Chapter 19: Thermal Physics

Question 1

What are the conditions for energy transfer to take place?

Answer 1

• one object exerts force on the other object and makes it move (work is done)
• one object is hotter than another object, so energy transfer by heating takes place by means of conduction, convection or radiation (due to temperature difference)

Question 2

What is internal energy?

Answer 2

The internal energy of an object is the sum of the random distribution of the kinetic and potential energies of its molecules.

The internal energy of an object is the energy of its molecules due to their individual movements and positions.

Question 3

What is thermal energy?

Answer 3

The internal energy of an object due to its temperature is sometimes called thermal energy.

Question 4

When does the internal energy of an object increase?

Answer 4

-Energy transfer occurs by heating the object

• work is done on the object i.e by electricity

Question 5

When does the internal energy of an object stay constant?

Answer 5

• There is no energy transfer by heating, and no work is done
• Energy transfer by heating and work done balance each other out

Question 6

What does the first law off thermodynamics state?

Answer 6

When work is done or energy is transferred by heating to an object, the change of internal energy of the object - the total energy transfer due to work done and heating

Question 7

What is a molecule?

Answer 7

The smallest particle of a pure substance that has characteristics of the substance.

Question 8

What is an atom?

Answer 8

The smallest particle of an element that is characteristic of that element.

Question 9

Describe molecules in a solid

Answer 9

The atoms and molecules are held tightly together due to the electrical charge of the protons and electrons in the atoms; the molecules in a solid vibrate randomly at fixed positions. The higher the temperature of the solid, the more the molecules vibrate. As energy is supplied, the kinetic energy of the molecules increases, causing them to vibrate so much that they break free from the solid structure and become free to move around; it has melted.

Question 10

Describe molecules in a liquid

Answer 10

The molecules move about at random contact with each other; the forces are not strong enough to hold the molecules in fixed positions. The higher the temperature of a liquid, the faster its molecules move. When its temperature increases, the kinetic energy of the molecules causes the molecules tobreak free and move away from each other effectively vaapourising.

Question 11

Describe molecules in a gas

Answer 11

The molecules move about randomly, but much further apart than a liquid, heating makes the molecule speed up and gain kinetic energy. the pressure increases. This means that they have more collisions with each other and the sides of the container and hence the pressure is increased.

Question 12

What is temperature?

Answer 12

The degree of hotness of the object. The hotter the more internal energy it has

Question 13

What is thermal equillibrium?

Answer 13

For any two objects that are at the same temperature, no overall energy transfer by heating will take place?

Question 14

What is a temperature scale?

Answer 14

These are fixed points, which are standard degrees of hotness that are accurately reproduced.

Question 15

What is the celsius scale, and how is it defined?

Answer 15

unit celsius

• ice point 0° C, which is the temperature of pure melting ice.
• Steam point is 100° C, which is the temperature of steam at standard atmospheric pressure.

Question 16

What is the absolute scale, and how is it defined?

Answer 16

• absolute zero 0K, which is the lowest possible temperature.
• triple point of water, 273.16K, which is the temperature at which ice, water, and water vapour co-exist in thermodynamic equilibrium

ice point is 273.15 and steam point is 373.15

Question 17

What is the conversion from celsius to kelvin

Answer 17

temp in ° C = absolute temperature in K - 273.15

Question 18

What is absolute zero?

Answer 18

The lowest possible temperature with minimum internal energy regardless of the substances the object consists of.

Question 19

What is specific heat capacity?

Answer 19

Specific heat capacity c, The energy needed to raise the temperature of unit mass of the substance by 1K without change of state. The unit JKg-1K-1

ΔQ =mc(T2 - T1)

ΔQ = mcΔT

Question 20

What is the equation for the inversion tube experiment?

Answer 20

c = gLn/ΔT

mcΔT = mgLn

Question 21

How can the Specific heat capacity of metal be measured?

Answer 21

c = IVt/mΔT

Question 22

How can the Specific heat capacity of liquid be measured?

Answer 22

Assuming no heat loss to the surroundings.

IVt = m1c1ΔT + mcal ccal x ΔT

Question 23

What is the relationship between density and states?

Answer 23

The density of a gas is less than the same substance of a liquid or solid state; this is due to the fact that the molecules of a liquid and of a solid are packed together in contact with each other.

Question 24

why can’t solids flow?

Answer 24

The atoms in a solid are locked together by strong force bonds from which the atom is unable to break free from. In liquid or gas, the molecules have too much kinetic energy, and the force bonds are not strong enough to keep the molecules fixed to each other.

Question 25

What is the Latent heat of fusion?

Answer 25

The energy needed to change the state of unit mass to melt a solid at its melting point is called the latent heat of fusion.

When a solid is heated at its melting point, its atoms vibrate
so much that they break free from each other. The solid, therefore,
becomes a liquid due to energy being supplied at the melting point.

Latent heat is released when a liquid solidifies. This happens
because the liquid molecules slow down as the liquid cools until the
temperature decreases to the melting point. At the melting point,
the molecules move slowly enough for the force bonds to lock the
molecules together. Some of the latent heat released keeps the
temperature at the melting point until all the liquid has solidified.
· Latent means hidden. Latent heat supplied to melt a solid may
be thought of as hidden because no temperature change takes
place even though the solid is being heated.
. Fusion is a word used for the melting of a solid because the solid
fuses into a liquid as it melts.

Question 26

What is the latent heat of vapourisation?

Answer 26

The energy needed changes the state of unit mass to vaporize a liquid at its boiling point.

When a liquid is heated at its boiling point, the molecules gain
enough kinetic energy to overcome the bonds that hold them close
together. The molecules therefore break away from each other
to form bubbles of vapour in the liquid. The energy needed to
vaporise a liquid is called latent heat of vaporisation.

Latent heat is released when a vapour condenses. This happens
because the vapour molecules slow down as the vapour is cooled.
The molecules move slowly enough for the force bonds to pull the
molecules together to form a liquid.
Some solids vaporise directly when heated. This process is called
sublimation

Question 27

Does it require more energy to vaporise or melt a substance?

Answer 27

Vaporise

Question 28

What is the specific latent heat of fusion?

Answer 28

The energy needed to change the state of unit mass from solid to liquid without a change of temperature.

Q = ml

Question 29

What is the specific latent heat of vapourisation?

Answer 29

The energy needed to change the state of unit mass from liquid to vapour without a change of temperature.

Q = ml

Question 30

Temperature - time graph

Answer 30

before the solid melts P = mcs (ΔT/Δt)s

Where (ΔT/Δt)s = P/mcs

after the solid melts P = mcL (ΔT/Δt)L

Where (ΔT/Δt)L = P/mcL