LAQs

Question 1

A student suggests that a parallel combination of springs could be a model for a parallel combination of resistors in a circuit.

Assess the validity of the students suggestion by considerting the effective stiffness of two identical springs in parallel. (6)

Answer 1

2 springs in parallel:
- Stretching force is shared between the springs
- Extension for a given force is half of one spring
- Parallel combination has twice the stiffness of one spring

2 resistors in parallel:
- 1/R_T = 1/R + 1/R
- Effective resistance is half
- So students suggestion is invalid

Question 2

A teacher quickly removed one end of a bar magnet from a suspended aluminium ring.
When the magnet is removed from the ring, the ring moves in the same direction as the magnet.
Explain, using the laws of electromagnetic induction, why the rings moves in the direction of the magnet. (6)

Answer 2

• Removing the magnet from the ring changes the magnetic FLUX
• this INDUCES AN EMF
• Emf causes a current in the ring
• which produces a magnetic field
• magnetic fields interact
• This opposes the change, causing an attractive force to act

Question 3

Explain, using ideas of momentum, why the pressure exerted by a gas increases as the temperature of the gas increases. (6)

Answer 3

• As temp increases, speed/KE of the atoms increases
• so momentum of the atoms increases
• Rate of collisions of atoms with the conatiner walls increases
• Rate of change of momemtum increases
• Rate of change of momentum is equal to the force
• Pressure is force over area and the force is greater

Question 4

A trolley is attached to the ends of two springs. When displaced from its equilibrium positions, the trolley moves with simple harmonic motion.
A student has a stopwatch and a metre rule available.
Explain the procedure that the student should follow to make an accurate determination of the time period of the trolley. (6)

Answer 4

• Time n oscillations and divide by n, where n is a large number
• Increasing the time reduces the uncertainty
• Repeat timing and calculate a mean
• Use a marker to indicate the reference position
• Use equilibirium position as reference position
• The trolley is moving fastest at this point, so the uncertainty in starting/stopping the stopwatch is least

Question 5

in curling;
Stone B is stationary. Stone A travels towards B and makes a direct hit with stone B. Both stones have mass m.
The collision is elastic. Just before the collision stone A has a velocity v. After the collision stone B moves off with velocity v.
Discuss how the relevant conservation laws apply to this collision. (6)

Answer 5

• ice surface so there is minimal friction
• momentum is conserved in the collision
• The momentum of stone A before the collsion equals the momentum of (A and) B after the collision
• Stone A must be at rest after the collision
• All of the kinetic energy of stone A must have been transferred to stone B
• Kinetic energy is conserved in an elastic collision.

Question 6

Radium has a half-life of 2000 years. A sample of radium can remain at a high temperature for a long period of time.
Explain how a sample of radium is able to release significant amounts of energy over a long period of time. (6)

Answer 6

• Rate of decay depends on the number of unstable nuclei in the sample (A = -lamdaN)
• Radium has a large half-life, so unstable nuclei are present for a long time
• When a nucleus decays there is a decrease in mass
• Energy is released according to E = mc²
• m is small but c is large, so a significant amount of energy is released
• Energy becomes kinetic energy of the atoms in the sample (so temp stays high)

Question 7

Answer 7

• Alternating p.d. causes and alternation current in the coil
• Current carrying conductor in a magnetic field excperiences a force
• Current is alternating, so force changes direction with current
• Loudspeaker forces the box into oscillation
• When the frequency of oscillation equals the natural frequency of the air in the box
• Maximum energy is transferred, and amplitude of vibration increases

Question 8

Explain why there is an exponential decrease in the rate of decay for a sample containing a large number of unstable nuclei.

Answer 8

• There is a fixed probability of an individual nucleus undergoin decay
• For a large number of unstable nuclei, there is a predictable pattern
• The fraction of nuclei decaying in the next second is equal to the decay constant (λ)
• activity = λN
• number of nuclei = N₀e^-λt
• rate of decay = A₀e^-λt

Question 9

Some fella not on a ferris wheel. W = weight, P = push from the seat.
The rider moves from the bottom to the top of the circular path.
Explain how the apparent weight experienced by the rider would change. (6)

Answer 9

• Rider experiences a resultant force acting towards the centre
• This force is constant, as velocity is constant
• At the bottom of the circle, P and W act in opposite directions, so P > W
• At the top of the circle, P and W act in the same direction, so P < W
• P is the weight the rider appears to have
• The rider would feel heavier at the bottom of the circle

Question 10

Answer 10

• Both circuits are suitable, as they allow readings of pd/current for the lamp
• For circuit 1, min pd = 0
  -For circuit 1, max pd = supply pd
• For circuit 2, adjusting the resistor changes the circuit resistance
• For circuit 2, minimum p.d. depends on the resistance of the variable resistor
• Circuit 1 is better, because bigger range (down to 0V)

Question 11

Explain how alpha particle scattering experiments support the nuclear model of the atom (6)

Answer 11

• Most particles are not deflected, as they pass through the foil
• This indicates that there is mostly empty space inside the atom
• Some were deflected by small angles
• This indicates a charged centre
• A very small proportion were deflected more than 90 degrees
• This indicates most of the mass is concentrated in the nucleus

Question 12

Neon bulb
When in use, the neon gas between the electodes emits EM radiation.
Explain why this happens when there is an electric current between the electrodes. (6)

Answer 12

• Electric current is a movement of electrons
• Electrons collide with neon atoms
• Energy is transferred to the neon atoms
• Electrons in the neon atoms are excited
• Electrons in the neon de-excite
• When the electron returns to a lower energhy state it emits a PHOTON

Question 13

Answer 13

• Ohm’s law requires current to be proportional to the pd
• The line should pass through the origin
• The correct line may be a curve
• The graph does not show Ohm’s law, so student’s statement is invalid.

