This chapter builds on the Gr 6 and 7 electric circuits work, and the previous chapter of this book. Up until now, we have only been looking at simple circuits. We will now examine the concept of series and parallel circuits. We will look at the difference between these two set-ups in circuits, specifically looking at the effects of adding resistors in series or in parallel and observing the change in brightness of bulbs. The use of ammeters has also been included in this chapter. However, if you do not have these instruments, you can simply do a qualitative study, using the brightness of the bulbs.

You are watching: What happens to the brightness of a bulb in a parallel circuit

You can also use the PhET simulations where learners can build their own circuits and test them out, observing the effects when they add or remove various components. These simulations will run directly within your browser from our website, www.curious.org.za. Here is a link to a guide (in pdf format) written by PhET in the use of some of the electric circuit simulations: phet.colorado.edu/files/teachers-guide/circuit-construction-kit-dc-guide.pdf

3.1 Series circuits (2.5 hours)

Tasks

Skills

Recommendation

Investigation: What happens when we add more resistors in series?

Investigating, hypothesising, following instructions, observing, interpreting, recording, analysing, writing, working in groups

CAPS suggested

Investigation: How does adding more cells in series affect the current?

Investigating, hypothesising, following instructions, observing, interpreting, recording, analysing, writing, working in groups

Suggested

Investigation: Testing the current strength

Investigating, hypothesising, following instructions, observing, interpreting, recording, analysing, writing, working in groups

Suggested

3.2 Parallel circuits (3 hours)

Tasks

Skills

Recommendation

Activity: Series or parallel?

Identifying, describing

Suggested

Investigation: How does adding resistors in parallel affect the current strength?

Investigating, hypothesising, following instructions, observing, interpreting, recording, analysing, writing, working in groups

CAPS suggested

Investigation: What happens to the current strength when cells are connected in parallel?

Investigating, hypothesising, following instructions, observing, interpreting, recording, analysing, writing, working in groups

Suggested

Investigation: Testing the current strength

Investigating, following instructions, observing, interpreting, recording, analysing, writing, working in groups

Suggested

Activity: Which metals offer the most resistance?

Following instructions, observing, interpreting, working in groups

CAPS suggested

3.3 Other output devices (0.5 hours)

Tasks

Skills

Recommendation

Activity: Sankey diagrams

Drawing, explaining

Suggested

Activity: History of electricity production

Research, summarising, working in groups, writing

CAPS suggested (can be done as homework task)

Activity: Careers

Research, writing

Optional


Are there different types of electric circuits? If all the light bulbs in a house are part of the same circuit, how can you switch one light off without the rest also turning off? What is a series circuit? What is a parallel circuit? What happens when you connect more components in series or in parallel?
In the last chapter, and in Gr 6 and 7, we have been looking at electric circuits. These have mostly been series circuits. What does this mean? And how else can a circuit be arranged?


A series circuit is one in which there is only one pathway for the electric current to follow. The components are arranged one after another in a single pathway. When we connect the components we say that they are connected in series. We have already seen examples of series circuits in the last chapter.

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A series circuit with one pathway for the current, from the negative to the positive terminal of the battery.

Ammeter

An ammeter is a measuring device used to measure the electric current in the circuit. It is connected into the circuit in series. The current is measured in amperes (A).


The ampere is named after André-Marie Ampère (1775-1836), a French mathematician and physicist. He is considered the father of electrodynamics, which is the study of the effect of electromagnetic forces between electric charges and currents.


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An ammeter.

What is the symbol for an ammeter? Draw it here.


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Do you think that an ammeter would have a high resistance or a low resistance to the current? Explain your choice.


Ammeters have an extremely low resistance because they must not alter the current they are measuring in any way.


A series circuit only provides one pathway for the electrons to follow. Let"s investigate what happens when we increase the resistance in a series circuit.


The aim of this investigation is to show the learners that adding more resistors in series causes the overall resistance of the circuit to increase and that this reduces the current strength.


This is a good opportunity for group work if you have enough equipment, but make sure that each learner is able to connect an ammeter correctly and is able to read the ammeter scale accurately. If you do not have sufficient equipment for all the learners, you can do this experiment as a demonstration. Perhaps give several learners an opportunity to come up to the front and help to connect the ammeters. If you do not have any ammeters then you can use the brightness of the bulbs to indicate current strength. The larger the current, the brighter the bulb will glow. This means that if the bulb glows brightly, it must have a large current moving through it. If the bulb is dimmer, it means that there is a smaller current flowing through it.

If you do not have the physical apparatus for this investigation but you do have internet access, use the PhET simulations found here: http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

The simulations are also useful because the ammeters (and voltmeters) commonly used in school laboratories are often not calibrated correctly or not serviced regularly and so often give slightly inaccurate results.


This is a learner-dependent answer. The hypothesis should relate the dependent and independent variables and make a prediction. The dependent variable will change as the independent variable is changed. Here is an example of a possible answer:

As the number of resistors increases, the current strength decreases.

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MATERIALS AND APPARATUS:

1,5 V cells 3 torch bulbs insulated copper conducting wires switch ammeter

It is important that the torch bulbs have the same resistance and are not randomly selected. The switch is not an essential part of this investigation. It can be left out of the circuit.