2 Thermal Physics Flashcards
1
Q
Gases and the absolute scale of temperature (Pressure Law, Boyleβs Law (pv = constant), Charlesβ Law)
Describe qualitatively, in terms of particles, the
effect on the pressure of a fixed mass of gas of:
(a) a change of temperature at constant volume
(b) a change of volume at constant temperature
A
a) change of temperature - increase of temperature, increase of pressure
b) change of volume - increase of volume, decrease of pressure
In a flexible sealed container, the amount of gas molecules are fixed.
If we keep the temperature constant, the only way to increase pressure is to reduce volume.
π·βπ/π½ π€βππ π ππ ππππ π‘πππ‘
π·π½=ππππππππ
Formula: π·πVπ = π·πVπ
In a rigid sealed container, both the volume and amount of gas molecules are fixed.
The only way to increase the gas pressure is by increasing its temperature.
π·βπ» π€βππ π ππ ππππ π‘πππ‘
π·/π»=ππππππππ
2
Q
i) Name the process by which thermal energy is transferred through a metal rod
ii) Describe how this process occurs
iii) observation: flame appears both above and below the gauze ;; observation: flame only appears below the gauze
How can these observations be explained? [4]
A
i) conduction
(ii) molecules at hot end vibrate more bc have more energy
energy/vibration transferred to neighbours/shared
iii) iron conducts heat slowly
gas hot enough to burn /
copper conducts heat rapidly
gas not hot enough to burn
3
Q
Evaporation VERSUS boiling
A
Both processes involve the change of state from liquid to gas.
Boiling is rapid and takes place in the entire body of liquid.
Boiling only occurs at the boiling point.
Evaporation is slow and takes place only on the surface of liquid. Evaporation occurs at any temperature between the melting and boiling point.
4
Q
What is latent heat? [2]
A
Latent heat = energy needed to change the state (gas, liquid, solid) of one kilogram of material
In Mark Scheme: βenergy required to change state with no change in temperatureβ
5
Q
What is latent heat of fusion?
Define: Latent heat of fusion (melting)
Unit?
A
the amount of heat energy required to change a substance from a solid state to a liquid state at its melting point, without changing its temperature.
Mark Scheme: energy needed to change state of substance, to melt, per kg
J/kg
6
Q
Latent heat of vaporisation? [2]
A
energy to change 1 kg from liquid to gas / gas to liquid (without changing its temperature)
7
Q
The rate of heating is 2.0 kW.
Calculate how much energy is supplied to the substance during the period 18 β 22 minutes
Formula? kW multiplied by time = energy
Rate of heating (W) x time (s) = energy (J)
A
2000 x 240
480,000 J
8
Q
The Gas Laws - Pressure & Volume (Constant Temperature)
If the temperature of a gas remains constant, the pressure of the gas changes when it isβ¦
A
- Compressed β decreases the volume which increases the pressure
- Expanded β increases the volume which decreases the pressure
9
Q
A change in pressure can cause a change in volume
A _____ can be used to remove the air from a sealed container
Describe change in volume to a tied up balloon when the pressure of the air around it decreases:
A
vacuum pump //
At normal air pressure the balloon has a low volume [bell jar open to the surroundings]
As the pressure in the bell jar decreases, the volume of the balloon increases
[air removed from bell jar by vacuum pump]
10
Q
Melting & Boiling
What are fixed points?
A
melting and boiling points of pure water at atmospheric pressure
Melts at 0, boils at 100
11
Q
Convert temperatures between kelvin and degrees Celsius
Eqn to recall
;; a temperature at which the particles in a gas exert no pressure - at this temperature they must no longer be moving, and hence not colliding with their container
This temperature is called absolute zero and is equal to -273 Β°C!
A
T (in K) = ΞΈ (in Β°C) + 273
Celsius temp + 273 => Kelvin
12
Q
Eqn for a fixed mass of gas at constant temperature,
including a graphical representation of this
relationship
A
pv = constant
13
Q
Eqn recall for finding vol/pressure given necessary info
relationship between the pressure and volume for a fixed mass of gas at constant temperature
A
P1V1 = P2V2
STATE FORMULA WHEN USING in test
VOLUME => m3
Pressure (Pa)
14
Q
kJ to J
1 Pa = 1 kg/m.s^2
1 kPa = ___ kg/m.s^2
A
x 1,000
1 Pa = 1 kg x m-1 x s-2
1 kPa = 1,000 kg/m.s^2
15
Q
Specific latent heat eqn
A
E = mL.
where E is the energy in joules (J), m is the mass in kilograms (kg), and L is the specific latent heat in joules per kilogram (J/kg).
