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Physics 2 - Mechancis, materials and waves > Thermal physics > Flashcards

Thermal physics Flashcards

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1
Q

Energy transfer between two objects takes place if

A

one object exerts a force on the other object and makes it move i.e. on object does work on the other; or if energy is transferred by heating because of a temperature difference between two objections (conduction, convection, radiation)

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2
Q

The internal energy of an object is

A

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

the sum of the random distribution of the kinetic and potential energies of its molecules

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3
Q

The internal energy of an object is increases because of

A

energy transfer by heating the object, or work done on the object (e.g. by electricity)

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4
Q

If the internal energy of an object stays constant then either

A

there is no energy transfer by heating and no work is one or energy transfer by heating and work done balance each other out

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5
Q

First law of thermodynamics

A

the change of internal energy of the object = the total energy transfer due to work done and heating (when work is done/energy transferred by heating)

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6
Q

A molecule is

A

the smallest particle of a pure substance that is characteristic of the substance

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7
Q

An atom is

A

the smallest particle of an element that is characteristic of the element

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8
Q

Structure of solid

A

In a solid, the atoms and molecules are held to each other by forces due to the electrical charges of the protons and electrons in the atoms. The molecules in a solid vibrate randomly about fixed positions. The higher the temperature of the solid, the more the molecules vibrate. The energy supplied to raise the temperature of a solid increases the kinetic energy of the molecules. If the temperature is raised enough, the solid melts. This happens because its molecules vibrate so much that they break free from each other and the substance loses its shape. The energy supplied to melt a solid raises the potential energy of the molecules because they break free from each other.

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9
Q

Structure of liquid

A

In a liquid, the molecules move about at random in contact with each other. The forces between the molecules are not strong enough to hold the molecules in fixed positions. The higher the temperature of a liquid, the faster its molecules move. The energy supplied to a liquid to rise its temperature increases h kinetic energy of the liquid molecules. Heating the liquid further causes it to vaporize. The molecules have sufficient kinetic energy to break free and move away from each other to become a gas.

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10
Q

Structure of gas

A

In a gas or vapour, the molecules also move about randomly but much further apart on average than in a liquid. Heating a gas or a vapour makes the molecules speed up and so gain kinetic energy.

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11
Q

Increasing the internal energy of a substance increases the

A

kinetic and/or potential energy associated with the random motion and positions of its molecules.

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12
Q

The temperature of an object is

A

a measure of the degree of hotness of the object. The hotter an object is, the more internal energy it has.

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13
Q

Thermal equilibrium when

A

two objects are of the same temperature so o overall energy transfer by heating will take place.

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14
Q

A temperature scale is defined in terms of

A

fixed points which are standard degrees of heat that can be accurately reproduced.

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15
Q

Celsius properties

A

ice point, 0, temperature of pure melting ice; steam point, 100, temperature of steam at standard atmospheric pressure

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16
Q

The absolute scale of temperature, kelvins properties

A

absolute zero 0K which is lowest possible temperature, the triple point of water 273K, which is the temperature at which ice, water and water vapour co-exist in thermodynamic equilibrium

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17
Q

Celsius –> Kelvins

A

+273.15

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18
Q

An object at absolute zero has

A

minimum internal energy as no object can have a lower temperature

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19
Q

Graph of gas pressure against temperature

A

Crosses y-axis at 0C or 273K and cuts x axis at -273C or 0K for any volume or type of gas

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20
Q

The specific heat capacity, c, of a substance is

A

the energy needed to raise the temperature of uni mass of the substance by 1K without change of state.

