Waves in matter Study Guide

Energy is never created or destroyed, only transferred from one form to another or from one store to another. This fundamental principle is known as the Law of Conservation of Energy. When a 'system' (which could be anything from a falling apple to a power station) undergoes a change, the way its energy is stored changes. We can describe these changes by identifying the initial and final energy stores involved. For instance, when an object falls, its gravitational potential energy store decreases, and its kinetic energy store increases. If there's air resistance, some energy is also transferred to the thermal energy store of the surroundings. Other common energy stores include chemical (in fuels, food, or batteries), elastic potential (in a stretched spring or squashed ball), magnetic (in magnets or electromagnets), and electrostatic (due to charges). Light and sound are also ways in which energy can be transferred. To fully describe energy changes, you need to state the initial energy store, the final energy store(s), and any energy transfers that occurred, often involving work done by forces or energy transferred by heating or by electric current. For example, a car braking involves the kinetic energy store of the car decreasing, and the thermal energy stores of the brakes and tyres (and the road) increasing due to friction.

Energy is a fundamental concept in physics, and understanding how it changes within a system is crucial. When a system undergoes a change, energy is often transferred to or from it. This energy transfer can manifest in several ways, leading to observable effects like changes in temperature, movement, or electrical activity.

One common way to change the energy of a system is by heating. When an object is heated, its internal energy increases. This increase in internal energy can cause its temperature to rise. The amount by which the temperature changes depends on the amount of heat energy supplied, the mass of the object, and a property called its specific heat capacity. Specific heat capacity is defined as the amount of energy required to raise the temperature of 1 kg of a substance by 1 °C. Substances with a high specific heat capacity require more energy to change their temperature significantly compared to those with a low specific heat capacity.

Another way to change a system's energy is by work done by forces. When a force acts on an object and causes it to move, work is done on the object. This work done transfers energy to the object, often increasing its kinetic energy (energy of motion) or potential energy (stored energy due to position or state). For example, pushing a box across a floor involves work being done on the box, increasing its kinetic energy and overcoming frictional forces.

Finally, work done when a current flows also involves energy transfer. Electrical components, such as resistors, convert electrical energy into other forms of energy, typically heat and light. When an electrical current flows through a component, work is done by the electrical forces, and this work done results in a transfer of electrical energy into the internal energy of the component, causing its temperature to increase, or into radiant energy (light) if it's a bulb. The amount of energy transferred depends on the current, voltage, and the time for which the current flows.