GCSE Physics: Energy Stores, Transfers, and Resources

Energy Stores and Systems

• Energy Transfer Basics     * Energy is never used up; it is only transferred between different energy stores and different objects.     * When energy is transferred to an object, the energy is stored in one of that object's energy stores.

• The Eight Energy Stores     1. Thermal energy stores: Also known as internal energy stores.     2. Kinetic energy stores: Energy associated with moving objects.     3. Gravitational potential energy stores: Energy associated with an object's position in a gravitational field.     4. Elastic potential energy stores: Energy associated with stretching or squashing an object.     5. Chemical energy stores: Energy stored in chemical bonds, such as in food or batteries.     6. Magnetic energy stores: Energy stored in magnetic fields.     7. Electrostatic energy stores: Energy stored in electric fields between charges.     8. Nuclear energy stores: Energy stored in the nucleus of an atom.

• Ways Energy is Transferred     * Mechanically: By a force doing work.     * Electrically: Work done by moving charges.     * By Heating: Transfer of thermal energy from a hotter to a cooler object.     * By Radiation: Transfer via waves like light or sound.

• Systems and Energy Change     * Definition of a System: A single object (e.g., the air in a piston) or a group of objects (e.g., two colliding vehicles).     * When a system changes, energy is transferred either into or away from the system, between objects in the system, or between different stores.     * Closed Systems: Systems where neither matter nor energy can enter or leave. The net change in the total energy of a closed system is always zero.

• Examples of Energy Transfer     * Boiling Water in a Kettle:         * The water is the system: Energy is transferred from the kettle's heating element to the water's thermal energy store by heating.         * Two-object system (element and water): Energy is transferred electrically to the element's thermal store, then by heating to the water's thermal store.     * Throwing a Ball: The initial force exerted by a person does work. Energy transfers from the chemical energy store of the person's arm to the kinetic energy store of the ball and arm.     * Dropped Ball: Accelerated by gravity. Gravitational force does work, transferring energy from the ball's gravitational potential energy store to its kinetic energy store.     * Car Braking: Friction between brakes and wheels does work. Energy transfers from the wheels' kinetic energy stores to the thermal energy store of the surroundings.     * Car Collision: The normal contact force between the car and a stationary object does work. Kinetic energy is transferred to the elastic potential and thermal energy stores of the car and object. Sound waves also carry energy away.

Kinetic and Potential Energy Stores

• Kinetic Energy Store     * Anything moving has energy in this store. Energy increases as speed increases and decreases as speed decreases.     * The amount of energy depends on the object's mass and speed.     * Equation: $E_k = \frac{1{}}{2{}}mv^2$     * Where $E_k$ is Kinetic energy in Joules ($J$), $m$ is mass in kilograms ($kg$), and $v$ is speed in meters per second ($m/s$).     * Example Calculation: A car of mass $2500\,kg$ traveling at $20\,m/s$.         * $E_k = \frac{1}{2} \times 2500 \times 20^2 = 500000\,J$

• Gravitational Potential Energy (GPE) Store     * Lifting an object in a gravitational field requires work, transferring energy to the GPE store.     * Depends on mass ($m$), height ($h$), and gravitational field strength ($g$).     * Equation: $E_p = mgh$     * Where $E_p$ is GPE in Joules ($J$), $m$ is mass in $kg$, $g$ is gravitational field strength in $N/kg$, and $h$ is height in meters ($m$).

• Falling Objects     * When an object falls, energy transfers from its GPE store to its kinetic energy store.     * Ideal Case (no air resistance): $\text{Energy lost from GPE store} = \text{Energy gained in kinetic energy store}$.     * Real World: Air resistance causes some energy to be transferred to thermal energy stores of the object and surroundings.

• Elastic Potential Energy Store     * Stretching or squashing transfers energy to this store.     * Valid so long as the limit of proportionality is not exceeded.     * Equation: $E_e = \frac{1{}}{2{}}ke^2$     * Where $E_e$ is elastic potential energy in Joules ($J$), $k$ is the spring constant in $N/m$, and $e$ is extension in meters ($m$).

