3.B field Induction

Question 1

U10PH5 Explain why charges accumulate on the upper and lower face of the conductor as shown.[2]

Answer 1

force on electrons is downwards [or electron deficiency on top] (1) due to Fleming’s LHR [or stating that current is to the right] (1)

Question 2

U10PH5 Where must the conductor be placed and how should it be orientated in relation to the solenoid to obtain the maximum Hall voltage?[2]

Answer 2

In the middle / inside [of the solenoid] (1) with front face ⊥ (1)[to axis of solenoid or B-field] [NB: “inside current” ×, “between the coils” ×]

Question 3

U10PH5 The hoop is in a magnetic field. Explain why an emf is induced in the hoop as it expands. [2]

Explain why the current flows anticlockwise in the diagram. [2]

Answer 3

Question 4

U10PH5B Explain briefly the role of the liquid helium in producing strong magnetic fields.[2]

Answer 4

Keeps superconductors at low temperature (1) so that high currents [are maintained] (1)

Question 5

U10PH5B Explain briefly why ‘contaminating the proton tubes with soot’ would be a problem (paragraph 24). [1]

Answer 5

(protons would ) collide with soot particles

Question 6

U10PH5B Some theoretical physicists believe that the large hadron collider might destroy the planet (paragraph 18) due to the formation of tiny black holes. The event horizon of a black hole is the distance from a black hole within which nothing can escape. This distance for a black hole formed from two protons is around 10^–54m. Explain why such a black hole would be unlikely to pull in the whole mass of the Earth rapidly. [2]

Answer 6

• Gravitational pull small (only 2 protons)
• Tiny probability of collision (with small object)
• Shrinks in size due to Hawking radiation

Question 7

U16PH5 State the laws of electromagnetic induction (Faraday’s law** and **Lenz’s law).

Answer 7

Farafay：Emf is (or proportional to) the rate of change of flux [linkage]

Lenz：Emf / current opposes (tends to) change to which it is due Accept equation with terms defined

Question 8

U16PH5 Use the laws of Faraday and Lenz to explain why the measured emf varies as shown in the graph opposite. [3]（5）

Answer 8

Question 9

U16PH5 A positron travels with velocity, v perpendicularly to a uniform magnetic field, B.Hence, explain why a positron produced at point P initially moving to the left will not travel with uniform circular motion. r=5.7 [m]

Answer 9

Field not uniform or field changes

Over 5.7 m or even though motion perpendicular to B or reference to the circle being too big

Question 10

C3 Explain briefly how the student used the Hall probe to obtain the data ln(B) v ln(A).

Answer 10

V_H=Bvd

Place Hall probe varying distances from the wire (1) Hall probe placed ⊥ to the field (1)

Question 11

C3 Does the graph confirm the relationship: Explain your reasoning.

Answer 11

)No mark for just stating Yes or No

Straight line (1)

Small scatter of points/low random error etc.

(1) Gradient close to -1 and Yes stated (1)

（origin,#,range）

Question 12

C3 A second student carries out the same experiment with a badly calibrated Hall probe that gives B-field readings that are consistently 40% too large. Explain how this would affect the graph shown on page 51. [3]

Answer 12

40% is incorporated into the constants i.e. μ₀, 2π, I (1)

So the intercept↑ will be greater (1)

The gradient~ will be the same (1)

Question 14

(a) (i) Explain why the reading on the balance is negative for the direction of current and magnetic field shown in the diagram. [3]

Answer 14

Force is upward on the magnet (1)

[F]LHR quoted and application of N3 (1)

Force is down on the wire (1)

Question 15

(i) Explain how the Hall voltage arises in the Hall probe shown. [3]

Answer 15

(i) Force on charge carriers due to mag field or [F]LHR (1)

Charge carriers go to one side of conductor / reference to build-up of charge (1)

Electric field set up or top is +ve / -ve depending on carriers (1)

Question 16

(ii) Discuss the effectiveness of the experiment in part (a) for confirming the relationship F=BIℓ and measuring the magnetic field strength, compared with using the Hall probe {see (a)(iii) and (b)}. [6 QER]

Answer 16

1. Line goes through origin.
2. Graph is a straight line.
3. Line passes through all the error bars.
4. Both values of B compared.
5. Hall probe is more precise or its uncertainty is smaller [accept accurate].
6. Sensible comment comparing methods e.g. method A is _graphica_l, method B required only one measurement or converse, method A is more prone to human error, method B doesn’t confirm F = BIl

Question 17

(i) Explain why a current is detected by the ammeter. [2]

Answer 17

Change in flux linkage or cutting flux (1) Complete circuit or implied (1)

Question 18

(ii) State why the current is independent of the horizontal component of the Earth’s magnetic field. [1]

Answer 18

) These are in the plane of motion/the vertical component is perpendicular to motion/these are not cut

Question 19

(c) State the direction of the current in the resistor and how you obtained this direction. [2]

Answer 19

Current down in resistor [or up in conductor or anticlockwise] (1) [F]RHR or [F]LHR (for electrons) or RH grip rule (1)

Question 20

(d) A student suggests that the opposing force due to the magnetic field on the moving conductor is negligible compared with other resistive forces. Is the student correct? Justify your answer with a calculation. [4]

Answer 20

Other force compared e.g. friction/drag (1)

Magnetic force small / resistive force is much bigger (1)

Because current small / B small (1)

Good calculation e.g. P = I 2R = 8.8 ×10-7 [W] or F = BIL = 1.6 × 10-8 [N] ecf on B or calculated values of B using I (1)