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PHYSICS Matters for GCE ‘O’ Level
Unit 6: Energy, Work and Power
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6.1 Energy In this section, you’ll be able to:
identify different forms of energy – kinetic energy, elastic potential energy, gravitational potential energy, chemical potential energy and thermal energy state the Principle of Conservation of Energy solve problems using the Principle of Conversation of Energy Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy What is Energy? Energy is the capacity to do work.
The SI unit of energy is the joule (J). Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Different forms of energy and energy conversions
There are many forms of energy. Examples include: Kinetic energy Potential energy Sound energy Electrical energy Thermal energy Light energy Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Kinetic Energy Moving objects have kinetic energy.
Kinetic energy can be used to do work. In windy places, wind is used to turn turbines that convert kinetic energy to electrical energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Potential Energy
Energy that is stored is known as potential energy. Potential energy can be converted to kinetic energy and vice versa. Potential energy exists in many forms. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Chemical Potential Energy
Food contains chemical potential energy which is converted from solar energy via photosynthesis. These can be converted to kinetic energy. How energy is transferred from the sun to humans and animals. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Chemical Potential Energy
Chemical potential energy is also stored in fossil fuels like coal and oil. A battery also stores chemical potential energy that can be converted to electricity. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Elastic Potential Energy
A spring or rubber band possesses elastic potential energy when it is compressed or stretched. This energy is converted to kinetic energy when the spring or rubber band is released. An archer makes use of the elastic potential energy stored in the bow to propel the arrows. A fully flexed bow stores about 300 J of energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Gravitational Potential Energy
An object has gravitational potential energy when it is raised to a certain height above the ground. When released, it falls and gravitational potential energy is converted to kinetic energy. When a ball is being dropped from a height, it falls and the gravitational potential energy it has is converted to kinetic energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Principle of Conversation of Energy
Energy can neither be created nor destroyed in any process. It can be converted from one form to another or transferred from one body to another, but the total amount remains constant. When energy is converted from one form to another, the total amount remains constant. 20 J energy in one form 20 J energy in another form Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Conversion of Energy Diver on a diving board
Stored chemical energy in the body of a diver allows him to exert a push to bend the diving board. This causes the bent diving board to store elastic potential energy which is then converted to kinetic energy that helps push the diver upwards. Elastic potential energy is converted to kinetic energy, helping to push the boy upwards. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Conversion of Energy Hammering a nail
A raised hammer possesses gravitational potential energy. When it falls, this energy is converted to kinetic energy which is used to do work in driving the nail into the wood block. Sound and thermal energy are also produced and released by the block, nail and hammer. When the hammer falls, gravitational potential energy is converted to kinetic energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Conversion of Energy Burning of Fuels
By burning fuels, the stored chemical energy in these fuels is converted to thermal and light energy. Burning charcoal in a barbecue pit emits a lot of thermal energy to cook food. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Conversion of Energy
In real life, energy is easily dissipated into the surroundings. This makes it difficult for us to compare the amount of energy before and after conversion in order to study the Principle of Conservation of Energy effectively. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Principle of Conservation of Energy and the ideal pendulum
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6.1 Energy Principle of Conservation of Energy and the ideal pendulum
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6.1 Energy Principle of Conservation of Energy and the non-ideal pendulum In the real world, frictional forces convert some of the total energy of a swinging pendulum to thermal energy. This thermal energy is dissipated to the surroundings and cannot be converted back into kinetic or gravitational potential energy of the pendulum. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy The pendulum eventually comes to a stop.
Principle of Conservation of Energy and the non-ideal pendulum The pendulum eventually comes to a stop. Height gained is lower than the original because some of the energy has been converted to thermal energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Efficiency From the Principle of Conservation of Energy, the total energy output by a machine must be equal to its energy input. In real life, energy output is always less than energy input as energy is dissipated, due to friction, or as a form of sound and thermal energy. This energy lost is considered wasted energy output. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Efficiency Energy input = useful energy output + wasted energy 100% input energy output useful Efficiency = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Key Ideas Energy is the capacity to do work.
