ELECTRICITY AND MAGNETISM

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ELECTRICITY AND MAGNETISM

• BY Tanisha Alexander
• 5A

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ELECTRICAL QUANTITIES
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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.

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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.

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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

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DIFFERENT FORMS OF ENERGY.

• Mechanical energy consisting of potential energy
  and kinetic energy.
• Gravitational potential energy.
• Elastic Potential Energy.

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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.

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ENERGY TRANSFORMATIONS

• EXAMPLE
• List four energy transfers that will take place
  at an oil burning power station.
• SOLUTION
• Chemical energy electrical energy
  mechanical energy light and heat energy

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Electromagnetic radiation the
photovoltaic cell electrical energy
Chemical energy the internal
combustion engine mechanical energy chemical
energy cells and batteries
electrical energy elastic potential energy
a catapult kinetic energy
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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).

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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.

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RELATIONSHIP BETWEEN VOLTAGE AND ENERGY

• Energy (E) charge (Q) x voltage (V)
• Energy current x time x
  voltage
• (joules)(amperes) x (seconds)
  (volts)
• E Q x V
• Therefore
• Q E/V
• And
• V E/Q

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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

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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

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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.

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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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THANK YOU FOR VIEWING