16
Q
Properties of Solids, Liquids & Gases
Solids
A
Solids have a definite shape and a definite volume
Solids cannot flow and are not compressible
17
Q
Liquids
A
Liquids have no definite shape but do have a definite volume
Liquids are able to flow to take the shape of a container but they are not compressible
18
Q
Gases
A
Gases have no definite shape and no fixed volume
Gases can flow to take the shape of their container and are highly compressible
19
Q
Changes of State
When a substance changes state, the number of molecules in that substance _____ change and so neither does its mass
The only thing that changes is its _____
Changes of state are physical changes and so they are _____
A
DOESNβT change
ENERGY
REVERSIBLE
20
Q
Arrangement & Motion of Particles
Solids
A
The molecules are very close together and arranged in a regular pattern
The molecules vibrate about fixed positions, high density
21
Q
Liquids
& Gases
A
The molecules are still close together (not reg)
The molecules are able to slide past each other, medium density
The molecules are widely separated - about 10 times further apart in each direction
The molecules move about randomly at high speeds, low density
22
Q
Intermolecular Forces and Motion of Particles
Solids
A
The molecules in a solid are held in place by strong intermolecular forces
They only vibrate in position
The distance between them is fixed
This gives the solid its rigid shape and fixed volume
23
Q
Liquids
A
The molecules in a liquid have enough energy to overcome the forces between them
They are still held close together
The volume of the liquid is the same as the volume of the solid
Molecules can move around (by sliding past each other)
This allows the liquid to change shape and flow
24
Q
Gases
A
The molecules in a gas have more energy and move randomly at high speeds
The molecules have overcome the forces holding them close together
Because of the large spaces between the molecules
The gas can easily be compressed and is also able to expand
Gases flow freely
25
While a substance is changing state, either
-Melting or freezing
- Boiling or condensing
Does the substance change temperature?
No, even though energy is being transferred to or away from the thermal energy store of the substance
26
At the boiling point, even if more thermal energy is added, does the water get hotter?
the liquid water does not get any hotter
internal energy is not rising
additional thermal energy goes into overcoming the intermolecular forces between the molecules of water, becomes gas
27
For condensation
For freezing
process is repeated backwards for cooling as energy/heat is transferred away
28
At the melting point, even if more thermal energy is added, does the water get warmer?
Again, the solid water does not get warmer
internal energy is not rising
additional thermal energy goes into overcoming the intermolecular forces btwn the molecules of the solid ice, becomes liquid
29
Heating is when ...
energy is transferred to the system and the kinetic energy of the molecules increases
30
Cooling is when ...
energy is transferred away from the system (or dissipated to the surroundings) and the kinetic energy of the molecules decreases
31
Describe condensation
The particles lose kinetic energy and move more slowly
They no longer have enough energy to overcome the intermolecular forces of attraction between molecules
The particles get closer together
They only have enough energy to flow over one another
The gas has condensed into a liquid with no change of temperature
32
Describe solidification
The particles lose kinetic energy and move more slowly
They no longer have enough energy to overcome the intermolecular forces of attraction between molecules
The particles get closer together
They only have enough energy to vibrate about their fixed position
The liquid has solidified into a solid with no change of temperature
33
Boiling vs Evaporation
Boiling is also a change in state from liquid to gas
Boiling happens only at the boiling point of the liquid
The change of state happens all through the liquid
34
the higher the pressure, the higher the ____
force exerted per unit area
35
What is Brownian Motion?
The Kinetic Theory of Matter, which simply says that ...
the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller particles which are often too small to see
All matter is made up of tiny particles
36
+ EXTENDED
When observing Brownian Motion, even w/ a microscope, only the microscopic particles can be seen. The pollen or smoke particles are seen to move
Smaller atoms and molecules, of water or air, are still too small to be seen.
