Energy Revision Notes

Energy Stores and Their Changes

When a system changes, energy is transferred from one store to another. You need to be able to identify these energy stores. Common energy stores include gravitational potential energy (due to position in a gravitational field), kinetic energy (due to motion), thermal energy (due to temperature), elastic potential energy (due to stretching or squashing), chemical energy (stored in chemical bonds), and electrical energy (due to electric current). During changes, such as an object accelerating, falling, or being heated, these stores will either increase or decrease.

Tracing Energy Transfers

Energy is always conserved. This means that if one energy store decreases, another must increase by an equivalent amount, though not always within the same system. For instance, when a pendulum swings downwards, its gravitational potential energy store decreases while its kinetic energy store increases. When it reaches the bottom, kinetic energy is at its maximum. As it swings upwards, kinetic energy decreases and gravitational potential energy increases again. Some energy is also transferred to thermal energy stores due to air resistance and friction at the pivot.

Common Situations and Energy Pathways

It is important to describe energy changes in everyday scenarios. For example: A ball being thrown upwards: kinetic energy store decreases as gravitational potential energy store increases. A battery powering a torch: chemical energy store in the battery decreases, transferring to electrical energy, which then transfers to light and thermal energy stores in the bulb. A kettle boiling water: electrical energy transfers to the thermal energy store of the water. A car braking: kinetic energy store decreases, transferring to the thermal energy stores of the brakes and tyres due to friction. Always aim to describe the initial store, the transfer mechanism, and the final stores.

Sources of Electrical Energy in the Home

Domestic devices predominantly obtain electrical energy from two main sources: batteries and the mains electricity supply (a.c.). Batteries store chemical energy, which is converted into electrical energy when connected to a circuit. They provide a direct current (d.c.) and are typically used in portable devices like remote controls, toys, and mobile phones, where mobility is essential or power requirements are low. The mains electricity supply, on the other hand, delivers alternating current (a.c.) at a much higher voltage and provides a continuous and more powerful source of energy for larger appliances such as refrigerators, washing machines, and televisions. The choice of energy source depends on the device's power requirements, portability needs, and intended use.

Energy Transfers in Battery-Powered Devices

In battery-powered devices, chemical energy stored within the battery is converted into electrical energy, which then flows through the circuit. This electrical energy is subsequently transferred into other useful forms by the device's components. For example, in a torch, electrical energy is converted into light energy by the bulb and some thermal energy as a byproduct. In a portable music player, electrical energy is converted into sound energy by the speakers. Devices like cordless drills convert electrical energy into kinetic energy to power the motor. While performing these conversions, some energy is always 'wasted,' typically as heat, due to inefficiencies in the components. However, this 'wasted' energy is still transferred to the surroundings.

Energy Transfers in Mains-Powered Devices

Appliances connected to the mains electricity supply receive a.c. electrical energy directly from the power grid. These devices are generally more powerful than battery-operated ones. A hairdryer, for instance, uses mains electrical energy to produce kinetic energy (to spin the fan) and a large amount of thermal energy (to heat the air). A washing machine converts electrical energy into kinetic energy for the drum's rotation, along with some thermal energy if it heats water. A television transfers electrical energy into light energy for the screen and sound energy for the speakers. In all these cases, the electrical energy undergoes a transformation into the desired output energy, with some inevitably being converted into less useful forms, primarily heat, due to the laws of thermodynamics and inherent inefficiencies.

Efficiency of Energy Transfer

The efficiency of a domestic device is a measure of how much of the input electrical energy is converted into useful output energy, rather than being 'wasted' as unwanted forms, most commonly heat or sound. For example, an energy-efficient light bulb converts a higher proportion of electrical energy into light energy compared to an older incandescent bulb, which produces a significant amount of heat. Devices such as electric heaters are designed to produce thermal energy as their primary output, so they are generally considered highly efficient for their intended purpose, even though they still lose some energy to their surroundings. Understanding efficiency helps in choosing appliances that consume less energy for the same output, leading to cost savings and reduced environmental impact.

What is Energy Dissipation?

