1.6 Electric circuits

Natural sciences

Grade 8

Energy: electicity, heat and light

Module 5

Electric circuits

An electric circuit consists of:

a source – cell (battery or cells)

conductors – wires

a switch – control point

other components like light bulbs, resistors and measuring instruments.

a Battery is a collection of cells.

Activity

To investigate the different components in a circuit and the symbols used to represent them [lo 2.3]

Assignment 1:

Draw a simple sketch of a single torch cell. One end has a protrusion, this is the positive pole. The opposite end is the negative pole. Add the labels “positive pole”, “negative pole”.

Now draw three more torch cells in a battery, as they would be placed in the shaft of a torch – what do you notice about the positive and negative poles.

This forms a battery and the cells are linked in series.

Assessment of Sketch

Were you able to draw the sketch and add labels correctly?

[LO 2.3]

Drawing a number of components is time consuming and cumbersome. Using symbols saves time. These symbols are used internationally!

following symbols are used:

A length of copper wire is represented by means of a line

A light bulb is represented by means of:

A cell (the long thin line is the +)

An ammeter (to measure the strength of the current)

A switch : Open:

Closed:

A current diagram will always contain these components (parts). When we place all the components one after the other they are in series.

The diagram is always drawn in the form of a square. A simple circuit diagram will be drawn as follows.

Components can also be linked in parallel, as shown below.

Assignment 2:

Now draw three light bulbs that are linked in parallel – make use of symbols.

In a parallel linkage, the current is divided. If one of the light bulbs breaks the others will continue to glow!

The light bulbs in you home are mostly in ________________________________.

Assessment of Sketch

Were you able to draw the sketch and add labels correctly?

[LO 2.4]

Assessment

LO 2: Constructing Science Knowledge:

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

This is evident when the learner:

• interprets information;

Memorandum

Assignment 1:

An electric circuit comprises:

1. An electrical point and closed circuits.

2. A source – cell or battery

conductors – wires

a switch – controlling point

other components like light bulbs, resistors and measuring instruments.

cellbattery

Positive cellpool

A Battery is a collection of cells.

Drawing a number of components may be a nuisance and time-consuming. You need symbols that could provide shortcuts. Scientists have fortunately devised these already and they are used internationally!

A light bulb is represented by means of:

A cell (the long thin line is the +)

An ammeter (to measure the strength of the current)

A switch

Assignment 2:

• A circuit diagram always contains these components (or parts) of the current.
• To begin, we arrange the components in line – this is known as "in series".
• The diagram is always drawn in the form of a square. A typical diagram is shown as:

A circuit diagram of three cells in series, an open switch, two light bulbs in series and an ammeter.

The concept of connection/linking in parallel may be difficult to understand. In the case of cells it can be shown as:

ELECTRICITY AND ITS USES IN THE HOME

THE LIGHT BULB:

• We'll begin with the light bulb. We have said that the light bulb represents ineffective utilisation of electric power. This is because 95 of the 100J are lost to heat and only 5J go towards light energy.
• We therefore need to think of a more economical means for lighting. But before we can investigate the issue further, we need to know what a typical light bulb consists of.
• The light bulb comprises a glass bulb filled with a particular gas, which isn't oxygen.
• The light bulb comprises a glass bulb filled with a particular gas, which isn't oxygen.

Why not? Wait and see. The wires inside a light bulb are wound up in small spirals. They are actually made of a particular metal filament (tungsten) that has resistance. This means that it tries to block the current and therefore becomes extremely hot. The heat is so intense that it begins to glow and becomes white hot! If the glass bulb were to be filled with oxygen, the intensity of the heat would melt everything. But the hollow is filled with a stable gas – Argon.

FLUORESCENT LIGHTS AND ECO-LAMPS:

Fluorescent lights (neon lights) are used in many offices and schools, as well as in some homes. They are filled with a particular gas and are lined with a chemical substance that emits light when an electric current passes through the tube. Fluorescent lights do not build up much heat, which reveals something of their effectiveness.

Eco-lamps: As we pay for our electricity, and also because electricity supply is limited, it is important for consumers to have electrical appliances that utilise electricity effectively. Fluorescent lights and eco-lamps do cost more, but they last much longer.