1
Module:
Energy
Time allocation: 10 hours
IJSO Training Course
Phase II
2Objectives
ü Introduce the concepts of heat, work, and internal energy. ü Discuss the main energy transformations that take place in a power station. ü Discuss the advantage and disadvantages of producing electrical energy using various energy resources, e.g., fossil fuel, nuclear fission, and some renewable energy resources, e.g., solar energy, hydroelectric power, etc. 31. Work, Energy and Power
ü When a force moves a body in the direction along which it acts, work is done on the body. ü Energy is the ability to do work, work is the process of converting energy. ü To calculate the work done by a moving force:Work done = force x distance moved in direction of force
4 ü That is, ü the unit of work or energy is the Joule (J).
W = F•s
1 J is the quantity of work done when a force of 1 N moves a body 1m, along its own line of action. Although force and displacement are both vectors, work (or energy) is a scalar quantity. Examples: Vectors: force, displacement, velocity Scalar: temperature, energy, speed Exercises: 1. How much work is done when a force of 4 kN moves a block 1000cm in the direction of the force? (40000J) 2. Find the work done in raising 85 kg of water through a vertical distance of 4m. (3332 J)7 If the direction of the displacement is not the same as the direction of the force, we use the component of the force which is parallel to the displacement. For example, when a body is caused to accelerate down an inclined plane by the force of gravity: 8 Therefore, the more general equation to calculate work done is , where q is the angle between the force and the displacement. W = F•s cosθ 9
2.
Kinetic Energy
A body in motion possesses kinetic energy. The kinetic energy possessed by a body depends on itsmassand itsspeed. Consider a body of mass m and speed v, its kinetic energy is given by :K.E. = ½ (mv 2 ) 10
3.
Potential Energy
Examples: 1. A mass in a gravitational field possesses gravitational potential energy. 2. A stretched spring possesseselastic potential energy. 3. A charged body in an electric field possesses electrical potential energy. 11Gravitational Potential Energy
Consider a ball of mass m being lifted from A to B, the increase in the gravitational potential energy is given by :where g = 9.8 ms 2 is the acceleration due to
gravity.
Gravitational P.E. = m g h
12 Note: The equation tells
only the change in G.P.E. of the body, and is valid for a short distance near the earth’s
13 Exercises: 1. What is the change in potential energy if a body of 7 kg on the ground (a) is lifted to 120 m. (b) is placed at the bottom of a vertical mine shaft 120m deep. (a) 8.2 kJ, (b) 8.2 k J 14 2. A car of mass 500 kg traveling at 20m/s has its speed reduced to 5m/s by a constant breaking force over a distance of 80m. (a) The cars initial kinetic energy (b) The final kinetic energy (c) The breaking force (a) 100 kJ, (b) 6.25 kJ, (c) 1172 N 15
Elastic Potential Energy
Consider a spring being stretched an extension is x, byHooke’s law, the restoring force is given by :F = kx 16 The elastic potential energy stored in the spring is given by the area under the curve: or it equals: Elastic P.E. = ½ (kx 2 )
4.
Power
Power is work done per second (or energy converted per second).
unit of power is theJoule per second, Js 1 . 1 Js 1 is
called 1 Watt (1W). Combining this equation with the definition of work we have: P = W/t P = Fv Exercises: 1. A constant force of 3kN pulls a crate along a level floor a distance of 15 m in 40s. What is the power used? 1125W 2. A hoist operated by an electric motor has a mass of 600 kg. It raises a load of 300 kg vertically at a steady speed of 0.5 m/s. Frictional resistance can be taken to be constant at 900 N. What is the power required? 4860W
19 3. A car of mass 1000 kg has an engine with power output of 45 kW. It can achieve a maximum speed of 110 km/h along the level. (a) What is the resistance to motion? (b) If the maximum power and the resistance remained the same what would be the maximum speed the car could achieve up an incline of 1 in 40 along the slope? (a) 1473 N, (b) 26.2 m/s 20
5. The Law of Conservation of Energy
Energy can never be destroyed, it is just converted from one form to another. Examples of Energy Conversions:1. A battery converts chemical energy into electrical energy.
2. An electric motor lifting an elevator converts electrical energy intogravitational potential energy.
21 3. When a moving vehicle uses its brakes to stop, thekinetic energy of the vehicle is converted to heatin the brakes.