Energy dissipation, often referred to as energy 'waste', is the process by which energy becomes spread out and less concentrated during an energy transfer. While the total amount of energy remains constant (due to the principle of conservation of energy), some of it changes into forms that are no longer useful for the intended purpose of the system. This often involves energy being transferred to the surroundings, rather than to the desired parts of the system. For instance, when a car brakes, kinetic energy is converted into thermal energy due to friction between the brake pads and the wheel, heating up the brakes and the surrounding air. This thermal energy is dissipated and cannot be used to power the car.

Common Forms of Dissipated Energy

The most common form of dissipated energy is thermal energy (heat). Friction, air resistance, and electrical resistance all lead to the production of heat, which then warms up the surroundings. Another common form is sound energy, which can be generated by moving parts, impacts, or rapid air displacements. Sound energy spreads out quickly and is rarely the intended useful output of a system (unless the purpose is to make sound). Light can also be dissipated if it is not directed efficiently or if it is generated as an unwanted byproduct (e.g., a non-light-producing appliance getting hot enough to glow faintly). Once energy is dissipated as heat or sound into the environment, it becomes much harder to harness and convert back into a useful form, therefore it is stored in less useful ways.

Examples of Energy Dissipation in Everyday Life

Many everyday examples illustrate energy dissipation. When you rub your hands together, the kinetic energy of your hands is converted into thermal energy due to friction, making your hands feel warm. This heat energy is dissipated into the air. When an electric fan operates, electrical energy is converted into kinetic energy to move the blades and circulate air. However, some electrical energy is converted into heat due to resistance in the motor wires and friction in the bearings. Additionally, the fan makes noise, which is sound energy dissipated into the room. Even walking involves energy dissipation as some of the chemical energy from your food is converted into heat due to muscle contractions and friction with the ground, making your body warm.

Why Dissipated Energy is 'Less Useful'

Dissipated energy is considered 'less useful' because it typically spreads out into the environment, becoming diffuse and difficult to collect or concentrate for further work. For example, the heat generated by a light bulb quickly disperses into the air of a room. This spread-out thermal energy has a lower 'temperature difference' relative to its surroundings, meaning it cannot easily be used to drive a heat engine or perform other tasks. The more spread out the energy, the lower its 'quality' or utility for performing work. Effective energy systems aim to minimise dissipation and maximise the useful energy transfer, thereby increasing efficiency. Minimising friction and using good insulation are common ways to reduce energy dissipation.

Kinetic Energy

Kinetic energy (KE) is the energy an object has due to its motion. It depends on the object's mass (m) and its speed (v). The formula to calculate kinetic energy is KE = 0.5 × m × v², where KE is measured in joules (J), mass in kilograms (kg), and speed in metres per second (m/s). An object at rest has zero kinetic energy. If the speed doubles, the kinetic energy quadruples because of the v² term in the formula. If the mass doubles, the kinetic energy doubles.

Gravitational Potential Energy

Gravitational potential energy (GPE) is the energy an object stores due to its position, specifically its height above a certain reference point, within a gravitational field. The formula for GPE is GPE = m × g × h, where GPE is in joules (J), mass (m) in kilograms (kg), gravitational field strength (g) in newtons per kilogram (N/kg), and height (h) in metres (m). On Earth, the value of g is approximately 9.8 N/kg (often approximated as 10 N/kg for simpler calculations). The higher an object is lifted, the more GPE it gains.

Elastic Potential Energy

Elastic potential energy (EPE) is the energy stored in an elastic material, such as a spring, when it is deformed (stretched or compressed). This stored energy is available to do work when the material returns to its original shape. The formula for EPE is EPE = 0.5 × k × x², where EPE is in joules (J), k is the spring constant in newtons per metre (N/m), and x is the extension or compression (change in length) in metres (m). The spring constant 'k' indicates the stiffness of the spring; a higher 'k' means a stiffer spring that stores more energy for the same extension. This is closely related to Hooke's Law.

Calculations and Units

When performing calculations for kinetic, gravitational potential, or elastic potential energy, it's essential to use the correct units for each variable to ensure the final energy value is in joules (J). Masses should always be in kilograms (kg), speeds in metres per second (m/s), heights and extensions in metres (m), and spring constants in newtons per metre (N/m). Always check the units provided in the question and convert them if necessary before plugging the values into the formulas. For example, if a mass is given in grams, convert it to kilograms by dividing by 1000.