Unit 1 Kinetic Theory and Heat Transfer

Question 1

What is the definition of ‘kinetic theory’?

Answer 1

Kinetic theory describes matter as very large numbers of atoms and molecules in constant motion.

Question 2

Describe the properties of solids

Answer 2

-particles are packed very closely together in a regular arrangement.
-they vibrate about a fixed positions and have strong forces of attraction between them (bonds).
-as a result they have a fixed shape and volume and a high density.

Question 3

Describe the properties of liquids

Answer 3

-particles are closer together, but not as close as they are in solids. The particles are mainly touching but some gaps have appeared in the structure.
-they have enough energy to move around and are not held in fixed positions; bonds are weaker than in solids.
-liquids have a fixed volume but take on the shape of the bottom of the container; they have a medium density.

Question 4

Describes the properties of gases

Answer 4

-particles are very far apart and can move around rapidly and randomly in any direction; very weak bonds.
-no fixed shape or volume- they completely fill their container.
-low density.

Question 5

What happens to the density of a solid substance when it changes to a liquid?

Answer 5

If a solid changes to a liquid, its mass would stay the same but (usually) its volume would increase since the distance between the particles in a liquid is greater than in a solid. This means that the density would decreases, as you are dividing the mass by a larger number.

Question 6

What happens to the density of a liquid substance when it changes to a gas?

Answer 6

If a liquid changes to a gas, its mass would stay the same but (usually) its volume would increase since the distance between the particles in a gas is greater than in a liquid. This means that the density would decreases, as you are dividing the mass by a larger number.

Question 7

What is the summary for density?

Answer 7

-In solids, since the particles are packed very close together they have a high density.
-In liquids, the molecules are close together, but not as close as they are in solids; they have a medium density.
-In gases, the particles are very far apart with large distances between them; they have a low density.

Question 8

What are the two factors which allow heat transfer most quickly?

Answer 8

1. The temperature difference is greatest
2. The surface area is greatest

Question 9

What are the three main methods of heat transfer?

Answer 9

Conduction, convection and radiation

Question 10

What is conduction?

Answer 10

Conduction is the transfer of heat energy through a substance by particle collisions (it occurs mainly in solids).

Question 11

How do solids conduct?

Answer 11

As the solid gets heated its particles absorb heat energy and convert this to kinetic energy and so they start to vibrate more vigorously. In other words, they vibrate faster and with greater amplitude, and so collide with neighbouring particles with more force and more often, which in turn causes these particles to vibrate more vigorously too, and in this way the energy is passed along/ through the solid in the form of kinetic energy.

Question 12

Why are metals better conductors than non-metals

Answer 12

Metals contain free electrons; which can move freely throughout the solid. When a metal is heated, the free electrons move from hot parts of the metal to colder parts, taking heat energy with them (electron diffusion). As they move through the metal the free electrons collide with atoms and in these collisions, the electrons transfer some of their energy to the atoms and cause them to vibrate more vigorously than before.
Metals conduct by both particle vibration and free electron movement

Question 13

Why are liquids and gases poor conductors?

Answer 13

Most liquids have no free electrons, so rely on passing vibrations from atom to atom, but the particles are further apart than in solids, meaning there are less particle collisions.

Particles in gases are too spread out to affect each other through particle collisions.

Question 14

What is convection?

Answer 14

Convection is the transfer of heat in a fluid (liquid or gas) due to the particles within the fluid moving around through it, as a result the density changes.

When a fluid is heated its particles gain kinetic energy and the fastest-moving particles in a hot region of the substance will move to a cooler region

Question 15

How are convection currents set up?

Answer 15

When you heat a liquid or gas its particles gain energy and begin to move faster and further apart. This means that the particles are spread out over a greater volume - the fluid expands, making the fluid less dense. The warmer, less dense fluid rises above its colder, denser surroundings. As the warm fluid rises, cooler fluid moves in to take its place, setting up a convection current.
CONVECTION CANNOT OCCUR IN SOLIDS since the particles within the solids are not free to move from place to place; they can only vibrate

Question 16

What is radiation?

Answer 16

Radiation is the transfer of heat in the form of electromagnetic waves. These waves are mainly infrared part of the electromagnetic spectrum.
Radiation is the fastest way in which heat is transferred (travels at the speed of light)

Question 17

Describe emission

Answer 17

Any object or body which is hotter than its surrounding radiates heat energy to them (emit).

Question 18

Describe absorption

Answer 18

All objects absorb radiant heat. When heat radiation falls on an object, some of it is reflected at the surface (like light getting reflected by a mirror) but some of it is absorbed.

Question 19

How does the temperature of an object affects its emission and absorption

Answer 19

If an object is hotter than its surroundings it emits more radiant heat than it absorbs, so its temperature decreases, whereas if an object is cooler than its surroundings it absorbs more heat energy than it emits, so its temperature increases.

Question 20

What is the effect of colour on absorption and emission?

Answer 20

Dark matt (rough) surfaces are good absorbers and emitters of radiant heat.
Light shiny (smooth) surfaces are poor absorbers and emitters of radiant heat.
Since radiant heat travels as waves similar to visible light, it can be reflected by light shiny surfaces.

Question 21

How is heat transfer minimised in a vacuum flask?

Answer 21

The inner flask is made up of a double-walled glass bottle with a vacuum between the two walls. The vacuum stops all heat transfer by conduction or convection through the sides.
The glass walls facing the vacuum are silvered. Their shiny surfaces reduce heat transfer by radiation.
The stopper is made of plastic and is often filled with cork or foam to reduce heat transfer by conduction through it. The cap also minimises heat loss by convection.
The outer plastic case and insulated foam supports primarily protect the inner, fragile flask against physical damage, but they are made of insulating materials to further help reduce heat transfer by conduction.

Question 22

What are thermograms used for?

Answer 22

Thermograms are useful for examining heat loss from a house, as they show the temperature distribution over its surface

Question 23

Why does a poorly insulated house cost more to heat?

Answer 23

It loses more heat energy so therefore has to be heated by radiators etc. which will cost money.

Question 24

Where is heat energy lost as conduction, convection and radiation (in the house)?

Answer 24

Conduction- walls, floor, roof and windows.
Convection- convection currents can transfer heat energy in the loft to the roof tiles, where it escapes by conduction. Cold air can also enter the house through gaps around doors and windows.
Radiation- walls, roof, doors and windows.

Question 25

Describe the methods used to reduce heat loss

Answer 25

Double glazing- multiple panes of glass which traps a layer of gas/air, or a vacuum between the panes. The air/gas between the glass minimises convection as there is little room for the air/gas to move. Since air/gas is a poor conductor there will be very little heat loss by conduction.

Cavity wall insulation- involves filling the gap between the outer and inner walls with insulating materials e.g. fibre glass, mineral wool or foam. Insulating materials are poor conductors so therefore reduce heat loss by conduction. It also prevents air circulating inside the cavity, therefore reducing heat loss by convection. Radiation can be reduced by using a special insulation board- backed with aluminium foil (shiny) which reflects heat back into the room.

Loft insulation- traps lots of air and so acts as an effective insulator to stop conduction of heat into the loft. This then reduces convection currents transferring heat energy in the loft to the roof tiles, where it escapes by conduction.

Draft excluders- under/around doors preventing the movement of cold air into the house by convection currents.

Curtains- traps air at windows so helps to reduce heat loss by conduction.

Carpet- reduces heat loss by conduction, through the floor.