electricity .pdf

Integrated Science Notes By Muhamad Nabeel Uddin

1 Copyright © Muhamad Nabeel Uddin®

CSEC Integrated Science All rights reserved

Electricity

Fuse/circuit breakers

It is a safety device that switches off an appliance if too much current flows through it. If too

much current if flowing through a fuse, the thin wires will melt (fuse blow) which turns off the

appliance.

Colour coding of fuse

3 Amps- Red

5 Amps- Black/Or dark blue

13 Amps- Brown

Wiring a plug

There are two types of plugs generally

(a) Two pin plug: these have a live and neutral wire.

(b) Three pin plug: has a live and neutral wire, as well as an earth wire. The earth wire is

a safety device. It prevents shock, and damage to devices if too much current builds

up.

Colour code of flex (complete the table below)

Flex colour Joined to

Brown

Blue

Green-Yellow

Overloading

If too many electrical appliances of high power ratings are switched on at the same time, they

will draw an extremely large amount of current from the circuits. This may cause overheating of

the wires and eventually lead to a fire if the insulation around the wire melts.

NB: Thick wire should be used for heavy duty machines and overhead wire cables. Suggest a

reason for this.




Power

The unit used to measure power of electrical devices is called watt. It is the amount of energy

that is supplied in a certain time.

Power (watts) = Current (A) * Voltage (V)

P= IV (NB: you should know how to change subject of formula)

Calculating amperage for fuse

Complete the table below

Appliance power (w) Mains voltage (v) Current (amps) Fuse you will use

1000 110

500 110

100 210

300 220

160 240

Energy consumption

Energy consumption (Joules) = Power (watts) * Time (seconds)

W= Pt

Q1. An Appliance had a power rating of 300 watts, and uses 120 v. Calculate the

(a) Current that the appliance will need (2.5A)

(b) Energy consumed in 20 mins. (360 J)

Q.2. A torch bulb is labelled 2.5 v, and used 0.3 A. Calculate

(a) Power of the bulb

(b) Energy consumed in 10 minutes




Energy consumption of electrical appliances (page 161 of text)

Energy consumption/transfer used by electrical appliances at home is measured in KWh

(KiloWatt-hours).

Energy used = Power in KW * Time in hrs

Cost = Number of units used (KWh)* price per each unit

Example: An electrical lamp needs 2 KW. It is switched on for about 3 hours. If was used for 4

hours. If GPL charges $45 per each KWh, how much does it cost to use the lamp?

Solution:

Energy used = 2KW * 3 hours

= 6 KWh

Cost = 6KWh * $45

= $ 270

Q1. A television needs 250 watts. It is switched on for 30 minutes. If each KWh costs $40, how

much does it cost to use the television?

Q2. A laptop uses 70 watts of power. Nabeel used his laptop for 3 hours every day in the month

of April. If GPL is charging $50 per KWh, calculate how much it would cost Nabeel to operate

his laptop for that one month period.

Q3. Complete table on page 162 of Integrated Science text :)

Q4. Go around your home, and estimate the cost of electricity and then compare your estimate

with GPL's bill total. (NB: GPL has a monthly rental charge included in your bill)

Energy conservation strategies

1. Electrical appliances not in use should be unplugged, to save power

2. Energy efficient appliances should be used e.g. energy saving fluorescent bulbs.

3. Faulty electrical appliances will consume more energy. E.g. If the door seal of a fridge or

freezer is faulty, then it will consume more energy.

Magnetic effect of electrical Current

Electromagnets are made when electricity flows through a circuit. These are temporary magnets,

which only lasts as long as current is flowing.

A permanent magnet is an object made from a material that is magnetized and creates its own

persistent magnetic field.




Reading of analog and digital meter

Light and Colour

White light (visible light) is made up of a combination of different colours. Using a prism, the

white light can be split into many colours. A prism is a triangular glass object that refracts or

bends light as it passes through it according to their wavelength. Why is it that rainbow is made

up of several colours? Rainbow is formed when light that passes through rain droplets are

diffracted. The colours observed in a rainbow are usually red, orange, green, blue, indigo, and

violet.

Primary colours

These are: Red, Blue, and Green

When primary colours are mixed together, they form secondary colours.

e.g. Red and Green --> yellow

Red and Blue ----> magenta

Red + Green + Blue--> White

We are able to see a colour of a pigment because it reflects only that part of the visible spectrum

of light and absorbs the rest (that we do not see). The reflected colour of light is what we are able

to see. E.g. We see leaves as Green, because the leaves DO NOT absorb Green Light (it reflects

it).

Artificial Lighting

-Fluorescent tubes

These are cylindrical glass tubes that contain mercury vapours. The inner lining of the tube is

coated with phosphor powder. When current flows through the electrodes, it shoots electrons into




the mercury vapor causing them to emit bluish light and ultraviolet radiation which we cannot

see. However, when the UV hits the phosphor powder coating, visible light is emited.

-Filament lamps

Filament lamps are an incandescent source of light (i.e. gives off light when heated). The lamp is

made of a very thin tungsten (metal) coil. The filament is enclosed in a glass case/bulb that is

filled with inert gas such as argon. When current passes through the tungsten filament, it heats up

to about 2500 0C and glows white hot and emits light and heat (heat is wasted energy).

Shadows

Shadows prove that light travels in a straight line. It is formed when an opaque object or material

is placed in the path of light rays. The shadow is the area that light cannot reach, because the

light rays have been blocked by the object.

Umbra

When the light source is small compared to the opaque object, then no light is able to pass

through or around the object. A very dark shadow is formed called umbra is formed.

Penumbra

It is formed when the light source is larger than the opaque object. Some of the light rays will be

able to pass the edge of the opaque object, and form a shadow with two parts; a dark part called

the umbra, and a lighter region surrounding it called the penumbra.

electricity .pdf

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