Heat

Question 1

What is heat (2)

Answer 1

-Heat is the energy that is transferred between objects because of a temperature difference
-objects are considered to be in thermal contact if heat can flow between them

Question 2

Explain 3 ways heat can be transferred between a hot object to a cooler one (3)

Answer 2

-conduction
-convention
-radiation

Question 3

State the definition of thermal equilibrium (1)

Answer 3

-when objects have the same temperature and no heat flows between them

Question 4

Explain how heat is exchsnged between a hot object and cold object (3)

Answer 4

-when a hot object is brought into thermal contact with a colder object heat is exchsnged
-the hot object cools down and it’s molecules slow down while the hot object warms up
-after sometime if being in thermal contact both objects come into thermal equilibrium

Question 5

State the zeroth law (1)

Answer 5

-if object A is in thermal contact with object B
-and object B is in thermal contact with object C
-then object A and C are in thermal equilibrium with no thermal contact

Question 6

At what temprature (Celsius) does water freeze (1)

Answer 6

0

Question 7

At what temprature (Celsius) does water boil (1)

Answer 7

100

Question 8

At what temprature (fahrenite ) does water freeze (1)

Answer 8

32

Question 9

At what temprature (farhenite) does water boil (1)

Answer 9

212

Question 10

State the conversion from C to F stating a formula (1)

Answer 10

Tf = 9/5 x Tc + 32

Question 11

State the conversion from F to C stating a formula (1)

Answer 11

Tc = 5/9 ( Tf - 32)

Question 12

State and explain the concept of absolute zero (2)

Answer 12

-absolute zero refers to the lowest possible temperature that a system or an object can reach
-it is the point at which the fundamental particle of a substance such as atom or molecules have minimum amount of motion or vibrations

Question 13

State the relationship between volume and temperature (3)

Answer 13

-cooling air from 100 to 0 reduces its volume
-volume of gas is inversely proportional to its temperature at constant pressure
-as the temprature approaches absolute zero the volume of the gas would theoretically be zero

Question 14

State the relationship between pressure and temprature (2)

Answer 14

-pressure in a gas is proportional to its temperature
-they all reach zero pressure at the same temperature called absolute zero

Question 15

The gas in a constant volume gas thermometer has a pressure of 80kPa at 0C . Assuming ideal gas behavior what is the pressure of a gas at 105C (2)

Answer 15

Use Charles law =
P1/T1 = P2/T2

Question 16

State the conversion from Celsius to kelvin

Answer 16

T = Tc + 273.15

Question 17

When rod 1 is heated by an amount of T it’s length increases by L. If rod 2 which is twice the length as rod 1 and made of the same material is heated by the same amount what happens to its length

Answer 17

The rod expands by 2L

Question 18

State the formula for the coefficient of linear expansion (1)

Answer 18

Change in length = coefficient of thermal expansion x initial length x change in temperature

Question 19

State and explain bimetallic strips (3)

Answer 19

-they are made by bonding two different metals with distinct coefficient of linear expansion
-as the strip is heated or cooled the metals change lengths by different amounts causing the strip to bend
-it is highly sensitive to temperature changes

Question 20

In a bimetallic strip one metal has a larger coefficient of expansion than the other, it is then heated, what effect does it have (1)

Answer 20

-when the bimetallic strip is heated, the metal with the higher coefficient of linear expansion will expand more than the one with the lower coefficient

Question 21

State one application of bimetallic strip (2)

Answer 21

Thermometer
-the shape changes with the temperature moving the needle to indicate the temperature

Question 22

State the formula for area expansion (1)

Answer 22

Change in area = initial area x (2 x coefficient of linear expansion) x change in temperature

Question 23

State the formula for volume expansion (1)

Answer 23

Change In volume = initial volume x (3 x coefficient of linear expansion) x change in temperature

Question 24

State and explain why ice is less dense than water (4)

Answer 24

-water molecules are polar, this polarity results in hydrogen bonding between water molecules
-in liquid state, water molecules are close together and exhibit hydrogen bonding
-when water freezes into ice, hydrogen bonding becomes more stable and the arrangement is open creating more space between molecules
-the open and hexagonal arrangement in ice results in a lower density which causes it to float in water

Question 25

Why do water pipes burst (1)

Answer 25

Water pipes burst due to the expansion of ice forming in them

Question 26

State the definition of kCal (1)

Answer 26

-one kilocalorie (kCal) is defined as the amount of heat needed to raise the temperature of 1kg of water from 14.5C to 15.5C

Question 27

How many Joules is one Cal

Answer 27

1 cal = 4.186J

Question 28

State the definition of BTU (1)

Answer 28

It is the energy required to heat 1 lb of water from 63F to 64F

Question 29

State the conversion from 1Btu to KCal and J (2)

Answer 29

1Btu = 0.252kCal = 1055J

Question 30

State the formula for heat (1)

Answer 30

Q = mgh

Question 31

State the definition of heat capacity (3)

Answer 31

-it is the amount of heat added to it divided by its rise in temperature -heat capacity depends on the mass Specific heat capacity = heat / temperature

Question 32

What is a calorimeter (1)

Answer 32

-it is a device used to measure the heat exchanged between an object and it’s surrounding

Question 33

What happens when an object is placed into a calorimeter, and how does heat transfer take place between the object and the water until they achieve thermal equilibrium (3)

Answer 33

-when an object is placed in the calorimeter heat transfer occurs between the object and the water until they reach thermal equilibrium -during this process the object either absorbs or releases heat causing a temperature change in the water -the heat transfer continuous until both, the object and the water reach the same temperature indicating thermal equilibrium -in an isolated system like calorimeter the total energy remains constant Q object + Q water = 0

Question 34

State the formula for object and water in calorimeter (1)

Answer 34

Q water = mass of water x specific heat capacity of water x (final tempt - intitial tempt) Q object = mass of object x specific heat capacity of object x (final tempt - intitial tempt) Final temperature = (mO x cO)+ (mW x cW x tW) / (mO x cO)+ (mW x cW)

Question 35

State the definition of conduction (2)

Answer 35

- It is the flow of heat directly through a physical material -in a solid material such as metal rod the particles vibrate and this vibration is passed along to neighboring particles

Question 36

State the relation between thermal conductivity and area, time, length, temperature (4)

Answer 36

-as area increases the thermal conductivity increases because wider rod provides more pathways for heat to travel -as time increases more heat can flow through the rod -as length increases the thermal conductivity decreases because larger rods provide more resistance for heat to flow -as temperature difference increases the rate of flow of heat increases

Question 37

State the formula between thermal conductivity and area, time, length, temperature (4)

Answer 37

Q = thermal conductivity x area x (temperature difference/ length) x time

Question 38

Why do you feel like the tile is colder than the carpet (2)

Answer 38

-tile has a much larger thermal conductivity than the carpet -more heat flows from your skin to the tile leaving your body cold

Question 39

State and explain on the topic of convention (3)

Answer 39

-convection involves the transfer of heat through the movement of fluids (liquids or gases) -when a fluid is heated it becomes less dense and rises such as warm air -as it moves it carries heat with it

Question 40

State and explain on the topic of radiation (2)

Answer 40

-radiation is the transfer of heat through electromagnetic waves such as infrared radiation -radiation does not require a material medium for heat transfer The amount of radiation emitted depends on the temperature and the nature of surface involved

Question 41

State and explain emissivity and state a formula (4)

Answer 41

-is a dimensionless number between 0 to 1 that characterizes how effectively an object radiates heat 1 = black color absorbs all heat 0 = white color does not emit any energy Radiated power = emissivity of an object x 5.57x10^-8 x area x (temperature)^4