Energy

1. Energy

2. Menu • The Ultimate Energy Source • Forms of Energy • Energy Transfers • How do we Measure Energy • Work Done • Power • Efficiency • The Cost of Electricity

3. Is there an Ultimate Energy Source?

4. Is there an ultimate energy source? The grass gets its energy from the Sun via photosynthesis. The Sun is the ultimate energy source The lamb gets its energy by eating the grass …. …. and WE get our energy by eating the lamb! Return to Menu

5. Forms of Energy

6. Gravitational Potential Energy This is energy when something is ‘out of position’. This rock climber has potential energy. The further she climbs – the more potential energy she would gain. Water held behind a dam has this type of energy too.

7. Chemical Potential Energy When a fuel burns, its energy is released by chemical reactions. The energy stored in the fuel is called chemical potential energy (or chemical energy for short). Batteries also store chemical potential energy.

8. Kinetic Energy This is energy due to motion. All moving objects have kinetic energy The faster they move (or the more mass they have) the more kinetic energy they have

9. Elastic Potential Energy A stretched rubber band can do work when released. It has elastic potential energy

10. Electrical Energy In circuits, the current is a flow of tiny charged particles called electrons. Electrons can transfer energy, for example from a circuit to a hairdryer. They have electrical energy.

11. Nuclear Energy An atom has a nucleus at its centre. This is made up of particles, held by strong forces. In some atoms the particles become rearranged or the nucleus splits and energy is released.

12. Thermal Energy When hot objects cool down, their atoms and molecules slow down and lose energy. This is known as thermal energy or heat.

13. Radiated Energy Examples of this are light from a lamp and radio signals from a transmitter.

14. Sound Anything that makes a noise has this form of energy. For example air compressions from a loudspeaker or other sound source. Return to Menu

15. Energy Transfers

16. What is the energy conversion in each of the solar cells? Electrical Energy Light (radiated)

17. What are the energy transfers in this match? Chemical Energy Light & Heat Sound is wasted

18. What energy conversions happen in a windmill? Kinetic Energy Electrical Energy

19. ‘Energy cannot be created or destroyed, but it can be changed from one form to another’ Chemical Energy Light (radiated) Energy wanted Heat (thermal) Energy wasted The wasted Heat Energy now heats up the surroundings. It is NEVER destroyed! Return to Menu

20. How do we Measure Energy

21. What is the unit for ENERGY? The Unit for all forms of energy is the Joule (J). The unit is named after the scientist in the picture. 1 kJ = 1,000J 1 MJ = 1,000,000J 1 J is equivalent to 1 kWh

22. The kiloWatt hour (kWh) is a unit of energy. In this case the meters are showing how much electrical energy has been used in a home Return to menu

23. Work Done

24. What is WORK DONE? When a force moves an object, ENERGY is TRANSFERRED and WORK IS DONE WD F x d Work Done = FORCE x DISTANCE

25. Question: Calculate the work done by a pupil who pulls a car tyre 5 metres with a force of 60 N. Answer: Work Done = FORCE x DISTANCE = 60 N x 5 m = 300 Nm We could also give our answer as 300 J ( As 1Nm = 1J) Return to Menu

26. Power

27. What is POWER? Power is the ‘rate of doing work’. A powerful machine is one that can transfer a lot of energy in short space of time. Work Done Power = Time Taken WD P x t

28. For electrical devices we can use the equation: Energy transformed (J) time taken (s) POWER = (W) Appliances such as toasters, irons & TVs have a power rating marked on them either in watts or kilowatts. 1 kiloWatt (kW) = 1000 watts 1 Watt = 1 Joule per second

29. Consider this kettle. In 5 seconds, the kettle takes 10,000 Joules of energy from the mains supply. What is its power? Energy Transformed Power = Time taken = 10,000 / 5 = 2000W (or 2kW)

30. Can you explain why this greyhound has a high power? Because he does a lot of work in a short time! Return to Menu

31. Conservation of Energy & Efficiency

32. What is the Principle of ‘Conservation of Energy’? ENERGY can never be CREATED or DESTROYED – it can only be CONVERTED from one form to another. Another important principle is: Energy is ONLY USEFUL when it is CONVERTED from one form to another.

33. Are energy conversions 100% efficient? NO device is 100% efficient and the wasted energy is dissipated usually as HEAT & SOUND Think of this washing machine. It gets noisy and hot when it is switched on to wash our clothes

34. What is EFFICIENCY? The efficiency of any device is defined as: Useful energy OUTPUT Efficiency = Useful energy INPUT Energy out Efficiency can be expressed as a fraction, decimal or percentage Efficiency x Energy In

35. Efficiency = Energy output Energy Input = 1,000 / 1,200 = 0.83 (or 83%) Calculate the efficiency of this computer 1200J of electrical energy is supplied to the computer. 200J of energy are wasted in the form of heat & sound. Return to Menu

36. The Cost of Electricity

37. How do we calculate the cost of electricity? This depends on: • The POWER RATING of the • appliance and • HOW LONG you leave it on for. Electricity is measured in “units”, also called “kilowatt hours” (kWh).

38. Calculate the cost of leaving a 600W computer on for 120 minutes. (The cost of each unit of electricity is 8 pence) Number of UNITS used = Power (in kW) x Time (in hours) = 0.6 kW x 2 hours = 1.2 units COST of Electricity = Number of Units used x price per unit = 1.2 units x 8 pence = 9.6 pence Return to Menu