Q4-WEEK 6-Heat, Work and Energy

Heat, Work
and
Energy
The production of heat is one
of the driving forces of human
activity. We use it in daily
chores, like cooking and
drying and ironing clothes. It is
also needed to greater extent
in commercial and industrial
applications especially in the
field of manufacturing where
many substances are made
with the use of enormous
amounts of heat.
So, understanding how heat works and how
it interacts with various objects is studied
since it will help in making many different
processes efficient.
Temperature in the simplest sense is a measure of
hotness or coldness of an object; an object that is
hot has a high temperature, and a cold object
has a lower temperature.
Heat on the other hand is energy transferred
from one object to another due to a
difference of temperature between the two
objects.
Energy is needed to
perform work. When
work is done on an surroundings
object, energy is
transferred to that system
object. The transfer of
energy to the object work
increases its internal
energy. The internal
energy will later
decrease as the object heat
dissipates it as to heat.
James Prescott Joule investigated in mechanical
equivalent of heat. He set up an apparatus in its
simplified form. The weight were made to fall
through a certain distance.
In his most famous experiment. Joule attached
some weights to strings and pulleys and
connected them to a paddle wheel inside an
insulated container of water. Then he raised
the weights to an appropriate height and
slowly dropped them. This friction
generated heat, and the temperature of the
water began to increase.
Using this device, he
found out that 4.194 J is
equivalent to 1 calorie.
A calorie is a unit of
energy that is
equivalent to amount
of energy needed to
raise the temperature
of 1g of water by 1°C,
ideally measured from
14.5°C to 15.5°C.
Conduction is the transfer
of energy from one
molecule to another by
direct contact. This
transfer occurs when
molecules hit against
each other, similar to a
game of pool where one
moving ball strikes
another, causing the
second to move.
Convection is the
movement of heat by a
fluid such as water or
air. The fluid (liquid or
gas) moves from one
location to another,
transferring heat along
with it. This movement
of a mass of heated
water or air is called a
current.
Radiation is the transfer of heat by
electromagnetic waves. When you stand in the
sun, you are warmed by the electromagnetic
waves that travels from the sun to Earth.
How is energy conserved
when you eat breakfast?
The chemical energy in
food will be converted
into mechanical energy
that enables you to do
work. But not all the
chemical energy is
converted to
mechanical energy,
some will be converted
in the form of heat.
First Law of
Thermodynamics
states that: The
change in internal
energy of a system is
equal to the
difference between
the heat taken in by a
system and the work
done by the system.
Heat transfer, a less organized process, is
driven by temperature differences. Work, a
quite organized process, involves a
macroscopic force exerted through a
distance.
Heat transfer into a system, such as when the Sun
warms the air in a bicycle tire, can increase its
temperature, and so can work done on the
system, as when the bicyclist pumps air into the
tire.
The law is expressed as:
∆U = Q – W
where,
Q = the amount of heat flowing into the
system
W =the net work done by the system
∆U = the change in the system’s internal
energy
∆U = Q – W
Q is positive (+) if the heat added to the system

Q is negative (-) if heat leaves the system
W is positive (+) if work is done by the system
W is negative (-) if work is done on the system
1. 3000 J of heat is added to a system and 2500 J
of work is done by the system. What is the change
in internal energy of the system?
Heat (Q) = +3000 Joule
Work (W) = +2500 Joule
The change in internal energy of the system :
ΔU = 3000-2500
ΔU = 500 Joule
Internal energy increases by 500 Joule.
2. 2000 J of heat is added to a system and 2500 J
of work is done on the system. What is the
change in internal energy of the system?
Heat (Q) = +2000 Joule

Work (W) = -2500 Joule
ΔU = Q-W
ΔU = 2000-(-2500)
ΔU = 2000+2500
ΔU = 4500 Joule
Internal energy increases by 4500 Joule.
The internal energy would
also increase if work were
done onto a system. Any
work or heat that goes into
or out of a system changes
the internal energy.
However, since energy is
never created nor
destroyed (thus, the first law
of thermodynamics), the
change in internal energy
always equals zero.
If energy is lost by the system, then it is absorbed by
the surroundings. If energy is absorbed into a
system, then that energy was released by the
surroundings.
Energy is Energy is
absorbed into absorbed by
a system the
surroundings
states that heat transfer occurs spontaneously from
higher- to lower-temperature bodies but never
spontaneously in the reverse direction.
The law states that it is impossible for any process to
have as its sole result heat transfer from a cooler to
a hotter object.
Unless people interfere, thermal energy
or heat naturally flows in one direction only: from
hot toward cold. Heat moves naturally by any of
three means. The processes are known
as conduction, convection and radiation.
When you touch a cup
of hot coffee, heat
flows from a hotter cup
to a cooler hand.
When you hold ice, heat again flows from
hot to cold, in this case, from the warmer
hand to the ice.
The response of the
nerves in the hand to
the arrival or
departure of heat
prompts the brain to
identify the cup of
coffee as being hot
and the ice as being
cold.
The reverse can be done, i.e., heat flows from a
colder object to a hotter object, with the use of a
heat pump. It is a device that allows heat to
transfer from a colder reservoir to a warmer
reservoir which is not a natural process.
Heat Pump is a device that pulls the energy out of
air for the purpose of either heating or cooling a
space. This would seemingly violate the Second law
of thermodynamics, but the key reason it doesn't is
because this heat transfer is not spontaneous; it
requires an input of energy to do so.
What is the difference between spontaneous and
non-spontaneous process?
In spontaneous process heat flows from higher
temperature to a lower temperature. It does not
require any external energy to occur.
In non–spontaneous process heat flows from lower
temperature to higher temperature, it needs
mechanical energy to occur.
Distinguish the process as spontaneous or non-
spontaneous process. Write S if spontaneous and
NS if non-spontaneous on the blank.
1. Melting of Ice
2. Rusting of Iron
3. Marble going down the spiral.
4. Going up hill
5. Keeping the food fresh from spoilage

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Heat, Work

Q is positive (+) if the heat added to the system

Heat (Q) = +2000 Joule

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