Chapter 14 Thermal physics

what does thermal equilibrium mean

there is no net flow of energy between 2 objects in contact - they are the same temperature

what is the internal energy of a substance

sum of the random distribution of kinetic and potential energies associated with the molecules of a system

why can internal energy never be 0

at the lowest temperatures kinetic energy of the molecules will be 0

but potential energy cannot be 0

why is absolute 0 the lowest limit for temperature

the internal energy of a substance is the lowest possible at absolute 0

due to potential energy of the molecules only

what happens at the triple point of a substance

the three phases of matter of that substance exist in thermal equilibrium

why is the celcius scale not accurate

the boiling point of water varies with temperature

what are 2 conlcusions that can be made from the brownian motion experiment using smoke

the air molecules move with random linear motion

the air molecules are much smaller than the smoke molecules

what is brownian motion

the random motion of particles suspended in a fluid due to their collision with randomly moving fluid molecules

how can brownian motion be demonstrated

using a lamp and a convex lens, shine a light through a smoke cell

use a microscope to observe the smoke

should see the smoke molecules moving linearly in random directions

why does volume of water increase as temperature increases

the molecules gain kinetic energy

their average speed increases

higher frequency of collisions

what happens to the internal energy of a substance as its temperature increases

internal energy increases because the average kinetic energy of the molecules increases

why does temperature not change when a substance is changing phase

the energy transferred to the substance does not increase the average kinetic energy of the molecules

it only increases the potential forces between the molecules

what is the order of potential energies in solids liquids and gases

most negative in solids

less negative in liquids

0J in gases (highest)

define specific heat capacity

the energy required to increase the temperature of 1kg of substance by 1 kelvin

what is the equation for specific heat capacity

E = mcΔtΘ

where E is the energy supplied to the substance

m is the mass of the substance

c is the SHC

ΔΘ is the change in temperature

kinetic model of solids

particles vibrate in a fixed ordered lattice

very little spacing between the molecules

kinetic model of liquids

particles flow past eachother

vibrating but not in fixed positions

larger spacing than in solids

kinetic model of gases

particles far apart

only exert forces on eachother during collisions

particles move in random linear motion

what happens to the vibrations of particles as temperature is increased

amplitude of vibrations increases

practical to work out specific heat capacity

Set up circuit as shown in diagram

Measure mass of substance using top pan balance

Measure V, I, and temperature at regular time intervals

Plot a graph of temperature on y axis and time on x axis

VI = mc Θ/t

Θ/t is the gradient so VI = mc x gradient

c = VI/m x gradient

To improve accuracy make sure heater is fully submerged

Make sure everything is insulated to minimise transfer of energy to surroundings

define specific latent heat

the energy required to change the phase of 1kg of substance without changing its temperature

define specific latent heat of fusion

energy required to melt 1 kg of substance at a constant temperature

define specific latent heat of vapourisation

energy required to vapourise 1 kg of substance at a constant temperature

equation for specific latent heat

E = mL

where E is the energy supplied to the substance

m is the mass of the substance

L is the SLH

practical to work out specific latent heat of fusion

Set up apparatus as shown in diagram

Use thermometer to make sure the ice is just at its melting point and not a lower temperature

use a stop watch to measure how long the heater is used

Calculate energy transferred to the ice using E = VIt

Measure the mass of the ice melted in the beaker using a top pan balance

To get an accurate value for the mass melted due to the heater set a control beaker with ice in a funnel and measure the mass of ice melted just due to room temperature. Subtract this mass from the mass in the beaker with the heater.

E = mL so L = E/m so L = VIt/m

practical to work out specific latent heat of vapourisation

Measure the mass of water in the collecting flask using a top pan balance

Use a thermometer to bring the heater to 100 degrees and ensure it stays at that temperature

Record V and I when the heater is at 100 degrees

Use a stop watch to measure how long it takes for all the water to evapourate

E = VIt

L= E/m so L = VIt/m

what are 3 key assumptions in the kinetic theory of gases

1. forces between particles are negligible except during collisions

2. collisions are perfectly elastic

3. large number of molecules are in random linear motion

explain the changes to the internal energy of molecules of a substance as it melts

1. no change in kinetic energy

2. as temperature is constant

3. the potential energy of the molecules increases

4. so the internal energy increases