Specific Heat Capacity

• Materials and Heat Retention     * Different materials require different amounts of energy to increase their temperature.     * Example: Water requires $4200\,J$ to warm $1\,kg$ by $1{^\circ}C$, while mercury requires only $139\,J$.     * Materials that need more energy to warm up also release more energy when cooling down; they "store" more energy.

• Definition of Specific Heat Capacity     * Specific heat capacity is the amount of energy needed to raise the temperature of $1\,kg$ of a substance by $1{^\circ}C$.     * Equation: $\Delta E = mc\Delta\theta$     * Where $\Delta E$ is the change in thermal energy in Joules ($J$), $m$ is mass in $kg$, $c$ is specific heat capacity in $J/kg{^\circ}C$, and $\Delta\theta$ is temperature change in ${^\circ}C$.

• Practical Investigation: Specific Heat Capacity of a Solid     1. Use a block of material (e.g., copper) with two holes for a heater and thermometer.     2. Measure the mass of the block and wrap it in insulation (e.g., newspaper) to reduce energy loss to surroundings.     3. Measure initial temperature and set the power supply to $10\,V$.     4. Turn on the power; current does work on the heater, transferring energy electrically to the heater's thermal store, then to the block's thermal store via heating.     5. Take readings of temperature and current ($I$) every minute for $10$ minutes. Current should remain constant.     6. Calculate power using $P = VI$, and energy using $E = Pt$ ($t$ in seconds).     7. Plot a graph of Energy Transferred ($J$) against Temperature (${^\circ}C$).     8. The gradient of the straight part is $\Delta heta/\Delta E$. Specific heat capacity $c = \frac{1}{\text{gradient} \times \text{mass}}$.     9. Liquid investigation: Place heater and thermometer in an insulated beaker with a known mass of liquid.

Conservation of Energy and Power

• Conservation of Energy Principle     * Energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.     * Dissipated Energy: Often called "wasted energy" because it is stored in a non-useful way (usually thermal energy stores).     * Mobile Phone System: Chemical energy from the battery is usefully transferred, but some is dissipated as thermal energy, making the phone feel warm.     * Closed System Example: A cold spoon in an insulated flask of hot soup. Energy moves from the soup's thermal store to the spoon's. Total net change in energy is zero.

• Power     * Power is the rate of energy transfer or the rate of doing work.     * Measured in Watts ($W$). One Watt = one Joule of energy transferred per second.     * Equations:         1. $P = \frac{E}{t}$ (Power = Energy transferred / time)         2. $P = \frac{W}{t}$ (Power = Work done / time)     * Powerful Machines: A machine that transfers a lot of energy in a short time. For two cars traveling the same distance, the one with the more powerful engine reaches the finish line faster.     * Example Comparison: Motor A lifts a performer in $50\,s$; Motor B takes $300\,s$ for the same work ($8000\,J$). Motor A is more powerful: $P = 8000 / 50 = 160\,W$.

Conduction and Convection

• Conduction     * Mainly occurs in solids.     * Definition: The process where vibrating particles transfer energy to neighboring particles.     * Mechanism: Heating increases the kinetic energy of particles, causing them to vibrate and collide. Collisions transfer energy between kinetic stores through the object to the surroundings.     * Thermal Conductivity: A measure of how quickly energy is transferred through a material. High conductivity means faster transfer.

• Convection     * Occurs only in liquids and gases.     * Definition: Where energetic particles move away from hotter to cooler regions.     * Mechanism: Particles are free to move. Heating a region causes particles to move faster and the space between them to increase, decreasing density. The warmer, less dense region rises above cooler, denser regions.     * Convection Current: Created by a constant heat source.     * Radiator Example:         1. Air near the radiator is heated by conduction.         2. Warm air becomes less dense and rises.         3. Cooler air moves in to replace rising air and is heated.         4. Heated air cools as it transfers energy to surroundings (walls/contents), becomes denser, and sinks.         5. The cycle repeats.

Reducing Unwanted Energy Transfers

• Lubrication     * Reduces frictional forces that dissipate energy.     * Lubricants like oil flow easily between surfaces to coat them.     * Streamlining also reduces air resistance.

