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# How the resistance of a thermistor changes with temperature keeping the current constant and recording the voltage as the temperature changes.

Extracts from this document...

Introduction

My Aims

I aim to find how the resistance of a thermistor changes with temperature keeping the current constant and recording the voltage as the temperature changes. I will then record this data and try to explain it using scientific knowledge and detailed reasoning.

Background information - Thermistors

Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) temperature coefficient. They work because at low temperatures, electrons are fixed onto atoms and so cannot move. As the electrons get hotter they receive enough energy to escape from their atoms, so the thermistor becomes a better conductor.

Materials that react like this are called semiconductors. Carbon, silicon are two materials whose resistance decreases as they get hotter. Thermistors can be used for the following functions:

• Temperature sensing, switching at temperatures ranging from 60°C to 180°C, e.g. protection of windings in electric motors and transformers.
• Solid state fuse to protect against excess current levels, ranging from several mA to several A (25°C ambient) and continuous voltages up to 600V and higher, e.g. power supplies for a wide range of electrical equipment.
• Liquid level sensor

Middle

Resistance =                                                 ( R=      )

Prediction

After research of this science I have found that the thermistor is made from a semi-conductible material which we have been told is an NTC ( negative temperature coefficient). In this case I believe that as the temperature increases the resistance will decrease as more electrons are freed and the material becomes a better conductor. The graph produced at the end of this experiment should therefore look like this:

Preliminary Experiment

Before the main experiment is carried out a preliminary experiment is needed to clarify several problems such as equipment choices and temperature ranges. Also I have to find the time it takes to complete a trial of the experiment so I can work out how many readings it is possible to take in the given time.

Method

I need to set up the experiment in the way (shown later) I had pre-determined which included both an ammeter and a voltmeter, first preparing the pot by wrapping foam round it for insulation. I will put 3 layers of the foam on attached with elastic and a layer (cut in a circle) on the bottom after deciding that the time needed for each trial did not require masses of insulation.

Conclusion

Risk Assessment

There is only element that provides a major risk in this experiment this being the extremely hot water. There is no real protection against this but if precautions and care is taken with regards to the surroundings and others working in the vicinity. If scalding occurs then the skin will have to be submersed in ice water to minimise the pain. Although water and electricity are not usually a good combination the fact that only 6V are passing round the circuit there is no real danger from the charge.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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