These light, fast-moving atoms and molecules collide with the larger microscopic particles
The collisions give the particles a little nudge, causing them to change their speed and directions randomly, each time they are struck by a molecule
The presence of the light, fast moving atoms and molecules is inferred from the motion of the microscopic particles
Can be seen: pollen, smoke particles
Pollen particle gets hit by air molecule
We cannot see them though
37
Gases & Absolute Temperature
An increase of 1 K is the same change as an increase of 1 Β°C
It is not possible to have a temperature lower than 0 K
meaning...?
This means a temperature in kelvin will never have a negative value
[also add 273 if beginning w Celc]
38
Boyleβs Law
If the temperature T of an ideal gas is constant, then Boyleβs Law is given by:
Graph looks like?
P proportional to 1/V
This means the pressure is inversely proportional to the volume of a gas
pressure on x-axis to volume graph, downward curve that's like IβΏ
39
Thermal expansion
When materials are heated, they _____ because _____
____ expand most, _____ expand least
EXPAND bc the molecules start to move around (or vibrate) faster, causing them to knock into each other & push each other apart
Gases most,
solids least
40
Thermal Expansion in Terms of Particles
Solids
Expand slightly
bc the low energy molecules cannot overcome the intermolecular forces of attraction holding them together
41
Liquids
Expand more than solids
bc the molecules have enough energy to partially overcome the intermolecular forces of attraction holding them together
42
Gases
Expand significantly
bc the high energy molecules have enough energy to completely overcome the intermolecular forces of attraction holding them together
43
Uses & Consequences of Thermal Expansion
Applications:
Thermometers rely on the expansion of liquids to measure temperature
Temperature-activated switches work when a bimetallic strip, consisting of two metals that expand at different rates, bends by a predictable amount at a given temperature
1. Thermostat in An Iron
2. Fire Alarm
44
Consequences:
The expansion of solid materials can cause them to buckle if they get too hot
This could include:
Metal railway tracks
Road surfaces
Bridges
Things that are prone to buckling in this way have gaps built in, this creates space for the expansion to happen without causing damage
45
Simple explanation: [Reminder molecules do not expand, SUBSTANCE does]
As heat is added,
As heat is added:
The increase in temperature...
Leads to an increase in kinetic energy, so that...
Molecules and atoms move more quickly...
And move apart
This separation of the the molecules makes the substance bigger
46
Internal energy is defined as:
The total energy stored inside a system by the particles that make up the system due to their motion and positions
47
Avg kinetic energy: Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles
Therefore, temp of material is...
The temperature of the material is related to the average kinetic energy of the molecules
48
This increase in kinetic energy (and therefore internal energy) can:
Cause the temperature of the system to increase
Or, produce a change of state (solid to liquid or liquid to gas)
49
As the container heats up, the gas molecules move faster
Faster motion causes higher kinetic energy and therefore ________________
higher internal energy
50
How much the temperature of a system increases depends on:
The mass of the substance heated
The type of material
The amount of thermal energy transferred in to the system
51
specific heat capacity, c, of a substance is defined as:
The amount of energy required to raise the temperature of 1 kg of the substance by 1 Β°C
52
If a substance has a low specific heat capacity...
it heats up and cools down quickly (ie. it takes less energy to change its temperature)
53
If a substance has a high specific heat capacity...