• Thermal Insulation in Houses     * Thick Walls: Low thermal conductivity slows the rate of energy transfer.     * Cavity Walls: Inner and outer walls with an air gap. Air reduces conduction. Cavity wall insulation uses foam to reduce convection in the gap.     * Loft Insulation: Reduces convection currents in the loft.     * Double-glazed Windows: Air gap between two glass sheets prevents conduction.     * Draught Excluders: Reduce convection around doors and windows.

• Investigating Insulation Effectiveness     1. Fill a sealable container with a known mass of hot water and measure initial temperature.     2. Seal and wait $5$ minutes; measure final temperature.     3. Repeat by wrapping the container in different materials (e.g., foil, newspaper) using the same mass and initial temperature.     4. The lower the temperature difference, the better the insulator.     5. Investigating thickness: Thicker layers result in smaller temperature changes.

Efficiency

• Efficiency Principles     * Devices are useful because they transfer energy from one store to another.     * Input energy that is not useful is "wasted" (usually dissipated as thermal energy).     * Efficiency Equations:         1. $\text{Efficiency} = \frac{\text{Useful output energy transfer}}{\text{Total input energy transfer}}$         2. $\text{Efficiency} = \frac{\text{Useful power output}}{\text{Total power input}}$     * Can be expressed as a decimal ($0.75$) or percentage ($75\%$).

• Device Performance     * No device is $100\%$ efficient, except for electric heaters. In heaters, all energy in the electrostatic store is transferred to the "useful" thermal energy store.     * Ultimately, all energy is transferred to thermal energy stores (e.g., an electric drill).     * Efficiency can be improved via lubrication, insulation, or streamlining.

Energy Resources and their Uses

• Non-Renewable Resources     * Fossil fuels (Coal, Oil, Natural Gas) and Nuclear fuel (Uranium, Plutonium).     * Characteristics: They will run out, damage the environment, but provide most current energy.

• Renewable Resources     * Sun, Wind, Water waves, Hydro-electricity, Bio-fuel, Tides, Geothermal.     * Characteristics: Will never run out, less environmental damage than non-renewables, but can be unreliable and provide less energy.

• Uses of Energy Resources     * Transport:         * Non-renewable: Petrol/diesel (oil) for cars; coal for steam trains.         * Renewable: Bio-fuels; electricity (if generated from renewables).     * Heating:         * Non-renewable: Natural gas for UK radiators; coal fireplaces; electric storage heaters.         * Renewable: Geothermal heat pumps; solar water heaters; burning bio-fuel.

Renewable Energy Details

• Wind Power     * Wind turbines use generators to produce electricity.     * Pros: No pollution, no fuel costs, minimal running costs, no permanent landscape damage.     * Cons: Spoils view, noisy, unreliable (stops with no wind or too much wind), high initial costs. You need $1500$ turbines to replace one coal station.

• Solar Cells     * Generate current directly from sunlight. Best for small scale (calculators) or remote areas.     * Pros: No pollution, reliable in sunny countries, free energy after high initial costs.     * Cons: Doesn't work at night, cannot increase output for extra demand, high manufacturing energy.

• Geothermal Power     * Uses energy from hot rocks or radioactive decay deep underground.     * Pros: Free, reliable, little environmental damage.     * Cons: Limited suitable locations, high building costs compared to energy output.

• Hydro-electric Power     * Uses falling water (usually requires flooding a valley with a dam).     * Pros: Immediate response to high demand, reliable (except in drought).     * Cons: Flooding destroys habitats and releases methane/$CO_2$ from rotting vegetation; unsightly reservoirs.

• Wave Power     * Small turbines around the coast.     * Pros: No pollution.     * Cons: Disturbs seabed/habitats, hazard to boats, unreliable.

• Tidal Barrages     * Big dams across river estuaries using the gravity of the Sun and Moon.     * Pros: No pollution, reliable (tides happen twice a day).     * Cons: Prevents boat access, alters habitats (wildlife like wading birds), variable energy (neap vs spring tides).

Bio-fuels and Environmental Impacts

• Bio-fuels     * Made from plant products or animal dung. Can be solid, liquid, or gas.     * Pros: Supposedly carbon neutral, reliable (short growth time).     * Cons: High refining costs, takes up land/water needed for food, deforestation causes habitat loss and emissions.

