Title: in Control Changing the Brightness of a Lamp Or the Speed of a Motor

Title: In Control – changing the brightness of a lamp or the speed of a motor

Introduction In this extended enquiry older children investigate how varying the length of resistance wire in a circuit affects the brightness of a bulb. They consider how an electric lamp dimmer can be made and decide how much wire they need to use to make an effective device. It can be used as part of a science and technology challenge where children make a model which includes a dimmer (variable resistor) to change the brightness of a light (e.g. in a table lamp) or the speed of a motor (e.g. on a buggy or fairground ride).

Lesson organisation This enquiry will require one hour for the initial investigations. A further one to two hours will be needed to design, make and test the dimmer. If this is part of a project additional time will be needed to design, make and evaluate the model which incorporates the dimmer. The initial investigation is carried out in groups of four, although larger groups may be needed if there are insufficient data loggers. The designing and making of the dimmers is an individual or paired activity.

Equipment and materials For group investigations each group will need  At least 2 1.5V batteries with holders  A bulb holder and a selection of bulbs of different voltage ratings  Standard plastic insulated wire with stripped ends or crocodile clips attached  High resistance wire (available from TTS and other suppliers). Note that it’s necessary to use this sort of wire which is known to work well in simple, single battery circuits. Fuse wire is not suitable.  Choice of rulers, metre sticks or tape measures  Data loggers if available  Shoe boxes or similar

Additional equipment, which may include motors, will be needed for the dimmer and circuit making. Depending on the children’s designs, dimmer construction may also require thick card, split pin paper fasteners, card tubes, lengths of thick dowel and suitable tools for making holes in the materials provided.

Technical notes and safety Use standard disposable batteries to minimise the risk of components becoming hot. Advice about types of battery can be found in CLEAPSS guide L112 Batteries & LV Units - Which to Buy? Remind children to match the batteries to the other components and avoid short circuits.

Measuring the distance along a piece of thin, bare resistance wire is easier if the wire is fixed between two paper fasteners at the ends of a length of thick card or two screws in a length of wood. One end can be connected into the circuit and the length can be varied by making contact at different points along the wire (see diagram 1). A ruler or tape measure can be temporarily fixed alongside the wire to provide a measuring scale.

When comparing brightness enclose the bulb in a box (a shoe box with a close fitting lid is suitable) to minimise the effect of background light and allow children to judge when it is no longer giving out any light. Children can observe in turn through a small peephole or a data logger can be enclosed in the box with the bulb.

This activity can help children to understand that electrical conductivity is a continuum; materials may be good or poor electrical conductors depending on their resistance. Resistance is not studied in detail in primary school but models and analogies can be helpful in developing understanding e.g. electricity represented as the flow of water through pipes with higher resistance (poorer conductivity) represented by narrower pipes which restrict the flow of water.

Procedure Part 1 Review with children the ways they already know to change the brightness of a bulb in an electrical circuit eg by changing the number of batteries or the number and type of components. Suggest that changing wires in a circuit would also affect the brightness. Ideas for questions to investigate could then come from the pupils or be prompted by the use of Concept Cartoons. Children use a fair test approach to investigate the effect of using wires of different length. Provide a selection of batteries and bulbs to allow children to demonstrate that they can match components and conduct a fair test. Evidence can be collected by making subjective judgements or by using the light sensor on a data logger.

Part 2 Then present children with the challenge of designing and making a dimmer for their circuit which will change the brightness of a bulb or the speed of a motor. To inform their designing they may need to investigate further, using knowledge gained from previous investigations, to find the optimum length of wire, number of batteries and number and type of other components for their circuit. The dimmer will need to be a manageable size. Children make their dimmer and test it in their circuit. If this is part of a technology project the circuit can then be added to their model.

Teaching notes If children will be using their dimmer in a model which includes a motor rather than a bulb the activities could be carried out using motors, with propellers to make it easier to visually judge the speed. However, if data loggers are available it is better to complete part 1 using bulbs as these allow measured data to be obtained. Children can confirm that there is a comparable effect for motors in the second part of the enquiry.

Further information The CLEAPSS guide L122p Simple Electric Circuits with Bulbs and Batteries, provides advice about components and how to teach simple circuits.

Ideas for making an electric lamp dimmer The simplest dimmer can be made from a strip of card or wood with resistance wire secured along it, as suggested for the initial investigation.

A more compact version is made by winding wire around a tube or thick dowel, secured on paper fasteners or at the back with tape. A rectangle of card cut with zig-zag bladed scissors along its long sides will also hold coiled wire.