rip 2.0

Question 1

Explain why this circuit will provide data for large V values but not for small V values

Answer 1

With the variable resistor set at zero, the p.d. across the resistor is zero, so p.d. across lamp is 2.4V / large.
With the variable resistor set at its maximum value, there is a p.d. across the variable resistor, so p.d. across the lamp is not small.

Question 2

explain how the wavelength of the light changes as light travels from glass to water.

Answer 2

The ray is refracted away from the normal, therefore the refractive index of water is less than the refractive index of glass or speed of light in water is greater than the speed of light in glass.
The frequency remains constant.
v = fλ and therefore the wavelength of light increases as it travels from glass to water.

Question 3

Explain how you can use an oscilloscope set to a time-base of 0.1 ms div-1 to check that the frequency of sound is 2.8 kHz.

Answer 3

Place a microphone close to loudspeaker and connect it to the oscilloscope.
Measure the number of divisions between neighbouring peaks of the signal. (AW)
The separation between the neighbouring peaks should be 3.6 divisions

Question 4

Explain how the arrangement shown in Fig. 18 produces an interference pattern along the line XY.

Answer 4

The sound is diffracted at each slit.
The diffracted waves interfere in the space beyond the slits.
There is loud sound / maxima / constructive interference when phase difference is zero or when path difference nλ.
There is quiet sound / minima / destructive interference when phase difference is 180°

Question 5

State what is meant by the decay constant of an isotope.

Answer 5

The decay constant is the probability of decay of a nucleus per unit time.

Question 6

Explain how the graph in Fig. 20 can be used to determine the half-life of protactinium.

Answer 6

1. A = A0e-λt
2. lnA = lnA0 - λt
3. A graph of lnA against t will be a straight line
   with gradient (-) λ
4. half-life = ln2/λ

Question 7

Describe the motion of the particle in terms of the force it experiences when the field is

(i) a magnetic field
(ii) an electric field.

Answer 7

i- The force is right angles to the motion / velocity.
The particle describes a circle in the plane of the paper.
ii- Particle experiences a force perpendicular to motion/velocity.
It moves to the right and either comes out or goes into the plane of the paper (in a parabolic path).

Question 8

State what is meant by induced nuclear fission.

Answer 8

The splitting of a (uranium) nucleus as a neutron is absorbed (into two fragment nuclei and neutrons).

Question 9

Explain the role of the moderator and the control rods in a nuclear reactor.

Answer 9

The moderator slows down the fast-moving neutrons.
The neutrons lose significant amount of their kinetic energy when colliding with moderator nuclei.
or
The moderator does not absorb the neutrons
The control rods absorb the neutrons.
The rate of fission reactions is less / reduced.

Question 10

Discuss how the actual value of the resistivity of the metal would differ from the value calculated

Answer 10

The actual resistance values will be smaller.
The gradient of the graph will be lower.
Hence resistivity of the metal will be smaller than the value calculated

Question 11

Explain the observations above and refine the circuit design so that the brightness of the lamp can be varied as the slider is moved from A to B.

Answer 11

The voltmeter has large or infinite resistance.
Hence the p.d across the lamp or current in the lamp is small or zero (and the lamp is not lit).
Refining design: remove voltmeter from the circuit or place the voltmeter across the lamp.

Question 12

The voltmeter has very high resistance and has a full scale deflection (f.s.d.) of 6.0 V.
Explain how the circuit works and use calculations to discuss a significant limitation of this design.

Answer 12

1. Total resistance decreases as temperature
   increases (allow reverse argument)
2. Current in circuit increases as temperature
   increases or p.d. is in the ratio of the resistance values
3. Therefore, the p.d. across resistor increases
   or p.d. across thermistor decreases.
4. The change in resistance is small when
   resistance of thermistor changes from 200 °C to 300°C
5. Change in voltmeter reading is too small over
   this range
6. Non-linear change of resistance with
   temperature.

Question 13

Explain how the stationary wave is formed on this stretched string

Answer 13

Waves are reflected at the pulley end.

This produces nodes and antinodes on the string.

