Heat and heating effect of electric current

Heat and Temperature – Principle of spontaneous combustion - Hygrometry- Rain,

Snow and Hail - Boyles Law - Charles Law - Expansion of solid and liquid-Anomalous
expansion of water-Transmission of heat - Conduction-convection-radiation- Laws
of thermodynamics- Carnot engine - Heating effect of electric current – *Joule’s law
of electric heating – Experiment to verify Joule’s law –– Applications of heating
effect – electrical fuse – arc lamps.

HEAT AND TEMPERATURE

Heat is a form of energy, while temperature is the degree of hotness or coldness of a

body. Heat is the energy transferred between a system and its surrounding by the virtue
of temperature difference. Heat flows from hotter body to colder body when in contact
until the temperature becomes equal. The bodies are said to be in equilibrium.

LAWS OF THERMODYNAMICS
Zeroth law

When a body ‘A’ is in thermal equilibrium with another body ‘b’, and also separately in
thermal equilibrium with a body ‘C’, then body ‘B’ and ‘C’ will also be in thermal
equilibrium with each other.

First law

The internal energy of a system has to be equal to the work that is being done on the
system, plus or minus the heat that flows in or out of the system and any other work
that is done on the system

ΔU = Δ Q – Δ W (or)
ΔQ = ΔU + Δ W

Where ΔU is the change in the internal energy, ΔQ is the heat added to the system, and
ΔW is the work done by the system.
First law of thermodynamics means that heat energy cannot be created or destroyed. It
can, however, be transferred from one location to another and converted to and from
other forms of energy.The most common practical application of the First Law is the heat
engine

Second law
The second law defines the direction in which a specific thermal process can take place.
The second law of thermodynamics states that it impossible for a self acting system in a
cyclic process unaided by external agency to make pass heat from one body to another at
high temperature.
The natural tendency of the heat is to flow from the high temperature to the low
temperature. It is not possible to extract heat from a cold body by cooling it further
without any external agency.

Entropy
The second law of thermodynamics is sometimes called the
law of entropy, as it introduces the important property called entropy. Entropy can be
thought of as a measure of how close a system is to equilibrium; it can also be thought of
as a measure of the disorder in the system.
Natural tendency of a physical system is to go towards higher entropy. Thus, entropy
always increases. It applies only to assembly of particles and not individual particles.

Third law
It states that any process cannot reach absolute zero temperature in a finite number of
steps and within a finite time. The third law of thermodynamics has two important
consequences:
It defines the sign of the entropy of any substance at temperatures above
absolute zero as positive, and
It provides a fixed reference point that allows us to measure the absolute
entropy of any substance at any temperature.
It impossible to make an heat engine with 100% as it is not possible to attain
absolute zero temperature.
According to third law the entropy of a perfect crystal at a temperature of zero Kelvin
(absolute zero) is equal to zero. At a temperature of zero Kelvin, the following
phenomena can be observed in a closed system:
• The system does not contain any heat.
• All the atoms and molecules in the system are at their lowest energy points.
• Therefore, a system at absolute zero has only one accessible microstate – it’s
ground state.

Principle of spontaneous combustion

Spontaneous combustion is an oxidation reaction that occurs without an external heat

source. The process changes the internal heat profile of the material leading to a rise in
temperature. This can eventually lead to open flame and burning of the material.
Spontaneous combustion of coal is a fire initiated by the oxidation of coal. Coal fires require
three basic elements to exist as shown in figure 1
 Figure : Fire triangle
The process leading to spontaneous combustion can be summarised as follows:
• Oxidation occurs when oxygen reacts with the fuel, i.e. coal
• The oxidation process produces heat
• If the heat is dissipated, the temperature of the coal will not increase
• If the heat is not dissipated then the temperature of the coal will increase
• At higher temperatures the oxidation reaction proceeds at a higher rate
• Eventually a temperature is reached at which ignition of the material occurs.
Heat dissipation depends on the thermal conductivity of coal and the surrounding
rock, on convection processes caused by wind and barometric changes in the atmosphere
and on the minor and major fracture density in the rock mass. The tendency of coal to self-
heat is a function of both intrinsic factors (coal type, geological setting and environmental
conditions) and extrinsic factors (mining related).