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21
Q

Unit of specific heat capacity

A

Unit J/kgK

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22
Q

To raise the temperature of mass m of a substance from temperature a to temeperature b

A

Q=mc(b-a)

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23
Q

Continuous flow heating

A

in an electric shower, water passes steaily through copper coils heated by an electrical heater. The water is hotter at the outlet than at the inlet. The electric energy supplied per second IV = mc(b-a)/t so for a solar heating panel, the energy gained per second by heating the liquid that flows through the panel is equal to mc(b-a)/t

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24
Q

The density of a gas is much less than the density of the same substance in the liquid or the solid state. This is because

A

the molecules of a liquid and of a solid are packed together in contact with each other. In contrast, the molecules of a gas are on average separated from each other by relatively large distances.

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25
Q

Liquids and gases can flow, but solids cannot. This is because

A

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

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26
Q

When a solid is heated at its melting point

A

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. The energy needed to melt a solid at its melting point is called latent heat of fusion

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27
Q

Latent heat is released when

A

a vapour condenses. This happens because the vapour molecules slow down s the vapour is cooled. The molecules move slowly enough for the force bonds to pull the molecules together to form a liquid.

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28
Q

Sublimation

A

when a solid vaporises directly when heated

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29
Q

More energy is needed to _____ a substance than to ____ IT

A

More energy is needed to vaporise a substance than to melt it

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30
Q

The specific latent heat of fusion of a substance is

A

the energy needed to change the state of unit mass of the substance from solid to liquid without change of temperature

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31
Q

The specific latent heat of vaporisation of a substance is

A

the energy needed to change the state of unit mass of the substance from liquid to vapour without change of temperature

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32
Q

Q=ml

A

(l is the specific latent heat with unit J/kg)

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33
Q

Energy transferred when its state changes

A

Q=ml

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34
Q

Energy transferred when its temperature changes

A

Q=mc(b-a)

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35
Q

Temperature time graph for a solid being heated

A

y=x for solid, then melting point, then y=c, then y=x for liquid, then boiling point, then y=x for gas

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36
Q

If the solid has a large specific heat capacity than the liquid, the rate of temperature rise of the solid is

A

less than that of the liquid

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37
Q

For a pure substance, the change of state is

A

at constant temperature

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38
Q

Pressure =

A

force per unit area that the gas exerts normally on a surface

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39
Q

Pressure is measured in

A

pascals, where 1 Pa = 1Nm^-2

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40
Q

The pressure of a gas depends on

A

its temperature, the volume of the gas container, and the mass of gas in the container

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41
Q

Boyle’s law states that

A

for a fixed mass of gas at constant temperature, pV = constant

42
Q

Graph of pressure (y) against 1/volume

A

y=mx

43
Q

Graph of pressure against volume is

A

a parabola that tends towards each axis. A higher temperature shifts the curve outwards

44
Q

A gas at very high pressure does not

A

obey Boyle’s law. The molecules are so close to each other than the molecules’ own volume becomes significant.

45
Q

Charles’ law

A

V/T is constant so y=mx

46
Q

Any change at constant pressure is called an

A

isobaric change

47
Q

When work is done to change the volume of a gas, energy must be transferred by heating to keep the pressure constant, and so the work done by the gas on a piston can be given by the equation

A

Work done = p(delta V)

48
Q

The pressure law

A

the relationship between pressure p and the temperature T, in kelvins, is: p/T = constant so y=mx

49
Q

Brownian motion

A

the motion of each particle is because it is bombarded unevenly and randomly by individual molecules. The particle therefore experiences forces due to these impacts, which change its magnitude and direction at random. So Brownian motion showed the existence of molecules and atoms

50
Q

The Avogadro constant, NA, is defined as

A

the number of atoms in exactly 12g of the carbon 12 isotope= 6.023*10^-23

51
Q

One atomic mass unit is

A

1/12th of the mass of a carbon 12 atom

52
Q

One mole of a substance consisting of identical particles is defined as

A

the quantity of substance that contains NA particles

53
Q

molarity is

A

The number of moles in a given quantity of a substance

54
Q

The unit of molarity is the

A

the mol

55
Q

The molar mass of a substance is

A

the mass of 1 mol of the substance. The unit of molar mass is kg/mol

56
Q

The number of moles in a mass of a substance =

A

mass of substance/molar mass of substance

57
Q

The number of molecules in a substance =

A

avagadros constant*mass of substance/molar mass of substance

58
Q

An ideal gas is a gas that

A

obeys Boyle’s law

59
Q

The three experimental gas laws can be combined to give

A

pV/T = constant for a fixed mass of ideal gas where T is the absolute temperature

