ch14 COPY Flashcards
how do you get impulse from a force time graph
the area under a force-time graph is equal to the impulse
motion of gas particles how do gas particles exert pressure
gas particles are in constant random brownian motion,
They continuously collide with each other and the walls of their container, causing a change in momentum (impulse) producing a force
this force results in pressure (p = F / A), which exerts a pressure on the containers
describe the pathing of gas particles
particles take a random path, they don’t travel in a straight line because they’re constantly changing direction due to collisions
displacement is proportional to the square root of number of steps
what is boyle’s law
provided that temp and amount of gas remains constant, pressure is inversely proportional to the volume
pV = constant
practical to look at boyle’s law
set up as shown, must have a trapped volume of gas whose pressure can be changed and whose volume measured
increasing pressure forces oil into the calibrated tube, while decreasing allows oil out the tube
by keeping temp constant, you can investigate the variation in volume with pressure
uncertainties: tube might fracture
relationship between pressure and number of molecules
provided that the volume is kept constant
the number of molecules is directly proportional to the pressure
how to work out number of molecules N
number of molecules N = moles n * avagadro constant Na
what is an ideal gas
a model for a gas where the particles are assumed to have negligible volume, there are negligible forces between particles except during collisiosn and the collisions are perfectly elastic,
what is charle’s law
at constant pressure, volume is directly proportional to absolue temperature T
what is the pressure law:
at constant volume, pressure is directly proportional to absolute temperature T
how to go from degrees to kelvin
kelvin = degrees celsius + 273
how to determine absolute 0
set up apparatus as shown
measure length L of the air column at different temperatures to see how the volume of enclosed gas is changing
you can mount the tube in a tall glass beaker of water whose temp can be changed
you must however be careful to ensure temp in water is similar to the temp in the enclosed air of the capillary tube
the data obtained can be used to find absolute temperature
ideal gas equations
pV = nRT
pV = NkT
p = pressure
v = volume
n = number of moles
N = number of molecules
R = gas constant
k = boltzmann constant
T = temperature (kelvin)
ideal gas law (molecules in a box)
what is the c² representing
pV = 1/3Nmc²
c² is for the mean squared
how to get value of molecular speeds
the value of molecular speeds is the square root of the mean square speed
this is the root mean square speed
RMS
combine pv = Nkt and pv = 1/3Nmc²
what does the result mean
NkT = 1/3Nmc²
NkT = 2/3N(1/2mc²)
3/2KT = 1/2 mc²
that for a certain temp, all molecukes of gases in a mixture of gases have the same kinetic energy
what is mean free path
how to use this to calculate displacement of a molecule
distance between collisions
displacement = square root of steps
displacement = distance travelled / mean free path
number of steps N = (distance travelled / mean free path)²
so displacement = N * mean free path
what can energy transfer lead to
a change in temperature
how to calculate change in internal energy U
change in U (internal energy) = W + Q
w = work done
q = heat it receives
what is specific thermal capacity
the increase in internal energy required to raise the temp of 1kg of a substance by 1K
equation to calculate change in internal energy with c
Q (ΔU) = mcΔT
how to determine Specific thermal capacity
set up as shown
you can find out work done by heater from appropriate readings
increase temp by about 10K, attach ammeter and voltmeter to calculate work done from W = VIT
warning: must minimise Q, the energy transferred thermally into or out the substance so that work done W = ΔU from which you can calc c
Why is the value you get for specific thermal capacity usually too high in experiments? How can you improve this?
Some of the energy from the heater will get transferred to the air and the container. To minimise this affect, start below and finish above room temperature to cancel out gains and losses. Some energy will also be lost due to resistance in the circuit
What equation do you get if you combine all three gas laws?
pV/T = constant
What does the graph of pressure against temperature look like for an ideal gas?
A straight line, intersecting the X axis at -273.15 Celsius, with a positive gradient
Explain why the volume of the box is inversely proportional to the pressure
Imagine you shrink the box. The particles have less distance to travel before they hit a wall, so you’ve increased the number of times the particles hit the walls of the box per second, which increases the total force on the wall. Because the box is now smaller, the area of the wall is smaller. So there is a greater force on a smaller area, meaning the pressure is greater
Explain why the pressure will be greater in a box, if the particle travels faster
The faster the particles, the larger its momentum, so the greater the impulse of the collision and the larger the force on the wall. The particle will also take less time to travel across the box and back again, and so will hit the walls more often. And as pressure = force/area, the pressure will be greater to
Explain root mean square speed
As the particles in a gas are moving in different directions, if you averaged their velocities you’d get zero
Instead, you take the average of the squared velocities. This quantity is called the mean square speed
The square root of this number gives you the speed of a typical particle, the root mean square speed
What is Newton’s second law in relation to gas particles?
The force exerted by a particle in a collision is equal to the rate of change of momentum of the particle
What does newtons second law mean about the force exerted on the wall depending on the speed of the particle?
If a particle collides with a wall of the container, the faster it is travelling, the more force it exerts on the wall
relationship between rms speed and density
Crms ∝ 1 / (sqr root) density
