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 electrical energy mechanical energy light and heat energy

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

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 = Q x V 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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