Question 14

Describe and explain how the capacitor plates A and B acquire opposite charges

Answer 14

Electrons in the circuit move in a clockwise direction and electrons are deposited on plate B.
(An equal number of) electrons are removed from plate A giving it a positive charge (of equal magnitude).

Question 15

Describe how the time constant of this circuit can be determined experimentally in the laboratory.

Answer 15

Connect a voltmeter or data-logger or oscilloscope across the resistor (or capacitor) or an ammeter in series with the resistor.
A stopwatch is started when the switch is opened and stopped when the p.d. or the current to decreases to 37% of its initial value.
The time constant is the time taken for the p.d. or the current to decreases to 37% of its initial value

Question 16

Describe the similarities and the differences between the gravitational field of a point mass and the electric field of a point charge.

Answer 16

Similarity
The field strength or force is dir prop to 1/separation2 or both produce a radial field.
Differences
Gravitational field is linked to mass and electric field is linked to charge.
Gravitational field is always attractive whereas electric field can be either attractive or repulsive

Question 17

Explain why the spheres are separated as shown

Answer 17

The charges repel each other (because they have like charges).
Each charge is in equilibrium under the action of the three forces: downward weight, a horizontal electrical force and an upwardly inclined force due to the tension in the string

Question 18

Describe an experiment that can be carried out to determine the half-thickness of lead and how you would use your results with Fig. 23.1 to determine the energy of a gamma photon from a radioactive gamma source in your laboratory

Answer 18

1. GM tube, counter or rate-meter and lead
   plates used
2. Micrometer or vernier calliper (to measure
   thickness of plates).
3. Measure counts for a specific time and hence
   the count-rate for each thickness of lead
4. Vary the thickness of lead and record the
   count-rates
5. Plot a graph of count-rate against thickness
   and determine the half thickness of lead
   .
6. Do not point source at person
7. Keep safe distance between you and source
8. Use tongs to handle source.
9. The counts are recorded over a long period of
   time
10. Background radiation taken into account.

Question 19

State the principle of superposition of waves.

Answer 19

(When two or more waves meet at a point in space) the

resultant (displacement) is equal to the (vector) sum of the individual displacements of waves (meeting at a point)

Question 20

The blue light is now replaced by a similar beam of red light.
State and explain the effect, if any, on the fringes observed on the screen.

Answer 20

Red light has longer wavelength and separation
between fringes increases
Separation between fringes justified in terms of x is dir prop to wavelength.
D and a are constants

Question 21

The instrument used to measure d has a zero-error. The measured d is much larger than the actual value. Discuss how the actual value of R compares with the value calculated above.

Answer 21

(The actual) R is large(r) because (the actual) d is

small(er)

Question 22

The e.m.f. of the variable supply is now slowly decreased from 4.2 V to 0 V.
Describe the effect on the current I in the 33 Ω resistor.

Answer 22

Any two from:
The current decreases up to 1.5 V
The current is zero at 1.5 V
The current changes direction / is negative when < 1.5V
The current increases below 1.5 V

Question 23

The group of students know that maximum power is dissipated in the variable resistor when R is equal to the internal resistance r of the cell.
Describe, with the help of a suitable circuit diagram, how the students may have determined P and R. Use Fig. 18.2 to estimate the internal resistance r of the cell and discuss any limitations of the data plotted by the group

Answer 23

 Correct circuit with (variable) resistor, ammeter and
voltmeter
 R changed to get different values for P
 R = V/I (using ammeter and voltmeter readings) or R
measured directly using an ohmmeter with the variable resistor isolated from the circuit or R read directly from a resistance box
 Power calculated using P = V2
/R or P = VI or P =I2R
 The value of r is between 1.0 to 3.0 ohm
 A smooth curve drawn on Fig. 18.2 (to determine r)
 A better approximation from sketched graph or r is
between 1.5 and 2.7 ohm
 Any attempt at using E = V +Ir, with or without the
power equation(s) to determine r - even if the value is incorrect
Limitations:
 ‘More data’ required
 Data point necessary at R = 2.0 ohm/ More data
(points) needed between 1 to 3 ohm
 No evidence of averaging / Error bars necessary (for
both P and R values)

Question 24

Define electric potential at a point in space.

Answer 24

(Electric potential) is the work done per (unit) charge in

bringing a positive charge from infinity (to the point).

