Chapter 14- Thermal

Question 1

define the triple point of a substance

Answer 1

a specific temperature and pressure where the three phases of matter can exist in thermal equilibrium

Question 2

what is the net transfer of thermal energy if one object is hotter than another

Answer 2

energy moves from the hotter object to the colder object

Question 3

what is meant by two objects being in thermal equilibrium

Answer 3

there is no net transfer of energy/at the same temperature

Question 4

what is the 0th law of thermodynamics

Answer 4

if A is in thermal equilibrium with B
and B is in thermal equilibrium with C
then A is in thermal equilibrium with C

Question 5

what are the 2 fixed points on the Celsius scale

Answer 5

• 0 is the freezing point of water

- 100 is the boiling point of water

Question 6

what are the important things about the Kelvin scale

Answer 6

0 is absolute 0
273 is the triple point of water
the increments are the same as in celcius
e.g. 0 celcius = 273 kelvin etc.

Question 7

what is the kinetic model

Answer 7

it is a model which describes how all substances are made of atoms or molecules arranged differently depending on phase

Question 8

summarise the particle layout for a solid

Answer 8

• particles are regularly arranged/packed closely
• strong electrostatic forces
• fixed positions
• particles are able to vibrate around their fixed positions so they have KE

Question 9

summarise the particle layout for a liquid

Answer 9

• particles are very close together but not in a regular arrangement
• particles can flow over each other
• more KE than solids
• weaker electrostatic forces than solids
• random motion of particles

Question 10

summarise the particle layout for gases

Answer 10

• negligible electrostatic forces
• fast and random motion
• more KE
• collisions between different particles
• collisions between particles and container walls leads to pressure

Question 11

what is the idea of Brownian motion and how did Einstein explain it

Answer 11

• Brownian motion is the idea that substances were formed of particles and the particles’ KE and collisions determines its motion
• its the idea of random motion

Einstein said:

• collisions between water molecules and pollen grains are elastic
• there is a transfer of momentum from the water molecules to the pollen grains

Question 12

how can Brownian motion be observed

Answer 12

• set up a light source going through a convex lens
• the light rays should pass through a smoke cell
• a microscope can be placed above the cell, at a right angle
• the smoke particles can be viewed as they scatter the light
• both the smoke particles and the air particles have random motion but only the smoke particles move slow enough to be viewed (they do have the same KE)

Question 13

what generally happens to density as you change phase

Answer 13

• density decreases from solid to liquid to gas

- this is because the particles have more KE so collide more frequently and take up more room

Question 14

define the internal energy of a substance

Answer 14

“the sum of the randomly distributed kinetic and potential energies of atoms or molecules within a substance”

Question 15

what are the two ways that internal energy is increased

Answer 15

increasing temperature, changing phase

Question 16

why does increasing the temperature of a substance increase the internal energy

Answer 16

the kinetic energies of the particles increases thus the internal energy increases

Question 17

why does changing phase increase the internal energy of a substance

Answer 17

the potential energies of the molecules increases,
because work is done on the molecules
they are rearranged and electrostatic attraction between them decreases

Question 18

why do solids and liquids have a negative potential energy

Answer 18

it requires energy to overcome the electrostatic forces between molecules, liquids are less negative than solids

Question 19

define specific heat capacity

Answer 19

“the specific heat capacity of a substance is the energy required per unit mass (kg) to change the temperature by a unit temperature (1K)”

Question 20

what is the equation for specific heat capacity

Answer 20

E = mc(delta(theta))
Energy = mass x SHC x change in temp

Question 21

what tend to be the units for SHC

Answer 21

J kg^-1 K^-1

Question 22

what is water’s SHC

Answer 22

4.2KJ kg^-1 K^-1

Question 23

what is the circuit required for the heater when determining SHC and SLH

Answer 23

• a heater in series with a cell, variable resistor and ammeter
• in parallel with a voltmeter

Question 24

outline the experiment to determine the SHC of a substance

Answer 24

• record the mass of a substance
• fully immerse a heater and place a thermometer into an insulated substance
• record initial temperature
• switch on the heater and start a timer simultaneously
• record the p.d. and current of the heater
• after a set time, stop the timer and record the change in temperature
• given E = VIt,
• C = VIt/m(delta(theta))