4. A nuclear reactor converts nuclear energy into heat. 5. Sound is caused by the vibration of air molecules. 22 Example: Now the ball is at rest at B and is falling from a height h. At B, the ball has gravitational potential energy only, i.e., E = mgh; At A, the elephant has kinetic energy only, i.e., E = ½ mv 2 23 By the law of conservation of energy, Therefore, the speed of the elephant at A is given by : m g h = ½ mv 2 v = √(2gh) 24 Exercises: 1. Describe the energy transformation for the following processes. (a) A block is released down a smooth inclined plane. (b) A bullet strikes a wooden block and is embedded inside. (c) Two blocks resting on a smooth horizontal surface held against a compressed, light spring are released. (d) Streams of water fall from the top to the bottom of a waterfall.
25 2. A cyclist and his bicycle has a mass of 90 kg. After 200 m he reaches the top of a hill, with slope 1 in 40 measured along the slope, at a speed of 3 m/s. He then free wheels the 200m to the bottom of the hill where his speed has increased to 7m/s. How much energy has he lost on the hill? 2610J 26
6.
Machines
Examples: Lever (left) and pulley (right) 27 Efficiency of a machine: Work input into a machine: Work output by a machine:Work input into a machine = Effort x distance moved by effort work input = E x d E
Work output by a machine = Load x distance moved by load work output = L x d L
´
28 Efficiency of a machine:
Mechanical advantage of a machine:
Efficiency (h) of a machine = (work input/ work output) x 100% h = [(L x d L )/(E x d E )]x 100%
M.A.= Load / Effort =L/E
Velocity ratio of a machine:
Efficiency of a machine can be rewritten as:
V.R. = distance moved by effort / distance moved by load =dE/dL
h = (M.A. / V. R.) x 100% Exercises: 1. A machine has an efficiency of 70%. The effort moves 5 m in lifting a load of 25 kg through 0.5 m. Find (a) the useful output of the machine, (b) the total input of the machine, (c) the effort required. (a) 122.5 J, (b) 175 J, (c) 35 N
31
2. A car of mass 800 kg, running at 55 km h 1 , consumes 5.25
liters of petrol per hour. The time taken for the car to decelerate from 70 km h 1 to 40 km h 1 when engaged in neutral gear is
found to be 14 s. (a) Calculate the deceleration of the car when its speed is 55 km h 1 . (b) What is the resistance acting on the car at this speed? (c) (i) What is the work output by the engine in 1 hour when the car is moving at km/hr? (ii) If 4.55 litres of petrol can release 1.7 x 10 8 J of energy, find the efficiency of the engine of the car.
(a) 0.595ms 2 , (b) 476.2 N, (c) (i) 2.62 x 10 7 J, (ii) 15.4%
32
7.
Some kinds of machines
The levers Scissors (left) and nutcrackers (right) 33 The pulleys V.R. = ______V.R. = ______V.R. = ______ 34The screw jack
V.R. = __________
35 8. Fossil fuels as a major source of energy Forms of energy can be classified as 1. Nonrenewable energy: those which are exhaustive, i.e., fossil fuels (石化燃料) and nuclear energy 2.Renewable energy: those which will not be exhaustive, e.g., solar power, water power, wind power, etc. 36 Fossil fuelsis commonly used today for electricity generation, and come in 3 principal forms: Coal (煤) , Natural Gas (天然氣) andCrude Oil (原油).37
Formation of Coal
Organic material made up primarily of carbon, varying amounts of hydrogen, oxygen, nitrogen and sulphur. It was formed millions of years ago in the Carboniferous Period 石炭紀時期 (~360 to 286 million years ago). As the trees and plants died, they sank to the bottom of the swamps of oceans, forming layers of a spongy material call peat(泥 煤). 38 Peat is a brownish material that looks like wood. Although it can be burnt as a fuel, it contains a lot of water, and therefore is very smoky when burnt. As the peat became buried beneath more sand, clay and other plant matter, the pressure and temperature increased and the water was squeezed out of it. Over many years the materials became compacted the carbon content increased, it turned into carbonrich coal. 39Formation of Oil
Fossil fuels: liquid forms crude oil, gaseous forms natural gas. Between 10 to 160 million years ago, marine plants and animals died and sank to the bottom, they were rapidly covered in mud, sand, and other mineral deposits. This rapid burial prevented immediate decay. 40 In a lackofoxygen environment, the organisms were slowly decayed into carbonrich compounds. These compounds mixed with surrounding sediments and formed source rock. As more layers were deposited on top of one another, pressure and heat acting on the source rock compressed the organic material into oil. As the movement of the earth, oil travels into rocks that have larger spaces, or pores, to hold it. Limestone and sandstone are two types of rocks with large pores, and they are called porous rocks. These reservoir rocks were trapped between impermeablecap rock, which can hold oil within the ground for many years.
