P1 Conservation and Dissipation of Energy - AQA Trilogy Physics

Energy Stores

Energy Stores

• kinetic - in a moving object

• elastic

• thermal

• nuclear

• chemical

• gravitational

• electrostatic - two separate electric charges that are attracting of repelling)

• magnetic

Energy Carriers

• mechanical

• electrical

• by heating

• by radiation

Conservation of Energy

• energy cannot be created or destroyed

Closed System

• no energy transfers take place out of or into the energy stores of the system

• energy can be transferred between energy stores in a closed system

• the total of the energy in the system if always the same before and after

Work Done

• energy transferred = work done

• work done (J) = force applied (N) x distance moved along the line of action of the force (m)

• W = Fs

Friction

• work done to overcome friction is mainly transferred to the thermal energy stores of the objects that rub together and to the surroundings

Gravitational Potential Energy

• gravitational potential energy store (J) = mass (kg) x gravitational field strength (N/kg) x change in height (m)

• E(p) = mg∆h

Kinetic Energy

• kinetic energy (J) = 0.5 x mass (kg) x speed² (m/s²)

• E(k) = 0.5 x m x v²

Elastic Potential Energy

• elastic potential energy (J) = 0.5 x spring constant (N/m) x extension² (m²)

• E(e) = 0.5 x k x e²

Useful Energy

• energy that is in the place and form wanted

Wasted Energy

• energy that is not transferred to the place or form wanted

• eventually transferred to the surroundings which becomes warmer

Efficiency

• efficiency = (useful output transferred by the device) / (total input supplied to the device)

• no device can be more that 100% efficient

HIGHER - Improving Efficiency

• friction between the moving parts causing heat → lubricant to reduce friction

• resistance of a wire causing wire to get hot when current passes through it → use wires with as little electrical resistance as possible

• air resistance causing energy to be transferred from the object to surrounding by energy from force being wasted → streamline the shapes of moving objects to reduce air resistance

• sound created by machinery → cut out the noise (e.g. tighten loose parts to reduce vibrations)

Power

• Power (W) = energy transferred to appliance (J) / time taken for energy to be transferred (s)

• P = E / t

• Power (W) = current² (A) x resistance (Ω)

• P = I² x R