THERMODYNAMICS Flashcards
What is meant by the temperature of a body
A measure of the mean, random KE of its vibrating atoms.
What is meant by heat
The flow of energy from a region of higher temperature to a region of lower temperature.
Specific heat capacity
The amount of energy needed to raise the temperature of 1 Kilogram of substance by 1 Kelvin.
What are the assumptions we make in the kinetic theory
- A gas consists of a very large number of molecules and are in continuous rapid motion - Brownian Motion.
- Collisions between molecules and between molecules and walls are perfectly elastic meaning KE in conserved. This must be true as Brownian motion observed continuous motion if KE was not conserved than they would gradually slow down
- The volume occupied by the gas molecules themselves compared to the container is negligible- as compressing a gas by a large amount is easy
- Intermolecular forces are negligible except during a collision as they are very far part and very small
- The duration of the collisions is negligible compared to the time spent between collisions as the molecules are far apart form one another
What is meant by absolute zero
The temperature at which the pressure and volume of an ideal gas becomes 0 as it has no Kinetic energy
What is the absolute temperature scale
The Kelvin or thermodynamic temperature scale with zero at 273 degrees.
Define fluid
A medium that can flow e.g. gases and liquids.
What is an Ideal gas
Gas that obeys the gas laws
Define Internal energy
The total kinetic and potential energy of a molecule.
Define Potential energy
Energy stored in the stretched or compressed bonds between molecules of a material
Define kinetic energy
Energy due to movement of molecules
What is mean square speed
The average of the squares of the speeds of the particles in a gas.
Define pressure
Force per unit area (Pa or Nm^-2).
What is the pressure law
For a constant mass of gas at constant volume, the pressure exerted by the gas is directly proportional to its absolute temperature
What is boyle’s law
For a constant mass of gas at constant temperature, the pressure exerted by the gas is inversely proportional to the volume it occupies
What is Charles law
For a constant mass of gas at constant pressure, the volume occupied by the gas is directly proportional to its absolute temperature
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What is the ideal gas law
Pv= NKT
Explain why the pressure of a gas increases when the volume decreases
- When the volume decreases, the frequency of collisions between the molecules of the gas and the container walls increases
- This causes a greater rate of change of momentum (because more collisions are happening per second)
- Force = rate of change of momentum
4· So the greater rate of change of momentum causes a greater force on the walls of the container
5· Pressure = force /area so the greater force causes a greater pressure on the walls of the container for the same area
Describe the energy transfers of a melting ice cube
- Energy from the surroundings is transferred to the ice cube
- The ice molecules gain potential and kinetic energy
- The gain in kinetic energy increases the temperature of the ice
- This continues until the ice reaches 0C
- The ice gains energy E=ml as it changes state to water that is zero degrees ,
- Kinetic energy is constant whereas potential energy increases
- The water then gains energy E=mcdelta T until thermal equilibrium is reached
Measuring the specific heat capacity of a liquid/ solid method:
- Measure the mass the block of aluminium/ mass of water
- (ensure there is appropriate insulation/lagging to prevent energy transfer from the material to the surroundings)
- Record the initial temperature
- Switch power supply on and start the stopwatch
- Record the current and potential difference
- After a fixed time (e.g. 4 minutes), switch power off and record highest temperature reached, use this to calculate the change in temperature Δθ
- Calculate the energy transferred E= IVt
- Calculate c = E/mΔθ
Sources of error in measuring the specific heat capacity of a liquid/ solid method
- Energy is absorbed by heater, larger c value.
- Energy lost to surrounding despite lagging, larger c value.
- Little energy lost to lagging/ expanded polystyrene cup, larger c value.
- Thermometer may have low resolution.
- Finding the SHC of a liquid means we have to find the energy taken by the container, however an expanded polystyrene cup is light and a good insulator.
Improving accuracy in measuring the specific heat capacity of a liquid/ solid method
- Use a data logger and temperature sensor
2. Repeat and take average
Specific Latent heat
The energy required to change 1 Kg of a substance from one state to another without change in temperature ( Q=ml)
where:
E= The heat energy(Joules)
m= Mass of substance (Kg)
l= Specific latent heat ( J kg-1 )
Specific Latent heat of Fusion
Energy required to change 1 kg of Solid to liquid.
Specific Latent heat of Vaporization
Energy required to change 1 kg of liquid into gas.
