Introduction and key concepts

Electric circuit

An electric circuit is a closed path (with no breaks or gaps) along which electrical charges (electrons) flow powered by an energy source.

Representing electric circuits

Components of electrical circuits

Some common elements (components) which can be found in electrical circuits include light bulbs, batteries, connecting leads, switches, resistors, voltmeters and ammeters. You will learn more about these items in later sections, but it is important to know what their symbols are and how to represent them in circuit diagrams. Below is a table with the items and their symbols:

Component	Symbol	Usage
light bulb		glows when charge moves through it
battery		provides energy for charge to move - conventional current flow from positive to negative through a circuit
switch		allows a circuit to be open or closed
resistor		resists the flow of charge
	OR
voltmeter		measures potential difference
ammeter		measures current in a circuit
connecting lead		connects circuit elements together

Circuit diagrams

Representing circuits

A physical circuit is the electric circuit you create with real components.

A circuit diagram is a drawing which uses symbols to represent the different components in the physical circuit.

We use circuit diagrams to represent circuits because they are much simpler and more general than drawing the physical circuit because they only show the workings of the electrical components. You can see this in the two pictures below. The first picture shows the physical circuit for an electric torch. You can see the light bulb, the batteries, the switch and the outside plastic casing of the torch. The picture is actually a cross-section of the torch so that we can see inside it.

Below is the circuit diagram for the electric torch. Now the light bulb is represented by its symbol, as are the batteries, the switch and the connecting wires. It is not necessary to show the plastic casing of the torch since it has nothing to do with the electric workings of the torch. You can see that the circuit diagram is much simpler than the physical circuit drawing!

Series and parallel circuits

There are two ways to connect electrical components in a circuit: in series or in parallel .

Series circuit

In a series circuit, the charge flowing from the battery can only flow along a single path to return to the battery.

Parallel circuit

In a parallel circuit, the charge flowing from the battery can flow along multiple paths to return to the battery.

The picture below shows a circuit with two resistors connected in series on the left and a circuit with two resistors connected in parallel on the right. In the series circiut, the charge path from the battery goes through every component before returning to the battery. In the parallel circuit, there is more than one path for the charge to flow from the battery through one of the components and back to the battery.

This simulation allows you to experiment with building circuits.

Circuits

1. Using physical components, set up the physical circuit which is described by the circuit diagram below and then draw the physical circuit:

   

2. Using physical components, set up a closed circuit which has one battery and a light bulb in series with a resistor.
   1. Draw the physical circuit.
   2. Draw the resulting circuit diagram.
   3. How do you know that you have built a closed circuit? (What happens to the light bulb?)
   4. If you add one more resistor to your circuit (also in series), what do you notice? (What happens to the light from the light bulb?)
   5. Draw the new circuit diagram which includes the second resistor.
3. Draw the circuit diagram for the following circuit: 2 batteries and a switch in series, and 1 lightbulb which is in parallel with two resistors.
   1. Now use physical components to set up the circuit.
   2. What happens when you close the switch? What does does this mean about the circuit?
   3. Draw the physical circuit.

Discussion : alternative energy

At the moment, most electric power is produced by burning fossil fuels such as coal and oil. In South Africa, our main source of electric power is coal-burning power stations.(We also have one nuclear power plant called Koeberg in the Western Cape). However, burning fossil fuels releases large amounts of pollution into the earth's atmosphere and contributes to global warming. Also, the earth's fossil fuel reserves (especially oil) are starting to run low. For these reasons, people all across the world are working to find alternative /other sources of energy and on ways to conserve /save energy. Other sources of energy include wind power, solar power (from the sun), and hydro-electric power (from water, e.g. dammed rivers) among others.

With a partner, take out the lists you made earlier of the item/appliances/machines which used electricity in the following environments. For each item, try to think of an alternative AND a way to conserve or save power.

For example, if you had a flourescent light as an item used in the home, then:

• Alternative: use candles at suppertime to reduce electricity consumption
• Conservation: turn off lights when not in a room, or during the day.

Topics:

• At home
• At school
• At the hospital
• In the city

Once you have finished making your lists, compare with the lists of other people in your class.