Using a Thermistor to Control Voltage + CP

Question 1

what can a thermistor in a potential divider circuit be used for

Answer 1

controlling the output voltage

Question 2

from the formula V out = V in x R2/ R1 + R2, with R1 being the thermistor, what would be the effect of the temperature increasing

Answer 2

• the resistance of the thermistor (R1) would decrease
• leading to a smaller denominator and therefore larger value for R2 / R1 + R2
• meaning the V out will increase for a given V in

Question 3

without using the formula, why would it make logical sense for the V out to increase of the resistance of the thermistor decreases

Answer 3

• the whole resistance of the circuit would decrease as a result
• and for a given V supplied by the battery, the current would increase
• and as V out = IR2, with a larger I and constant R2
• V out would increase

Question 4

what is the focus of core practical 12

Answer 4

to use a thermistor in a potential divider circuit to activate a thermostatic switch

Question 5

what circuit is set up for this practical

Answer 5

• you have your battery with resistor 1 and 2 in series with it
• resistor 1 being the thermistor and resistor 2 being an ohmic resistor
• with a voltmeter placed in parallel to resistor 2

Question 6

what is firstly done in this experiment

Answer 6

an ohmmeter is used to measure the resistance of the thermistor (R1) at room temperature

Question 7

what is done to the thermistor after this

Answer 7

• still being connected to the circuit

- it is placed in a beaker of water with a thermometer inside

Question 8

how is the normal resistor picked

Answer 8

• it is picked such that its resistance is half that of the thermistors resistance at room temperature
• R1 = 1/2 R2

Question 9

what is the main part of the experiment

Answer 9

• a heat source, like a bunsen burner, is placed under the beaker to heat the water up
• the output voltage, V out, for temperatures ranging from room temp to about 60C are measured and recorded

Question 10

what graph would you plot with the data you have

Answer 10

a graph of V out against temperature

Question 11

what would the graph be useful for

Answer 11

• identifying the output voltage for specific temperatures
• it can also allow you to calculate the resistance of the thermistor at different temperatures and how much it changes per unit change of temp

Question 12

how can circuits like these be practically used

Answer 12

• the circuit could be designed to operate a switch when the output from the circuit reaches a premeditated value
• for example if the switch was set to operate at an output voltage corresponding to 40C
• the circuit could be used to switch on a warning lamp when the temperature reached 40C

Question 13

what trend should be discovered with the results

Answer 13

an increase in the temperature should result in a increase in the output voltage

Question 14

why is that trend discovered, keeping in mind where the voltmeter is

Answer 14

• with a thermistor, an increase in temperature results in a decrease in its resistance
• using R = V/I, with a constant pd from the supply, the current would need to be increasing for this to occur
• this is due to more charge carries per cubic meter btw
• with an increased current flowing through the circuit, the ohmic resistor which has a constant resistance would experience an increase in the pd across it
• shown by R = V/I, where an increase in current must also lead to a proportional increase in pd as resistance remains the same
• or V = IR, where an increase in current with a constant R leads to an increased V