Question 14

Answer 14

2 out of 3 pairs:
- Let the pendulum swing back and to before starting the stopwatch
- The first swing may be affected by the student pushing

• Use a marker at O
• Easier to determine when it passes O
• Time more oscillations
• Longer time reduces uncertainty

Question 15

Part (a) (ii): calculate increase in GPE as the satellite is raised to its orbital height.

A student suggests that giving the satellite more energy than that required in (ii) would result in the satellite orbiting at a greater height and with a greater speed.
Assess the validity of the student’s suggestion. (four)

Answer 15

• This would bring the gravitational potential energy closer to zero
• This would mean that the satellite would orbit at agreater height as 𝐺𝑃𝐸 ∝ 1/𝑟
• To remain in orbit the centripetal acceleration must equal the gravitational field strength at the orbit
  height
  Or Since gravitational force smaller, 𝑚𝑣^2/𝑟
  would be reduced
• 𝑣^2 = 𝐺𝑀𝑟 and satellite would orbit at lower speed

OR

• Satellite will have more kinetic energy at the original orbit height
• The centripetal force will be too small to keep it in this orbit
• The satellite would be travelling too fast, so it would move to a higher orbit
• 𝑣^2 = 𝐺𝑀𝑟
  and satellite would orbit at lower speed

Question 16

Answer 16

• High precision means a small spread of values
• High accuracy means close to the true value

Two from:
- A is precise but not accurate
- B is accurate and precise
- C is neither
- D is accurate but not precise

Question 17

A student released a ping pong ball in front of a metre rule and used a phone camera to record the motion of the ball. The phone camera captures 60 images per second.
The ball reached its terminal velocity as it passed the metre rule.
Explain how the terminal velocity of the ball could be determined. (four)

Answer 17

• Identify when the distance fallen between frames has become constant
• Measure the distance fallen over a number of frames
• terminal veolocity = distance/time
• Time between frames = 1/60 s

Question 18

Answer 18

• A standing wave is set up in the tube
• Amplitude is maximum where constructive interference occurs (antinodes)
• Amplitude is minimum where destructive interference occurs (nodes)
• Sand is displaced from max amplitude to min amplitude (from antinodes to nodes)

Question 19

The average intensity of radiation from the sun at the Earth’s surface is much less than the intensity incident at the top of the Earth’s atmos-phere.
EXPLAIN BITCH (four)

Answer 19

• Radiation passing through the atmosphere is absorbed
• Radiation is reflected from the top of the atmosphere
• Only half of the Earth’s surface has radiation from the sun at one time
• Intensity of radiation is greater at the equator than at the poles (not even slightly what the question asked but ok)

Question 20

Assess the extent to which the student’s data supports the relationship h = k/r
(four)

Answer 20

• Only four points, which isn’t enough to form a firm conclusion
• Range of points in insufficient to form a firm conclusion
• the relationship suggests a straight line through the origin
• The points do lie in a straight line

Question 21

“The [not ferris wheel] is perfectly safe without the need for safety harnesses for the riders.
Centrifugal force ensures that the riders remain in their seats at all stages in the ride.”
Assess the validity (four)

Answer 21

four of five:
- The riders are most at risk of falling out at the top
- They will not fall out if reaction force > 0
- The faster that the ride rotates, the larger the contact force needed to maintain the circular path
- There is a minimum speed
- The centripetal force keeps riders in their seats (not centrifugal)

Question 22

Explain why the most massive stars in a young cluster have the greatest luminosities

Answer 22

four of six:
- Most massive stars experience greater gravity
- core temp and density is greatest
- rate of fusion is greatest Or power generated is greatest
- temp is greatest
- surface area is greatest
- power radiated is greatest according to Stefan’s law

Question 23

Answer 23

four of five:
- Difficult to measure from the same position on the spring each time
- Ruler may move
- May be parallax error
- Only taken one measurement for each mass (repeats)
- Hence not accurate

Question 24

Explain how the student could modify his method in order to obtain more accurate values for the extensions of the spring.

Answer 24

five of six:
- Attach a pointer to the bottom of the spring
- Bring ruler closer
- Take measurements as masses are added and removed
- Calculate mean extensions
- Ensure ruler is vertical (set square, spirit level)
- Read at eye level

(1 and 2 are holistic)??

Question 25

The student timed a pendulum with a stopwatch as it moved from one extreme of the oscillation to the other.
Explain how the procedure used by the student to determine T could have been improved. (5)

Answer 25

• Time a number of oscillations and divide by number of oscillations
• This increases the total time Or This reduces the effect of reaction time
• Time from the mid-point
• Use a marker to identify the mid-point Or pendulum is fastest at the mid-point
• Reduces uncertainty

Question 26

Explain the shape of the graph.

Answer 26

• Amplitude increases to a maximum at 2 Hz
• This is the natural frequency of the trolley
• When driving frequency = natural frequency, there is a maximum transfer of energy
• Resonance