An hour lesson on the physics of energy

1. Energy From Airplanes to Atoms Sam Nolan

2. Aims

3. What is Energy ? How would you describe someone who has energy ? Energy is a physical property possessed by an object, and is a measure of the capability of an object to “make something happen”. To “make something happen” energy is usually transferred to another object Energy is measured in Joules

4. Energy Conversion at Extremes

5. Energy per Gram

6. Power Power is the rate energy is transferred from one form to another Energy Power Time Energy is in Joules Time is in Seconds Power is in Watts

7. Energy per Gram TNT releases its energy within 0.0000001 seconds, whereas Chocolate Chip Cookies take around 100 seconds, which has most power ?

8. Energy per Gram

9. TNT TNT undergoes a violent chemical reaction It can release all its Chemical Energy very rapidly (few 0.0000001 seconds) It has huge power as it can convert its energy so rapidly

10. Light Bulbs A 60 Watt Lightbulb converts electrical energy to heat and light 60 Watts means 60 Joules every second.

11. Energy per Gram

12. Energy per Gram

13. Joules versus Calories There are 4200 Joules in 1 Kilo Calorie (kcal) Typically we consume 2000-2500 kcal a day In Strenuous Exercise we burn 360 kcal/hour 1 Bottle of Pop contains 160 kcal

14. Conservation of Energy The Principle of Conservation of Energy: “Energy can neither be created or destroyed merely changed from one form to another”

15. Conservation of Energy In a human, food (chemical energy) is used to produce: thermal energy electrical energy kinetic energy What happens in a battery ? What happens in your car ? What happens in

16. Useful Energy and Waste Heat Think of a car. For every 100 kJ (100 000 J) energy contained in the petrol: 60 kJ is lost as heat up the exhaust and radiator; 40 kJ is turned into movement. Of that 40 kJ: 35 kJ goes to driving the wheels 0.5 kJ to power the electrics; 1 kJ to power the steering; 3.5 kJ to power the air conditioning. Of the 35 kJ to drive the wheels, some will be lost in friction. Less than 35 % of the energy we put into the car actually ends up in moving the car along the road. However none of the energy has been destroyed. It has simply been turned into other forms of energy.

17. Recycling & Efficiency Any energy that is not useful is wasted. It is possible to harvest some of the waste energy to make it useful, for example: the heater in a car keeps us warm on cold days; We can calculate the efficiency of a machine (like our car) by comparing the useful energy output with the total energy input: Efficiency = useful energy output x 100% total energy input

18. Efficiency Example In a torch, for each 100J of chemical energy supplied by the battery, only 5J are output as useful light energy. What is the efficiency of the torch?

19. What is Kinetic Energy ? Kinetic energy is proportional to the mass of the body, e.g. a tennis ball and a bowling ball Kinetic energy is proportional to the speed of the body squared Its units are Joules (M in kg, V in ms-1)

20. Examples Around Us Anything that has motion has kinetic energy from the large scale

21. Examples Around Us To the very small scale

22. Practice Exercise “The Importance of Speed Limits”

23. Why Safe Driving is Important A car weighing 1000kg, travelling at 13 metres per second (30 mph) has a kinetic energy of How much kinetic energy does the same car travelling at 18 metres per second (40 mph) have ? Why is this important ?

24. Why Safe Driving is Important A car weighing 1000kg, travelling at 18 metres per second (40 mph) has a kinetic energy of The greater the kinetic energy the bigger the energy expended in a collision

25. Practice Exercise “The Physics of Playing Pool”

26. The Physics of Playing Pool Mass of Each Ball = 250g Both Balls are at rest to begin with What are their kinetic energies ?

27. The Physics of Playing Pool Mass of Each Ball = 250g Both Balls are at rest to begin with What are their kinetic energies ? We Strike The White Ball and it moves at 0.5 m/s. What is its kinetic energy ?

28. The Physics of Playing Pool Mass of Each Ball = 250g Both Balls are at rest to begin with What are their kinetic energies ? We Strike The White Ball and it moves at 0.5 m/s. What is its kinetic energy ? What can we say about the total energy just before and just after the collision ?

29. The Physics of Playing Pool Mass of Each Ball = 250g Kinetic Energy Before= Kinetic Energy After After the collision will the white ball be moving faster or slower than before the collision ?

30. The Physics of Playing Pool Mass of Each Ball = 250g After the collision will the white ball be moving faster or slower than before the collision ? Before the collision the white ball has a speed of 0.5 m/s, after this is reduced to 0.2 m/s. What is the speed of the black ball then?

31. Answer Speed of black ball is 0.46 m/s What have we not considered ?

32. Gravitational Potential Energy

35. Practice Exercise “ The Life of a Ski Jumper”

36. Changing Kinetic Energy Skier Starts at Rest What is their kinetic energy here ? Mass 100 kg

37. Changing Kinetic Energy The skier starts to move. Does the skier have more kinetic energy at the top of the ramp or at the bottom ? Why ? Where has the skier obtained kinetic energy from ? Skier, Mass 100 kg

38. Changing Kinetic Energy What is the potential energy of the skier at the top of the ramp ? What is their kinetic energy at the bottom ? How much energy have they gained from gravity ? What is their speed at the bottom of the ramp ? Mass 100 kg 100m

39. Other Examples

40. Dropping Things I drop a weight of mass 1 kg from a height of 10 metres, assuming g=10m/s2 how fast is it falling when it hits the ground ? If 95% of its energy is lost as heat and sound how high will it bounce back ?

41. Pendulums Pendulums work on the principle of conservation energy between kinetic energy and gravitational potential energy. If the pendulum pictured is released from 1 metre above its rest position what will its speed be when it reaches the bottom of its swing ? 1 metre

42. What have we covered ?

43. What have we covered What is Energy ? Energy is a physics property possessed by an object, and is a measure of the capability of an object to “make something happen” Most energy conversions don’t give 100% efficiency, most often energy is lost as heat and sound. What is the formula for Kinetic Energy ? Kinetic Energy is the Energy Associated with motion Kinetic Energy = ½ MV2 Gravitational potential energy mgh For falling objects Kinetic Energy=Potential Energy See Homework Exercise

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