Question 25

how can this SHC experiment be improved

Answer 25

• plot a graph of temperature (on Y) against time (on x)
• given VIt = MC(delta(theta))
• this means delta(theta) = p/mc (t)
• thus SHC = power/(m)(gradient)

Question 26

what to do where you mix two different substances and they reach thermal equilibrium

Answer 26

• where 2 substances of different initial temps are mixed
• final temps are =
• known SHC of 1
• known masses’

we assume the energy transferred by both is equal
this implies m1c1theta1 = m2c2theta2
this means if 5 are known a 6th can be calculated

Question 27

define specific latent heat

Answer 27

“the specific latent heat of a substance (L) is the energy required to change the phase per unit mass (usually kg) while at a constant temperature”

Question 28

what is the equation for specific latent heat

Answer 28

E = Lm

Question 29

what is the symbol and term for the transition of

solid –> liquid

Answer 29

specific latent heat of fusion Lf

Question 30

what is the symbol and term for the transition of

liquid –> gas

Answer 30

specific latent heat of vaporisation Lv

Question 31

why does changing state require energy

Answer 31

although the temp doesn’t change, the potential energy increases

Question 32

how to experimentally find specific latent heat of fusion

Answer 32

• weigh the beaker first
• place ice in a funnel with a heater as in SHC experiment a thermometer
• make sure the temperature is 0 degrees C
• just as it starts to melt, switch on the heater and start a timer
• record p.d. and current
• once all ice has melted, switch off timer and heater
• measure mass of water in beaker
• given E = VIt = Lf x mass
• use Lf = VIt/mass

Question 33

how to experimentally find specific latent heat of vaporisation

Answer 33

• do the same thing as before but use a different setup

- inner flask with heater-> outer flask-> condenser-> beaker

Question 34

why is SLH (vaporisation) usually higher than SLH (fusion)

Answer 34

• there is a bigger difference in internal energies between liquid and gas than solid and liquid

Question 35

combining SLH and SHC

Answer 35

SLH and SHC might need to be combined where a substance undergoes a very large temperature change and the substance melts or boils in between
e.g. ice from -40 to 100
(SHC for -40 to 0) + (Lf) + (SHC for 0 to 100) + (Lv)

Question 36

what is absolute zero

Answer 36

a temperature of 0K

the minimum internal energy a substance can have and the lowest possible temperature

Question 37

is there any increase of potential energy when a substance’s temperature increases

Answer 37

only a small increase

Question 38

when heating a sample of a substance to calculate specific latent heats, why may the substance not start changing state immediately

Answer 38

• it may need to heat up (increase temperature) first before it can change phase

Question 39

why may the temperature of an object not start heating up immediately when the power to a heater is switched on/ why is there sometimes a flat line on a temp-time graph even if its not changing state

Answer 39

the heater is being heated/work is done on the heater

Question 40

what is the difference between internal energy of a solid/liquid and internal energy of an ideal gas

Answer 40

• internal energy of an ideal gas is purely kinetic as the molecules have no potential energies
• internal energy of a solid/liquid is both kinetic and potential energies

Question 41

what is an important detail of wording in the SHC experiment

Answer 41

heater is FULLY IMMERSED in water/oil/substance

Question 42

what are some sources of uncertainty in the SHC experiment and how can these be reduced

Answer 42

• non-uniform temperature distribution, to solve stir as the substance heats (if liquid)
• heat loss to surroundings - insulate substance or start the experiment the same amount below room temperature as you will finish it
• take into account the energy required to heat container/beaker

Question 43

if you are describing the motion/energy of particles in objects that are in different phases but at the same temperature what should we note

Answer 43

• the particles still have the same KE because they are at the same temperature

Question 44

what should we do if there’s two experiments for SHC or SLH where one is heated only by the surroundings and one has a heater

Answer 44

• find the difference in mass heated/melted by the two experiments
• THIS is the mass heated by the heater