Waves in matter Revision Notes

Battery Energy Transfer

Batteries store chemical potential energy. Through chemical reactions inside the battery, this stored energy is converted into electrical energy. This electrical energy is then supplied to the device, usually as direct current (DC). Common devices using batteries include remote controls, portable radios, and mobile phones. When the chemical reactants are used up, the battery's voltage drops, and it eventually runs out. Batteries provide portable power but have a limited energy capacity.

Mains Electricity Energy Transfer

Mains electricity is supplied as alternating current (AC) at a high voltage, typically 230 V in the UK. This electrical energy is generated at power stations and continuously delivered to homes via the national grid. Devices plugged into mains sockets receive a constant supply of electrical energy. Examples include televisions, toasters, and washing machines. Many mains-powered devices contain components like transformers to modify the voltage and rectifiers to change AC to DC if needed for internal electronics.

Energy Transformations in Devices

Both battery-powered and mains-powered devices transform electrical energy into other useful forms. For instance, a torch converts electrical energy into light and some thermal energy. A fan converts electrical energy into kinetic energy (of the fan blades) and sound energy. A heating element in a kettle or toaster converts electrical energy mainly into thermal energy. A television converts electrical energy into light and sound energy. Understanding these conversions helps explain how devices perform their functions.

Key Differences: Batteries vs. Mains

The primary difference lies in the energy source characteristics. Batteries offer portability and deliver DC, but they have a finite charge and must be replaced or recharged. Mains electricity provides a continuous, high-voltage AC supply, making it suitable for high-power or long-duration use appliances. It eliminates the need for battery changes but restricts the device's mobility. Safety features are also crucial for mains connections due to the higher voltage.

Energy Stores

Energy exists in various forms, often referred to as 'stores'. The main energy stores to consider are: kinetic (energy of movement), gravitational potential (energy due to height), elastic potential (energy stored in stretched or compressed objects), chemical (energy in chemical bonds, like in food or fuel), thermal (internal energy of an object due to the movement of its particles), magnetic (energy in magnetic fields), and electrostatic (energy due to electric charges). Understanding these stores is crucial for describing energy changes.

Energy Transfers

Energy cannot be created or destroyed, but it can be transferred between stores or between objects. The primary mechanisms for energy transfer are: by heating (conduction, convection, radiation), by work done by forces (e.g., friction, pushing), and by electrical currents. For example, when you switch on a lamp, electrical energy is transferred to light and thermal energy stores.

Describing Energy Changes in a System

When a system changes, its energy is converted from one store to another, or transferred from one system to another. To describe these changes fully, you need to identify: 1. The initial energy store(s). 2. The final energy store(s). 3. The process(es) by which the energy is transferred (e.g., work done by forces, heating). For example, a ball thrown upwards: chemical energy (from person's muscles) is converted to kinetic energy, then to gravitational potential energy as it rises, then back to kinetic energy as it falls. Some energy is also transferred to thermal energy of the surroundings due to air resistance.

Examples of Energy Changes

Consider common situations: a phone battery powering a phone involves chemical energy converting to electrical energy, then to light and sound energy. A car accelerating uses chemical energy (fuel) converted to kinetic energy (car) and thermal energy (engine, exhaust). A diver on a high board has gravitational potential energy, which converts to kinetic energy as they dive, then to kinetic and sound energy (splashing) and thermal energy (water resistance) in the water. Remember to always account for energy conservation.

What is Energy Efficiency?

Energy efficiency quantifies how effectively a device or process converts supplied energy into useful energy output. No process is 100% efficient due to the inevitable generation of wasted energy, typically as heat, which dissipates into the surroundings. This wasted energy cannot be reused for its intended purpose. The concept aligns with the conservation of energy, where total energy input equals the sum of useful energy output and wasted energy output.

Calculating Energy Efficiency

The formula for calculating energy efficiency is straightforward: Efficiency = (Useful energy output / Total energy input) × 100%. Alternatively, if you are given power values instead of energy values, the formula is: Efficiency = (Useful power output / Total power input) × 100%. Both energy and power must be in consistent units (e.g., Joules for energy, Watts for power) for the calculation to be correct. The result is always expressed as a percentage.

Interpreting Efficiency Values

Efficiency values range from 0% to 100%. A higher percentage indicates greater efficiency, meaning more of the input energy is converted into the desired useful form. For instance, an electric motor that is 90% efficient converts 90% of the electrical energy into kinetic energy (movement), with the remaining 10% typically wasted as heat and sound. Understanding efficiency helps in comparing devices and designing systems to minimise energy waste and reduce environmental impact.

Kinetic Energy (Ek)

Kinetic energy is the energy an object possesses due to its motion. Any moving object has kinetic energy. The magnitude of kinetic energy is directly proportional to the object's mass and the square of its speed. This means that doubling the speed of an object quadruples its kinetic energy. The formula for calculating kinetic energy is: Ek = 0.5 × m × v², where Ek is kinetic energy in joules (J), m is mass in kilograms (kg), and v is speed in metres per second (m/s). For example, a car with a mass of 1000 kg moving at 20 m/s has Ek = 0.5 × 1000 kg × (20 m/s)² = 0.5 × 1000 × 400 = 200,000 J.

Gravitational Potential Energy (Ep)

Gravitational potential energy is stored energy an object has due to its position within a gravitational field, specifically its height above a reference point. The higher an object is, the more gravitational potential energy it has. This energy can be converted into kinetic energy if the object is allowed to fall. The formula for gravitational potential energy is: Ep = m × g × h, where Ep is gravitational potential energy in joules (J), m is mass in kilograms (kg), g is gravitational field strength (typically 9.8 N/kg on Earth), and h is height in metres (m). For instance, a 5 kg object lifted 10 m above the ground has Ep = 5 kg × 9.8 N/kg × 10 m = 490 J.

Elastic Potential Energy (Ee)

Elastic potential energy is the energy stored in an elastic object, such as a spring or rubber band, when it is stretched or compressed. This energy is stored when work is done to deform the object from its natural shape. When released, this stored energy can do work. The amount of elastic potential energy depends on the spring constant (k), which measures the stiffness of the spring, and the extension or compression (x) from its original length. The formula for elastic potential energy is: Ee = 0.5 × k × x², where Ee is elastic potential energy in joules (J), k is the spring constant in newtons per metre (N/m), and x is the extension or compression in metres (m). For example, a spring with a spring constant of 200 N/m stretched by 0.1 m stores Ee = 0.5 × 200 N/m × (0.1 m)² = 0.5 × 200 × 0.01 = 1 J.