ch14

Question 1

how do you get impulse from a force time graph

Answer 1

the area under a force-time graph is equal to the impulse

Question 2

motion of gas particles how do gas particles exert pressure

Answer 2

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

Question 3

describe the pathing of gas particles

Answer 3

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

Question 4

what is boyle’s law

Answer 4

provided that temp and amount of gas remains constant, pressure is inversely proportional to the volume

pV = constant

Question 5

practical to look at boyle’s law

Answer 5

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

Question 6

relationship between pressure and number of molecules

Answer 6

provided that the volume is kept constant

the number of molecules is directly proportional to the pressure

Question 7

how to work out number of molecules N

Answer 7

number of molecules N = moles n * avagadro constant Na

Question 8

what is an ideal gas

Answer 8

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,

Question 9

what is charle’s law

Answer 9

at constant pressure, volume is directly proportional to absolue temperature T

Question 10

what is the pressure law:

Answer 10

at constant volume, pressure is directly proportional to absolute temperature T

Question 11

how to go from degrees to kelvin

Answer 11

kelvin = degrees celsius + 273

Question 12

how to determine absolute 0

Answer 12

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

Question 13

ideal gas equations

Answer 13

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)

Question 14

ideal gas law (molecules in a box)

what is the c² representing

Answer 14

pV = 1/3Nmc²

c² is for the mean squared

Question 15

how to get value of molecular speeds

Answer 15

the value of molecular speeds is the square root of the mean square speed

this is the root mean square speed

RMS

Question 16

combine pv = Nkt and pv = 1/3Nmc²

what does the result mean

Answer 16

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

Question 17

what is mean free path

how to use this to calculate displacement of a molecule

Answer 17

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

Question 18

what can energy transfer lead to

Answer 18

a change in temperature

Question 19

how to calculate change in internal energy U

Answer 19

change in U (internal energy) = W + Q

w = work done

q = heat it receives

Question 20

what is specific thermal capacity

Answer 20

the increase in internal energy required to raise the temp of 1kg of a substance by 1K

Question 21

equation to calculate change in internal energy with c

Answer 21

Q (ΔU) = mcΔT

Question 22

how to determine Specific thermal capacity

Answer 22

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

Question 23

Why is the value you get for specific thermal capacity usually too high in experiments? How can you improve this?

Answer 23

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

Question 24

What equation do you get if you combine all three gas laws?

Answer 24

pV/T = constant

Question 25

What does the graph of pressure against temperature look like for an ideal gas?

Answer 25

A straight line, intersecting the X axis at -273.15 Celsius, with a positive gradient

Question 26

Explain why the volume of the box is inversely proportional to the pressure

Answer 26

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

Question 27

Explain why the pressure will be greater in a box, if the particle travels faster

Answer 27

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

Question 28

Explain root mean square speed

Answer 28

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

Question 29

What is Newton’s second law in relation to gas particles?

Answer 29

The force exerted by a particle in a collision is equal to the rate of change of momentum of the particle

Question 30

What does newtons second law mean about the force exerted on the wall depending on the speed of the particle?

Answer 30

If a particle collides with a wall of the container, the faster it is travelling, the more force it exerts on the wall

Question 31

relationship between rms speed and density

Answer 31

Crms ∝ 1 / (sqr root) density