Thermal physics ppt

Thermal Physics
Heat
Temperature
Specific Heat Capacity
Specific Latent Heat

Key words: heat energy, temperature, specific
heat capacity

By the end of this lesson you will be able
to:
Describe and explain the terms heat
energy and temperature.
State that the same mass of different
materials needs different quantities of
heat energy to change their temperature
by one degree celsius.
Carry out calculations involving specific
heat capacity.

Heat and temperature
Are heat and temperature the same
thing? How would you explain to someone
else the difference between the two?

Can you use a thermometer to measure
heat?

What are
thermometers
used for?

This cup of
coffee has been
made from
freshly boiled
water.
Temperature?
Heat?

Both cups of
coffee have
been made from
freshly boiled
water (twice as much
water has been used)

Heat?
Temperature?

Which is hotter…
and which contains more heat?

Heat
Heat is a form of energy.
It is a scalar quantity.
It is measured in joules (J).

Temperature
Temperature is an indication of
how hot or cold an object is.
It is a scalar quantity.
It is measured in degrees celsius (°C).

What happens when we heat a
substance?
Virtual Int 2 Physics – Mechanics & Heat – Heat - Introduction

When we heat the air in a
room, the kinetic energy of
the molecules increases.
The air molecules move
faster.

When we heat snow, the
vibration of the molecules
increases and it eventually
melts.

When we heat water, the
molecules vibrate faster.

substance?

When we heat a
substance it
becomes hotter
or changes state.

How much heat is needed to raise
the temperature of an object?
It might depend on…

Virtual Int 2 Physics – Mechanics & Heat – Heat – Specific Heat Capacity

How much heat is needed to raise
the temperature of an object?

As heat energy increases,
temperature increases.

E h αT

As mass increases, more
heat energy is required to
reach the same
temperature

Ehα m

It depends on the material!
Each material has its own
specific heat capacity (c).
Virtual Int 2 Physics – Mechanics & Heat –

specific heat capacity of a
material is the amount of heat energy
The

required to change the temperature of

1kg of the substance by 1°C.

E h = cm∆T


E h = cm∆T
Heat energy (J)

mass (kg)

Specific heat capacity

Change in
temperature (°C)

Units of Specific Heat Capacity
(J)

Eh
c=
m∆ T

Specific
heat
capacity
J/kg °C

(kg) (°C)

Virtual Int 2 Physics – Mechanics & Heat – Heat – Specific He

Activity 10
To measure the specific heat capacity of
different metals.
Find the power of the immersion heater.
What will you need to know?
How can you use the joulemeter and
stopclock to calculate the power of the
immersion heater?

Immersion heater
Energy used (J): 23200 J
Time (s): 12 x 60 = 720 s

E 23200
=
= 32W
Power (W): P =
t
720

What will you need to know to find the specific heat
capacity of the metal block?
Use the apparatus to find the specific heat capacity.
You must find the mass of the block in kg first.
Metal:
Mass (kg):
Power (W):
Time taken (s):
Temperature change (°C):
Eh (by calculation):

Units of Specific Heat Capacity
Remember E = Pt (J)

Eh
c=
m∆ T
Specific
heat
capacity
J/kg °C

(kg) (°C)

Check your value against the data sheet.
How does it compare?
Explain any difference:

RADIATOR

Why is water used as the coolant?

Because water has a high specific
heat capacity, it can take away a
lot of energy without boiling away.

Key words: heat energy, change of state,
specific latent heat

By the end of this lesson you will be able
to:
State that heat is gained or lost by a
substance when its state is changed.
State that a change of state does not involve
a change in temperature.
Carry out calculations involving specific latent
heat.
Carry out calculations involving energy, work,
power and the principle of conservation of
energy.

Cooling Curves
Virtual Int 2 Physics – Mechanics & Heat – Heat – Cooling Curves

Why does the temperature of the stearic
acid stop decreasing after a time?
The acid (red line) is changing from a
liquid to solid.
Heat energy is given out, without a change
in temperature.

substance?

When we heat the air in a room, the
kinetic energy of the molecules increases.
The air molecules move faster.

When we heat water, the molecules
vibrate faster.

When we heat snow,
the vibration of the
molecules increases
and it eventually
melts.

The temperature does not
always rise!

substance?
The temperature does not
always rise!
Where does the heat energy go?