General idea to rmbr:
it heats up and cools down slowly (ie. it takes more energy to change its temperature)
the LARGER the number, the LESS the substance will increase in temperature for a given amount of heat
54
Eqn
β³E = mcβ³ΞΈ
Where:
ΞE = change in thermal energy, in joules (J)
m = mass, in kilograms (kg)
c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg Β°C)
ΞΞΈ = change in temperature, in degrees Celsius (Β°C)
55
Investigating Specific Heat Capacity
Aims of the Experiment
to determine the specific heat capacity of a substance, by linking the decrease of one energy store (or work done) to the increase in temperature and subsequent increase in thermal energy stored
56
Independent variable =
Dependent variable =
Control variables =
IV => Time, t
DV => Temperature, ΞΈ
CV's => Material of the block
Current supplied, I
Potential difference supplied, V
57
Equipment, Purpose
1. 1kg block of metal le.g. aluminium), or, a beaker containing a known mass of water
2. Thermometer
3. Immersion heater
4. Power supply
5. Stopwatch
6. Voltmeter
7. Ammeter
1. Substance to calculate the specific heat capacity
2. To measure the temperature rise of the substance
3. To heat the substance
4. To supply power to the heater
5. To measure the time taken for the substance to heat up by a certain temperature
6. To determine the potential difference through the heater
7. To determine the current from the power supply to the heater
58
[Resolution of measuring equipment:
Thermometer = 1 Β°C
Stopwatch = 0.01 s
Voltmeter = 0.1 V
Ammeter = 0.01 A]
Steps:
Start by assembling the apparatus, placing the heater into the top of the block
Measure the initial temperature of the aluminium block from the thermometer
Turn on the power supply and start the stopwatch
Whilst the power supply is on, the heater will heat up the block. Take several periodic measurements, eg. every 1 minute of the voltage and current from the voltmeter and ammeter respectively, calculating an average for each at the end of the experiment up to 10 minutes
Switch off the power supply, stop the stopwatch and leave the apparatus for about a minute. The temperature will still rise before it cools
Monitor the thermometer and record the final temperature reached for the block
59
thermal energy supplied to the block can be calculated using
E = IVt = current x voltage x time
Where:
E = thermal energy, in joules (J)
I = current, in amperes (A)
V = potential difference, in volts (V)
t = time, in seconds (s)
60
change in thermal energy eqn
ΞE = mcΞΞΈ
61
Notes for all practicals
Evaluating the Experiment
Systematic Errors:
Make sure the voltmeter and ammeter are initially set to zero, to avoid zero error
Random Errors:
Not all the heat energy supplied from the heater will be transferred to the block, some will go into the surroundings or heat up the thermometer
This means the measured value of the specific heat capacity is likely to be higher than what it actually is
To reduce this effect, make sure the block is fully insulated
A joulemeter could be used to calculate energy directly
This would eliminate errors from the voltmeter, ammeter and the stopwatch
Make sure the temperature value is read at eye level from the thermometer, to avoid parallax error
The experiment can also be repeated with a beaker of water of equal mass, the water should heat up slower than the aluminium block
=> Safety Considerations
Make sure never to touch the heater whilst it is on, otherwise, it could burn skin or set something on fire
Run any burns immediately under cold running water for at least 5 minutes
Allow time for all the equipment, including the heater, wire and block to cool before packing away the equipment
Keep water away from all electrical equipment
Wear eye protection if using a beaker of hot water
62
Explaining about insulators
The insulator contains trapped air, which is a poor conductor of heat
Trapping the air also prevents it from transferring heat by convection
This reduces the rate of heat loss from the object, meaning that it will stay warmer for longer
63
Other things to watch out for:
Heat does NOT rise (only hot gases or liquids rise)
Shiny things do NOT reflect heat (they reflect thermal radiation)
Black things do NOT absorb heat (they absorb thermal radiation)
Correct to say:
- Hot gases or liquids rise
- Shiny things reflect thermal RADIATION
- Black things absorb thermal RADIATION
64
Experiments Demonstrating Thermal Conductors
Define: Good thermal conductors
E.g.
Bad thermal conductors (also called insulators) are...
solids which easily transfer heat
For example; a metal pan or a ceramic tea cup
solids which do not transfer heat well
e.g., a woolen blanket or layers of cardboard or paper
65
A liquid is heated in a beaker.
The density of the liquid changes as its temperature increases. This causes energy to be transferred throughout the liquid.
How does the density change and what is this energy transfer process?
density DECREASES as liquid is heated
energy transfer process: CONVECTION
66
Convection
e.g. process involving convection
the flow of heat through a fluid from areas of a higher temperature to areas of a lower temperature by movement of the fluid itself.
e.g. hot air rising to the top of a cool room
67
A student suggests some uses for containers made from good thermal conductors and for
containers made from poor thermal conductors.
In which row are both suggested uses correct?
good thermal conductor: transferring thermal energy
quickly to a cold liquid
poor thermal conductor: keeping a hot liquid at
a high temperature
68
Thermal energy travels through space from the Sun to the Earth. Space is a vacuum.
How is thermal energy transferred from the Sun to the Earth?
by radiation only
69
A cupboard is placed in front of a heater. Air can move through a gap under the cupboard.
