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Energy is the ability to do work or make a change.
4. A body which is moving has kinetic energy.
5. Raise a book & put it on a high shelf. Your energy is stored in the book. The stored energy is potential energy.
When the book falls down, its potential energy is converted into kinetic energy.
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Mechanical energy = potential energy only.
Mechanical energy of the ball at B = Mechanical energy only.
Potential energy increases when the objects weight or height increase.
Potential energy = weight x height.
Example: compare the potential energy of 2 balls, the weight of each is 1 Newton & the first is placed at a height of 1m & the 2nd is placed at a height of 2 m.
Solution:
Potential energy (1) = 1 Newton x 1 m
= 1 Joule
Potential energy (2) = 1 Newton x 2 m
= 2 Joules
Potential energy of the 2nd object is double the potential energy of the 1st object.
Kinetic energy increases if the mass or the speed of the moving object increase.
The following rule calculates kinetic energy .
A solved example:
Calculate the kinetic energy of a ball that has
a mass =0.5 kg & moves with a speed 3 m/s.
Solution:
Kinetic energy = ½ x 0.5 x (3)2
= 2.25 Joules.
I. Choose the correct answer:
1. An object of 20 N weight is placed on a shelf of height 5 m. its potential energy = .J
(50 100 150)
2. A truck & a car move with the same speed. The car has kinetic energy than/ as truck.
(less more the same)
3. The ..energy of water at the top of waterfalls is maximum.
(kinetic potential mechanical)
4. 
The kinetic energy of a falling ball
gradually.
(doesnt change increases decreases)
5. An object is thrown upwards, therefore its velocity ...
(increases decreases doesnt change)
II. Solve the following problems:
1. Calculate the mechanical energy of a book whose weight = 10 Newton & is placed on a shelf 3m high.
2. Calculate the mechanical energy of a ball of weight 3 Newton , its mass = 0.3 kg , its thrown up with speed 3 m/s to a height 4m.
A pendulum is a ball tied with a string & fixed at a point. When the ball is pushed, it moves to the right & then back to the beginning point, then to the left.
 |
A B
At points A&B , the potential energy of the pendulum is maximum.
As it moves away from point A, its energy is changed to kinetic energy.
The mechanical energy of the pendulum is the same at each point.
Forms of energy:
| Forms of energy | Sun |
| 1. Light | Sun |
| 2. Sound | Bell Piano |
| 3. Chemical | Fuel Battery Food |
| 4. Heat | Heater |
| 5. Kinetic | A moving car wind |
| 6. Potential | Stretched bow to through an arrow |
| 7. Electric | Produced by an electric generator |
| 8. Nuclear | Released by a nuclear bomb |
Activity 1:
Simple cell
1. In a glass beaker, put 2 metal plates separately
(zinc & copper).
2. Fill half the beaker with diluted acid.
3. Connect the 2 plates externally with an electric current.
4. Put a voltammeter in the circuit to measure the electric current.
Ø Observation :
The pointer of the voltammeter moves to indicate the electric current is passing through the circuit.
Ø Conclusion:
Chemical energy in the simple cell is transformed into electric energy.
Activity 2:
Prepare an electric circuit like the one in the figure, close the circuit using the switch , open the circuit using the switch.
Ø Observation :
The bulb light up when the circuit is closed. The bulb is turned off when the circuit is open & electricity isnt passing in the circuit.
Ø Conclusion:
In the electric bulb (lamp) electric energy is converted into light energy.
Energy is transformed from one form to another as in the following table.
| Device | Energy used | Energy produced |
| 1. Electric bell | Electric | Sound |
| 2. Burning fuel in a car | Chemical | Mechanical & heat |
| 3. Electric generator (dynamo) | Mechanical(kinetic) | Electric |
| 4. Radio | Electric |  Sound
|
| 5. Electric heater | Electric | Heat |
Modern technology has many harms, some are mentioned in the following table.
| Technology | Harms |
| 1. Transportation vehicles | Their exhaust pollutes the air. Breathing polluted air harms humans health . |
| 2. Chemical pesticides | They are sprayed to kill harmful insects but harm useful ones as well. They also pollute the air. |
| 3. Nuclear weapons used in war. | a. Spread harmful radiations that harm humans health. b. Kill a large number of humans. |
| 4. Cell phone transmission towers. | Their effect on health is being studied . |
 |
Nuclear weapons
I. Fill in the blanks:
1.
| Device | Energy it uses | Energy it produces |
| 1. Simple cell | ||
| 2. Electric motor | ||
| 3. Solar batteries cell in your calculator | ||
| 4. Car engine | ||
| 5. | Electric | Light |
| 6. | Electric | Sound |
2.
| Device | Energy used | Energy produced |
1.  Television
| ||
2.  Cellular phone (mobile)
| ||
3.  Sewing machine
| ||
4.  Alarm clock
|
II.
|
Choose the correct answer:
1. The mechanical energy of the ball at position A =
(kinetic energy only potential energy only kinetic energy x potential energy)
2. The work done to raise a book to a high shelf is stored in the book as energy.
(heat potential chemical)
3. The dynamo produces energy.
(chemical kinetic electric)
III. Write the scientific term:
| 1. Energy is neither created nor destroyed. | |
| 2. The sum of potential energy & kinetic energy is constant. |
IV. Explain some of the harms of using nuclear weapons in wars.
V. Use the opposite figure to fill in the blanks.
1. The kinetic energy = the potential energy at position(s) .
2. The kinetic energy is maximum at position(s) .
3. The potential energy is maximum at
1. Ice cubes are placed in a glass of juice to make the juice cooler.
Heat is transferred from the high temperature juice to the lower temperature ice. The ice melts & the juice is cooler.
2. Rub your hands together & notice that they become hot. As the speed of rubbing increases so does the heat energy.
3. 
When the cyclist presses the brakes to stop the bicycle, the tyres rub against the ground & become heated friction produced heat energy.
How is heat transferred through different mediums ?
1. Through solids:
Ø 
Leave a metal spoon in a cup of hot tea. Touch the end of the spoon outside the cup. It feels hot because heat was transferred in the spoon by conduction.
2. Put some tea dust in a beaker that contains water. Heat the beaker with Bunzen burner. Observe: how tea dust moves up then falls down in circles.
Ø Conclusion:when heated, the molecules of liquids move up & are replaced by cooler water, falling down. This way of heat transfer is called convection.
Heat is transferred in liquids & gases by convection.
3. Heat is transferred from the Sun through space to the Earth by radiation.
Air conditioners are placed in a high place in the room because cold air is denser than warmer air, therefore cold air moves downwards. Convection distributes cold air in the room.
Heaters are usually placed on the floor because the density of hot air is low, therefore hot air rises up. The heat is distributed in the room by convection.
Date: 2015-12-24; view: 1220
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| To complete h. to let someone have or do something | | | Some domestic devices which produce heat are listed in the following table. |