Heat Energy – SS2 Physics Lesson Note (Second Term)

Heat is a form of energy due to a temperature difference, measured in joules (J). Temperature is the degree of hotness or coldness of a body, measured in degrees Celsius (°C) or kelvin (K). Thermometers measure temperature using changes in the physical properties of materials, and they are fixed at two reference points, 0°C and 100°C.

Behavioural objectives

By the end of the lesson, students should be able to:

• define heat and temperature and state their units;
• state the upper and lower fixed points and explain the fundamental interval;
• name the types of thermometer, their thermometric properties and their advantages;
• describe the clinical thermometer and the working of the platinum resistance thermometer, the thermocouple and the constant-volume gas thermometer;
• explain the absolute scale of temperature and absolute zero.

What is heat?

Heat is a measure of the internal energy of a body. It is a form of energy due to a temperature difference. It is measured in joules (J).

What is temperature?

Temperature is the degree of hotness or coldness of a body. The unit of temperature is the degree Celsius (°C) or the kelvin (K).

How is temperature measured?

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

What are the fixed points?

The upper fixed point is the temperature of steam from pure water boiling at a standard atmospheric pressure of 760 mm of mercury. It is 100°C.

The lower fixed point is the temperature of pure melting ice at the standard atmospheric pressure of 760 mm of mercury. It is 0°C.

What are the temperature scales?

The difference in temperature between the upper and lower fixed points is called the 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, temperature is also measured in degrees Celsius and degrees Fahrenheit.

What are thermometers?

Thermometers are instruments used to measure temperature. They are named after the thermometric properties used in the measurement of temperature.

Each thermometer makes use of the change in a physical property of the material it is made of, to indicate temperature change.

Liquid-in-glass thermometer

The liquid-in-glass thermometer depends on the uniform expansivity of the liquid used with temperature change. Any liquid used as a thermometric liquid must:

• be a 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;
• not wet glass;
• expand uniformly.

Clinical thermometer

The clinical thermometer has a constriction and a short range (35°C to 43°C). The narrow constriction prevents the mercury from flowing back into the bulb immediately after the thermometer has been removed from the patient’s body.

Platinum resistance thermometer

This thermometer depends on the variation in the electrical resistance of a conductor with temperature, as expressed below:

(R_t – R₀) / (R₁₀₀ – R₀) = t / 100

• R_t = resistance at t°C
• R₀ = resistance at 0°C
• R₁₀₀ = resistance at 100°C

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 E = a + bt + ct², where a, b and c are constants.

Constant-volume gas thermometer

This thermometer 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

What is the absolute scale of temperature?

Temperature has no direction but has magnitude, which depends on the scale being used. The absolute scale of temperature is thermodynamic because it gives the idea of the lowest possible temperature, or absolute zero, with the value of -273°C (0 K).

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. As we remove heat from a substance, its temperature drops and so does its kinetic energy, until the kinetic energy becomes zero. At that point the molecules remain stationary.

What is the Celsius scale?

The Celsius scale is the most commonly used scale. It is based on two fixed points: the lower fixed point (0°C) and the upper fixed point (100°C). The gap between these points is called the fundamental interval.

Evaluation

1. Define heat and state its unit.
2. State the upper and lower fixed points of a thermometer.
3. Why does a clinical thermometer have a constriction?
4. What is absolute zero?

Assignment

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 the ice point for a constant-volume gas thermometer is 4.81 × 10⁴ Pa, while that of the steam point is 6.48 × 10⁴ Pa. What temperature will this thermometer indicate at 50°C?