Controlling spontaneous combustion

Effective control of spontaneous combustion can be achieved by using a combination of
techniques. The control measures that can be applied in South African collieries can be
listed in three groups:
Control measures to reduce or eliminate oxygen from the process
• Sealing agents
• Dozing over
• Buffer blasting
• Cladding of the highwall.

Control measure to reduce the temperature and hence the reaction rate
• Water cannons onto the highwall and in front of the dragline and during coaling
• Nitrogen injection into old workings
• Carbon dioxide injection into old workings.

Removal of the fuel

• Excavation of hot or burning material.
The efficacy of these control measures is dependent on individual situations such as mining
layouts and the extent of the spontaneous combustion problem.
HYGROMETRY
The meaning of HYGROMETRY is a branch of physics that deals with the measurement of
humidity especially of the atmosphere.

Types of Precipitation
Rain
Rainfall occurs when the liquid water falls from the clouds. Water droplets in the clouds
merge together and eventually fall due to increased weight.

Snow
Snowfall occurs at places with low temperatures. The ice crystals in the clouds stick
together to form snowflakes. These snowflakes fall as snowfall when they become heavy
enough to fall under gravity. Snowfall with large snowflakes occurs at places where the
temperature is slightly above freezing point and the air is moist. Powdery snow occurs in
places with very cold, dry air.

Hail
Hailstones are formed within a thunderstorm. The thunderstorm clouds must have a great
vertical height for hail to form. Also, the upper portion of the cloud must have a below-
freezing temperature.

Boyle's Law
Boyle's Law states that the pressure (P) of a gas is inversely proportional to the volume (V).
This law is valid as long as the temperature and the amount of gas are constant. Any units
will work here:
PV = k
The constant, k will depend on the number of moles and the temperature.
 Charles's Law
Charles's Law states that the Volume (V) of a gas is directly proportional to the
temperature (T). This law is valid as long as the pressure and the amount of gas are
constant. The temperature must be an absolute temperature:
V/T = k (constant)
The constant, k, will depend on the number of moles and the pressure.

Expansion of Solids, Liquids and Gases

The separation of atoms and molecules is more when the temperature increases. This is
thermal expansion of that material.
Thermal expansion is defined as a phenomenon which is observed in solids, liquids, and
gases. In thermal expansion, an object or a body expands on the basis of application of heat
(temperature). Thermal expansion is the tendency of a body to change its dimension which
are either in length, area, density, or volume due to heat When a substance is heated the
kinetic energy of the substance increases.
Thermal Expansion Types
There are three types of thermal expansions which we will learn.
1. Linear Expansion
2. Volume Expansion
3. Area Expansion
Linear Expansion
When the length changes because of heat then it is known as linear expansion.
l/lo α T or l/lo=LT
Here,
l= Length Change, L= Coefficient of length expansion, lo= Original Length,T= Difference in
temperature
Coefficient of Linear Expansion is defined as rate of change of length per unit temperature.
A=dl/dT
Volume Expansion
When the volume changes because of heat then it is known as volume expansion.
v/v0=VT
v= Volume change, V= Coefficient of volume expansion, v0= Original Volume
T= Difference in temperature
The Coefficient of Volume Expansion is defined as the rate of change of unit volume per
unit temperature.
Area Expansion
When the area changes because of heat then it is known as area expansion.
a/a0 =AT
Here,
a= Area change, A= Coefficient of Area expansion, a0 = Original Area
T= Difference in temperature
Coefficient of Thermal Expansion
Coefficient of Thermal Expansion is defined as a relative expansion of a material which is
divided by the change in temperature.
 Thermal Expansion of Solids
For solids, thermal expansion is expressed as a change in height, thickness, and length.
Whereas for both liquids and gases, the coefficient of volumetric expansion is more useful.
In general, if the material is a fluid (liquid or gas), we can determine it in terms of volume
change.
Applications of thermal expansions
There are many applications of thermal expansions some of which are as follows
1. Thermal expansion used in thermometers to measure the temperature.
2. Thermal expansion is used for the removal of tight lids.
3. Rubber spacers are required for windows with metal-frame.
Mercury Thermometer
Thermal expansion is the basic principle on which a thermometer works. The mercury in
the reservoir which is present at the bottom of the thermometer is immersed in a cold or
hot object. After that the mercury in the thermometer expands and contracts and changes
the level which is indicated on the thermometer.

