# Heat and Temperature physics: Introduction

This article is written for the Tnpsc exam for the topic ‘Heat’, in General studies. Science part (Physics). This note is for quick revision purpose only, kindly refer to Samacheer Kalvi Book.

The measurement of warmness(hotness) or coldness of a substance or object is called its temperature. Temperature is also a measure of the average kinetic energy of particles in an object.

Temperature is related to how fast the atoms in a substance/object are moving. Figure 1: Kinetic energy during a change in temperature in gases

## Temperature Units

Degree Celsius, Fahrenheit, Kelvin

## Degree Celsius

It is written as °C and read as Degree. Ex 43° C, Celsius is also called Centigrade.

## Fahrenheit

Fahrenheit is written as °F. For Example 70 °F.

## Kelvin

Kelvin is written as K, For example, 175 K, SI unit of Kelvin is K.

## Measuring Temperature

The temperature of the object is approximated with the kinetic energy of the substances. The high temperature means molecules inside objects move at a faster rate.

Molecules in any substance are very small to analyze and calculate their Kinetic energy to measure the temperature.

There is a need to use an indirect method to calculate the temperature. Any objects expand when heated, shrink when cooled using this property temperature of objects is calculated. This principle is employed in thermometers.

## Thermometer

The liquid inside the thermometer expands when heated and contracts when cooled using this property, the thermometer is used to measure the temperature.

There are various kinds of a thermometer, the common one is a thin glass tube with some liquid.

## Fahrenheit Scale

It is termed after the name of German Physicist Daniel Gabriel Fahrenheit. In the Fahrenheit Scale, the Freezing point of water is taken as 32°F and a boiling point of 212 °F.

Thermometers that use Fahrenheit are calibrated from 32°F to 212 °F. Fahrenheit is mostly used to measure human body temperature.

## Kelvin Scale

This scale is termed after Lord Kelvin. Kelvin is the S.I unit of measuring temperature. Kelvin is written as K also called as Absolute Scale as it is from absolute zero temperature.

“Kelvin is defined as 1/273.16 of the triple point temperature”.

## Conversion of Celcius into Fahrenheit and Kelvin

(F – 32)/ 9  = C / 5, where F – Fahrenheit, C- Celsius, K-Kelvin

K = 273.15 +C

## Some Facts on Heat and Temperature

The average body temperature of humans is 37 °C. The maximum and minimum temperature of the previous day is reported by a thermometer called Maximum-Minimum thermometer in weather Stations.

1032 Kelvin is the earliest temperature during Big Bang.

“373.15 Kelvin is the boiling point of water”.

329.85 Kelvin is the hottest temperature ever recorded on earth naturally.

310.15 Kelvin is the average human temperature.

273.15 Kelvin is the freezing point of water.

178.45 Kelvin is the coldest natural temperature ever recorded on the earth.

1 Kelvin is the temperature Boomerang Nebula maintains which is the coldest known temperature on earth.

0 Kelvin is the absolute zero temperature.

The conversation of Celsius to Kelvin is easy but the conversation of Fahrenheit to Kelvin is not easy. To solve this issue Rankine Scale.

R = F + 459.67

## Effects of Heat

Each molecule exerts a force of attraction on other molecules and possesses potential energy.

This sum of the kinetic and potential energy is called the internal energy of the molecules. This internal energy when flowing out is called Heat Energy.

This energy is more in hot objects and less in cold objects.

When a substance is heated things happen are Expansion, Change in temperature, Change in the state, Chemical change, etc.

### Expansion

When heated molecules gain energy and vibrate and force other molecules apart due to this expansion takes place.

This is the reason some gap is left between railway tracks. During summer the tracks expand by heating. Expansion is maximum in Gases followed by liquids and then solids.

### Change in Temperature

When heat energy is added to the substance, the kinetic energy of the particles increases, and particles move at higher speed causes a temperature rise.

When cooled, heat is removed and the molecules slow down and lose heat, and the temperature falls.

### Change in State

When a solid is heated turns into liquid and then gas. Example: Ice Cube when heated turns into water and then water vapor.

Reverse in the state when the heat is removed.

### Chemical Changes

Heat plays a major role in Chemical reactions as heat is a form of energy. Some chemical reaction needs heat to begin and continue.

Such as Cooking, burning of wood, etc.

“Heat increases the speed of the chemical reaction”.

## Transfer of Heat

Heat does not stay in one place or the place we put. Heat goes from one place to another.

The cold thing gets hot and hot things get cold. Heat gets transferred from one place to another till its temperature reaches equilibrium.

The heat gets transferred from the object of higher temperature to the object of lower temperature. Heat transfer takes place in three ways as Conduction, Convection, and Radiation.

## Specific Heat Capacity

The land is cool in the morning and hot during the day. But water in the lake or sea is almost during the whole day as well in the afternoon.

Both the land and water bodies such as lakes or sea are subjected heat of the Sun, Uniformly but they react to the heat differently.

It is because of the difference in their properties. The conclusion we come to know is the amount of heat energy absorbed or lost by the object or body is determined by:

Mass of the body, Change in temperature of the body, and Nature of the material of the body.