• Impact of Non-Renewables     * Global Warming: Burning fossil fuels releases $CO_2$.     * Acid Rain: Sulfur dioxide from coal and oil. Can be reduced by extracting sulfur/cleaning emissions.     * Landscape: Coal mining (especially open-cast) and power plants spoil the view.     * Pollution: Oil spills affect marine mammals and birds.     * Nuclear Hazards: Radioactive waste is dangerous to dispose of; risk of major catastrophe (e.g., Fukushima).

Trends in Energy Use

• Historic and Current Trends     * 20th Century: UK electricity use increased with population and appliance usage.     * 21st Century: Use is slowly decreasing due to efficiency and conservation.     * Most electricity currently comes from fossil fuels and nuclear.     * UK Goal: renewable resources to provide $15\%$ of yearly energy by 2020.

• Shift Toward Renewables     * Triggered by environmental awareness and knowledge that non-renewables will run out.     * Governmental targets and public pressure force energy providers and car companies to adapt.

• Limitations of Change     * Economic: Building renewable plants is expensive; costs are passed to consumers via taxes or bills.     * Political/Ethical: Arguments over where to place wind farms.     * Reliability: Research is still needed to improve the reliability of renewable sources.     * Personal Cost: Electric/hybrid cars and solar panels are still expensive for many individuals.

Questions & Discussion

• Q1: A $2.0\,kg$ object is dropped from a height of $10\,m$. Calculate the speed of the object after it has fallen $5.0\,m$, assuming there is no air resistance. ($g = 9.8\,N/kg$)     * Answer Strategy: Loss in GPE = Gain in KE ($mgh = \frac{1}{2}mv^2$).
• Q1: Find the final temperature of $5\,kg$ of water, at an initial temperature of $5{^\circ}C$, after $50\,kJ$ of energy has been transferred to it. Specific heat capacity of water is $4200\,J/kg{^\circ}C$.
• Q1: A motor transfers $4.8\,kJ$ of energy in $2\,minutes$. Calculate its power output.     * Answer Strategy: $P = E / t$. Convert $4.8\,kJ$ to $4800\,J$ and $2\,mins$ to $120\,s$. $4800 / 120 = 40\,W$.
• Q1: Describe how energy is transferred through a solid by heating. What is the name of this effect?     * Answer: Conduction. Vibrating particles transfer kinetic energy to neighbors through collisions.
• Q1: Explain how cavity wall insulation reduces the amount of energy transferred out of a house.     * Answer: The air gap reduces conduction; the foam filling the gap reduces convection currents.
• Q1: A motor in a remote-controlled car transfers $300\,J$ of energy into the car's energy stores. $225\,J$ are transferred to the car's kinetic energy stores. Calculate the efficiency of the motor.     * Answer: $225 / 300 = 0.75$ or $75\%$.
• Q2: A machine has a useful power output of $900\,W$ and a total power input of $1200\,W$. In a given time, $72\,kJ$ of energy is transferred to the machine. Calculate the amount of energy usefully transferred by the machine in this time.     * Answer Strategy: Efficiency = $900 / 1200 = 0.75$. Useful energy = $0.75 \times 72\,kJ = 54\,kJ$.
• Q1: Write down whether each of the following are renewable or non-renewable energy resources: a) Tidal power, b) Natural gas, c) Nuclear power, d) Bio-fuel.     * Answer: a) Renewable, b) Non-renewable, c) Non-renewable, d) Renewable.
• Q1: Explain why geothermal power is more reliable than wind power.     * Answer: Geothermal energy is available constantly from earth's internal heat, whereas wind is dependent on weather conditions.
• Q1: Give one negative environmental impact of wave power.     * Answer: Disturbing the seabed or marine habitats.
• Q1: Give two benefits of power plants that use fossil fuels.     * Answer: Reliability (can respond to demand) and cost-effectiveness.
• Q2: Describe the environmental impact of using oil as an energy resource for generating electricity.     * Answer: Burning releases $CO_2$ (global warming) and potential for damaging oil spills.
• Q1: Give two reasons we currently do not use more renewable energy resources in the UK.     * Answer: High building costs and reliability issues.