Heat Flashcards
What is heat (2)
-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
Explain 3 ways heat can be transferred between a hot object to a cooler one (3)
-conduction
-convention
-radiation
State the definition of thermal equilibrium (1)
-when objects have the same temperature and no heat flows between them
Explain how heat is exchsnged between a hot object and cold object (3)
-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
State the zeroth law (1)
-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
At what temprature (Celsius) does water freeze (1)
0
At what temprature (Celsius) does water boil (1)
100
At what temprature (fahrenite ) does water freeze (1)
32
At what temprature (farhenite) does water boil (1)
212
State the conversion from C to F stating a formula (1)
Tf = 9/5 x Tc + 32
State the conversion from F to C stating a formula (1)
Tc = 5/9 ( Tf - 32)
State and explain the concept of absolute zero (2)
-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
State the relationship between volume and temperature (3)
-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
State the relationship between pressure and temprature (2)
-pressure in a gas is proportional to its temperature
-they all reach zero pressure at the same temperature called absolute zero
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)
Use Charles law =
P1/T1 = P2/T2
State the conversion from Celsius to kelvin
T = Tc + 273.15
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
The rod expands by 2L
State the formula for the coefficient of linear expansion (1)
Change in length = coefficient of thermal expansion x initial length x change in temperature
State and explain bimetallic strips (3)
-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
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)
-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
State one application of bimetallic strip (2)
Thermometer
-the shape changes with the temperature moving the needle to indicate the temperature
State the formula for area expansion (1)
Change in area = initial area x (2 x coefficient of linear expansion) x change in temperature
State the formula for volume expansion (1)
Change In volume = initial volume x (3 x coefficient of linear expansion) x change in temperature
State and explain why ice is less dense than water (4)
-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
Why do water pipes burst (1)
Water pipes burst due to the expansion of ice forming in them
State the definition of kCal (1)
-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
How many Joules is one Cal
1 cal = 4.186J
State the definition of BTU (1)
It is the energy required to heat 1 lb of water from 63F to 64F
State the conversion from 1Btu to KCal and J (2)
1Btu = 0.252kCal = 1055J
State the formula for heat (1)
Q = mgh
State the definition of heat capacity (3)
-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
What is a calorimeter (1)
-it is a device used to measure the heat exchanged between an object and it’s surrounding
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)
-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
State the formula for object and water in calorimeter (1)
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)
State the definition of conduction (2)
- 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
State the relation between thermal conductivity and area, time, length, temperature (4)
-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
State the formula between thermal conductivity and area, time, length, temperature (4)
Q = thermal conductivity x area x (temperature difference/ length) x time
Why do you feel like the tile is colder than the carpet (2)
-tile has a much larger thermal conductivity than the carpet
-more heat flows from your skin to the tile leaving your body cold
State and explain on the topic of convention (3)
-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
State and explain on the topic of radiation (2)
-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
State and explain emissivity and state a formula (4)
-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
