Energy, work & power

1. Energy, work & power

2. Energy • the ability to do work or cause change • typically expressed in units of joules (J) • can be transferred from one object to another • two general types: • Potential • Kinetic

3. Work and energy When a force causes a body to move through a distance, energy is transferred and work is done. work done = energy transferred. Both work and energy are measured in joules (J).

4. Power (P) Power is a measurement of how quickly work is done. power = work done time taken P = W t power, P is measured in watts (W) work done, W is measured in joules (J) time, t is measured in seconds (s) One watt is the same as one joule per second.

5. 0.45 kg 0.45 kg Potential Energy (PE) • stored energy that an object has due to its position or chemical composition • Types: • Gravitational – results from vertical position or height • Formula: PE = mgh • Elastic – results from stretching or compressing Which soccer ball has more gravitational potential energy? Explain your reasoning.

6. 2 m/s 3 m/s 0.45 kg 0.45 kg Kinetic Energy (KE) • energy of motion • depends on mass and velocity • Formula: KE = ½ mv2 • increases as mass or velocity increases and decreases as mass or velocity decreases What is the difference between speed and velocity? Which soccer ball has more kinetic energy? Explain your reasoning.

7. Gravitational potential energy Gravitational potential energy(GPE) is the energy stored in an object when work is done in moving the object upwards. GPE = mass x g x height GPE = m x g x h GPE is measured in joules (J) mass, m is measured in kilograms (kg) gravitational field strength, g is measured in newtons per kilogram (N/kg) height, h is measured in metres (m)

8. Kinetic energy Kinetic energy is the energy possessed by a body because of its speed and mass. kinetic energy = ½ x mass x (speed)2 KE = ½ x m x v2 kinetic energy, KE is measured in joules (J) mass, m is measured in kilograms (kg) speed, v is measured in metres per second (m/s)

9. Falling objects If there is no significant air resistance then conservation of energy results in gravitational potential energy being converted into kinetic energy as an object falls. gain in KE = loss of GPE m v1 ½ h v2 gpe = mgh ke = 0 h gpe = ke gpe = ½ mgh ke = ½ mv12 gpe = 0 ke = ½ mv22 ke = mgh

10. Measuring a person’s power 1. Measure the weight, W of person using weighing scales. 2. Measure the time taken for the person to run up a flight of stairs of height, h 3. Work done = weight x height = W x h = W x n x s 4. Power of the person = work done / time taken = (W x n x s) / t s total stairs height, h = n x s stairs of n steps person of weight,W

11. Example calculation Weight of person, W = 800N Time taken, t = 3.0 seconds Stairs: number of steps, n = 12 height of step = 0.20m total stair height, h = 12 x 0.20m = 2.4m Work done = weight x height = 800N x 2.4m = 1920J Power = 1920J / 3.0s = 640W s total stairs height, h = n x s stairs of n steps person of weight,W

12. Forms of Energy • Mechanical • Chemical • Strain • Thermal(heat) • Electrical • Nuclear • Light • Sound

13. Mechanical Energy • energy that moves objects • the total energy of motion and position of an object • may be in the form of potential energy, kinetic energy, or both • Example: • If a student were to lift and/or drop a stack of textbooks, mechanical energy would be involved

14. O H C OH H C HO H C H C OH C H OH H OH C H Chemical Energy • type of potential energy stored in the chemical composition of matter • depends on the types and arrangement of atoms in a substance • i.e. • A bond between a hydrogen atom and an oxygen (H-O) atom will release more energy than one between two carbon atoms (C-C) ≈464,000 J ≈347,000 J Glucose

15. Sound Energy • an example of mechanical energy that results from the vibration of particles in a solid, liquid, or gas • can be impacted by temperature and pressure • must have a medium (usually air) to travel through - cannot travel through empty space • sound in a vacuum This person is listening to someone telling a secret. How are the sound waves being generated?

16. Thermal Energy • results from the movement (kinetic energy) of particles in matter • when particles move faster they have more thermal energy than when they move slower • particles of a substance that are farther apart have more thermal energy than if they were closer together • depends on the number of particles in a substance Which box of particles has more thermal energy? Why?

