Exam Review: Topic 06 – Fields Reagan IB Physics

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Practice Test: 41 marks (55 minutes) Additional Problem: 19 marks (27 minutes)

1. A spherical planet of uniform density has three times the mass of the Earth and twice the average radius. The magnitude of the gravitational field strength at the surface of the Earth is g. What is the gravitational field strength at the surface of the planet?

A. 6 g B. C. D.

(Total 1 mark) 2. In Newton’s universal law of gravitation the masses are assumed to be

A. extended masses. C. point masses.

B. masses of planets. D. spherical masses. (Total 1 mark)

3. An electron passes the north pole of a bar magnet as shown below.

What is the direction of the magnetic force on the electron?

A. Into the page C. To the left

B. Out of the page D. To the right (Total 1 mark)

4. A spacecraft travels away from Earth in a straight line with its motors shut down. At one instant the speed of the spacecraft is 5.4 km s–1. After a time of 600 s, the speed is 5.1 km s–1. The average gravitational field strength acting on the spacecraft during this time interval is

A. 5.0 × 10–4 N kg–1 C. 5.0 × 10–1 N kg–1

B. 3.0 × 10–2 N kg–1 D. 30 N kg–1 (Total 1 mark)

5. Two isolated point charges, –7 µC and +2 µC, are at a fixed distance apart. At which point is it possible for the electric field strength to be zero?

(not to scale) (Total 1 mark)

6. A long straight wire carries an electric current perpendicularly out of the paper. Which of the following represents the magnetic field pattern due to the current?

(Total 1 mark)

g32 g

43 g

23

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7. An electron is moving in air at right angles to a uniform magnetic field. The diagram below shows the path of the electron. The electron is slowing down.

Which of the following correctly gives the direction of motion of the electron and the direction of the magnetic field?

Direction of motion Direction of magnetic field

A. clockwise into plane of paper B. clockwise out of plane of paper C. anti-clockwise into plane of paper D. anti-clockwise out of plane of paper

(Total 1 mark)

8. The weight of an object of mass 1 kg at the surface of Mars is about 4 N. The radius of Mars is about half the radius of Earth. Which of the following is the best estimate of the ratio below?

A. 0.1 B. 0.2 C. 5 D. 10 (Total 1 mark)

9. Three positive point charges of equal magnitude are held at the corners X, Y and Z of a right-angled triangle. The point P is at the midpoint of XY. Which of the arrows shows the direction of the electric field at point P?

(Total 1 mark)

Earth of massMars of mass

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10. An electron travelling in the direction shown by the arrow X, enters a region of uniform magnetic field. It leaves the region of field in the direction shown by the arrow Y.

The direction of the magnetic field is

A. in the direction of X.

B. into the plane of the paper.

C. in the opposite direction to X.

D. out of the plane of the paper. (Total 1 mark)

11. The radius of a charged spherical conductor is R. Which of the following graphs best shows how the magnitude of the electrical field strength E varies with distance r from the centre of the sphere?

(Total 1 mark)

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12. An electron enters the vacuum between two oppositely charged plates with velocity v. The electron is followed by an alpha particle moving with the same initial velocity as the electron. A uniform magnetic field is directed out of the plane of the paper.

The electron’s path is undeflected. The path of the alpha particle will be

A. deflected out of the plane of the paper.

B. undeflected.

C. deflected upward.

D. deflected downward. (Total 1 mark)

13. This question is about gravitational fields.

(a) Define gravitational field strength.

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................................................................................................................................... (2)

(b) The gravitational field strength at the surface of Jupiter is 25 N kg–1 and the radius of Jupiter is 7.1 × 107 m.

(i) Derive an expression for the gravitational field strength at the surface of a planet in terms of its mass M, its radius R and the gravitational constant G.

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......................................................................................................................... (2)

(ii) Use your expression in (b)(i) above to estimate the mass of Jupiter.

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......................................................................................................................... (2)

(Total 6 marks)

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14. This question is about force fields.

(a) Outline what is meant by a field of force.

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...................................................................................................................................... (2)

(b) Five particles A to E are each placed in a different type of field. Complete the table to identify the nature of the field in which each particle is situated.

Particle Charge on particle

Initial direction of motion of particle

Direction of force on particle Type of field

A uncharged stationary in direction of field .....................................

B negative along direction of field

opposite to direction of field .....................................

C positive normal to direction of field

normal to direction of field .....................................

D positive normal to direction of field

in direction of field .....................................

E uncharged opposite to direction of field

in direction of field .....................................

(5) (Total 7 marks)

15. This question is about electric fields and electric circuits.

(a) Two parallel, charged metal plates A and B are in a vacuum.

At a particular instant an electron is at point P.

On the diagram, draw

(i) the electric field pattern due to the plates. (3)

(ii) an arrow to represent the direction of the force on the electron at P. (1)

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(b) The acceleration of the electron at P is 8.8 × 1014 m s–2. Determine the magnitude of the electric field strength at the point P.

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...................................................................................................................................... (3)

(c) The electric potential energy of the electron changes by 1.9 × 10–17 J as it moves from one plate to the other. Show that the potential difference between the plates is 120 V.

......................................................................................................................................

...................................................................................................................................... (1)

(d) A resistor R and a filament lamp L are connected in series with a battery. The battery has an emf of 12 V and internal resistance 4.0 Ω. The potential difference across the filament of the lamp is 3.0 V and the current in the filament is 0.25 A.

(i) Define emf and describe the concept of internal resistance.

emf:

...........................................................................................................................

...........................................................................................................................

Internal resistance:

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........................................................................................................................... (2)

(ii) Calculate the total power supplied by the battery.