60
Q

The molar gas constant, R =

A

R = pV/T = 8.31J/Kmol for an ideal gas at absolute zero with a pressure of 101kPa

61
Q

A graph of pV against temperature for n moles is

A

a straight line through absolute zero and has gradient equal to nR

62
Q

So the combined gas law can be written as

A

pVm = RT where Vm = volume of 1 mol of ideal gas

63
Q

Ideal gas equation

A

pV = nRT for n moles of ideal gas, where V = volume of the gas at pressure p and temperature T in kelvins

64
Q

The mass of a substance is equal to

A

its molar mass * the number of moles

65
Q

n =

A

pV/RT

66
Q

Density of an ideal gas of molar mass M, p =

A

p = nM//V = pM/RT

67
Q

In the equation pV = nRT, substituting the number of moles n in give

A

In the equation pV = nRT, substituting the number of moles n =N/NA gives pV=NkT where the Boltzmann constant k is R/NA and N is the number of molecules

68
Q

For an ideal gas at constant pressure, its density is

A

inversely proportional to its temperature

69
Q

k =

A

1.38*10^-23J/K

70
Q

Boyles law explanation

A

the pressure of a gas at constant temperature is increased by reducing its volume because the gas molecules travel less distance between impacts at the walls due to the reduced volume. Therefore, there are more impacts per second, and so the pressure is greater

the pressure of a gas at constant temperature is increased if the volume is reduced because the gas molecules travel less distance between impacts at the walls due to the reduced volume hence there are more impacts per second and so greater pressure

71
Q

Pressure law explanation

A

the pressure of a gas at constant volume is increased by raising its temperature. The average speed of the molecules is increased by raising the gas temperature. Therefore, the impacts of the molecules on the container walls are harder and more frequent so the pressure is raised.

the pressure of a gas at constant volume is increased by raising the temperature. The average speed of the molecules is increased by raising temperature so the impacts of the molecules on the container walls exert more force and are more frequent hence the pressure (F/A) increases

72
Q

The root mean square speed of molecules =

A

the square root of (the sum of the squares of the individual molecules/number of molecules)

73
Q

If the temperature of a gas is raised

A

its molecules move faster, on average. The root mean square speed of the molecules increases. The distribution curve becomes flatter and broader because the greater the temperature the more molecules there are moving at higher speeds.

74
Q

Graph of number of molecules with speed v against speed v is

A

a normal distribution curve. Low temperature means the peak is stretched to the left. High temperature causes the peak to extend to the right.

75
Q

For an ideal gas consisting of N identical molecules, each of mass m, in a container of volume V, the pressure p of the gas is given by the equation

A

pV = Nm(C(rms)^2)/3 where C(rms) is the root mean square speed of the gas molecules

76
Q

For an ideal gas, its internal energy is due only to

A

the kinetic energy of the molecules of the gas

77
Q

The mean kinetic energy of a molecule of a gas =

A

total kinetic energy of all the molecules/total number of molecules = 0.5mC(rms)^2, the higher the temperature of a gas, the greater the mean kinetic energy of a molecule of the gas

78
Q

For an ideal gas at absolute temperature T

A

the mean kinetic energy of a molecule of an ideal gas = 1.5kT
where k=R/N(A)

79
Q

The total kinetic energy of one mole =

A

NA *1.5kT = 1.5RT

80
Q

The total kinetic energy of n moles of an ideal gas

A

= n*1.5RT = 1.5nRT

81
Q

The total kinetic energy of n moles of an ideal gas =

A

1.5nRT - the internal energy for n moles of an ideal gas at temperature T (in kelvins)

82
Q

The specific heat capacity is

A

the energy needed to raise the temperature of 1kg of that substance by 1K without changing state

83
Q

The specific latent heat fusion of a substance is the energy needed to

A

change the state of unit mass of the substance from solid to liquid without change of temperature.