Anomalous Expansion of Water

A common observation seen in the behaviour of the substances is that they expand when
heated as the density decreases and vice versa when the material is cooled. This is how
substances generally react to heat. Let us now look at how water behaves when heated.
The general tendency of cold water remains unchanged until 4o C. The density of water
gradually increases as you cool it. When you reach 4oC, its density reaches a maximum.
What water does next will astound you. When you cool it further to make some ice, i.e. 0oC,
water expands with a further drop in temperature, meaning the density of water decreases
when you cool it from 4oC to 0oC. The below graph explains this behaviour.

The effect of this expansion of water is that the coldest water is always present on the
surface. Since water at 4oC is the heaviest, this water settles on the bottom of the water
body and the lightest, i.e. the coldest layer, accumulates on the top layer. So in the winter,
the top of the water is always the first to freeze over. Since ice and water both are bad
conductors of heat, this top layer of ice insulates the rest of the water body from the cold of
the winter, thereby protecting all the life in the water body. Now you can truly comprehend
how essential the anomalous properties of water are for life.

Transmission of Heat
Heat energy can be transferred from one body to the other or from one location in a body to
the other. Study of the techniques and methods adopted to transfer heat energy is known as
‘Heat Transfer’. To facilitate heat transfer between 2 bodies there needs to be a temperature
difference between them.This means that these bodies must be a 2 different temperatures one
higher than the other to allow heat to flow from one body to the other.
This means that no heat transfer occurs between 2 bodies which are at the same temperature.
At the same time, it is very important to note that heat only flows from a body at higher
temperature to a body at a lower temperature. Although this may look obvious, this law is
very important from the point of view of thermodynamics.

Let us say that you have prepared a cup of tea for yourself. The tea is very hot say at 80°C and
so you leave it in a room with a temperature of 25 ° C for some time to cool down. This is the
first law of heat transfer. Heat transfer will only take place between 2 bodies when they have a
substantial temperature difference.
Now, after some time you come back to find that the tea in the cup has cooled down to say
50°C and you have a sip of the same. This is the second law of heat transfer. Heat will only
flow from a body at higher temperature to a body at a lower temperature. It is not possible to
have a scenario where the heat flows from the room at 25° c to the cup of tea at 80° C and heat
it even further.
These techniques and methods discussed below are observed in nature and thus have been
generalized for all things while under consideration for the purpose of the study. However, no
observation against these has been ever recorded or observed thus establishing their
credibility as truthful and applicable at all times.
Heat transfer takes place in 1 of the three ways namely: Conduction, Convection and Radiation
We will discuss each of these methods in detail.

Modes of Heat transmission

Conduction
Process of conduction of heat from a body to the other due to the transfer of heat without the
actual movement of molecules. The molecules vibrating at their mean positions. The bodies
through which the heat transfer must be in contact with each other. There is no actual
movement of matter while transferring heat from one location to the other.
Conduction occurs usually in solids where molecules in the structure are held together
strongly by intermolecular forces of attraction amongst them and so they only vibrate about
their mean positions as they receive heat energy and thus pass it to the surrounding
molecules by vibrations.
Convection
Convection is the mode of heat transfer which occurs mostly in liquids and gases.In this
method, heat transfer takes place with the actual motion of matter from one place within the
body to the other. Often when we boil water we have seen bubbles and currents develop in
the water on careful observation.
This is an apt example of the convection process. The hot water at the bottom becomes lighter
and moves upwards forcing the cold and denser water at the top to come down and thus get
heated up.

