Physics Kenya
Work, Energy, Power and Machines
Work, Energy, Power and Machines
Work, Energy, Power and Machines
Forms of energy and energy transformations
The Concept of Energy
Energy can be defined as capacity of doing work. It is the ability of doing work. In order for work to be done there must be a property which enables the act of doing work.
A person cannot move a box without having enough energy to do the work. Likewise, it is not possible for a car to move from one place to another without possessing some amount of energy to enable its movement.
S.I Unit of Energy
Energy and work done are closely related because energy is the property which make it possible for a certain work to be done. Therefore, Energy has the same SI unit like that of work, and that is Joules (J).
Different Forms of Energy
There are different forms of energy such as:
- Chemical energy
- Heat energy
- Light energy
- Sound energy
- Electrical energy
- Nuclear energy
- Solar energy
Difference between Potential Energy and Kinetic Energy
Potential energy and Kinetic energy are two forms of mechanical energy. Mechanical energy is the kind of energy which is associated with the position and motion of a body.
Potential energy – this is the energy possessed by the body due to its position. Its SI unit is Joules.
Potential energy of the body depends on its mass m, gravitational strength g, and the height h.
Suppose a body of 5 kg is at 2m above the ground and left to fall freely, the potential energy possessed by it before falling down is;
Kinetic energy – is the energy possessed by the body due to its motion. Its SI unit is Joules.
Kinetic energy of the body depends on its mass m and velocity, v.
A ball weighing 2kg kicked to attain velocity of 20 m/s possesses the Kinetic energy of;
The Transformation of Energy
The notion of energy is that energy is changed from one form into different forms using transducers.
Transducer is a device used to transform energy from one form to another.
For example:
- Battery converts chemical energy into electrical energy.
- A generator converts mechanical energy into electrical energy.
- A motor converts electrical energy into mechanical energy.
| ORIGINAL ENERGY | TRANSDUCER | ENERGY OBTAINED |
| Chemical energy | Battery | Electrical energy |
| Electrical energy | Motor | Mechanical energy |
| Chemical energy | Generator | Electrical energy |
| Solar energy | Solar panel | Electrical energy |
| Chemical energy | Combustion engine | Mechanical energy |
| Sound energy | Microphone | Electrical energy |
| Electrical energy | Heater | Heat energy |
| Electrical energy | Speaker | Sound energy |
Activity 1
To demonstrate pressure of potential energy.
Materials and Apparatus
- A heavy stone
- A bucket full of water
- A strong inelastic rope; and
- Smooth pulley
Procedures:
- Collect the heavy stone, using a strong rope tie it to a bucket of water
- Pass the rope over smooth pulley fixed to a support.
- Hold stationary the heavy stone at height “h” above the ground
- Release the stone
Results and observations:
- When the stone released the bucket of water will start to rise.
- The stone is said to have potentials energy because of its position above the ground.
- Lifting a body of mass “m” to a height “h” above the ground requires work to be done against gravity. Work = Mgh
Example 1
A ball of mass 0.5 kg is kicked vertically upwards and rises to a height of 5m. Find the potential energy by the ball.
- Data given:
- Mass of the ball (Mb) = 0.5 kg
- Height (h) = 5m
- Gravitation force (g) 10N/kg
- Potential energy (P.E) = ?
- Solution:
- Potential energy (P.E)= mgh
- = 0.5kg x 10N/kg x 5m = 25 NM
- 1NM= 1Joules
- Potential energy (P.E) = 25 Joules.
Activity 2
Aim: to investigate the law of conservation of energy by a simple pendulum.
Materials and Apparatus:A pendulum bob and light inelastic string.
Producers
- Pull the bob of a simple pendulum in position A so that it is at height “h” above position B.
- Release the bob so that it swings to position C via the lowest position B and back to A.
- Consider the figure below:
Observation
When the bob is at position A, it possesses potential energy only due to the height “h” which is equal to “Mgh”.
As it swings downwards to position B, the height decreases, and as the result it loses potential energy.
The bob has Vmax and hence K.Emax at B. The height at B is zero, thus the P.E is zero.As it swings towards C, the P.E increases and reaches its maximum again in position C, where the Kinetic Energy is zero. At position D, the energy of the bob is party potential and party Kinetic.
Sources of energy: Renewable
Sources of energy: non renewable
Law of conservation of energy
Work, energy and power
The Concept of Power
Power is defined as the rate of which work is done. It measures the rate at which energy has been consumed. It is the measure of the rate at which the energy is changing.
The S.I Unit of Power
The SI unit of power is Joule per second (J/s) which is also known as Watts (W).
Other units of power are kilowatts (kW), megawatts (MW) and horsepower (hp).
Watt is famously used for measurement of electrical power.
Kinetics and potential energy
Simple machines
Problems on work, energy, power and machines
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