Which row describes the temperature, and the direction of movement, of the air in the gap?
cool, towards heater
; As air in front of heater gets warmer and move upward, to fill that gap fresh air moves through gap towards heater.
70
One method of heat transfer involves the energy travelling at a much greater speed than in other methods.
name?
radiation
[also travels thru vacuum]
71
The air in a room is heated by a heater. The diagram shows the circulation of the air in the room.
Which statement about the air that is heated is correct?
AIR HEATED => air EXPANDS
& becomes LESS dense.
72
In a refrigerator, the cooling unit can be fitted either at the top or at the bottom. In an oven, the heater can be fitted either at the top or at the bottom.
Which row shows the best position for the cooling unit and the best position for the heater?
cooling unit: top
heater: bottom
73
A girl is outdoors. She warms her hands by holding them near a fire, as shown.
How does the heat from the fire reach her hands?
& A heating engineer fits a heater to the ceiling of an office so that workers in the office are kept warm.
How does thermal energy reach the workers below the heater?
radiation only
74
Ice is trapped by a metal gauze at the bottom of a tube containing water.
The water is heated strongly at the top, but the ice only melts very slowly.
Why does the ice melt so slowly?
Water is a poor conductor of heat.
75
Which statement about thermal radiation is correct?
A It can only occur in a vacuum.
B It involves movement of electrons through a material.
C It involves movement of atoms.
D It is infra-red radiation.
D. It is infra-red radiation.
76
Why does convection take place in a liquid when it is heated?
Liquids expand when they are heated.
77
Food is kept in a cool-box which uses two ice packs to keep it cool.
Where should the ice packs be placed to keep all the food as cool as possible?
both at the top of the box
78
Why is the cooling unit placed at the top?
Cold air falls and warm air is displaced upwards.
79
A substance loses thermal energy (heat) to the surroundings at a steady rate.
The graph shows how the temperature of the substance changes with time.
What could the portion PQ of the graph represent?
LIQUID COOLING β in 2nd slant in a cooling curve from left
80
A beaker of liquid is left on a laboratory bench. There is an electric fan in the laboratory causing a draught over the liquid.
The liquid evaporates.
Which row shows two changes that will both cause the liquid to evaporate more quickly?
change to surface area of the liquid ;; INCREASE
change to speed of fan ;; increase
81
Which processes occur in a metal to cause thermal conduction?
ELECTRON TRANSFER
LATTICE VIBRATION
82
black surface better for...
better absorber/heats more fast
better emitter
cools more quickly
83
One end of a copper rod is heated.
What is one method by which thermal energy is transferred in the copper rod?
Free electrons transfer energy from the hotter end to the cooler end.
84
Evaporation causes the temperature of the remaining liquid to ______.
DECREASE
85
A liquid is evaporating. The liquid is not boiling. How will it evaporate?
Only molecules with enough energy can escape, and only from the liquid surface.
86
Gases - decrease in volume, of course increase in pressure, does speed of gas molecules change?
no change
87
What causes the random, zig-zag movement (Brownian motion) of smoke particles suspended in
air?
air molecules colliding with smoke particles
88
A sealed bottle of constant volume contains air.
The air in the bottle is heated by the Sun.
What is the effect on the average speed of the air molecules in the bottle, and the average distance between them?
speed increases,
avg distance stays the same
89
Air in a sealed syringe is slowly compressed by moving the piston. The temperature of the air stays the same.
Pressure does what bc what?
The pressure of the air increases because its molecules now hit the syringe walls more
frequently.
[When a gas is compressed, the molecules will hit the walls of the container more frequently
This creates a larger overall net force on the walls which increases the pressure]
90
Very small pollen grains are suspended in a beaker of water. A bright light shines from the side.
Small, bright dots of light are seen through a microscope. The dots move in rapidly changing, random directions
What are the bright dots?
pollen grains being hit by water molecules
;; (The water particles or water molecules themselves are invisible under the microscope because they are very, very small)
91
Small smoke particles suspended in air are viewed through a microscope.
The smoke particles move randomly.
What does this show?
The air consists of fast-moving molecules.
92
A swimmer feels cold after leaving warm water on a warm, windy day.
Why does she feel cold even though the air is warm?
The water on her skin evaporates quickly and cools her skin.
93
Some liquid is poured into a metal dish on a wooden table. The dish, the liquid, the table and the air around the dish are all at the same temperature.
