02/12/2018 Energy

The ULTIMATE energy source 02/12/2018 The sun is the ultimate source of all our energy. For example, we often get energy from beef: BEEF comes from COWS, COWS eat GRASS, GRASS gets energy from the sun by PHOTOSYNTHESIS

Gravitational potential The 9 types of energy 02/12/2018 Type 3 example sources Type 3 example sources Heat Kinetic (movement) Nuclear Sound Light Chemical Electrical Gravitational potential Elastic potential

Electricity Light + heat Energy changes 02/12/2018 To describe an energy change for a light bulb we need to do 3 steps: 1) Write down the starting energy: 3) Write down what energy types are given out: 2) Draw an arrow Electricity Light + heat What are the energy changes for the following…? An electric fire A rock about to drop An arrow about to be fired

Conservation of Energy 02/12/2018 In any energy change there is ALWAYS some “waste” energy: e.g. a light bulb: Electricity Light + heat In this example HEAT is wasted – it’s given to the surroundings. Waste energy is virtually always heat and it’s always given to the surroundings.

Energy Transfer diagrams 02/12/2018 Consider a light bulb. Let’s say that the bulb runs on 100 watts (100 joules per second) and transfers 20 joules per second into light and the rest into heat. Draw this as a diagram: “Input” energy “Output” energy 20 J/s light energy 100 J/s electrical energy 80 J/s heat energy (given to the surroundings)

Example questions Consider a computer: Consider a kettle: 02/12/2018 Consider a computer: 150 J/s electrical energy 10 J/s wasted sound 20 J/s wasted heat Useful light and sound How much energy is converted into useful energy? What is the computer’s efficiency? Consider a kettle: Work out each energy value. What is the kettle’s efficiency? Sound energy Wasted heat Heat to water 2000 J/s electrical energy

Efficiency 02/12/2018 Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it. Efficiency = Useful energy given out by the device Energy put into it e.g. if 2000 joules of electrical energy are put into a kettle and 500 joules of heat energy are gained from it, its efficiency is 500/2000 x 100% = 25% x100%

Some examples of efficiency… 02/12/2018 5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it? A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go? A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?