Thermal Physics

Question 1

define internal energy

Answer 1

sum of randomly distributed kinetic energy and potential energy of all particles in a body

Question 2

how can internal energy of a system be increased?

Answer 2

heating, thermal energy transfer

doing work, energy transfer as a result of a force moving

Question 3

what is the first law of thermodynamics?

Answer 3

internal energy of a system increases if a system is heated or work is done on the system by an external force (or decreased of cooled or does work against external force)

Question 4

define specific heat capacity and what a high specific heat capacity means for a material

Answer 4

energy required to raise 1kg of the material by 1’C/1K without any change of state

high specific heat capacity = lots of energy myst be transferred to raise temperature

Question 5

how can specific heat capacity be determined by electrical method?

Answer 5

• immersion heater placed into a cavity of a block of material and thermometer placed in other
• block heated for measured length of time and temperature change recorded
• power of heater calculated
• energy transferred (Q) by heater calculated from power x time
• c = Q / (m•dT)

Question 6

how could accuracy of electrical method to determine specific heat capacity be improved?

Answer 6

use of thick insulation around block, using ammeter/thermometer with higher resolution, heating for longer as greater dT reduces percentage uncertainty

Question 7

what are the changes in internal energy as a metal block is being heated?

Answer 7

increased kinetic vibrational energy of atoms + as block slightly expands (work done on object by external force) potential energy of atoms increase

Question 8

why is an induction hob better than a flame/conventional ring?

Answer 8

no heat transferred to surroundings, and when pan removed heating effect stops

Question 9

how can specific heat capacity be determined through the method of mixtures?

Answer 9

•heat a solid object to 100’C in boiling water
•quickly transfer object to insulated beaker of room temperature water
•Energy transferred from brass to room temperature water: Q=m(solid)•c(solid)•(100-max temp room temp water reached)
•assuming no energy lost to surroundings, energy gained by water: Q=m(water)•c(water)•(max temp room temp water reached - initial temp of room temp water)
•therefore as Q values are equal: c(solid) =
m(water)•c(water)•(max water temp - initial) /
m(solid)•(100 - initial temperature)

Question 10

how would final temperature of water be affected if specific heat capacity by method of mixtures experiment carried out in uninsulated cups?

Answer 10

final temperature higher as, if water cooler than room temperature, water is heated by surroundings as well as hot solid

Question 11

what is the main cause of inaccuracy in the determination of specific heat capacity by method of mixtures?

Answer 11

As water warms after solid placed in it, some of its energy will be lost through evaporation. Some will also be transferred through sides and base as insulation cannot be perfect. This is a systematic error which means the final temperature of water is too low.

Question 12

how does dropping an object raise its temperature?

Answer 12

IF IT DOESN’T BOUNCE all of its kinetic energy on reaching the ground is converted to internal energy which raises temperature of object

Question 13

how can specific heat capacity be determined by doing work?

Answer 13

INVERSION TUBE METHOD
•on inverting tube, lead shot gains additional gravitational potential energy, and as it falls gravitational potential energy converted to kinetic energy
•work done by bung to bring shot to rest = gravitational potential energy
•mcdT=Nmgh therefore c = Ngh / dT

Question 14

is the value obtained for specific heat capacity from inversion tube method likely to be an underestimate or overestimate?

Answer 14

OVERESTIMATE
•not all lead shot will fall full distance
•some lead shots slide down sides so less work done than Nmgh
•some internal energy lost to bung and air

as c = Nmgh / mdT, if actual Nmgh value is smaller than theorised, c will be an overestimate

Question 15

why is water used as the fluid in many cooling systems?

Answer 15

water has high specific heat capacity therefore lots of excess thermal energy can be removed from engine without water temperature getting too high

Question 16

how can mass of liquid flowing through a system per second be calculated?

Answer 16

density of liquid x volume flow per second

Question 17

how can rate of energy transfer be calculated?

Answer 17

rate of mass flow x c x dT

rate of mass flow = density x rate of volume flow

Question 18

define specific latent heat of vaporisation

Answer 18

energy required to change 1kg of a liquid into 1kg of a gas with no temperature change

Question 19

define specific latent heat of fusion

Answer 19

energy required to change 1kg of a solid into 1kg of liquid with no temperature change

Question 20

what is boyle’s law?

Answer 20

volume of a fixed mass of gas at constant temperature is inversely proportional to its pressure

V ~ 1/p

Question 21

what is charle’s law?

Answer 21

volume of a fixed mass of gas at constant pressure is directly proportional to its KELVIN temperature

V ~ T

Question 22

what is the pressure law?

Answer 22

for a fixed mass of gas at constant volume, the pressure is directly proportional to the KELVIN temperature

p ~ T

Question 23

define absolute zero

Answer 23

atoms in a substance have effectively zero kinetic energy

Question 24

what is avagadro’s law?

Answer 24

equal volumes of gas, containing the same temperatures + pressures contain the same number of molecules

V ~ N

Question 25

define avagadro’s constant/ 1 mole

Answer 25

number of atoms in 12g of carbon-12

Question 26

when can pV / T = pV / T be used?

Answer 26

number of moles remains constant

Question 27

equation for work done by a gas expanding at constant pressure

Answer 27

W = p•dV

Question 28

what is brownian motion?

Answer 28

observable random movements of particles caused by the high speed thermal motion of liquid/gas molecules

Question 29

why do particles larger than 1 micro meter not display brownian motion?

Answer 29

they are hit by many molecules at once so imbalance of molecules hitting certain area is insignificant

Question 30

how does an increase in temperature effect brownian motion of particles?

Answer 30

increase in rapid erratic motion as air molecules are moving faster

Question 31

derive a term for average molecular kinetic energy

Answer 31

• total random translational Ek = sum of individual Ek = 1/2mc(1)^2 + 1/2mc(2)^2 +…+ 1/2mc(N)^2
• average translation Ek per molecule = [1/2mc(1)^2 + 1/2mc(2)^2 +…+1/2mc(N)^2] / N
• OR 1/2m*[(c(1)^2 + c(2)^2 +..+ c(3)^2) / N]
• (Crms)^2 = (c(1)^2 + c(2)^2 +…+ c(3)^2) / N
• average translational Ek per molecule = 1/2m(Crms)^2
• for monoatomic gas, vibrational and rotational Ek is negligible so average molecular kinetic energy = 1/2m(Crms)^2

Question 32

derive kinetic theory equation

Answer 32

• molecule moving in 3 directions so C^2 = U^2 + V^2 + W^2
• molecule collides with RHS only need to observe x-component of velocity(u) so momentum change = mu - (-mu) = 2mu
• this molecule will hit RHS again after travelling to other side of and back in time = 2L / u
• number of collisions with RHS in 1sec = u/2L
• molecule momentum change per sec=mu^2/L
• force = rate of change of momentum=mu^2/L
• F.total = m/L*[u(1)^2 + u(2)^2 +…+ u(N)^2]
• (Urms)^2=u(1)^2+u(2)^2+…+u(N)^2 so F.total = Nm(Urms)^2 / L
• area=L^2, p=F/A so p=Nm(Urms)^2 / V (L^3)
• (Crms)^2 = (Urms)^2 + ( Vrms)^2 + (Wrms)^2
• (Urms)^2 = (Vrms)^2 =(Wrms)^2=1/3(Crms)^2
• pV = 1/3Nm(Crms)^2

Question 33

what are the assumptions for kinetic theory model

Answer 33

• gas molecules have negligible volume, Point particles
• no intermolecular forced of Attraction
• gas molecules move in Random direction
• all collisions Elastic
• Time between collisions is significantly larger than collision duration