To investigate how the temperature affects the resistance of a thermistor.

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Gail Wingham

Physics Sc1 – Thermistor Experiment

Aim: To investigate how the temperature affects the resistance of a thermistor

Background Information:

The resistance determines the difficulty in which energy can pass through a circuit. It is measured in Ohms (Ω) and the equation for this is;

Resistance (Ω) = Voltage (V)

                            Current (I)

A thermistor is a type of variable resistor that’s resistance is affected by heat and in my investigation I am experimenting with the manor in which temperate affects the resistance in a thermistor.

In Physics by Patrick Fullick it states that;

[a thermistor] allows more current to flow as the potential difference across it increases”

This is the exact opposite behaviour of a filament lamp which as the potential difference increases the current is also allowed to increase but as the potential difference begins to reach a certain level the resistance increases and so the rate at which the current increases slows down. In a thermistor the current increases as the potential difference across it increases. This is because as the voltage increases it creates more heat and the thermistor reacts to this by decreasing its resistance and therefore allowing the current to increase.

In an ordinary resistor the resistance increases as the voltage increase. This is because as the potential difference across it increase, the friction of the moving particles heats up the particles in the resistor causing them to vibrate more. So as the particles vibrate, they obstruct the electrons that are trying to pass through more frequently and so a greater resistance is built up.

In an electrical current it is only the electrons on the outer shells that move. These electrons are know as bands because as there are so many atoms grouped together the only electron that have freedom to jump from one atom to another are the electrons on the top and bottom outer shells. The voltage in a circuit gives the electrons the energy to move and as more voltage is applied and the potential difference across the atoms increases it gives the atoms that are not in the outer shells the energy to push against the force from the nucleus and effectively jump into the band of moving electrons in the outer shells.

This applies to the band theory which explains why metals conduct electricity so easy, why insulators conduct virtually no electricity whatsoever and the behaviour of a thermistor.

Between the conduction band (the band of freely moving electrons) and the valence band (the band on inner shell electrons) there is a gap that is know as the forbidden gap. No electrons can move through this gap and the only way electrons can pass from the valence band to the conduction band is by gaining enough energy to jump the gap.

In metals the forbidden gap between the bands is extremely small and so it takes an extremely small amount of energy for the electrons to jump the gap as so electricity can flow very easily through all metals – this is why they conduct so well.

In insulators the gap between the conduction band and the valence band is extremely large and almost no amount of energy given to the electrons will be enough for them to jump to the conduction band. This means that electricity cannot pass through these substances and explains why they do not conduct electricity.

A thermistor is a semi-conductor. Semi-conductors have a small gap and although there are not many electrons flowing through the conduction band as the potential difference increases in a circuit (along with the temperature in the semi-conductor) more energy is going into the valence band. This energy allows electrons to jump the gap and flow along the conduction band. The higher the potential difference the hotter the component will get and the more electrons that will jump into the conduction band. This is why when a thermistor is heated the resistance decreases.

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From the information above I predict that as I increase the temperature of the thermistor by heating it in a beaker of oil in a water bath, the resistance, measure in Ohms (Ω), will decrease. This is because a thermistor is a semi-conductor and so as the particles in the thermistor are heated the electrons gain more energy to jump the gap between the valence band and the conductive band and so more electrons are in the conductive band meaning there is more conduction. In turn I also predict the lower the temperature of the thermistor the higher ...

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