Thermal Phyiscs Flashcards
How are particles in solids arranged
Particles vibrate about fixed positions in a regular lattice. They’re held in position by strong forces of attraction.
Remember to talk about amplitude and lattice
How are particles is liquids arranged
Particles are constantly moving around and are free to move past one another, but are attracted to each other.
How are particles in gases arranged
Particles are free to move around with constant random motion and no fixed shape.
There are no forces of attraction between particles in an ideal gas.
What is the kinetic model of matter
The idea that solids, liquids and gases are made up of tiny moving or vibrating particles is called the kinetic model of matter.
What did Robert Brown notice about particle of pollen in water. What is Brownian
Robert Brown noticed that tiny particles of pollen suspended in water moved with a random motion.
This type of movement (random) of any particle suspended in a fluid is known as Brownian motion.
How can Brownian motion be observed in the lab
You can observe Brownian motion in the lab:
• Put some smoke in a brightly illuminated glass jar and observe the particles using a microscope.
• The smoke particles appear as bright specks moving haphazardly from side to side, and up and down.
What did the Brownian motion of particles provide evidenced for?
This provided evidence for the existence of atoms or molecules in the air (that have energy) - (the kinetic model of matter).
Why does Brownian motion occur
The randomly moving water particles were hitting the pollen particles unevenly, causing this motion. He explained that these collisions were elastic and resulted in a transfer of momentum from the water molecules to the pollen grains.
What is internal energy
Internal energy is the sum of the kinetic and potential energy of the particles within a system.
Why does kenetic energy depend on
The kinetic energy of a particle depends on its mass and speed. Through kinetic theory, the average kinetic energy is proportional to temperature — the hotter the temperature, the higher the average kinetic energy.
What does potential energy depend
Potential energy is caused by interactions between particles and is based on their positions relative to each other.
Do all particles have the same energy
energies are randomly distributed amongst the particles.
When you heat a substance how does its energy change
When you heat a substance, you increase its temperature - thereby increasing the kinetic energy of the particles within it and its internal energy.
When a substance changes phase, describe how it’s energy and temperature change
When a substance changes phase, its internal energy changes, but its kinetic energy (and temperature) doesn’t. This is because the change of phase is altering the bonds and therefore potential energy of the particles.
How do you convert from degrees to kelvin
+273
What happens at 0K
At 0 K all particles have the minimum possible internal energy - everything theoretically stops
How is the absolute Scale different to the Celsius scale
The Celsius scale uses the freezing and boiling points of water (0 °C and 100 °C) to make a temperature scale
While the absolute scale does not depend on the properties of any particular substance,
What is absolute zero
0k
What is another name for the absolute scale
The thermodynamic scale
What can you say if body A and body B are both in equilibrium with body C
If body A and body B are both in thermal equilibrium with body C, then body A and body B must be in thermal equilibrium with each other.
What is the rule about thermal energy
Thermal energy is always transferred from regions of higher temperature to regions of lower temperature.
A, B and C are three identical metal blocks. A has been in a warm oven, B has come from a refrigerator and C is at room temperature, what happens when you put the blocks together
Suppose A, B and C are three identical metal blocks. A has been in a warm oven, B has come from a refrigerator and C is at room temperature.
Thermal energy flows from A to C and C to B until they all reach thermal equilibrium and the net flow of energy stops. This happens when the three blocks are at the same temperature.
What does thermal equilibrium
When two objects are in thermal equilibrium there is no net flow of thermal energy between them. This means any objects in thermal equilibrium must be at the same temperature.
What is the specific heat capacity of a substance
The specific heat capacity (c) of a subostance is the amount of energy needed to raise the temperature of 1 kg of the substance by 1 K (or 1°C).
What is an equation to find energy change
energy change = mass x specific heat capacity x change in temperature
E= mc Δ θ
E is joules
m is in kg
θ is temperature in K or °c
c is Jkg^-1K^-1
Practical on specific heat capacity
How could you measure the specific heat capacity of a block using the method of mixtures
See bottom of CGP pg 104
What is the latent heat of fusion (in terms of particles )
To melt a solid, you need to break the bonds that hold the particles in place. The energy needed for this is called the latent heat of fusion.
What is the latent heat of vaporization (in terms of particles
Similarly, when you boil or evaporate a liquid, energy is needed to pull the particles apart completely. This is the latent heat of vaporisation.
What is the definition of specific latent heat of fusion or evaporation?
The specific latent heat (L) of fusion or vaporisation is the quantity of thermal energy required to change the state of 1 kg of a substance, while at a constant temperature.
What is the equation uses to calculate specific latent heat
energy change = mass of substance changed x specific latent heat
E=mL
How is the latent heat of vaporization or fusion written?
L ᵥ or L (subscript f)
How do you measure the specific latent heat of solid or liquid
For a solid, e.g. ice:
1) Put a heating coil and equal masses of ice in two funnels above beakers.
2) Turn on one heating coil for three minutes. Record the energy transferred in the three minutes.
Don’t turn on the other coil — it’s there so you can measure how much ice melts due to the ambient temperature of the room.
3) At the end of the three minutes, measure the mass of water collected in the beakers. Subtract one from the other to get the mass of ice, m, that melted solely due to the presence of the heater.
4) E = mL, so to find the specific latent heat of fusion for water just divide the energy supplied by the
mass of ice that melted: L = E ÷ m
For a liquid, you can do a very similar experiment - use a heating coil to boil water in a beaker, then divide the energy transferred by the coil while the water is boiling by the change in mass of the water.