The temperature of the liquid now starts to decrease.
What could cause this temperature decrease?
evaporation of the liquid
94
The diagram shows four labelled changes of state between solid, liquid and gas.
Which changes need an energy input?
P = solid to liquid
& Q = liquid to gas
95
A thermometer bulb is covered by a piece of damp absorbent cloth.
Air at room temperature is blown across the damp cloth.
What happens to the thermometer reading?
It falls.
96
The molecules of a substance become more closely packed and move more quickly.
What is happening to the substance?
A gas is being heated and compressed.
97
MCQ: Evaporation occurs when molecules escape from a liquid surface into the air above it.
During this process the temperature of the liquid falls.
Why does the temperature of the liquid fall? //
SAQ: Two containers made of insulating material contain the same volume of water at room
temperature. The containers do not have lids. The volume of liquid in each container gradually decreases.
After a certain time, the temperature of the water has decreased to below room
temperature.
Explain, in terms of molecules, why the temperature has decreased.
The molecules with the highest energies escape into the air. //
Faster, more energetic water molecules evaporate
Slower, less energetic molecules remain (so temperature is lower)
98
A beaker contains 0.500kg of water at a temperature of 3.0Β°C. The beaker is heated, and the
internal energy of the water increases by 21.0kJ.
The specific heat capacity of water is 4200J /(kgΒ°C).
What is the temperature of the water after it has been heated?
13.
H = mc(T_2 - T_1)
where m = mass of the body
c = Specific heat capacity of the body
T_2 = final temperature of the body
T_1 = initial temperature of the body
99
MCQ: Gases can be compressed, but liquids cannot.
Which statement explains this difference? //
SAQ: Explain, in terms of molecules, why it is possible to compress a gas, but not a liquid. [2]
Molecules in a gas are further apart than molecules in a liquid. //
Gas molecules are far apart
Molecules of liquid are close together and are touching
100
The diagram shows a quantity of gas trapped in a cylinder. The piston is pushed in slowly and the
gas is compressed. The temperature of the gas does not change.
Which graph shows the relationship between the pressure and the volume of the gas?
[B] - pressure-volume downward curve that's like IβΏ
101
One of the containers is wide and shallow. The other container is narrow and deep.
Predict which container has the greater rate of cooling. Explain your answer.
Water in wide container
bc it has water with larger surface area
Rate of evaporation higher,
OR higher chance of evaporation
102
A quantity of gas is contained in a sealed container of fixed volume. The temperature of the
gas is increased.
State, in terms of molecules, two reasons why the pressure of the gas increases
Molecules hit walls more often/ more frequently
Molecules hit walls with greater force/ impulse/ harder
103
Suggest why it may be necessary to release helium from the balloon as it rises even
higher.
to reduce the pressure inside the balloon
104
A student carries out an experiment to find the relationship between the pressure p and the
volume V of a fixed mass of gas. The table contains four of her sets of measurements.
p / kPa 250 500 750 1000
V / cm3 30.0 15.2 9.8 7.6
(i) Use the data in the table to suggest the relationship between the pressure and the
volume in this experiment. Explain how you reach your conclusion.
P Γ V values are about 7500 [OR If pressure doubles, volume halves]
(so) PV = constant
105
A bubble of gas escapes from the mud at the bottom of the lake and rises to the surface.
Place one tick in each row of the table to indicate what happens to the volume, the mass and the density of the gas in the bubble. Assume that no gas or water vapour enters or
leaves the bubble.
Volume of bubble ...
Mass of gas ...
Density of gas ...
Volume of bubble increases
Mass of gas stays the same
Density of gas decreases
106
Use the idea of momentum to explain how the molecules exert a force on the walls of the
cylinder.
collisions with walls causes change in momentum (of molecules) [1]
FORCE NEEDED TO CHANGE MOMENTUM
107
The piston is moved so that the new length of cylinder occupied by the gas is 40cm. The
temperature of the gas is unchanged.
Explain, in terms of the behaviour of the molecules, why the pressure has changed.
(molecules) collide with walls more often [1]
greater force per unit area [1]
108
State two ways in which the molecular structure of a liquid is different from the molecular
structure of a solid.
