Thermal Physics -> Chapter 1 (Thermometry)

Briefly define the term "heat".

It's the effective flow of energy from a place of high temperature to a place of low temperature.

Briefly describe the concept of thermal equilibrium.

When two bodies are in thermal contact, a flow of heat occurs, and their temperatures become equal. Then the heat flow stops. The two bodies are said to be in thermal equilibrium.

What is meant by the term 'absolute zero of temperature'?

This may be defined as the temperature at which atoms have minimum or zero energy.

Briefly explain the zeroth law of thermodynamics.

If two bodies A and B are in thermal equilibrium with another body C separately, then A and B are also in thermal equilibrium.

Briefly define temperature.

The property which decides whether a body is in thermal equilibrium with another body

In every thermometer, a property changes with the temperature. Give examples for this.

Volume change in a liquid

Change in length of a solid

Change in pressure of a gas in a constant volume

Change in volume of a gas at constant pressure

Change in resistance of a conductor

Change of colour of a hot object

Briefly describe a method of defining temperature scales and give an example of it.

One method of defining temperature scales is using two fixed temperatures, which can be produced again and again. For example, in both Celsius and Fahrenheit temperature scales, the melting point and boiling point of water under atmospheric pressure are used as the two fixed points.

Explain why the Celsius temperature scale is also known as the centigrade temperature scale.

In the Celsius temperature scale, the melting point of water under atmospheric pressure has been defined as 0°C, and the boiling point of water has been defined as 100°C. The gap between these two points has been divided into 100 divisions, which is why the Celsius temperature scale is also known as the centigrade temperature scale.

What is the melting point of water at atmospheric pressure, and the boiling point of water at atmospheric pressure in Fahrenheit?

32°F and 212°F, respectively.

What is the relationship between Celsius and Kelvin?

0°C = 275K

State some important features of thermometric properties

Has to be a single value function of temperature

To be a continuous function of temperature

To be a linear function of temperature as much as possible

Briefly define "triple point of water"

The temperature at which pure water, water vapour and ice all exist together in thermal equilibrium.

What is the triple point of water?

273.16 K or 0.01°C

State the relationship between the change in thermometric property and the corresponding temperature difference.

Theta= (Xtheta - X0/ X100 - X0) X 100 , where Xtheta -> value of the thermometric property at theta, X0 -> value of thermometric property at 0°C, X100 -> value of thermometric property at 100°C

State the relationship between the absolute temperature (Kelvin) and the triple point of water.

T = (XT/XTr) X 273.16, where T-> unknown temperature that you want to find in Kelvin, XT -> Value of a thermometric property at the unknown temperature of T, and XTr -> The value of the same thermometric property at the triple point of water

What is the thermometric property used in mercury thermometers and alcohol thermometers?

The expansion of liquid when heated in a capillary tube.

State the 3 characteristics of a thermometer.

Accuracy

Sensitivity

Response time

Briefly define the accuracy of a thermometer.

Accuracy indicates the exact temperature of a substance.

Briefly describe the relationship between the accuracy of a thermometer and the volume of the liquid in the thermometer.

If the amount of liquid in the thermometer is large, it needs more heat to expand so the thermometer will only indicate the remaining temperature, decreasing its accuracy.

What makes a thermometer sensitive?

If a thermometer can accurately measure even a small temperature change, it's a sensitive thermometer.

State the criteria for a glass thermometer to be sensitive, and briefly describe each one.

1.) The volume of the bulb should be large

When the volume is large, the expansion due to the increase in temperature is also large. Therefore, it indicates a greater length, so for a small temperature change, there is a noticeable change. But if the bulb is unnecessarily large, it takes more energy and time to get heated, which is a disadvantage because it may indicate a lower value for the temperature and decrease the response time of the thermometer.

2.) The radius of the capillary tube should be smaller

For a given increase in volume, the length of the liquid column is high.

3.) The coefficient of volume expansion of the thermometric liquid should be high

For a given temperature, the expansion of volume is directly proportional to the coefficient of volume expansion of that liquid. For example, the coefficient of volume expansion of alcohol is 6 times the coefficient of volume expansion of mercury. Hence, an alcohol thermometer is more sensitive than a mercury thermometer.

What makes a thermometer a quick-responding thermometer?