Q α mΔT

Q = mCΔT, Where Q is the quantity of heat energy, ΔT is the temperature change, C is the specific heat capacity and m is the mass of the body.

The Specific heat capacity of a substance is represented the quantity of heat needed to increase the temperature of 1kg of the substance by 1°C or 1K.”

Jkg-1 K-1 is the SI unit of Specific Heat Capacity.

Among all the substances, water has the highest specific heat capacity and its value is 4200 J/Kg0 C.

Due to the highest specific heat capacity of water, it absorbs large heat to get heated. Due to this property water is used as a coolant in car radiators and factories to keep the machinery and spare parts cool.

It is the same reason that water stays cool throughout the day in the sea or lake.

Water in various forms has different specific heat capacities. Liquid Water = 4200 JKg-1 K-1 , Solid Water or Ice  = 2100 JKg-1 K-1 , Gaseous Water or Steam  = 460 JKg-1 K-1

## Heat Capacity or Thermal Capacity

“Heat capacity or thermal capacity is defined as the quantity of heat energy needed to increase the temperature of the body by 1°C”.

Heat Capacity or Thermal Capacity = Quantity of heat required / Rise in temperature.

C’= Q/t, Where C is the heat capacity, Q is the quantity of heat required and t is the temperature rise.

SI unit of heat capacity is J/K. It is also expressed in cal/°C, kcal/oC or J/°C.

• Note: Specific heat capacity is represented by C and Heat Capacity is represented by  C’. And also this C is not Celcius, it is Capacity.

## Change of State

Three forms of matter are solid, liquid and Gas. The things in the universe exist in these three forms.

The Change of state is defined as change from Solid to liquid, liquid to gas, gas to a liquid, liquid to solid, etc. The change of state depends on temperature, pressure, transfer of heat.

## Melting-Freezing

The process in which a solid is converted to a liquid by absorbing heat is called melting or fusion. The temperature at which solid changes to liquid is called Melting Point.

The reverse of Freezing is Melting. The process in which liquid changes to solid is called Freezing.

The temperature at which liquid changes to solid is called Freezing Point. The Melting point and Freezing point of water is 0 °C.

## Boiling-Condensation

The process in which liquid is converted into gas is called boiling or vaporization. The temperature at which liquid changes to gas is called the Boiling point.

The process in which gas is converted into a liquid by releasing heat is called condensation. The temperature at which a vapor changes liquid is called the condensation point.

The boiling point and condensation point of water is 100 °C.

## Sublimation (solid turns to gas)

The process in which solid directly convert to gas, without being converted to a liquid state is called Sublimation. Examples of Sublimation are Iodine, Dry Ice, Naphthalene Balls, etc.

Simply,

• Solid to Gas is Sublimation
• Gas to Liquid is Condensation
• Liquid to Solid is Freezing
• Gas to Solid is Deposition
• Water is the only substance on the earth which is found in all the states such as solid, liquid and gas.

## What is latent heat?

Latent Heat is called Hidden Heat. Ice till becomes liquid has a temperature of 0°C when heated.

Water, till becomes vapors, has a temperature of 100°C when heated.

Latent heat is the quantity of heat energy absorbed or released by a substance when there is a change in its physical state without any change in its temperature”.

### Latent heat of fusion and vaporization

#### Latent Heat of Fusion definition

Heat energy is absorbed by solid during melting, and an equal amount of heat energy is released by the liquid when freezing, without any temperature change is called the latent heat of fusion.

#### Latent Heat of Vaporization definition

A liquid absorbs the heat energy during vaporization, and an equal amount of heat energy is liberated by the vapor during condensation, without any temperature change is called the latent heat of vaporization.

## Specific Latent Heat

Amount of heat energy absorbed or released by a unit mass of substance during a change of state without causing any temperature change.

SI is  J/Kg.

## Some point to know about Heat

The technique used to measure the amount of heat involved in a physical or a chemical process is known as calorimetry.

The calorimeter is a device used to measure the quantity of heat absorbed or released by a substance. The calorimeter is used to measure the heat capacity of liquids.

The first ice-calorimeter was used in 1782 by Antoine Lavoisier and Pierre-Simon Laplace to calculate the heat generated by different chemical changes.

The thermostat is used various electrical appliances as a sensor or controller, that used a switch on or switch off when the device or the thermal environment reaches a certain temperature.

The thermostat is used in appliances such as refrigerators, Air conditioners, Oven, Room Heater, etc.

Thermos Flask or Vaccum Flask is insulating storage used to keep contents such as tea or coffee hot or cold.

Vacuum Flask is thermal insulated, has double walls where is the gap between two walls is the vacuum. The inner wall of the flask is silvered. The vacuum prevents conduction and convection the substance does not lose its temperature.

## Isothermal Process

The Process that causes a change in the system and change to the system such as evaporation, melting, etc, at a constant temperature. i.e. there is no change in the temperature of the system.

## Isothermal process example in daily life

The reaction inside the refrigerator, Melting of Ice at zero degree Celsius, evaporation of rivers, seas, etc