We must supply energy to
change a solid to a liquid or a
liquid to a gas.
Virtual Int 2 Physics – Mechanics & Heat – Heat – Latent Heat (Fusion)

Melting Ice

Notice that energy is supplied to the ice but the
temperature remains at 0 °C.
167 kJ of energy is required to melt the 0.5 kg of ice
(to turn the solid to liquid) at 0 °C.
How much energy would be required to melt 1 kg of ice
at 0 °C?
334 kJ of energy. This is the specific latent heat of
fusion of water.

Specific Latent Heat of Fusion

When we heat a solid, it will melt and become a liquid.
At this point, energy is required but the temperature
does not rise.

The specific latent heat of fusion of a
substance is the heat energy required to
change 1kg of solid at its melting point to
1 kg of liquid – without a change in
temperature.

Specific Latent Heat of Fusion
The specific latent heat of fusion of a
substance is the heat energy required to
change 1kg of solid at its melting point to
1kg of liquid – without a change in
temperature.

Heat energy (J)

E h = ml
mass (kg)

Specific latent
heat

Units of Specific Latent Heat
(J)

Eh
l =
m
Specific latent
heat J/kg

(kg)

Boiling Water
Virtual Int 2 Physics – Mechanics & Heat – Heat – Latent Heat (Vaporisation)

Notice that energy is supplied to the water but the
temperature remains at 100 °C.
1130 kJ of energy is required to evaporate 0.5 kg of
liquid (to turn 0.5 kg to gas) at 100 °C.
How much energy would be required to evaporate 1 kg of
water at 100 °C?
2260 kJ of energy. This is the specific latent heat of
vaporisation of water.

Specific Latent Heat of
Vaporisation
Virtual Int 2 Physics – Mechanics & Heat – Heat – Latent Heat (Vaporisation)

When we heat a liquid, it will evaporate and become a
gas. At this point, energy is required but the
temperature does not rise.

The specific latent heat of vaporisation
of a substance is the heat energy
required to change 1kg of liquid at its
boiling point to 1kg ofvapour (gas) –
without a change in temperature.

Specific Latent Heat of
Vaporisation
The specific latent heat of vaporisation
of a substance is the heat energy
required to change 1kg of liquid at its boiling
point to 1kg of gas – without a change in
temperature.

Heat energy (J)

E h = ml
mass turned into
vapour (kg)

Specific latent
heat

Units of Specific Latent Heat
(J)

Eh
l =
m
Specific latent
heat J/kg

Remember m
is the mass
turned into
vapour.

(kg)

A solid is continuously heated until it
eventually reaches the gas state.
temperature / °C
The
What’s
temperature
happening of
the solid is
here?
rising

The
What’s
temperature
happening of
There’s no
the liquid is
What’s
here?
change in
rising
happening
There is a
temperature
here?
change
because there What’s of state
from liquid
happening to
is a change of
gas so
state – the solid here? no
change in
is melting
temperature
All the liquid has
turned
What’s to gas
and the gas
happening
temperature
here?
begins to rise
time / s

Evaporation

How does sweating
keep us cool?

Cooling by melting
The ice requires
energy to melt. It
takes this energy
from the bottles of
beer, keeping them
cool.

Principle of Conservation of Energy

Energy cannot be created or destroyed –
simply transferred from one form to
another.

Doing work to lift an object
When work is done to lift an object – the
object gains potential energy.
Energy is neither created nor destroyed,
simply transformed from one form into
another.

Potential to Kinetic Energy
At the top of the
rollercoaster, the cart has
potential energy. This is
converted to kinetic energy.
Energy is neither created
nor destroyed, simply
transformed from one form
to another.
In reality, some energy is
“lost” due to friction – this
means it is converted to a
form which isn’t useful
(heat).

Kinetic Energy to ?
What energy
transformation takes
place the bring a
moving car to a halt?
How can the force
required to do this
be calculated?

Conservation of Energy
The principle of conservation of energy
also applies when we use electrical energy
to heat substances.

Example
Calculate the time taken for a 500 W
heater to melt 2 kg of ice at 0 °C.

Think – we are talking about change of
state without a change of temperature
therefore latent heat.

What do I know?
Latent heat of fusion of ice l = 3.34x105 J/kg
P = 500 W
m = 2kg
t=?
Energy required to melt the ice = ?
Time taken to melt the ice = ?

What do I know?

Latent heat of fusion of ice l = 3.34x105 J/kg
P = 500 W
m = 2kg
t=?

E h = ml

E h = 2x 3.34x 10

5

E h = 6.68x 105 J
E
t =
P
6.68x 10 5
t =
500
t = 1336s

Conservation of energy tells us
that the energy required to melt
the ice must be the electrical
energy provided by the heater.

Thermal physics ppt

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Thermal physics ppt

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