frac{\text{Useful Output}}{\text{Total Input}} $$
Efficiency can be improved by reducing friction (lubrication) or heat loss (insulation). Multiply by 100 to convert to a percentage.
Q1: A bulb has a total input of 50W and a useful light output of 10W. Calculate efficiency.
$$ 10 / 50 = 0.2 \, \text{or} \, 20\% $$
Q2: Name one way to increase the efficiency of a machine with moving parts.
Lubrication (using oil/grease) to reduce friction and thermal dissipation.
| Resource | Renewable? | Pros/Cons |
|---|---|---|
| Fossil Fuels | No | Reliable, but CO2 emissions. |
| Wind/Solar | Yes | No pollution, but unreliable (weather). |
| Nuclear | No | No CO2, massive energy, but radioactive waste. |
Q1: State one major disadvantage of using Nuclear Power.
It produces radioactive waste that is difficult and dangerous to store for long periods.
Q2: Why is Wind Power considered "unreliable"?
Because it depends on the weather; if there is no wind, no electricity is generated.