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Energy Conversion and Conservation

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- You will be able to identify and describe conversions from one type of energy to another.
- You will be able state the law of conservation of energy.

The change from one form of energy to another.

Forms of Energy

Chemical

Sound

Radiant

Electrical

Mechanical

Magnetic

Thermal

Nuclear

- Often a series of energy conversions is needed to do a task.
- For example, Strike a match:
- The mechanical energy used to scratch the match is converted to thermal energy.
- The thermal energy causes the match to release stored chemical energy
- The chemical energy is then converted to thermal energy and to the energy you see as light.

- In a car engine another series of conversions occurs.
- Electrical energy produces a hot spark.
- The thermal energy of the spark releases chemical energy in the fuel.
- When the fuel burns, this chemical energy in turn becomes thermal energy.
- Thermal energy is converted to mechanical energy used to move the car, and to electrical energy that produces more sparks.

- One of the most common conversions is the conversion of potential energy to kinetic energy.
- When you stretch a rubber band, you give it elastic. potential energy.
- If you let it go, the rubber band flies across the room.
- When the rubber band is moving, it has kinetic energy.
- The potential energy of the stretched rubber band is converted to the kinetic energy of the moving rubber band

- Any object that rises or falls experiences a change in its kinetic and gravitational potential energy. Look at the orange in the figure.
- When it moves, the orange possesses kinetic energy.
- As it rises, it slows down. Its kinetic energy decreases. But because its height increases, its potential energy increases.
- At the highest point in its path, it stops moving. At this point, it no longer possesses kinetic energy, but it possesses potential energy.
- As the orange falls, the entire energy conversion is reversed—kinetic energy increases while potential energy decreases. potential energy to kinetic energy.

- There is a conversion between potential and kinetic energy on a large scale at Niagara Falls.
- The water at the top of the falls has gravitational potential energy because it is higher than the bottom of the falls.
- But as the water falls, its height decreases and so it loses potential energy.
- At the same time, its kinetic energy increases because its velocity increases.
- Thus potential energy is converted into kinetic energy.

- As a pole vaulter runs, he has kinetic energy because he is moving.
- When he plants his pole to jump, the pole bends. His kinetic energy is converted to elastic potential energy in the pole.
- As the pole straightens out, the vaulter is lifted high into the air. The elastic potential energy of the pole is converted to the gravitational potential energy of the pole vaulter.
- Once over the bar, the vaulter’s gravitational potential energy is converted into kinetic energy as he falls to the safety cushion below.

- A continuous conversion between kinetic energy and potential energy takes place in a pendulum.
- At the highest point in its swing, the pendulum in the figure has only gravitational potential energy.

- As the pendulum starts to swing downward, it speeds up and its gravitational potential energy changes to kinetic energy.
- At the bottom of its swing, all its energy is kinetic energy.
- Then, as it swings to the other side and slows down, it regains gravitational potential energy, and at the same time loses kinetic energy.
- At the top of its swing on the other side it again has only gravitational potential energy. And so the pattern of energy conversion continues.

- If you set a pendulum in motion, do you think it will remain in motion forever?
- No, it will not. Does that mean that energy is destroyed over time?
- The answer is no.
- The law of conservation of energy states that when one form of energy is converted to another, no energy is destroyed in the process.

- So the total amount of energy is the same before and after any process. All energy can be accounted for!!

- So what happens to the kinetic energy of the pendulum?
- As the pendulum moves, it encounters friction at the pivot of the string and from the air through which it moves.
- When an object experiences friction, the motion (and thus the kinetic energy) of the atoms or molecules increases.
- This means its thermal energy increases. So the mechanical energy of the moving pendulum is converted to thermal energy.
- The pendulum slows down, but its energy is not destroyed.

The fact that friction converts mechanical energy to thermal energy should not surprise you. After all, you take advantage of such thermal energy when you rub your cold hands together to warm them up. The fact that mechanical energy is converted to thermal energy because of friction explains why no machine is 100 percent efficient. Now you know that mechanical energy is converted to thermal energy in a machine.

- You might have heard of Albert Einstein’s theory of relativity. Einstein’s theory included a small change to the law of conservation of energy.
- He explained that energy can sometimes be created—by destroying matter!
- This discovery means that in some situations energy alone is not conserved.
- But scientists say that matter and energy together are always conserved.
- Just as different forms of energy can be converted to one another, matter can sometimes be converted to energy.

- Calculate the Potential Energy:
- A 250 N book bag raised 5.6m off the ground.
- Formula to use: PE= w*h
- PE= 250N*5.6m
- PE= 1400 Joules

- Formula to use: PE= w*h
- A 28.6 N object being held 128cm above the floor.
- Formula to use: PE= w*h
- Remember 128cm equals 1.28m
- PE= 28.6N*1.28m
- PE= 36.608 Joules

- Formula to use: PE= w*h
- A 629 Kg car at the top of a small hill, 4,122mm tall.
- Formula to use: PE= mgh
- Remember 4,122mm equals 4.122m
- PE= 629Kg * 9.8m/s2 * 4.122m
- PE= 25,408.8324 Joules

- Formula to use: PE= mgh