Thermal Physics Flashcards
Heat
Defined as thermal energy that’s transmitted from one body to another
- While an object is capable of containing thermal energy (due to random motion of its molecules) it doesn’t contain heat
- Heat is energy in transit
- Temperature is a measure of the concentration of an object’s internal thermal energy
Calorimetry
- The change in temperature that a substance experiences upon a transfer of heat depends on the identity and amount of the substance present
- Q= mcΔT
- When an object absorbs or loses heat, either its temperature will change or the phase of the object will start to change but not both
- An ice cube melting remains at 0oC for the melting process, only when ice is melted will any additional heat cause temperature to rise
- Q= mL
- L is the latent heat of transformation
- Q= mL
- An ice cube melting remains at 0oC for the melting process, only when ice is melted will any additional heat cause temperature to rise
Heat Transfer
Three principle modes by which energy can be transferred:
- Conduction
- Convection
- Radiation
Conduction
Highly agitated atoms of a “hot” object bump into the atoms of another object, making them vibrate more rapidly thus heating the object
Convection
The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat
Radiation
Radiant energy from the sun’s fusion reactions is transferred across millions of kilometers of essentially empty space via electromagnetic waves
- Absorption of the energy carried by these light waves defines heat transfer by radiation
Thermal Expansion
- When a substance undergoes a temperature change, it changes in size
- The change in size of a substance due to a temperature change depends on the amount of the temperature change and the identity of the substance
- ΔL= αLoΔT
- α is the coefficient of linear expansion of the material
- Nearly all substances have a positive value of α, which means they expand upon heating
- α is the coefficient of linear expansion of the material
- ΔV= βVoΔT
- β is the coefficient of volume expansion of the substance
- Nearly all substances have a positive value of β, which means they expand upon heating
- Water is an example of a negative value of β, water expands as it turns into ice
- Nearly all substances have a positive value of β, which means they expand upon heating
- β is the coefficient of volume expansion of the substance
The Kinetic Theory of Gases
Molecules of a gas move freely and rapidly, in a chaotic swarm
- A confined gas exerts a force on the walls of its container because the molecules are zipping around inside the container, striking the walls and rebounding
- The magnitude of the force per unit area is called pressure
- P= F/A
- SI Unit for pressure is pascal (N/m2)
- P= F/A
- The magnitude of the force per unit area is called pressure
Ideal Gas Law
- Ideal gases exhibit certain properties
- The volume of the gas molecules is negligible compared with that of the container that holds them
- They experience no electrical forces
- Undergo elastic collisions
- Universal gas constant: 8.31 J/mol•K
- Kavg= 3/2kBT
- Tells that the average translational kinetic energy of the gas molecules is directly propoertional to the absolute temperature of the sample
- vrms= √(3RT/M)
- Root-mean-square gives type of average speed given temperature of gas from a wide range of speeds found in a gas at a given temperature
Thermodynamics
Study of energy transfers involving work and heat, and the resulting changes in internal energy, temperature, volume, and pressure is called thermodynamics
The Zeroth Law of Thermodynamics
When two objects are brought into contact, heat will flow from the warmer object to the cooler one until they reach thermal equilibrium
- If objects 1 and 2 are each in thermal equilibrium with object 3, then objects 1 and 2 are in thermal equilibrium with each other
The First Law of Thermodynamics
Energy (in the form of heat) is neither created nor destroyed in any thermodynamic system
- Mathematical equation corresponding to law: ΔU= Q - W
- ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system
- U depends on the state of the system
- ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system
Work Done by Gas During Expansion
An insulated container filled with an ideal gas rests on a heat reservoir (something acting as heat source or heat sink), the container is fitted with a snug, but frictionless, weighted piston that can be raised or lowered
- The confined gas is the system, and the piston and heat reservoir are the surroundings
- PV=nrT connects state variables, pressure, volume, and temperature
- W= PΔV
- Work is done on or by the system when the piston is moved and the volume of the gas changes
- When piston pushed up work is positive, when piston is pushed down, volume decreases and W is negative
- W is positive when the ystem does work against its surroundings, and W is negative when the surroundings do work on the system
- The equation W= PΔV assumes that pressure P does not change during the process, if P does change then the work is equal to the area under the curve in the P-V diagram; moving left to right gives a positive area (and positive work), while moving right to left gives a negative area (and negative work)
Isothermal and Adiobatic
A process where the temperature remains constant is referred to as isothermal
If there is no heat exchanged between the system and its surroundings the process is adiabatic
The Second Law of Thermodynamics
The total amount of disorder- the total amount of entropy- of a system plus its surroundings will never decrease
- Entropy of system can decrease (e.g. freezing) but it will always be at the expense of a greater increase in entropy in the surroundings
- When water freezes it’s entropy decreases however it releases heat energy into environment, increasing entropy of surroundings
- The total entropy of the system + surroundings will have increased
- When water freezes it’s entropy decreases however it releases heat energy into environment, increasing entropy of surroundings