Energy can be converted from one form to another. The Principle of Conservation of Energy states that energy can neither be created nor destroyed in any process. It can be converted from one form to another or transferred from one body to another but the total amount remains constant. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy C D B A E Test Yourself 6.1 Answer:
1. A softball player throws a ball into the air and catches it on the way down. State the energy conversions that take place. K.E partly K.E, partly G.P.E G.P.E A B C D E Answer: When the ball left the player’s hand (at A), it has kinetic energy. As it rises up (at B), part of the K.E. is converted into gravitational potential energy. At the point of maximum height (at C), the ball has grav. P.E. As it is falling down (at D), part of the grav. P.E. is converted into K.E. When the ball reaches the player’s hand (at E) it will have only K.E. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Test Yourself 6.1 2. A cyclist pedals up to the top of a hill at a steady speed. What kind of energy is being used to do work against gravity? State the type of energy the cyclist has at the top of the hill. When the cyclist moves downhill without pedaling, what type of energy does he gain? The cyclist uses his stored chemical energy. The chemical energy is converted into gravitational potential energy as he rises up the hill. When he moves downhill, the G.P.E. is converted into kinetic energy. Without him pedaling, he is gaining K.E as he moves downhill. Answer: Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.1 Energy Test Yourself 6.1 3. Using the Principle of Conservation of Energy, explain: What happens to the stored chemical energy of a dry cell when it is connected to a light bulb? Answer: The chemical energy in the dry cell is converted into electrical energy that drives a current round the circuit. The light bulb converts the electrical energy into light energy as well as thermal energy. Why a ball released from rest at a certain height above the floor bounces up to a lower and lower height until it finally comes to a stop? Every time the ball hits the floor, some of the kinetic energy of the ball is converted into sound energy and heat energy. Hence, the ball will rise to a lower height after each bounce. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Learning Outcomes In this section, you will be able to:
Understand the concept of work and apply the relationship W = F s to solve problems Apply the relationships to solve problems 1 h g m E and v 2 k = p Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Work Done Definition: Work done by a constant force on an object is given by the product of the force and the distance moved by the object in the direction of the force. W = F s where W = the work done (in J), F = the constant force (in N) s = the distance moved in the direction of the force (in m) Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work The SI unit of work is the joule (J).
Definition: One joule (J) is defined as the work done by a force of one newton (N) which moves an object through a distance of one metre (m) in the direction of the force. one joule = one newton one metre 1 J = 1 N m Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Example of work being done: Lady pushing a pram
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6.2 Work No work is being done when:
1. The direction of the applied force and the direction in which the object moves are perpendicular to each other. A man carrying a load while walking. No work is done on the load in the upward direction as the load is only moving horizontally. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work No work being is being done when:
2. The force is applied on the object (such as the wall or the pile of books) but the object does not move. Boy pushing against a solid wall. A girl holding a heavy pile of books in a stationary position does no work. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work How is energy related to work and force?
We need energy to move an object, run and climb stairs. To move a stationary object, we need to apply force to them. For a moving object, we also need to apply force to increase its speed. Hence, work is done when we move a stationary object or make a moving object move faster. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Mechanical Energy There are two types of mechanical energy:
1. Kinetic energy 2. Gravitational potential energy A roller coaster uses a motor-and-chain system to pull the riders up the first hill before letting gravity take over the rest of the ride. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Kinetic energy and work done
A moving body has kinetic energy. When a force moves an object, it does work and the object gains kinetic energy. Kinetic energy is defined as: ) s m (in body the of speed and kg) mass J), energy kinetic E where 1 – k = v 2 mv Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Gravitational potential energy and work done
Potential energy is stored energy Gravitational potential energy (G.P.E) is the energy a body has due to its position To find G.P.E. of an object near surface of Earth, we need to consider its mass and its height above the ground. An object of mass m raised to a height h above ground level possesses G.P.E. of mgh. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Gravitational potential energy and work done
Gravitational potential energy is defined as: m) (in height ) kg N strength field nal gravitatio where 1 – = J), energy potential E p h g kg) body the of mass m Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Energy, Work and Power Figure 6.23
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6.2 Work Key Ideas 1. Force, work and energy are interrelated.