17. Electromagnetic Energy • transmitted through space in the form of electromagnetic waves • light, electricity, and magnetism are representative of electromagnetic energy • can travel through empty space Image taken from: http://zebu.uoregon.edu/~imamura/122/lecture-2/em.html

18. + + + + Nuclear Energy • found in the nucleus of an atom • is released when an atom’s nucleus breaks apart (fission) or when the nuclei of two atoms come together (fusion) • Example: • Nuclear fission takes place in a nuclear power plant while nuclear fusion takes place in the Sun. Is nuclear fission or nuclear fusion taking place? Explain your reasoning. Tritium Deuterium helium neutron

19. Identify the Form of Energy Thermal (burner increases movement of H2O molecules) Mechanical (moving gears) Electromagnetic (radio waves) Chemical (food) Electromagnetic (electricity)

20. 6CO2 + H2O C6H12O6 + 6O2 Light Energy O2 O2 light energy O2 O2 CO2 O2 O2 CO2 CO2 CO2 CO2 CO2 H2O H2O H2O H2O H2O H2O Energy Conversions • a change from one form of energy into another • energy can be converted into any other form and is often converted into more than one form • most of the wasted or unwanted energy in a conversion is attributed to heat (friction) • Example • Electromagnetic energy (in the form of light) from the Sun is converted, by plants, into chemical energy in the form of glucose What is an example of an energy conversion you have experienced in your own life? C6H12O6

21. Law of Conservation of Energy • states that energy can be neither created nor destroyed • the total amount of energy in a closed system is the same • energy can be changed from one form to another, but all of the different forms of energy add up to the same total amount of energy PE = 24 J KE = 0 J PE = KE = 12 J 12 J A seagull steals a sandwich and drops it from a height of 7 m before eating it. What would be the sandwich’s approximate PE and KE as it falls to the ground if air resistance is negligible? 0 J PE = KE = 24 J

22. Identify the Energy Conversions 2. ElectromagneticEnergy (electricity moving through the wires) Wires The apparatus to the right was placed into a bell jar. What energy conversions take place when it is operating? Battery Buzzer 1. Chemical energy (in the battery) 3. Sound Energy (noise coming from the buzzer)

23. Energy transfer in a television An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy. For example, in a television: light electrical sound heat

24. Energy transfer in a car engine What are the main energy transfers in a car engine? kinetic chemical sound heat

25. How is energy wasted? This cordless drill transfers chemical energy in the battery into movement, sound and heat energy. The movement energy can be used to do useful work, but the sound and heat energy cannot. Energy that does not do useful work is called wastedenergy. Appliances usually waste energy as sound and heat. In fact, all electrical devices produce a small amount of heat energy when they are used, which is normally wasted.

26. Which devices waste energy as sound? If a device makes noise but is not designed to make noise, then energy is being wasted as sound. Examples of devices that waste energy as sound include:

27. Which devices waste energy as heat? If a device gets warm but it is not designed to get warm, then energy is wasted as heat. Examples of devices that waste energy as heat include:

28. What is energy efficiency? Efficiency is a measure of how good a device is at changing energy from one form to another. All devices waste energy, so the efficiency of a device is never 100%. The more efficient a device is the less energy is wasted. An energy-efficient light bulb is more efficient than a filament bulb because it wastes less energy. It converts a greater proportion of the energy that it is supplied with into light energy and not heat.

29. Why is energy efficiency important? New electrical appliances display information about how energy efficient they are. This helps consumers to identify which appliance will use the least energy and cost the least to run. Energy efficiency is not just about saving money, it’s also about saving the planet. Appliances that are more efficient need less electricity, and so less fossil fuel needs to be burnt in fossil-fuel power stations. This is important for reducing pollution and the effects of climate change.

30. How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: useful output energy total input energy energy efficiency = • Useful energy is measured in joules (J). • Total energy is measured in joules (J). • Energy efficiency does not have any units. • It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100.

31. Credits • www.simpson-physics.com • www.physicsclassroom.com • www.googlephysicsimages.com • www.edurite.com • www.learnnext.com • www.funtoot.com