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........................................................................................................................... (1)

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(iii) Calculate the power dissipated in the external circuit.

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........................................................................................................................... (2)

(iv) Determine the resistance of the resistor R.

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........................................................................................................................... (3)

(Total 16 marks)

Additional Problems

16. A current is established in a coil of wire in the direction shown.

The direction of the magnetic field at point P is

A. out of the plane of the paper.

B. into the plane of the paper.

C. to the left.

D. to the right. (Total 1 mark)

17. A small sphere X of mass M is placed a distance d from a point mass. The gravitational force on sphere X is 90 N. Sphere X is removed and a second sphere Y of mass 4M is placed a distance 3d from the same point mass. The gravitational force on sphere Y is

A. 480 N. B. 160 N. C. 120 N. D. 40 N. (Total 1 mark)

18. This question is about electric and gravitational fields

(a) State, in terms of electrons, the difference between a conductor and an insulator.

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...................................................................................................................................... (1)

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(b) Suggest why there must be an electric field inside a current-carrying conductor.

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...................................................................................................................................... (3)

(c) The magnitude of the electric field strength inside a conductor is 55 N C–1. Calculate the force on a free electron in the conductor.

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...................................................................................................................................... (1)

(d) The electric force between two point charges is a fundamental force as is the gravitational force between two point masses. State one similarity between these two forces and one difference (other than the fact that one applies to charge and the other to mass).

Similarity: ...................................................................................................................

......................................................................................................................................

Difference: ..................................................................................................................

...................................................................................................................................... (2)

(e) The force on a mass of 1.0 kg falling freely near the surface of Jupiter is 25 N. The radius of Jupiter is 7.0 × 107 m.

(i) State the value of the magnitude of the gravitational field strength at the surface of Jupiter.

........................................................................................................................... (1)

(ii) Calculate that the mass of Jupiter is about 1.8 × 1027 kg.

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........................................................................................................................... (2)

(Total 10 marks)

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19. This question is about electric charge.

(a) A plastic rod XY is held at end X. The end Y is rubbed with a piece of cloth and, as a result, the end Y becomes electrically charged.

The procedure is now repeated using a copper rod and it is found that the copper rod remains electrically neutral. Explain these observations in terms of the properties of conductors and insulators.

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...................................................................................................................................... (5)

(b) Two plastic rods each have a positive charge +q situated at one end. The rods are arranged as shown.

Assume that the charge at the end of each rod behaves as a point charge. Draw, in the shaded area on the diagram, the electric field pattern due to the two charges.

(2) (Total 7 marks)

Exam Review: Topic 06 – Fields Reagan IB Physics

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Mark Scheme

1. C

2. C

3. B

4. C

5. D

6. D

7. D

8. A

9. B

10. D

11. D

12. B

13. (a) the force exerted per unit mass; on a point (small) mass; 2

(b) (i) use of and

combine to get 2

(ii)

substitute to get M = 1.9 × 1027 kg; 2 [6]

14. (a) region/area/volume (of space); where a mass/charge experiences a force; 2

(b)

Particle Charge on particle

Initial direction of motion of particle

Direction of force on particle Type of field

A

B

C

D

E

uncharged

negative

positive

positive

uncharged

stationary

along direction of field

normal to direction of field

normal to direction of field

opposite to direction of field

in direction of field

opposite to direction of field

normal to direction of field

in direction of field

in direction of field

gravitational; electric; (accept electrostatic) magnetic; electric; (accept electrostatic) gravitational;

5 [7]

15. (a) (i)

uniform field equal spacing of lines; edge effect; direction; 3

(ii) as shown; 1

(b) combine F = qE and F = ma;

mFg = ;2R

MmGF =

;2RMGg =

;2

GgRM =

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to get E = ;

E = 5.0 × 103 N C–1/V m–1; 3

(c) V = ;

= 120 V 1

(d) (i) 3.0 W; 1

(ii) power dissipated in battery = (0.252 × 4.0) = 0.25 W; power dissipated in circuit = (3.0 – 0.25) = 2.8 (2.75) W; 2

(iii) power dissipated in lamp = (3.0 × 0.25) = 0.75 W; power dissipated in resistor = (2.75 – 0.75) = 2.0 W;

resistance = 32 Ω; 3

or

resistance of lamp =12 Ω; 12 = 0.25 (R + 16); R = 32 Ω;

or

V across R = 8.0V;

R = ;

= 32 Ω; 3 [16]

Additional problems

16. B

17. D

18. (a) a conductor contains “free” electrons and insulators do not / OWTTE; 1

(b) to have a current electrons must be accelerated/move along the wire; and so a (electric) force must act on them; this is provided by the electric field; 3

(c) 8.8 × 10–18 N; 1

(d) similarity: both follow an inverse square law;

difference: gravitational force is always attractive/is much weaker than electric force / electric force can be repulsion/is much stronger than gravitational force; 2

(e) (i) 25 N kg–1; 1

qma

19

17

106.1109.1

−

−

×

×

⎟⎠

⎞⎜⎝

⎛=

225.00.2

25.08.0

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(ii) M = ;

= ;

= 1.8 × 1027 kg 2 [10]

19. (a) in the plastic there are no free electrons;

(but) electrons can be transferred to/from the cloth (by friction) leaving an imbalance of charge on the rod / OWTTE; electrons can move freely in copper; electrons transferred from/to the cloth from/to the rod; because the body is a conductor; will flow to/from Earth leaving the rod neutral; 5 max

(b)

at least four field lines (minimum two per rod) to show overall shape of pattern; direction of lines all away from poles; 2 Ignore all working outside region. Any field lines crossing loses first mark even if accidental.

[7]

GR 225

11

142

107.6100.725−×

××