Heat Energy and Temperature

• Heat
• Temperature
• Measurement of temperature
• Thermometers

Heat

Heat is a measure of total internal energy of a body. It is a form of energy due to a temperature difference. It is measured in Joule, J

Temperature

Temperature is the degree of hotness or coldness of a body. The unit of temperature is in degree Celsius (⁰C) or Kelvin (K)

Measurement of temperature

Temperature is measured by using thermometers. Thermometers have two reference temperatures or fixed points called the upper fixed point and lower fixed point.

The upper fixed point is the temperature of steam from pure water boiling at standard atmospheric pressure of 760mm of mercury. It is 100⁰C

The lower fixed point is the temperature of pure melting ice at the standard atmospheric pressure of 760mm of mercury. It is 0⁰C

Temperature scales

The difference in temperature between the upper and lower fixed points is called fundamental interval of a thermometer. The calibration of this interval depends on any of the three scales below:

1. Celsius scale
2. Fahrenheit scale
3. Kelvin or Absolute scale

The S. I. Unit of temperature is the Kelvin.  However, it is also measured in degree Celsius and Fahrenheit.

Thermometers

Thermometers are instruments used to measure temperature. Thermometers are named using the thermometric properties employed in the measurement of temperature. Each one makes use of the change in the physical properties of materials they are made of, to indicate temperature change.

1.       The liquid-in-glass thermometer: The liquid-in-glass thermometer depends on the uniform expansivity of the liquid used with temperature change.  Any liquid that will be used as a thermometric liquid must be good conductor of heat, be easily seen in glass, have a high boiling point, have a low freezing point, have a low specific heat capacity, must not wet glass and must expand uniformly.

2.       The clinical thermometer: This has a constriction and it has a short range (35⁰C – 43⁰C).  The narrow constriction prevents the mercury from flowing back into the bulb immediately after the thermometer has been removed from the patient’s body. 3.       Platinum resistance thermometer: This thermometer depends on the variation in the electrical resistance of a conductor with temperature

3. The Thermocouple: A thermocouple consists of two different metals joined together by a circuit containing a galvanometer.  The working of a thermocouple depends on the variation of the electromotive force (e.m.f) between junctions.  The equation of the relationship is

, where a, b and c are constants

4. Constant – volume gas thermometer: This depends on the variation in the pressure of a gas at constant volume with changes in the temperature of the gas.

The equation is: P1/T1 = P2/T2

ABSOLUTE SCALE OF TEMPERATURE

Temperature has no property of direction but has magnitude or size, which depends on the scale being used.

The absolute scale of temperature is thermodynamic scale because it gives us the idea of the lowest possible temperature or absolute zero with the value of –273⁰K.

The absolute zero is the lowest possible temperature below which nothing can be cooled since temperature is the measure of the average or mean kinetic energy of the molecules of a substance.  It follows that as we subtract heat from a substance, its temperature drops and hence its kinetic energy until it eventually becomes zero under which the molecules remain stationary.

The Celsius seal is the most commonly used scale.  This scale is based on two fixed points – the lower fixed point (O⁰C) and the upper fixed point (100⁰C).  The gap between these points is called the fundamental interval.

CLASSWORK 1

1. Define (i) ice point (ii) steam point
2. Give five properties of  thermometric liquid
3. The length of mercury thread when it is at 0⁰C, 100⁰C and at an unknown temperature θ is 5mm, and 125mm respectively.  Find the value of θ

ASSIGNMENT 1

SECTION A

1. An un-graduated thermometer reads 2.0cm and 112.0cm at ice and steam points respectively. Determine the true temperature in Kelvin, when the thermometer reads 5.0cm (a) 303.0K (b) 300.0K (c) 278.0K (d) 30.0K (e) 30.3K
2. Clinical thermometer differs from other mercury in glass thermometers because it has I. a constriction II. A wide range III. A short range IV. A narrow bore (a) I and II only (b) I and III only (c) III and IV only (d) I, II and III only (e) I, III and IV only
3. A platinum resistance thermometer has a resistance of 4Ω at 0⁰C and 12Ω at 100⁰C. Assuming that the resistance changes uniformly with temperature, calculate the resistance of the temperature when the temperature is 45⁰C (a) 6.0Ω (b) 6.5Ω (c) 7.6Ω (d) 8.4Ω (e) 16.0Ω
4. The purpose of constriction in a clinical thermometer is to (a)prevent the mercury from expanding beyond the bulb (b) prevent the mercury from falling back into the bulb until required (c) enable the mercury to expand slowly (d) serve as the lower limit of the scale to be read (e) none of the above
5. Mercury has an advantage over other liquids as thermometric liquid because it (a) has low expansivity (b) has higher conductivity (c) vaporizes easily (d) relatively low freezing point (e) none of the above

SECTION B

1. What is temperature?
2. Distinguish between temperature and heat
3. Give three advantages of mercury over alcohol as a thermometric liquid
4. The pressure at ice point for a constant volume gas is 4.81×10⁴Pa. while that of the steam point is 6.48×10⁴Pa. what temperature will this thermometer indicate at 50⁰C