Convert a galvanometer into a voltmeter of range 0 3 volts. Study the variation of resistance of a thermister with temperature

Apparatus

Thermister (  ), test tube, thermometer, slide wire bridge, resistance box, galvanometer, rheostat, cell, tapping key and connecting wires.

Pic?

Theory

A thermister is a heat sensitive resistor made from semiconductors. Usually thermisters have negative temperature coefficient of resistance, i.e., the resistance of a thermister decrease when its temperature is increase and rises when its temperature falls.

The temperature coefficient of resistance of a wire is defined as the fraction of its resistance at 0C by which its resistance changes per degree centrigrade rise in its temperature. If R₀ is the resistance of the wire at 0C and R_t is its resistance at t” C, the temperature coefficient of resistance is given by

a =     or = (1+a t)……… (4.1)

The temperature coefficient of resistance of a wire can be determine by measuring its resistance R₁ and R₂ at temperature t1 “C and t₂” C respectively. For these resistances we can write.

R₁ = R₀ (1+t₁) …………… (4.2)

R₂ = R₀ (1+t₂) …………. (4.3)

Dividing Eq (4.2) by (4.3)

=   or …….(4.4)

The resistance of the thermister at various temperature is determined by using a slide wire bridge which has been discussed in detail in experiment No.1

Procedure

1. Clean the end of connecting wires with sand paper and arrange the apparatus as shown in fig. (a) Where B is a battery, K is a tapping key and G is a galvanometer whose one terminal is connected to point D and the other terminal to a jockey which can made to touch the wire of the slide wire bridge at any point. A resistance box R.B is connected in the gap LN and the thermister in the gap MT of the slide wire bridge. The thermister is enclosed in a test tube which is placed in a beaker containing water whose temperature can be noted by the thermometer T.
2. Wait for some time (say about 5 minutes) so that the thermister in the test tube attains the temperature of water in the beaker. Determine the resistance R_th of the thermister by following the step No. 3 and 4 of experiment No 1. Also note the reading of the thermometer T.
3. Start heating water in the beaker and continue stirring it till temperature is say 10 above the first reading. Adjust the flame of the burner so that the temperature of water remains constant for say 5 minutes. Note the constant temperature and again determine the resistance of the thermister.
4. Similarly take five to six reading by raising the temperature of water in step of 10 till the water begins to boil.
5. Plot a graph R_th the resistance of the thermister and its temperature t and taking R_thalong y-axis. Then graph will not be exactly a straight line.
6. Plot a graph between R_rh the resistance of the thermister and its temperature R₁ and R₂ corresponding to any two nearby temperature t₁ and t₂ respectively. Calculate the value of, the temperature coefficient of resistance in the temperature range from t₁ and t₂ by Eq. (4.4).

Similarly calculate the value of in three different ranges of temperature.

Observation &calculation

Precautions

1. The test tube containing thermister should be well immersed in water and should not touch the sides or bottom beaker.
2. The reading of the thermometer should be taken the temperature indicated by its remains constant for a few minutes.
3. Other precautions are the same as given in Experiment NO 1.

Viva voice

Q.1    What is mean by resistance?

Ans. The resistance is define as the property which opposes the flow of current through a conductor. It is equal to the ratio of potential difference between the ends of a conductor to the current flowing through it.

Q.2       Name the factors on which the resistance of a wire depends?

Ans.     (i)Length,(ii) area of cross section,(iii) nature and(iv) temperature.

Q.3       Define temperature coefficient of resistance?

Ans.     For answer see theory of experiment no.4.

Q.4       why does the experimental graph between the resistance and temperature of the thermister not a straight line?

Ans.     the relation R_t = R₀ (1+t) shows that the graph between the resistance R_t of a conductor say a thermister and its temperature should be a straight line, but the experimental graph shown in fig 4.b is not a straight line. The reason is that the temperature of the thermister change externally as well as internally. In other words its resistance depends on the surrounding temperature and also on self-heating due to the current passing through it. It is only the surrounding temperature which is measured by the thermometer.

Q.5       what is mean by thermistor?

Ans.     Thermistor is a temperature dependent resistor, made up metal. Oxide semi conductor material. Often a thermistor with negative temperature coefficient are used to determine very low value of resistance up to 10k.