1. liquid molecules not in fixed positions
2. solid molecules
arranged regularly in a lattice
109
Explain, in terms of energy, the process which takes place as a solid at its melting point
changes into a liquid at the same temperature.
heat required to break bonds (between molecules)
110
The animal derives energy from its food to maintain its body temperature.
State the energy change that takes place.
chemical (energy in body) converted to thermal / internal (energy)
111
Explain, in terms of molecules, the effect on the pressure of the gas if it was not given time to cool to its original temperature. [3]
molecules would move faster/have more KE
more (frequent)/harder collisions with walls
pressure increases
112
The area of the piston is 5.5 Γ 10β3 m2 (0.0055 m2).
Calculate the force exerted by the gas on the piston when the pressure is 800 kPa.
= 4400 N
Pressure = force/area
800 x 1000 = 800,000 kg/(m.s^2)
800000 x 0.0055 = 4400 N
113
Smoke particles are introduced into a glass box containing air.
Light shines into the box so
that, when observed through a microscope, the smoke particles can be seen as bright points of light.
Describe the motion of the smoke particles and account for this motion in terms of the air
molecules. [4]
random movement;
sudden changes of direction;
collide with smoke particles;
air molecules move randomly;
114
Fig. 5.1 shows a quantity of gas in a cylinder sealed by a piston that is free to move.
The temperature of the gas is increased. State what happens, if anything,
1. to the piston,
2. to the pressure of the gas.
1. moves to the right
2. remains constant
115
The piston is now fixed in place and the temperature of the gas is increased further.
Explain, in terms of the behaviour of molecules, what happens to the pressure of the gas. [2]
(pressure of the gas) increases
more frequent collisions (of gas molecules) with walls
116
The solar panel in Fig. 4.1 is designed to heat water.
A person is deciding whether to install solar panels on her house.
List and explain three pieces of information she needs to consider in order to make her
decision.
which direction roof faces
household needs / whether
cost of panel / installation
whether roof is shaded
117
The Sun releases energy as a result of nuclear fusion.
State the meaning of nuclear fusion.
nuclei join together, accept hydrogen for nuclei
to produce a different element / helium (and energy)
118
Fig. 4.1 shows a cross-section of a double-walled glass vacuum flask, containing a hot liquid.
The surfaces of the two glass walls of the flask have shiny silvered coatings.
Explain why the rate of loss of thermal energy through the walls of the flask by radiation is
very low.
surface/silver (of walls) is good reflector/poor absorber (of radiation)
surface/silver (of walls) is poor emitter (of radiation)
119
Suggest, with reasons, what must be added to the flask shown in Fig. 4.1 in order to
keep the liquid hot.
add a lid
to reduce/prevent (loss of heat by) convection
made of insulator
120
Explain
(i) why her hands are not heated by convection,
(ii) why her hands are not heated by conduction.
(i) warm air rises/moves up (not sideways)
(ii) air (between plate and hands) is a poor conductor/does not conduct
121
Fig. 5.1 shows two identical metal cans, open at the top, used in an experiment on thermal energy. The outside of can A is polished and the outside of can B is painted black.
The cans are heated to the same temperature. Predict and explain the relative rates of
loss of thermal energy by infra-red radiation from the two cans.
black can/B loses heat energy quicker/ cools faster
black radiates/emits more
122
Thermal Expansion Of Water.
1.Water will expand when it _____
2. Density of ice is _____ than density of water.
Water freezes => expands
Density of ice => LESS than water
123
In a rigid sealed container, both the volume and amount of gas molecules are fixed.
The only way to increase the gas pressure is by increasing its temperature.
π·βπ» π€βππ π ππ ππππ π‘πππ‘
so
π·/π»=ππππππππ
124
The same quantity of thermal energy is supplied to two solid objects X and Y. The temperature
increase of object X is greater than the temperature increase of object Y.
Which statement explains this?
A X has a lower melting point than Y.
B X has a lower density than Y.
C X has a lower thermal capacity than Y.
D X is a better thermal conductor than Y.
C) X has a lower thermal capacity than Y.
125
specific heat capacity
Energy required to raise temperature of 1kg of a substance by 1 degree Celsius
126
Total internal reflection [2]
all light is reflected
no light is refracted
β’ (occurs) when light travels in a more dense medium towards a (boundary with a) less dense medium
