14 - Thermal Physics

Question 1

When two objects are in thermal equilibrium…

Answer 1

there is no net transfer of thermal energy between them (both objects same temperature)

Question 2

two objects in thermal contact with each other

Answer 2

If two substances are in thermal contact, and one is hotter than the other, there will be a net flow of
thermal energy from the hotter object to the cooler object. The hotter object will cool down and
the cooler one will warm up, until they are at the same temperature, where there will be no net
energy transfer

Question 3

zeroth law of thermodynamics

Answer 3

if two objects are in thermal equilibrium with a third, then all three objects are in thermal equilibrium with each other.

Question 4

temperature in degrees celsius to kelvin

Answer 4

0℃=273K
0K=-273℃

Question 5

absolute 0

Answer 5

is equal to 0K (lowest possible temperature)
molecules stop moving - zero kinetic
minimum internal energy provided solely by potential energy

Question 6

kinetic model for solids

Answer 6

regularly arranged, packed close together
strong electrostatic forces of attraction
fixed positions
they vibrate so have kinetic energy

Question 7

kinetic model for liquids

Answer 7

atoms are still close together
more kinetic energy than solids
flow past each other

Question 8

kinetic model for gases

Answer 8

more kinetic energy than liquids
atoms much further apart
move past each other
negligible electrostatic forces of attraction
random speed and directions.
(collisions with the container are responsible for pressure gas exerts on its container)

Question 9

Brownian motion

Answer 9

proved by viewing smoke cells under microscope and observing random motion path
caused by the air molecules striking the larger smoke particles - (random transfer of momentum)

Question 10

density

Answer 10

effected by spacing between particles

Question 11

define internal energy

Answer 11

the sum of the randomly distributed kinetic and potential energies of atoms within a substance

Question 12

why is internal energy not 0 even if atoms are not moving

Answer 12

kinetic energy of atoms may be 0 but there is still electrostatic potential energy between particles

Question 13

temp effect on internal energy

Answer 13

increasing temperature increases the average kinetic energy of atoms, moving faster and increasing internal energy of the substance
(during solid / liquid / gas phase)

Question 14

other way of increasing internal energy

Answer 14

when a substance changes phase, temperature does not change nor does kinetic energy but electrostatic potential increases

Question 15

internal energy change while changing state

Answer 15

potential energy increases, kinetic no change
temp stays same as thermal energy is being used to overcome electrostatic bonds between molecules
internal energy increases

Question 16

electrostatic potential during solid liquid gas phase

Answer 16

SOLID - electrostatic force large negative value
LIQUID - potential negative as energy required to break bonds
GAS - potential zero due to negligible electrical forces between atoms

potential lowest in solids, highest in gas (0J)

Question 17

define specific heat capacity

Answer 17

energy required per unit mass to change the temperature by 1K
units Jkg-1K-1

Question 18

metals tend to have…

Answer 18

low specific heat capacities, water has high

Question 19

specific latent heat of fusion

Answer 19

energy required per unit mass to change phase of substance from solid to liquid
E=mL

Question 20

specific latent heat of vaporisation

Answer 20

energy required per unit mass to change phase of substance from liquid to gas
E=mL

Question 21

when a substance reaches its melting point…

Answer 21

it requires energy to change phase from solid to liquid. the energy transferred to the substance increases the internal energy without increasing temperature

Question 22

method to find specific latent heat of fusion

Answer 22

use heating circuit
use thermometer to make sure ice is at melting point
measure pd across heater, current in heater and time for which heater is used, temp make sure constant
E=ItV
measure ice melted - collected in funnel
ItV=mL

Question 23

method to find specific latent heat of vaporisation

Answer 23

use electric heater and condenser to collect and measure mass of liquid that changes phase
measure pd across heater, current in heater and time for which heater is used, temp make sure constant
ItV=mL

Question 24

suggest why Lv is higher than Lf for most substances

Answer 24

there is a much larger difference between the internal energy of a gas and a liquid than between a liquid and a solid

Question 25

when water freezes…

Answer 25

energy is transferred to the surroundings, as water in its solid phase has less internal energy than when in its liquid phase