important experiments (all topics p1 & p2)

Question 1

outline the stopping voltage set up

Answer 1

set up a circuit with a variable supply, ammeter, resistor and two separate metal plates under incident em radiation

Question 2

capacitor circuit

Answer 2

parallel circuit with a switch across the two loops. cell and capacitor in one loop. resistor and (same) capacitor in the second.

Question 3

explain the stopping voltage experiment

Answer 3

• photons arriving at the metal plate cause photoelectrons to be emitted (if above threshold frequency) and crossing to the second metal plate
• as the voltage of the supply is decreased it decreases the potential over the two plates. the photoelectrons must do work to overcome this.
• the voltage at which the photoelectrons can no longer reach the other plate and turn around is the stopping voltage

Question 4

stopping potential

Answer 4

The stopping potential is defined as the potential necessary to stop any electron (or, in other words, to stop even the electron with the most kinetic energy) from reaching the other side

Question 5

what does the stopping voltage tell you

Answer 5

eV = 1/2mv^2
you can work out the work function from this
(W = QV)

Question 6

verifying coulombs law

Answer 6

1. charge insulating rods and measure their charge using a coulomb meter
2. place one rod on the top tap balance and zero the scale
3. bring the second rod close to the first and measure the change in mass
4. repeat, changing either radius of rod or charges

Question 7

finding the internal resistance of a power supply

Answer 7

1. measure the terminal pd
2. vary the current by changing the load resistance
3. plot V agaisnt I the gradient is internal resistance, the y intercept is the emf

Question 8

finding the resisitivty

Answer 8

1. measure the diameter of the wire
2. measure the voltage and current for various lengths of the same wire
3. calculate the resistances
4. plot resistance against length the gradient is resistivity/area

Question 9

Boyles law experiment

Answer 9

1. slowly use the foot pump to increase the pressure
2. record the volume for different pressure level
3. plot p agaisnt 1/v, if the graph is directly proportional it follows boyles law

Question 10

Charles Law experiment

Answer 10

• record the volume and temperature of the cold water
• heat the water and record the temperatures
• plot volume agaisnt temperature. If the graph is straight line and approximately crosses the x-axis at -273 degrees it follows Charles law

Question 11

Gay Lussacs experiment

Answer 11

1. heat is applied to the cylinder
2. measure the temperature and volume at regular intervals
3. plot P agaisnt T if its a straight line graph then it obeys the law

Question 12

How does gay lussacs and charles experiments give evidence for an absolute zero temperature

Answer 12

They should be directly proportional however they are not and both intersect the x axis at -273. This gives evidence for the kelvin temperature scale where 0K is absolute zero.

Question 13

Rutherford’s scattering experiment

Answer 13

A stream of alpha particles from a radioactive source were fired at a very thin gold foil sheet. The number of alpha particles at different angles was recorded

Question 14

Rutherford’s scattering experiment conclusions

Answer 14

• most (fast charged) alpha particles went straight through therefore an atom in mostly empty space
• some alpha particles deflected at an angle greater than 90 therefore part of the atom must be more massive than the alpha particle this is the nucleus
• alpha particles were repelled therefore the nucleus must be positively charged
• since atoms are neutral overall so electrons must be in the outside of the atom

Question 15

trigonometric parallax

Answer 15

The star is viewed from two positions at 6 month intervals
The change in angular position of the star against background of fixed stars
is measured
Trigonometry is used to calculate the distance to the star
The diameter/radius of the Earth’s orbit about the Sun must be known

Question 16

distance to stars standard candles

Answer 16

the standard candles flux/intensity is measured. Use the inverse square law I = L/4πd^2 to calculate the distance.

Question 17

how to find plancks constant?

Answer 17

• set up a potential divider circuit with a paralell section with different coloured LEDs, an ammeter and a voltmeter
• measure the voltage and record the wavelength (read from packet)
• plot a graph of v agaisnt 1/λ
• the gradient equals Vλ
• substitute E = eV into E =hc/λ input values and rearrange to get h

Question 18

what experiment determines the work function of different materials and the value of h?

Answer 18

stopping voltage experiment

Question 19

What does the graph from the stopping voltage experiment show?

Answer 19

gradient = h
F0 (x intercept) = threshold frequency
y intercept = work function

Question 20

What does the y intercept from the stopping voltage experiment show?

Answer 20

the voltage needed to stop an electron being liberated by light of 0 frequency and so 0 energy (the work

Question 21

What axises are plotted from the stopping voltage experiment?

Answer 21

y = stopping voltage 
x = frequency

Question 22

locating the CoM of an object

Answer 22

1. put holes in the object and hang it from a nail with a plumline and a weight of it
2. hang the shape from the three pivots
3. trace along the plumline with a pen
4. where the lines cross is the CoM

Question 23

if you change charge when verifying coulombs law the charge force graph should be

Answer 23

a straight line through the origin

Question 24

if you change radius when verifying coulombs law the radius force graph should be

Answer 24

a 1/x curve

Question 25

measuring the resisitivity circuit

Answer 25

potential divider circuit with voltmeter and ammeter

Question 26

measuring internal resistance circuit

Answer 26

voltmeter in paralell over supply

resistor and ammeter in series

Question 27

measuring internal resistance graph

Answer 27

Voltage y axis
Current x axis
gradient internal resistance
y intercept emf

Question 28

measuring the young modulus

Answer 28

1. measure the diameter of the wire
2. thread the wire over the pulley, mark a point on the wire
3. add masses and record how much it extends by
4. measure stress and strain for these values and plot stress against strain, YM is the gradient

Question 29

how is the photoelectric effect shown?

Answer 29

by a gold leaf electroscope

Question 30

how does a gold leaf electroscope show the photoelectric effect

Answer 30

a charge is given to the electroscope, like charges repel so the gold leaf lifts up. if photons of the right frequency liberate the electrons the gold leaf will drop.

Question 31

if the p.d. in a stopping voltage experiment is increased what happens?

Answer 31

electrons are accelerated faster as they move in the same direction as the current

Question 32

if the pd. in a stopping voltage experiment is decreased what happens?

Answer 32

the battery is more effective than the photoelctric effect therefore the electrons are slowed and start to move backwards.