Radiation
Radiation is another form of heat transfer. It does not require any medium and can be used for
transfer of heat in a vacuum as well. This method uses electromagnetic waves which transfer
heat from one place to the other. The heat and light from the sun in our solar system reach our
planet using radiation only.
In fact, radiation is the most potent method of heat transfer. In winters when we sit near a fire
we feel warm without actually touching the burning wood. This is possible by radiation only.

Heating effect of electric current

When an electric current is passed through a conductor, it generates heat due to the
hindrance caused by the conductor to the flowing current. The work done in overcoming
the hindrance to the current generates heat in that conductor.
Ex: The electric iron, kettle, toaster, heater, etc.
JOULE’S LAW OF HEATING
Joule in 1841 experimentally studied the heating effect of electric current. He found that
the amount of heat
It states that, when a current ‘I ' passes through a conductor of resistance ‘r’ for time ‘t’ then, H
produce in a conductor due to the flow of current,
(i) is directly proportional to the square of the current flowing through the conductor i.e.
H∝I2
(ii) is directly proportional to the resistance across the conductor, H∝R
(iii)is directly proportional to time for which the current is passed, H∝t
Mathematically,
H∝I2 RT
Experimental Verification of Joule's Law of Heating
Joule's Law of heating
According to Joule's Law of heating, heat generated in a conductor is given by:
H=VIt=I2Rt
Apparatus
Lagged beaker or calorimeter with a lid, heating coil, battery or low voltage power
supply, rheostat, ammeter or multimeter, thermometer, stopwatch, balance.
Procedure
1.Put sufficient water in a calorimeter to cover the heating coil. Set up the circuit as
shown.
2.Note the temperature.
3.Switch on the power and simultaneously start the stopwatch. Allow a current of 0.5 A
to flow for five minutes. Make sure the current stays constant throughout; adjust the
rheostat if necessary.
4.Note the current, using the ammeter.
5.Note the time for which the current flowed.
6.Stir and note the highest temperature. Calculate the change in temperature
7.Repeat the above procedure for increasing values of current I, , taking care not to
exceed the current rating marked on the rheostat or the power supply. Take at least six
readings.
8.Plot a graph of Temperature (T) and I

Analysis:
Draw a graph, on graph paper, of Rise in Temperature versus Current squared. Start both
axes at zero and draw a "best fit" line through the data points. From the graph we can find
the Heating rate of the coil

S. Current (I)2 Temperature

No.
Applications of Joule heating
(i) Electric heating device

Electric iron, electric heater, electric toaster are some of the appliances that work on the
principle of heating effect of current. In these appliances, Nichrome which is an alloy of
nickel and chromium is used as the heating element for the following reasons.

1. It has high specific resistance

2. It has high melting point
3. It is not easily oxidized

(ii) Fuse wire

Fuse wire is an alloy of lead 37% and tin 63%. It is connected in series in an electric circuit.
It has high resistance and low melting point. When large current flows through a circuit
due to short circuiting, the fuse wire melts due to heating and hence the circuit becomes
open. Therefore, the electric appliances are saved from damage.

(iii) Electric bulb

Since the resistance of the filament in the bulb is high, the quantity of heat produced is also
high. Therefore, the filament is heated to incandescence and emits light. Tungsten with a
high melting point (3380oC) is used as the filament. The filament is usually enclosed in a
glass bulb containing some inert gas at low pressure.

Electric arc and electric welding also work on the principle of heating effect of current.

In some cases such as transformers and dynamos, Joule heating effect is undesirable.
These devices are designed in such a way as to reduce the loss of energy due to heating.