Energy, Work & Power: Types of Energy• The following are some examples of

types of energy:• Internal energy• Gravitational potential energy = mgh• Kinetic energy = ½mv2

• Heat energy• Electrical energy• Magnetic energy• Chemical Potential energy• Solar energy• Nuclear energy• Geo-thermal energy

Internal Energy

• This is the sum of Potential Energy and Kinetic Energy of the molecules.

• Potential Energy of the molecules is related to the energy needed to separate the molecules.

• Kinetic Energy is related to the motion of the molecules and is temperature dependent.

Internal Energy• Example:• During the change of state, the kinetic

energy of the molecules remain constant (temperature constant).

• So the heat energy absorbed is used to separate the molecules thus increasing the potential energy of the molecules.

• (Change of state refers to changing from solid to liquid or from liquid to gaseous state. If the change is reversed, then energy is given out.)

• Kinetic Energy (KE)– energy due to the motion of an object

• KE = ½mv2

• Where m = mass, v = velocity• Unit = J• NOTE: • Energy is a scalar quantity

Kinetic Energy

• Gravitational Potential Energy (GPE) – energy possessed by an object due to its height (usually with reference to the earth’s surface)

• GPE = mgh

• where m = mass; g = acceleration due to gravity; h = height

• Unit of energy is the joule, symbol J

Gravitational Potential Energy

• Most of the time heat or thermal energy is wasted in the process of doing work.

• It is usually lost to the surroundings.

• Heat or thermal energy is only useful when it is directly used to heat up substances or objects

Heat or Thermal Energy

Law of Conservation of Energy

• Law of Conservation of Energy states that energy cannot be created or destroyed. It can only be converted from one form to another.

• That is the total energy of a system is always constant.

Example:• In the previous physics practical:

h

• The marble initially have potential energy = mgh (m = mass of marble, g = acceleration due to gravity and h = height above the bench)

• When the marble roll down the ruler it has kinetic energy = mv2 where v = final speed of the marble

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2

• It was found that the initial energy (Gravitational potential energy, mgh) is not equal to the final energy (kinetic energy, mv2)

• The reason being there is some energy lost in terms of heat energy and sound.

• So Gravitational potential energy is transformed to kinetic energy + heat energy + sound energy

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Work Done

• All energy can be used to do work.

• Work is done when there is motion in the direction of the force applied to the object.

• Work done is defined as the magnitude of the force acting on the object multiply by the distance moved by the object in the direction of the force.

Work Done

• Definition:• Work Done = Force x distance• Note: The Force must be in the same

direction as the distance moved

• Unit of work is the same as that of energy, the joule (J)

• Unit in base quantities is kg m2 s-2

• Work done is a scalar quantity.

Work Done• Concept:

• Work is done by the force acting on the object only if the object moves in the direction of the force.

• The following are some examples of no work being done although energy is used.

A person pushing on a wall does no work because the wall does not move.

Examples 1:

Work Done

Force exerted by man on the wall does no work

Work Done• Here is another example of no work done.

Here although the man move but his direction of motion is perpendicular to the force applied so no work is done by the force applied.

Direction of motion perpendicular to the direction of the applied force.

Force applied by man to carry object does no work because the object does not move in the direction of the force.

Work Done

Force, FForce, F

Distance, d, moved by the object.

Work Done = F x d

Work is done only if the force F moves the object in the direction of F. If the distance moved is d, then

Work Done

F

F

Distance, d, moved by the object.

If F is inclined at an angle to the horizontal then work done by F is Work Done by F = F cos

x d.

Note: F sin does no work. Why?

F cos

F cos

F sin

F sin

Power and Efficiency• Power P, is defined as the rate of doing

work or work done per unit time.

• P = Work Done/time

• Unit of power is the watt (W).• 1 W = 1 J/s• Power is a scalar quantity.

Power and Efficiency• Efficiency measures the amount of work

obtained against the the amount of work put in.

• This applies particularly to machines.• The amount of work put in is always not

equal to the amount of work obtained from a machine as there is no such thing as a 100% efficient machine.

• Efficiency = (Power Out/Power In) x 100% • = (Work Out/Work In) x 100%