Imagine a graph of temperature over time. Explain how you could calculate the energy transferred at each interval…
This graph has four distinct sections before turning into a gas. The energy transferred to the substance in each section can be calculated using either the specific heat capacity or specific latent heat equation:
1 heating the solid to its melting point, E = mc(solid) Δ θ
2 melting the solid at constant temperature, E = mL(f)
3 heating the liquid to its boiling point, E = mc(liquid) Δ θ
4 boiling the liquid at constant temperature, E= mL(v)
The total energy can then be determined by adding up the energy transferred in each section.
What are the three laws obeyed by an ideal gas?
Boyle’s Law: pV = constant
At a constant temperature, the pressure p and volume V of a gas are inversely proportional.
The higher the temperature of the gas, the further the curve is from the origin.
The Pressure Law: p ÷ T = constant
At a constant volume, the pressure p of a gas is directly proportional to its absolute temperature T.
Charles’s Law, says that at a constant pressure, the volume V of a gas is directly proportional to its absolute temperature T.
What would a graph of pressure again temperature (in degree Celsius) look like?
CGP pg 106
What is an experiment to investigate pressure and volume (boyle’s law)
CGP pg 106
What is an experiment to estimate absolute zero (pressure law)
CGP Pg 106
What is the equation of state?
Combining all three gas laws gives:
pV/T = constant
What is the constant in equation of states dependent on ?
The constant in the equation depends on the amount of gas used.
What is the equation of state of an ideal gas
pV=nRT
p is pressure (Pa),
V is volume (m^3)
T is temperature (K)
n is amount of gas in moles (see below)
When does the equation of state of an ideal gas work well?
This equation works well (i.e., real gases approximate to an ideal gas) for gases at low pressures and fairly high temperatures.
What is the symbol of avagrdos constant
N (subscript)A
How can you calculate the number (N) of particles in an amount of gas of “n” moles
N = nN(Subscript)A
What is the Boltzmann constant
The Boltzmann constant, k, is given by k = R/N ₐ
You can think of the Boltzmann constant as the gas constant for one particle of gas, while R is the gas constant for one mole of gas.
What is a version of the equation of state that involves the Boltzmann constant
pV = NkT
you combine N = nN ₐ, and k = R/N ₐ you’ll see that Nk = nR which can be substituted into the equation of state to give this alternative form (in terms of number of particles N, rather than moles, n).
How can newtons laws be used to explain the pressure that a gas causes
The particles of the gas are free to move around with constant random motion. There are no forces of attraction between the particles, so according to Newton’s 1st law, they continue to move with constant velocity until they collide with another particle or the box itself.
When they collide with the walls of a container the container exerts a force on them, changing their momentum as they bounce off the wall.
When a single atom collides with the container wall elastically, its speed does not change, but its veloeity changes from +ums-‘ to -ums-. The total change in momentum is -2mu.
The atom bounces between the container walls, making frequent collisions. According to Newton’s second law, the force acting on the atom is F = Δ p / Δt , where Δ p = -2mu and t is the time between collisions with the wall. From Newton’s third law, the atom also exerts an equal but opposite force on the wall.
A large number of atoms collide randomly with the walls of the container. If the total force they exert on the wall is F, then the pressure they exert on the wall is given by p= F/A, where A is the
cross-sectional area of the wall.
What is an equation for the pressure of an idea, gas
—
pV=1/3 Nmc^2
— Where c^2 is the mean square speed (mean of the squared speeds of all the particle)
What does the pressure exerted by a gas depend on
1) The volume, V, of the container — increasing the volume of the container decreases the frequency of collisions because the particles have further to travel in between collisions. This decreases the pressure.
2) The number of particles, N - increasing the number of particles increases the frequency of collisions between the particles and the container, so increases the total force exerted by all the collisions.
3) The mass, m, of the particles - according to Newton’s 2nd law, force is proportional to mass, so heavier particles will exert a greater force.
4) The speed, c, of the particles — the faster the particles are going when they hit the walls, the greater the change in momentum and force exerted.
What are the assumptions made about idea gases in kenetic theory
1) The gas contains a large number of particles.
2) The particles move rapidly and randomly.
3) The volume of the particles is negligible when compared to the volume of the gas.
4) Collisions between particles themselves or between particles and the walls of the container are perfectly elastic.
5) The duration of each collision is negligible when compared to the time between collisions.
6) There are no forces between particles except for the moment when they are in a collision.
When are the assumption about ideal gases wrong
Real gases behave like ideal gases as long as the pressure isn’t too big and the temperature is reasonably high (compared with their boiling points).
—
What is c^2?
What is the root of this?
—
1)c^2 is the mean square speed and has units m^2s^-2.
2) it is the average of the squared speeds of all the particles, so the square root of it gives you the typical speed.
3) This is called the root mean square speed or, usually, the r.m.s. speed. It’s often written as C ᵣₘₛ.
The unit is the same as any speed — ms^-1
Draw and describe a Maxwell-Boltzman distribution
CGP pg 110
Draw the speed distribution of a gas particles (one at a high temperature and one at a low temperature
What can be said as the average temperature increases?
Pg 110
What happens to the average speed of particles at a constant temperature
1) As a result of the collisions, energy will be transferred between particles.
2) Some particles will gain speed in a collision and others will slow down.
3) Between collisions, the particles will travel at constant speed.
4) Although the energy of an individual particle changes at each collision, the collisions don’t alter the total energy of the system.
5)
So, the average speed of the particles will stay the same provided the temperature of the gas stays the same.
How can you find the kinetic energy of one gas particle?
Therefore how do you find the internal energy of an ideal gas
CGP 111
Orange and green boxes
How are kinetic energy and internal energy related to absolute temperature
The equations above show that average kinetic energy and internal energy are both directly proportional to the absolute temperature — a rise in absolute temperature will cause an increase in the kinetic energy of the particles, meaning a rise in internal energy.