Crude oil
It has various components, which do not have the same boiling points. Atoil refineries, crude oil is split into various types of products by heating.43 It then made into many different products the clothes, the toothbrush, the plastic bottle, the plastic pen. Almost all plastic comes originally from crude oil. The products include gasoline, diesel fuel, aviation or jet fuel, home heating oil, oil for ships and oil to burn in power plants to make electricity. 44
Natural gas
Mostly made up of a gas called methane (CH4), which is made up of 1 carbon and 4 hydrogen atoms. Lighter than air, highly flammable(易燃)Colourlessandno odor. It is mixed with a chemical that gives a strong rottenegg odor before being sent to storage tanks. 45
Safety Note
If you smell that rotten egg smell in your house, get out of the house quickly. Don't turn on any lights or other electrical devices. A spark from a light switch can ignite the gas easily. 46Saving Fossil Fuels
Fossil fuels take millions of years to make. We are using up the fuels that were made more than 300 million years ago before the time of the dinosaurs. So, it's better not to waste fossil fuels. They are not renewable. We can save fossil fuels by conserving energy. 47 9.Alternative energy source: Nuclear power Nuclear energy is not renewable. Matter and energy can't be created nor destroyed, but they can be changed in form. Matter can be changed into energy. That is the massenergy equation: . This equation says mass and energy are equivalent. In which, E [energy] equals m [mass] times c 2 , where c is the speed of light.Nuclear energy
E=mc 2 48 When a neutron bombards on a heavy nucleus, e.g., uranium 235, it splits into several smaller fragments. 2 or 3 neutrons are emitted spontaneously. This process is called nuclear fission. The sum of the masses of these fragments is less than the original mass. This 'missing' mass (~ 0.1 %) has been converted into energy according to the massenergy equation.49 Nuclear chain reactionrefers to a process in which neutrons released in fission produce an additional fission in further nucleus. If each neutron releases one more neutron, then the number of fissions doubles each generation. In that case, in 10 generations there are 1,024 fissions. The rate of the reaction can be controlled (nuclear power) or uncontrolled (nuclear weapons). 50
Controlled nuclear reaction
Nuclear power stations work much the same way as fossil fuelburning stations, except that a nuclear fission inside a nuclear reactor makes the heat instead. Modern reactors use enriched uranium235 as fuel. Natural uranium is only 0.7% uranium235; the rest is uranium238. 51 Neutrons smash into the nucleus of the uranium atoms, which split the atom and release energy in the form of heat. Pressurized water is pumped through the reactor to take the heat away, and the hot gas then heats water to make steam. The steam drives turbines which drive generators. 52 Control rods(made of boron or graphite) are to absorb extra neutrons in order to control the rate of nuclear reaction. If the reactor gets too hot (nuclear reaction is too fast), the control rods are lowered into the reactor to absorb neutrons. Hence, the reaction rate is decreased. If the reaction is slow down, the control rods are raised. More neutrons crash into uranium atoms and hence more energy is generated.55
Advantage of using nuclear power
It is relatively not expensive to build a nuclear power plant. Produces huge amounts of energy from small amounts of fuel. Does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect. Small amounts of waste are produced. 56Disadvantage of using nuclear power
Radioactive waste is produced. It must be sealed up and buried for many years to allow the radioactivity to die away. Terrorism may increase in scale. Reactors could be a target being attacked with bombs. Much effort (e.g., money) is paid for safety measures. If it goes wrong, a nuclear accident can be a major disaster. 5710. Renewable energy sources
Solar energy is free, renewable and available to everywhere in the world. No waste or pollution is produced, few environmental problems are created. Exempt from rising energy prices. Good for remote locations.Solar power
58 However, Produces a small energy output per surface area of solar cell. Not good for largescale production. It requires thousands of mirrors or cells that take up a large area of land. It’s relatively expensive to build solar power stations. Does not work at night or in a cloudy day. 59Hydroelectric power
The efficiency is high and the running costs are low. No waste or pollution produced. More reliable than other renewable energy resources such as wind or solar power. Water can be stored above the dam ready to cope with peaks in demand. 60 However, Very expensive to build a dam, but many dams are also used for flood control, so building costs can be shared. Building a large dam will flood a very large area upstream, causing problems for animals that used to live there. Water quality and quantity downstream can be affected, which can have an impact on plant life.61
