Chapter 10: Thermal Physics

Question 1

Thermal Expansion

Answer 1

the increase in the volume of a material when its temperature rises

Question 2

Melting point

Answer 2

the fixed temperature at which a solid melts to become a liquid.

Question 3

Boiling point

Answer 3

the fixed temperature at which a liquid changes into a gas. (at constant pressure).

Question 4

Thermostats

Answer 4

Thermostats are used to control the temperature of devices such as ovens and irons.

Question 5

Why do the states of matter expand on heating? (4)

Answer 5

-When they are heated they gain thermal energy and have greater average kinetic energy,
-Their vibrations (in solids) and their movement starts to take up more space,
-They also start to push each other farther apart
-So there is an expansion in all directions.

Question 6

Why do bridges have gaps? (4)

Answer 6

-They have gaps to allow for expansion,
So when they contract or expand on a cold or hot day,
-The concrete does not get damaged, or crack.
-Rollers may be used at one end so that movement can take place.

Question 7

Why are overhead cables left slack?

Answer 7

To allow for the contraction that could happen on a very cold day.

Question 8

Bimetallic Strip (4)

Answer 8

-A bimetallic strip is designed to bend as it gets hot.
-It is made of 2 metals joined firmly together,
-One metal expands much more than the other.
-As the strip is heated, this metal expands causing the strip to bend.

Question 9

Why do glass cups break when hot water is poured into them? (3)

Answer 9

-It is due to the inner surface of the glass expanding rapidly,
-Before the thermal energy has passed through to the outer surface.
-The force of expansion cracks the glass.

Question 10

Which state of matter expands the most to least?

Answer 10

Gas → Liquid → Solid

Question 11

Exceptions of which matter expands fastest
Liquid-(2)

Answer 11

Paraffin expands very rapidly during heating.
Petrol expands very rapidly that it may overflow.

Question 12

Why can’t solids expand much?

Answer 12

It is difficult for the particles of a solid to push their neighbours aside, so the solid does not expand much.

Question 13

Why do gases expand more? (4)

Answer 13

-Due to the particles of gas being able to move about randomly and because of their average kinetic energy,
-Making it easier for them to push the walls of their container further apart,
-This then causes more force to be exerted due to a change in momentum finally causing pressure,
-That causes the gas to take up more space.

Question 14

How does the state change happen? (3)

Answer 14

-Changes in the chemical potential energy in the bonds between the molecules and atoms,
-Causes energy that needs to be provided to break the bonds and change the substance,
-Energy is also needed to overcome the attraction between the particles.
-When these changes are reversed energy is given out.

Question 15

When a state loses energy- (2)

Answer 15

-Particles slow down and reform the bonds holding the particles together and, (liquid to solid)
-The particles are drawn together. (gas to liquid)

Question 16

pure and other substances

Answer 16

-A pure substance has a clear melting and boiling point.
-Not all substances are heated, some burn or break down into simpler substances before they can change state.

Question 17

boiling vs evaporating

Answer 17

Boiling
Happens throughout the liquid.
Occurs at the boiling point of a substance only.
The liquid bubbles.

Evaporation
Happens on the surface of the liquid…
Due to some particles being able to escape the fastest from the surface.
Occurs at all temperatures
A liquid can evaporate without bubbles.

Question 18

How does increasing temperature speed up evaporation? (3)

Answer 18

Particles have higher average kinetic energy.
More particles will have enough energy to escape.
The liquid will evaporate quickly.

Question 19

How does increasing surface area speed up evaporation? (3)

Answer 19

More particles are closer to the surface of the liquid
They can escape more easily.
Liquid can evaporate more quickly

Question 20

How does blowing air across the surface speed up evaporation? (3)

Answer 20

A draught is moving air.
When the particles escape they are blown away so they cannot fall back into the liquid.
Liquid can evaporate more quickly.

Question 21

howdoes fridge keep stuff cold? .

Answer 21

A liquid is compressed and then squired through a narrow hole so the pressure is reduced and it evaporates.
This draws thermal energy from inside the fridge into the liquid.
The liquid is pumped out of the fridge to the pipes back of the fridge where it is compressed and condenses
Releasing thermal energy to the surroundings