Download introduction to electrical quantities

ELECTRICITY AND
MAGNETISM
BY: Tanisha Alexander
5A
ELECTRICAL
QUANTITIES
POWER P AND ENERGY,
E OR W
• Energy is the ability of an object to
do work, therefore work is done or
things happen whenever energy
transfers take place.
• Energy and Work are measured in
Joules (J).
• The mathematical equation for work
is: Work = Force x displacement.
EXAMPLES OF SITUATIONS
WHERE WORK IS DONE OR
ENERGY IS NEEDED.
• A Shopper pushing a cart down the
aisle in a supermarket.
• An athlete throwing a javelin.
• A police officer chasing a thief.
• A mother chasing after her baby
• N.B: No work is done when a boy
pushes a wall because there is no
displacement in this situation.
BRIEF EXAMPLE AND SOLUTION OF A
QUESTION USING THE
MATHEMATICAL FORMULA FOR
WORK.
• 1)
A 30N force is applied horizontally
causing a block to be displaced
horizontally 5m along a frictionless floor.
How much work is done?
• SOLUTION:
• W = F x d cos^ = F x d, since ^ = 0
–
= (30 x 5) Nm = 150J
DIFFERENT FORMS OF
ENERGY.
• Mechanical energy consisting of
potential energy and kinetic
energy.
• Gravitational potential energy.
• Elastic Potential Energy.
PRINCIPLE OF
CONSERVATION OF
ENERGY.
• The Principle of Conservation of
Energy states that energy can
neither be created nor
destroyed but can only be
transformed from one form to
another.
ENERGY
TRANSFORMATIONS
• EXAMPLE:
• List four energy transfers that will take place at an
oil burning power station.
• SOLUTION:
• Chemical energy
mechanical energy
electrical energy
light and heat energy
Electromagnetic radiation
electrical energy
the photovoltaic cell
Chemical energy
the internal combustion engine
mechanical energy
chemical energy
electrical energy
cells and batteries
elastic potential energy
energy
a catapult
kinetic
POWER
• Power is work done or energy converted per unit
time.
• Power = force x displacement/ time taken = energy
transformed/ time taken.
• Power is measured in watts (W)
• Large amounts of power are measured in kilo
watts (kW) OR megawatts (MW).
VOLTAGE
• Voltage measures the work done by a battery or
other sources in driving 1 coulomb of charge
between two points in an electric circuit.
• The unit for voltage is the volt.
• The volt is defined as one joule per coulomb.
RELATIONSHIP BETWEEN
VOLTAGE AND ENERGY
Energy (E) = charge (Q) x voltage (V)
Energy = current
x
time
x voltage
(joules)=(amperes) x (seconds)
(volts)
E=QxV
Therefore:
Q = E/V
And
V = E/Q
RELATIONSHIP BETWEEN
VOLTAGE AND POWER.
• Power is the rate of energy transfer or the rate at
which work is done.
• P = E/t = (Q x V)/t = (I x t x V)/t = IV
• Therefore:
• P = VI
WORKED EXAMPLE TO
DEMONSTRATE BOTH
RELATIONSHIPS.
• The starting motor of a car draws a current of 60
amperes from a 12 volt battery for 7 seconds.
Calculate a the charge flowing, b the power
dissipated and c the energy transferred.
• Solution:
• A. charge = current (A) x time (s) = (60 x 7) As =
420C
• B. power = current x voltage = (60 x 12)W = 720 W
• C. energy = power x time = 720 x 7 =5040 J
THE NEED FOR REDUCING WASTAGE
OF ELECTRICAL ENERGY SOME
GUIDELINES THAT CAN HELP.
• There is limited reserves of fossil fuels, hence
there is the need to conserve.
• Use fluorescent lamps rather than incandescent
lambs, since less electrical energy is required.
• Use solar rather than electrical or gas fired water
heaters to conserve electrical energy.
• High efficiency refrigeration and a/c units.
QUIZ
• 1) Define energy?
• 2) What is the mathematical equation for work
done?
• 3) List the three main types of energy.
• 4) State the Principle of Conservation of Energy.
• 5) Define power, voltage and the volt.
• 6) List the energy transfers that will take place at a
hydroelectric plant.
• 7) A 20N force is applied horizontally causing a
block to be displaced horizontally 6m along a
frictionless floor. How much work is done?
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