Energy

Explanation

• A system is one object or a group of objects being considered. When a system changes, energy is transferred between its stores or between the system and its surroundings.
• Describe the change by naming the store that decreases, the store that increases and the transfer pathway: mechanically, electrically, by heating or by radiation.
• Energy stores include kinetic, thermal, gravitational potential, elastic potential, chemical, magnetic, electrostatic and nuclear.
• Total energy is conserved, so increases in stores equal the decrease elsewhere.
• Energy is not used up; it may become dissipated into less useful thermal stores.

Explanation

• Calculate energy in kinetic, elastic potential and gravitational potential stores using $E_k=\dfrac{1}{2}mv^2$, $E_e=\dfrac{1}{2}ke^2$ and $E_p=mgh$.
• Use kilograms, metres per second, newtons per metre, metres and the supplied value of $g$.
• The elastic equation applies while the spring obeys the linear force–extension relationship, and $e$ is extension rather than total length.
• If a question states that transfers are complete with no dissipation, equate the decrease in one store to the increase in another.
• Speed and extension are squared, so doubling either makes the corresponding energy four times larger.

Explanation

• Specific heat capacity is the energy required to raise the temperature of $1\,\text{kg}$ of a substance by $1\,{}^\circ\text{C}$.
• Use $\Delta E=mc\Delta\theta$, where energy is in joules, mass in kilograms, specific heat capacity in $\text{J/(kg }{}^\circ\text{C)}$ and temperature change in degrees Celsius.
• In the required practical, measure mass, supply energy with an electrical heater using $E=Pt$, and record the temperature rise.
• Insulate the block, ensure good thermal contact and repeat readings because energy transferred to the surroundings otherwise makes the calculated value inaccurate.

Explanation

• Power is the rate of energy transfer or the rate of doing work. Use $P=E/t$ or $P=W/t$, with energy or work in joules and time in seconds, to obtain power in watts.
• One watt means one joule transferred each second.
• Two devices can transfer the same amount of energy but have different powers: the device that completes the transfer in less time has greater power.
• Conversely, two devices operating for the same time transfer different energies if their powers differ.
• Power is a rate, not an energy store or a total amount of energy.

Explanation

• Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed. In a closed system, the total energy does not change.
• Dissipated energy spreads into thermal stores of the surroundings and becomes less useful, rather than disappearing.
• Reduce unwanted mechanical transfers with lubrication, which lowers friction.
• Reduce transfer by heating with thermal insulation, including thicker walls and materials that trap air.
• A material with a high thermal conductivity transfers energy through it rapidly; increasing wall thickness reduces the transfer rate for the same material and temperature difference.

Explanation

• Efficiency is the fraction of total input energy transferred usefully: $\text{efficiency}=\dfrac{\text{useful output energy}}{\text{total input energy}}$.
• The same ratio can use useful power output and total power input.
• A decimal efficiency lies between $0$ and $1$; multiply by $100$ only when a percentage is required.
• If efficiency is $0.72$, then $72\%$ of the input is useful and $28\%$ is dissipated.
• Improve efficiency by reducing unwanted transfers, such as using lubrication to reduce friction or thermal insulation to reduce heating of the surroundings.

Explanation

• Renewable resources are replenished as they are used: biofuel, wind, hydroelectricity, geothermal, tides, sunlight and water waves. Fossil fuels and nuclear fuel are non-renewable.
• Compare resources for transport, heating and electricity generation using reliability, response to demand, environmental effects and cost.
• Wind and solar output vary with conditions, while fossil-fuel stations can respond to demand but release greenhouse gases and pollutants.
• Nuclear generation has low operational carbon emissions but produces radioactive waste.
• Patterns of use reflect availability, technology, economics and political, social or ethical choices; science identifies impacts but does not make every decision.