If any thermometer can read the required temperature quickly, that thermometer is known as a quick-responding thermometer.

State the criteria that are needed for a glass thermometer to be quick-responding and briefly describe each criterion.

1.) The volume of the bulb should be smaller. When the volume of the bulb is smaller, it takes only a small time to reach the required temperature.

2.) The liquid should have a higher heat conductivity.

When a liquid has a higher heat conductivity, the whole liquid in the bulb reaches the required temperature much faster.

3.) The walls of the bulb should be thin.

4.) The bulb should have a cylindrical shape instead of a spherical shape.

While a sphere has the smallest possible surface area for any given volume, for the same amount of liquid, an elongated cylindrical bulb exposes more surface area to the surrounding environment. More surface area means the liquid inside absorbs or releases heat more quickly, lowering the response time.

Give examples of various types of thermometers.

Mercury-in-glass thermometer

Constant-volume gas thermometer

Constant-pressure gas thermometer

Platinum resistance thermometer

Thermocouple thermometer

State the advantages of a mercury-in-glass thermometer.

Easy to use and easy to transport.

Readings can be taken directly.

Low cost.

Heat transfers quickly to the mercury since mercury is a good conductor.

Since mercury is opaque, it can easily be seen, and it also has the quality of not wetting the glass wall of the capillary tube.

State the disadvantages of a mercury-in-glass thermometer.

There may be errors due to the non-uniformity of the bore of the capillary tube.

Mercury in the bore is not at the same temperature as that in the bulb.

Due to the vapour pressure of mercury, errors could be present.

Errors may occur due to permanent deformation of the bulb in long-term usage.

Accuracy is not of a higher degree.

Briefly describe how a bimetallic thermometer works and is made.

When the temperature of a metal rod increases, it expands, but this expansion is very small, so it can be used as a thermometer. A piece of bimetal, which is made out of two different metals, can be used for this purpose. One end of a spring, which is made out of bimetal, is fixed, and an indicator is attached to the other end. Then it can be used as a thermometer.

What is the thermometric property used in a constant-volume gas thermometer?

The change in pressure of a constant volume of gas with the temperature is the thermometric property.

Briefly describe what a thermocouple thermometer is and how it works.

A junction of two different metals (e.g. copper-iron, nickel-nichrome, platinum-platinum/rhodium, or copper-constantan) develops a small thermoelectric e.m.f. (the Seebeck effect) when heated. Two junctions are used: a cold junction held at 0 °C in melting ice, and a hot junction placed at the temperature being measured. Since the e.m.f. is only a few millivolts, it is read with a sensitive millivoltmeter, or more accurately with a potentiometer.

What are the common metal pairs used in a thermocouple?

1. Copper and iron

2. Nickel and nichrome

3. Platinum and platinum-rhodium alloy

4. Copper and constantan

What is the temperature range of a thermocouple?

−200 °C to 1400 °C

State the advantages of a thermocouple thermometer.

1. The thermal capacity of the junction is very small. Therefore, it can be used to measure varying temperatures because of its quick response.

2. Suitable for measuring the temperature of a small body or a small amount of liquid.

3. Easy to construct.

4. Able to take direct measurements when a calibrated millivoltmeter is used.

State three disadvantages of a thermocouple

1.) A potentiometer (needed for accurate readings) is hard to use precisely

2.) Difficult in practice to keep the cold junction exactly at 0 °C

3.) The e.m.f.-temperature relationship becomes clearly non-linear above about 400 °C, which limits accuracy at higher temperatures

What is a thermistor?

An electronic device whose electrical resistance changes with temperature.

How is a thermistor used to measure temperature?

The change in resistance is converted into a change in voltage or current, which is fed into an electronic circuit that produces a digital temperature reading.

What are the two types of thermistors?

NTC (Negative Temperature Coefficient): resistance decreases as temperature rises. This is the type used in most practical thermistors.

PTC (Positive Temperature Coefficient): resistance increases as temperature rises.

State the advantages of using a thermistor.

A thermistor is 20 times more accurate than a normal resistance thermometer.

Since the resistance is in kilohms, the resistance of the connecting wires is neglected.

Since the heat capacity is small, it's quick-responding, and it doesn't affect the temperature.