Question 9
(a)(i) Differentiate between heat and temperature.
(ii) Define the lower fixed point and upper fixed point on the Celsius scale of
temperature.
(b) State the feature that ensures that a clinical thermometer:
(i) keeps the measured temperature for reading;
(ii) is sensitive;
(iii) has quick reaction.
(c) (i) Explain linear expansivity.
(ii) A brass rod and an iron rod differ in length by 28 cm by 20oC. What should be the original length of the iron rod for the difference in length to remain the same when both rods are heated to 90.oC? [Linear expansivity of brass = 1.9 x 10-5 K-1; Linear expansivity of iron = 1.2 x 10-5 K-1]
Observation
Part (a) candidates performance was fair.
Part (b) candidates performance was low.
Part (c) candidates performance was poor.
The expected answers are:
9.(a) (i) Distinction between heat and temperature
Heat is the total internal energy of molecular motion in a substance, while temperature is a measure of the degree of hotness (or coldness) of a body.
OR
Heat is the energy transferred from one point to another due to temperature difference, while temperature is a measure of the degree of hotness (or coldness) of a body.
(ii) Definition of lower fixed point and upper fixed point on the Celsius scale of temperature
The lower fixed point is the temperature of pure melting ice (at standard atmospheric pressure).
The upper fixed point is the temperature of dry steam from water boiling at standard atmospheric pressure.
(b) The feature that ensures that a clinical thermometer
(i) keeps the measured temperature for reading
Kink/constriction
(ii) is sensitive
Fine/narrow bore
(iii)has quick reaction
Thin glass small bulb
(c) (i) Explanation of linear expansivity
The fractional change/rise in length of an object per degree change/rise in temperature measured in per kelvin (K-1)
(ii) Determination of the original length of the iron rod
Since brass expands faster than iron,
lbrass = liron – 28
expansion of iron = liron aironDq
= liron x 1.2 x 10-5 x (90 – 20)
expansion of brass = lbrass abrassDq
= lbrass x 1.9 x 10-5 x (90 – 20)
expansion of iron = expansion of brass
liron x 1.2 x 10-5 x 70 = (liron – 28) x 1.9 x 10-5 x 70
liron =
= 76 cm