2. Work done W by a constant force F is given by the product of the force F and the distance moved in the direction of the force, i.e. W = F s. 3. The SI unit of work is the joule (J), which is the same as the SI unit of energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Key Ideas 4. No work is done when
The direction of the applied force and the direction in which the object moves are perpendicular to each other The force is applied on the object but the object does not move. 5. Moving objects have kinetic energy. The kinetic energy of an object of mass m in kilograms and speed v in m s–1 is given in joules by the expression: 2 k 1 E mv = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Key Ideas 6. An object of mass m kg at height h has gravitational potential energy given by Ep = mgh where g is the gravitational field strength (10 N kg–1). 7. Potential energy can be converted to kinetic energy and vice versa. The total energy in a system is fixed. If all the gravitational energy is converted to kinetic energy or all the kinetic energy is converted to gravitational potential energy, the equation is true. 2 1 mv mgh = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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Distance moved in the direction of force
6.2 Work Test Yourself 6.2 1. (a) Define the joule. Answer: One joule is defined as the work done by a force of one newton which moves an object through a distance of one metre in the direction of the force. (b) Complete the table by filling in the missing quantities (in bold). Force exerted Distance moved in the direction of force Work done (i) N 10 m 200 J (ii) 0.1 N 1 J (iii) 0.04 N 20 m 0.8 J (iv) 500 N 7200 m 3.6 106 J Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Test Yourself 6.2 G.P.E 30 m 5 m K.E Answer:
2. A block of mass 4 kg slides from rest through a distance of 30 m down a frictionless slope, as shown in the diagram. What is the kinetic energy of the block at the bottom of the slope? 5 m 4 kg 30 m G.P.E K.E Answer: At the top, the block has G.P.E G.P.E = m g h = 4 10 5 = 200 J At the bottom, the G.P.E is converted into K.E. Hence, the K.E of the block at the bottom is 200 J. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work Test Yourself 6.2 Answer: 1 2 vf = vi ÷ ø ö ç è æ = mv 1 4 v
3. If the speed of a springboard diver decreases by half on entering the water, by how much will his kinetic energy decrease? Answer: Let the initial speed of the diver just before he hit the water be vi , and the final speed after he entered the water be vf . Since speed is decreased by half, i.e. 1 2 vf = vi ÷ ø ö ç è æ = 2 i mv 1 4 v m K.E Final 2 i mv 1 K.E Initial = Hence, the final K.E is now one quarter of the initial K.E. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.2 Work 5 kg G.P.E Test Yourself 6.2 10 m
4. A package of 5 kg is lifted vertically through a distance of 10 m at a constant speed. Taking acceleration due to gravity to be 10 m s–2, what is the gravitational potential energy gained by the package? 5 kg 10 m G.P.E Answer: Gravitational P.E = m g h = 5 10 10 = 500 J Hence, the package gained 500 J of gravitational potential energy. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Learning Outcomes In this section, you will be able to:
Recall and apply the relationship to solve problems. taken time done work power = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power s) (in taken time t and J) converted energy E done work W
What is power? Power is defined as the rate of work done or rate of energy conversion. s) (in taken time t and J) converted energy E done work W power P where = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power s J W second one joule watt =
The SI unit of power is the watt (W). One watt (W) is defined as the rate of work done or energy conversion of one joule per second. 1 - s J W second one joule watt = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Key Ideas Power is the rate of work done or energy converted. The SI unit of power is the watt (W). One watt is the rate of work done at 1 joule per second. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Test Yourself 6.3 1. (a) Define the watt. Answer:
One watt is defined as the rate of work done or energy conversion of one joule per second. (b) What is meant by power? Answer: Power is defined as the rate of work done or rate of energy conversion. Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Test Yourself 6.3 1. (c) In the following situations, calculate the power involved. (i) A force of 50 N moves through a distance of 10 m in 5 s. Answer: = W 100 5 10 50 t s F P (ii) An object of mass 1 kg is lifted up vertically through m in 10 s. Answer: W 5 10 1 t mgh E P = Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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6.3 Power Test Yourself 6.3 2. An electric motor in a washing machine has a power output of 1.0 kW. Find the work done in half an hour. Answer: Given Power P = 1.0 kW = 1000 W and 1 2 time t = = 0.5 60 60 = 1800 s hour J 10 1.8 1800 1000 t P W 6 = Hence, the work done W = 1.8 106 J Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
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