1. Lecture 17 Changing Forms of Energy Ozgur Unal
NIS – PHYSICAL SCIENCE
Lecture 17
Changing Forms of Energy
Ozgur Unal

2. Changing Forms of Energy
Consider a hair dryer.
What forms of energy are involved in it?
In a hair dryer, electrical energy is converted into heat, kinetic energy, sound energy etc.
Electrical energy being used comes from power plants.
Examples of power plants: Thermal power plants, nuclear power plants, hydroelectric power plants etc.

3. Changing Forms of Energy
More examples of energy transformation:
Light bulbs:
Burning fuel:

4. Conversion Between Mechanical and Potential Energy
Potential energy (PE) and kinetic energy (KE) can be converted to each other.
The GPE of a falling object is converted into KE.
When you stretch a rubber band and let it go, you convert elastic PE into KE.

5. Mechanical Energy
In order to understand the conversion between KE and PE, it is helpful to identify the mechanical energy of a system.
Mechanical energy = Potential energy + Kinetic Energy
ME = KE + PE
In other words, mechanical energy is energy due to the position and the motion of an object or the objects in a system.

6. Mechanical Energy Falling objects:
As an object falls down, its PE gets smaller and its KE increases.
In other words, PE is converted into KE.
How about ME? Does it get smaller? Does it increase?
Total ME does not change. It stays the same!

7. How is the energy transformation in projectile motion?
As the object rises up its speed (KE)
decreases, however due to the
increased height the PE gets bigger.
Mechanical energy stays the same!

8. Energy Transformation in a Swing
Consider the pendulum shown to you.
Analyze the energy transformation taking place in the pendulum.

9. Lecture 18 The Law of Conservation of Energy Ozgur Unal
NIS – PHYSICAL SCIENCE
Lecture 18
The Law of Conservation of Energy
Ozgur Unal

10. Law of Conservation of Energy
If we ignore the effects of friction, the mechanical energy of an object or a system of objects is constant.
This is true for other forms of energy.
Energy can change from one form to another, but the total amount of energy never changes.
The law of conservation of energy states that energy cannot be created or destroyed.
In other words, the total amount of energy in the universe does not change.

11. Law of Conservation of Energy
Does the law of conservation of energy hold every time?
Is there any exceptions to this law?
Consider the motion of the ball.
Inıtially it has kinetic energy, because it moves. But eventually it stops.
Where does the energy go? Is it being destroyed?
Similar to the fading motion of the bob in a pendulum, the ball’s KE is transformed into thermal energy due to friction.
Bend the copper wires continuously for a minute, then touch the wire by your lowe lips and sense the thermal energy accumulated in the wire.
DO NOT SWALLOW THE WIRE!

12. Law of Conservation of Energy
Example: Consider a simple pendulum. The bob in the pendulum has a mass of 2 kg. It is released to swing from a height of 25 cm from the lowest level. What is the maximum speed of the bob during its motion?
Example: A 4kg-object is thrown up from a height of 10 meters. If the initial speed of the object is 9m/s, what is the speed of the object when it reaches the ground?

13. Energy Conservation in Food Chain
How do living things get the energy they need?
Where does that energy come from?
Consider the food chain below:
Grass > Rabbit > Fox
The radiant energy (sunlight) coming from the Sun is used by plants.
Consumers get the energy they need by eating plants and other consumers.
Energy is transformed and conserved in a food chain!
How about the Sun?
Is it a limitless souce of energy?
Is energy created in the Sun?

14. Converting Mass into Energy
Have you every seen
this person before?
Have you every seen this equation?
E = m * c2
What does this equation tell you?
c is the speed of light, c = 3 x 108 m/s
Mass can be converted into energy!!
Matter and energy mean the same thing.
This is what fuels the Sun.
Let’s take a closer look at what’s going on in the Sun.

15. Nuclear Reactions
In a type of nuclear reaction, called nuclear fusion, nuclei of atoms fuse together to form heavier nuclei.
The mass difference
between the reacting
nuclei and the newly
formed nuclei is
converted into energy.
Sun is fueled by the nuclear fusion of Hydrogen nuclei into Helium nucleus.

16. Nuclear Reactions There is nuclear reaction, called
nuclear fission, in which a heavy
nucleus breaks apart to form smaller
nuclear by releasing enormous
amounts of energy.

17. Nuclear Reactions
In both nuclear reactions, nuclear fusion and nuclear fission, the total energy is conserved.
To see this, you need to consider the energy equivalent of mass according to
Einstein’s famous
formula.

18. Nuclear Reactions
Example: 4 H nuclei fuse to form a He nuclei in the process called nuclear fusion. The masses of H nucleus and He nucleus are x kg and x kg, respectively. What is the amount of energy released in this reaction?
Energy equivalent of one H nucleus: EH = mH * c2
Energy equivalent of one He nucleus: EHe = mHe * c2
Total energy before reaction: 4*EH = 4*mH * c2
Total energy after reaction: EHe + Ereleased = mHe * c2 + Ereleased
Total energy is conserved:
Ebefore = Eafter
4*mH * c2 = mHe * c2 + Ereleased
Ereleased = c2(4mH - mHe)