CHAPTER 4

Work, Energy, and Power

WORK

Work is done when a force moves an object to which it

acts in the direction of the force

Work = force x distance moved by force in the direction of

the force

Unit of work = Joule (J) or (force in newton x distance

moved in metre)

WORK DONE BY EXPANDING GAS

As gas expands, it does work by breaking down the masonry.

Work done = pressure x change in volume

When gas expands, work is done by the gas. If the gas contracts,

then work is done on the gas.

Unit of work (J) = pressure (pascals/Pa) x changes in volume (m3)

ENERGYEnergy is the ability to do work

Energy Notes

Potential energy Energy due to position

Kinetic energy Energy due to motion

Elastic or strain energy Energy due to stretching of an object

Electrical energy Energy associated with moving electric charge

Sound energy A mixture of potential and kinetic energy of the particles in the wave

Wind energy A particular type of kinetic energy

Light energy Energy of electromagnetic wave

Solar energy Light energy from the sun

Chemical energy Energy released during chemical reaction

Nuclear energy Energy associated with particles in the nuclei of atoms

Thermal energy Sometimes called heat energy

ENERGY

Potential energy is the ability of an object to do work

as a result of its position or shape

Work done = force x distance moved

= mxgxh

m- mass g- gravitational force

h- height of the distance moved

ENERGY

Kinetic energy is energy due to motion.

Ek=½mv2

m- mass v- velocity

ENERGY CONVERSION AND CONSERVATION

Law of energy conversation:

Energy cannot be created or destroyed. It can only

be converted from one form to another.

EFFICIENCY

Efficiency gives measure of how much total energy may be

used and is not ‘lost’

Efficiency = useful work done ÷ total energy input

DEFORMATION OF SOLIDS

Deformation – change of shape

Tensile – stretching of an object

Compressive- pressing of an object

Deformation

tensile

compressive

HOOKE’S LAW

Hooke’s law stated that proved the elastic limit is

not exceeded, the extension of a body is proportional

to the applied load.

F= kΔL

F- force k – elastic constant (Nm-1)

ΔL- extension

STRAIN ENERGY

Strain energy is the energy store in a body due to

change of shape

Strain energy W= ½k(ΔL)2

or

W= ½kx2

THE YOUNG MODULUS

Young modulus is the constant that a particular

material has that enable us to find extensions

knowing the constant and the dimension of the

speciment.

Young modulus is = Stress ÷ Strain

STRAIN

Strain is the ratio of two lengths, the extension and

the original, and thus it does not have unit.

Stress= extension ÷ original length

STRESS

The strain produced within an object is caused by

stress.

Tensile stress is the changes in length of the object

Stress= Force ÷ area normal to force

The unit of stress is (Nm-2) also known as Pascal

(Pa)

SPECIFIC HEAT CAPACITY

Specific heat capacity is the numerical value which

a substance needed to raise the temperature of unit

mass of substance by one degree.

Q= mcΔt

Q- heat m- mass c- specific

heat capacity Δt- temperature change

THERMAL CAPACITY

Thermal capacity is the numerical value of a body

needs to raise the temperature of the whole body by

one degree

Q=CΔt

Q- heat energy C- thermal capacity

SPECIFIC LATENT HEAT

Specific latent heat is the numerical value of the

quantity of heat energy required to convert unit

mass of solid to liquid (fusion) or liquid to gas

(vaporization) without any change in temperature.

Q=mL

L – specific latent heat

EXCHANGES OF HEAT ENERGY

Law of conservation of energy stated that energy

applies in heat energy gained by the colder object is

equal to heat loss by the hotter object.

Energy gained= Energy lost

POWER

Power is the rate of doing work and it’s a scalar

quantity

Power = work done ÷ time taken

Power= force x speed

Unit of power is watt

KILOWATT HOUR

One kilowatt hour is the energy expended when

work is done at the rate of 1 kilowatt for a time of 1

hour.

MOMENT OF A FORCE

The turning effect of a force is called moment of

force

The moment of force is defined as the product of

the force and the perpendicular distance of line of

action of the force from the pivot.

COUPLES

A couples consist of two forces equal in magnitude

but opposite in direction of whose lines of action do

not coincide.

The torque of a couple is the product of one of the

forces and the perpendicular distance between the

forces.

PRINCIPLE OF MOMENT

The principle of moment stated that for a body to

be in rotational equilibrium, the sum of the clockwise

moment about any point must equal the sum of

anticlockwise moment about the same point.

CENTRE OF GRAVITY

The centre of gravity of an object is the point at

which the whole weight of the object may he

considered to act.

EQUILIBRIUM

Equilibrium :• Sum of all forces in any direction must be 0• Sum of moment of the forces about any point must

be 0

RESOURCES

International A/AS Level Physics by chris mee,

mike crundell, brian arnold, and wendy brown,

published at 2008.

THE END

Jessica L

Grade 11 - Sci