physics controlled assessment - Investigating Thermistors

this is the low control part of my physics controlled assessment. I investigated the change in voltage across a thermistor as the temperature changed.

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  • Created on: 01-04-13 17:12
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Physics Controlled Assessment
Potential dividers are an important part of our modern lives; we encounter them from day to day, in
radios, toys and even in dimmer lights. In a circuit a potential divider divides the voltage equally
between two resistors, which are connected in series. The two resistors may have fixed values or one
may be a LDR, a thermistor or other input device.
The two voltages across the two resistors are
related by a simple relationship:
The voltage across a resistor within a potential
divider can be worked out by using this equation:
By looking at these equations we can see that
the rule is, when the resistance across one
resistor increases the resistance across the
other must decrease proportionally as the voltage
input must equal the sum of the voltages across the two resistors.
A further rule that applies within a potential divider is if one of the resistances in a voltage divider
increases, then the voltage across that resistor also increases. Therefore, if plotted onto a graph, the
general trend would be a positive correlation between the voltage and resistance.
Strand: S (a)
There are several different types of resistors that I could use effectively in my experiment:
Variable resistor ­ A variable resistor is used in circuits to vary the magnitude of resistance passing
through the resistor. The resistance across the variable resistor, that is available to me, can be altered
manually by turning a nob that is connected to the variable resistor. Through background knowledge I
know that if you increase the resistance across the variable resistor the voltage will also increase.
Thermistor ­ A thermistor is a component that has a resistance that changes with temperature. There
are two types of thermistor. Those with a resistance that increase with increasing temperature and
those with a resistance that decreases with increasing temperature. The amount by which the
resistance decrease as the temperature decreases is not constant and it varies with temperature.
LDR (light dependent resistor) - An LDR is a component that has a resistance that changes with the
light intensity that falls upon it. It has a resistance that falls with an increase in the light intensity falling
upon the device. Similarly to a thermistor, the amount by which the resistance decreases as the light
intensity increases is not constant and it varies with the light intensity.
Factor that I chose:
For my experiment I have chosen to use a thermistor and a constant resistor connect in series. I believe
that this is the best circuit to use because it will be easy to control and collect data from. A thermistor

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I have decided not to use the LDR in my experiment because I believe that it will be difficult to measure
and control the light intensity. I believe this because it will be very difficult to control the amount of light
that will be hitting the LDR at any given moment as the background light intensity constantly
fluctuates. The fluctuation in light intensity will therefore result in a fluctuation in the voltage across
the LDR, which will not give me very accurate results.…read more

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­ I have chosen to use a resistor, which has quite a large resistance because
it will take a greater share of the input voltage. This means that the change in voltage across the
thermistor will be greater and more significant as the temperature changes. This will give me results that
are easier to read and analyse.…read more

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By doing this I will be able to collect accurate results as the temperature of
the water would decrease slowly and in a controlled manner meaning that the components within the
thermistor would be very close to the same temperature as the water that they are in.
As the temperature of the water comes nearer to room temperature the rate at which it will be cooling
down will decrease.…read more


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