Vaporisation –> liquid to gas

84
Q

Lowest possible temperature is called

A

absolute zero (0K = -273C), at 0K, all particles have the minimum possible kinetic energy – the particles are point masses. K = C + 273

85
Q

Ideal gas equation

A

pV = nRT for n moles and as pV = NkT for N molecules

p= Pressure (Pa, Nm-2) V= Volume (cm3 , dm3 , m3 ) T= temperature (K, °C) R= Molar gas constant = 8.31 J/Kmol n= Number of moles

86
Q

Number of particles in a mass of gas, N =

A

n*NA

87
Q

Avogadro constant NA =

A

6.023*10^23

88
Q

Boltzmann constant k

A

= R/NA = 8.31/6.02310^23 = 1.3810^-23J/K

89
Q

Combine N=nNA and k= R/NA

A

Nk = nR

90
Q

The molar mass of a substance is the mass of

A

1 mole of that substance

91
Q

The molecular mass of a substance is the mass of

A

1 molecule of that substance

92
Q

Assumptions made leading to derivation of 3pV=Nm(C(rms))^2

A

Randomly moving particles with range of velocities and direction

Particles are point masses – negligible volume

Perfectly elastic collisions

No intermolecular interaction between particles

Time of collision with container is less than time of flight between impacts

The root mean squared value is equal to the root mean value. Average molecular kinetic energy: 0.5mC(rms)^2 = 1.5Kt = 1.5(RT/NA)

Mean kinetic energy of a molecule of an ideal gas = 1.5kT

Total kinetic energy of n moles of an ideal gas = 1.5nRT

93
Q

Boyle’s law: constant, explanation, equation, graphs

A

Temperature is kept constant (isothermal)

pV = constant

Graph of p against V is y=1/x
Graph of p against 1/V is y=x

at constant temperature, the pressure and volume of the gas are inversely proportional

94
Q

Charles’ law: constant, explanation, equation, graphs

A

Pressure is kept constant (adiabatic)
V/T = constant

Graph of V against T is y=x

at constant pressure, the volume of a gas is directly proportional to its absolute temperature

95
Q

Pressure law: constant, explanation, equation, graphs

A

Volume is kept constant

Graph of P against T is y=x

at constant volume, the pressure of a gas is directly proportional to its absolute temperature

96
Q

The inversion tube experiment

A

tube, lead shots, glass rod replaced by thermometer: the gpe of an object falling in a tube is converted into internal energy when it hits the bottom of a tube. The tube is inverted each time the shots hit the bottom of the tube and the temperature of them is measured initally and after the inversions. For a tube of length L and inversions n, the loss of gpe = mgLn = the gain of internal energy = mc(b-a). Assuming all gpe lost is transferred to the internal energy of the lead shots we can rearrange to find c, the specific heat capacity.

97
Q

How a liquid turns into a gas

A

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.

98
Q

Latent heat

A

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/hidden 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.

99
Q

Brake pads

A

The brake pads of a moving vehicle become hot if the brakes are applied for long enough time. The work done by the frictional force between the brake pads and the wheel heats the brake pads which gain energy from the kinetic energy of the vehicle. The temperature of the brake pads increases as a result, and the internal energy of each brake pad increases.

100
Q

Molecules in an ideal gas have a …. The speed of an individual molecule changes when it … The distribution stays the same as long as …

A

Molecules in an ideal gas have a continuous spread of speeds. The speed of an individual molecule changes when it collides with another gas molecule. But the distribution stays the same as long as the temperature is the same.

Physics 2 - Mechancis, materials and waves (10 decks)

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