Energy Work And Power

Energy Stores & Transfers

• Energy is a property stored or transferred, measured in **joules (J)**.
• A **system** is an object or group of objects; a change in a system involves energy transfer.
• **Energy stores** include: kinetic, gravitational potential, elastic, magnetic, electrostatic, chemical, nuclear, thermal.
• **Energy transfer pathways**: mechanical (force), electrical (charge flow), heating (particle collision), radiation (electromagnetic waves).
• Example: a battery powering a torch transfers energy electrically from the **chemical store** of the battery to the **thermal store** of the bulb.

Kinetic Energy

• **Kinetic energy** is the energy an object has due to its mass and speed: $E_k = \frac{1}{2}mv^2$.
• Kinetic energy is directly proportional to mass ($E_k \propto m$) and to the square of speed ($E_k \propto v^2$).
• Doubling speed quadruples kinetic energy; doubling mass doubles kinetic energy.
• Always square the speed when calculating $E_k$.

Gravitational Potential Energy

• **Gravitational potential energy** is the energy an object has due to its height in a gravitational field: $\Delta E_p = mg\Delta h$.
• Work is done against weight to lift an object, transferring energy to its gravitational potential store.
• Gravitational field strength on Earth is $g = 9.8\,\text{N/kg}$.
• Round final answers to the lowest number of significant figures in the input values.

Conservation of Energy

• **Energy cannot be created or destroyed**, only transferred from one store to another.
• In a closed system, total energy is constant: total energy $in =$ total energy out.
• **Dissipated energy** is spread out to the surroundings (often as thermal energy) and is usually wasted.
• Energy flow diagrams show stores (labels) and transfers (arrows); the total energy is conserved.

Work Done

• **Work is done** when a force moves an object over a distance in the direction of the force: $W = Fd = \Delta E$.
• Work done and energy transferred are equivalent: $1 N m = 1 J$.
• No work is done if the object does not move (e.g., pushing against a wall).
• Example: a bird flying does work against air resistance (drag).

Power

• **Power** is the rate of work done or energy transferred: $P = \frac{W}{t} = \frac{\Delta E}{t}$.
• Power is measured in **watts (W)**; $1 W = 1 \frac{J}{s}$.
• Common power ratings: torch 1 W, light bulb 100 W, large power station 10 GW.
• A more powerful machine does the same work in less time.

Efficiency

• **Efficiency** is the ratio of useful energy (or power) output to total energy (or power) input.
• Efficiency $= (useful$ energy output / total energy $input) \times 100\%$.
• Efficiency $= (useful$ power output / total power $input) \times 100\%$.
• Efficiency has no units; it can be a decimal (0–1) or percentage (0–100%).
• Example: a typical thermal power station is about 30% efficient; 70% of energy is wasted.

Energy Resources

• **Solar cells** convert sunlight directly into electricity (photovoltaic effect); **solar panels** heat water using infrared radiation.
• **Wind turbines** transfer kinetic energy of wind to electricity; efficiency ~50%.
• **Fossil fuels** (coal, oil, gas) are non-renewable; burning releases CO₂ and SO₂.
• **Biofuels** are renewable and carbon-neutral in principle, but have lower energy density.
• **Nuclear fission** splits large nuclei to release energy; used in power stations.
• **Hydroelectric**, **wave**, and **tidal** power use water movement to turn turbines.
• **Geothermal** energy uses heat from Earth's core; renewable but location-dependent.

Nuclear Fission & Fusion

• **Nuclear fission**: splitting a large nucleus into two smaller nuclei, releasing energy.
• **Nuclear fusion**: joining two small nuclei to form a larger nucleus, releasing huge energy (occurs in stars).
• Fission is used in nuclear power stations; fusion requires extremely high temperatures and pressures.

Energy pyramid showing energy transfer and loss at each trophic level.