Question 25

Evaluate the information from Fig. 22.1 and the analysis of the data from the experiment. No further calculations are necessary.

Answer 25

 The straight line misses one error bar / anomalous
point ringed or indicated
 Too few data points plotted
 The triangle used to calculate the gradient is (too)
small
 Some plots should have been repeated / checked
 No error bars for current
 ‘Not regular intervals’ (for current)
 No origin shown
Evaluation of analysis:
 The value of B is close to the accepted value
 The difference of only 7%
 No absolute or percentage uncertainty in B shown
 Worst-fit line or maximum / minimum gradient line
could have been used to determine the (absolute or percentage) uncertainty in B
 F against I graph should be a straight line or
 BL = gradient (any subject)

Question 26

Use Faraday’s law of electromagnetic induction to explain why the filament lamp is lit.

Answer 26

There is a changing / fluctuating (magnetic) field / flux
(linkage)
(magnetic) field / flux (linkage) in core and secondary (coil)
Statement of Faraday’s law: e.m.f. (induced) dir prop to rate of change of (magnetic) flux linkage

Question 27

The alternating voltage supply is replaced by a battery and an open switch in series.
The switch is closed. The lamp is lit for a short period of time and then remains off.
Explain this observation.

Answer 27

Idea of changing / increasing (magnetic) field / flux / current (in primary) at the start
Eventually current and flux (linkage) are constant, therefore no e.m.f.

Question 28

Explain why high temperatures are necessary for fusion reactions to occur in stars.

Answer 28

The protons / nuclei repel each other

At high temperature) particles have more KE and hence can get close (enough to fuse

Question 29

Use your knowledge and understanding of potential divider circuits to explain the shape of the graph shown in Fig. 17.2. Include in your answer the maximum and minimum values of the resistance of the LDR.
Describe how the student can determine the frequency of the rotating plate.

Answer 29

 The ‘trace’ is because of light reaching and not reaching LDR
 Resistance of LDR varies with (intensity) of light
 In light:
o resistance of LDR is low
o p.d. across LDR is low
o p.d across resistor (or V) is high
o current in circuit is large
 In darkness:
o resistance of LDR is high
o p.d. across LDR is high
o p.d across resistor (or V) is low
o current in circuit is small

Question 30

Describe how you can demonstrate in the laboratory that the reflected light is plane polarised.

Answer 30

Use a polaroid / polarising filter

Rotation will change intensity

Question 31

Explain why the electromotive force (e.m.f.) induced across the ends of the coil is a maximum at the times when B = 0.

Answer 31

The gradient is maximum / maximum rate of change of B / maximum rate of change of flux (linkage)

Question 32

Use Fig. 22 to determine the capacitance C of the capacitor. Describe how the student can then use this value of C to determine a value for ε.
In your description, mention any additional measurements required on the capacitor.

Answer 32

 C = A/d
 A = area (of overlap) and d = separation.
 Use ruler to measure the side / radius / diameter (and hence the area A)
 Ensure total overlap of plates.
 Measure the thickness / d of paper using micrometer
 Take several readings of thickness and determine an
average value for d

Question 33

Briefly describe and explain an experiment that can be carried out to confirm the beta-minus radiation emitted from the lead nuclei.

Answer 33

Aluminium (sheet placed between source and detector)
The count (rate) reduces
or
Magnetic / electric field used
Electrons identified from correct deflection / motion in field

Question 34

ripple tank

Answer 34

vibrating bar connected to power pack

lamp

Question 35

Why is frequency dependent on wavelength, for stationary wave?

Answer 35

The speed is constant; and the frequency has to be an integer multiple of 2f0 from f0 (e.g. f0, 3f0, 5f0, etc…). The wavelength depends on the length of the tube, so the wavelength has to decrease with each harmonic.

Question 36

systematic error- why is gradient correct

Answer 36

all measurements vary by same amount

same gradient just shifted

Question 37

control rods material

Answer 37

boron

Question 38

nuclear fission

total energy=

Answer 38

number of molecules x energy for a reaction

Question 39

Kirchhoff’s second law

Answer 39

sum of emf= sum of pd in closed loop

energy is conserved