AQA A2 Physics Electric Fields - Exam QA

ELECTRIC FIELDS

TEST 1

A2-Level Grade

For this paper you must have:RulerPencil and RubberScientific calculator, which you are expected to use when appropriate

InstructionsAnswer all questionsAnswer questions in the space providedAll working must be shown

InformationThe marks for the questions are shown in brackets

PHYSICS

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(a) State what is represented by gravitational field lines.

___________________________________________________________________

(b) Figure 1 shows the gravitational field above a small horizontal region on the surface of theEarth.

Figure 1

Suggest why the field lines converge over a small area at K.

___________________________________________________________________

(c) A ball travelling at constant speed passes position J moving towards position K in Figure 1.

Assume friction is negligible.

Explain any change in the speed of the ball as it approaches K.

___________________________________________________________________

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(d) Figure 2 shows lines of force for the electric field surrounding two charged objectsL and M.

Figure 2

Explain why the lines of force shown in Figure 2 cannot represent a gravitational field.

___________________________________________________________________

(e) State which object L or M has a charge with the greater magnitude.

object ____________________

State which object L or M has a positive charge.

object ____________________

(f) Draw, on Figure 2, an equipotential line that passes through point N.Do not extend your line beyond the given field lines.

(Total 9 marks)

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An α particle with an initial kinetic energy of 4.9 MeV is directed towards the centre of a gold

nucleus of radius R which contains 79 protons. The α particle is brought to rest at point S, a

distance r from the centre of the nucleus as shown in the diagram below.

(a) Calculate the electric potential energy, in J, of the α particle at point S.

electric potential energy = ___________________ J

(b) Calculate r, the distance of closest approach of the α particle to the nucleus.

r = ___________________ m

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(c) Determine the number of nucleons in the gold nucleus.

R, radius of the gold nucleus = 7.16 × 10−15 m

R0 = 1.23 × 10−15 m

number of nucleons = ________________

(d) The target nucleus is changed to one that has fewer protons. The α particle is given thesame initial kinetic energy.

Explain, without further calculation, any changes that occur to the distance r. Ignore anyrecoil effects.

___________________________________________________________________

(Total 10 marks)

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The figure below shows a system that separates two minerals from the ore containing them usingan electric field.

The crushed particles of the two different minerals gain opposite charges due to friction as theytravel along the conveyor belt and through the hopper. When they leave the hopper they fall 4.5metres between two parallel plates that are separated by 0.35 m.

(a) Assume that a particle has zero velocity when it leaves the hopper and enters the regionbetween the plates.

Calculate the time taken for this particle to fall between the plates.

time taken = _____________s

(b) A potential difference (pd) of 65 kV is applied between the plates.

Show that when a particle of specific charge 1.2 × 10–6 C kg–1 is between the plates itshorizontal acceleration is about 0.2 m s–2.

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(c) Calculate the total horizontal deflection of the particle that occurs when falling between theplates.

horizontal deflection = _____________m

(d) Explain why the time to fall vertically between the plates is independent of the mass of aparticle.

___________________________________________________________________

(e) State and explain two reasons, why the horizontal acceleration of a particle is different foreach particle.

___________________________________________________________________

(Total 12 marks)

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(a) State, in words, Coulomb’s law.

___________________________________________________________________

(b) The graph shows how the electric potential, V, varies with , where r is the distance from a

point charge Q.

State what can be deduced from the graph about how V depends on r and explain why all

the values of V on the graph are negative.

___________________________________________________________________

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(c) (i) Use data from the graph to show that the magnitude of Q is about 30 nC.

(ii) A +60 nC charge is moved from a point where r = 0.20 m to a point where r = 0.50 m.Calculate the work done.

work done ____________________ J

(iii) Calculate the electric field strength at the point where r = 0.40 m.

electric field strength ____________________ V m−1

(Total 10 marks)

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(a) Figure 1 shows a negative ion which has a charge of –3e and is free to move in a uniformelectric field. When the ion is accelerated by the field through a distance of 63 mm parallel

to the field lines its kinetic energy increases by 4.0 × 10–16 J.

Figure 1

(i) State and explain the direction of the electrostatic force on the ion.

______________________________________________________________

(ii) Calculate the magnitude of the electrostatic force acting on the ion.

magnitude of electrostatic force ____________________ N

(iii) Calculate the electric field strength.

electric field strength ____________________ NC–1

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(b) Figure 2 shows a section of a horizontal copper wire carrying a current of 0.38 A.

A horizontal uniform magnetic field of flux density B is applied at right angles to the wire inthe direction shown in the figure.

Figure 2

(i) State the direction of the magnetic force that acts on the moving electrons in the wireas a consequence of the current and explain how you arrive at your answer.

______________________________________________________________

(ii) Copper contains 8.4 × 1028 free electrons per cubic metre. The section of wire inFigure 2 is 95 mm long and its cross-sectional area is 5.1 × 10–6 m2.Show that there are about 4 × 1022 free electrons in this section of wire.

(iii) With a current of 0.38 A, the average velocity of an electron in the wire is5.5 × 10–6 m s–1 and the average magnetic force on one electron is 1.4 × 10–25 N.

Calculate the flux density B of the magnetic field.

flux density ____________________ T

(Total 10 marks)

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The diagram below shows an arrangement to demonstrate sparks passing across an air gapbetween two parallel metal discs. Sparks occur when the electric field in the gap becomes largeenough to equal the breakdown field strength of the air. The discs form a capacitor, which ischarged at a constant rate by an electrostatic generator until the potential difference (pd) acrossthe discs is large enough for a spark to pass. Sparks are then produced at regular time intervalswhilst the generator is switched on.

(a) The electrostatic generator charges the discs at a constant rate of 3.2 × 10–8 A on a daywhen the minimum breakdown field strength of the air is 2.5 × 106 V m–1. The discs have acapacitance of 3.7 × 10–12 F.

(i) The air gap is 12 mm wide. Calculate the minimum pd required across the discs for aspark to occur. Assume that the electric field in the air gap is uniform.

pd ____________________ V

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(ii) Calculate the time taken, from when the electrostatic generator is first switched on,for the pd across the discs to reach the value calculated in part (a)(i).

time ____________________ s

(b) The discs are replaced by ones of larger area placed at the same separation, to give alarger capacitance.

State and explain what effect this increased capacitance will have on:

(i) the time between consecutive discharges,

______________________________________________________________

(ii) the brightness of each spark.

______________________________________________________________

(Total 7 marks)

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A conducting sphere holding a charge of +10 μC is placed centrally inside a second unchargedconducting sphere.

Which diagram shows the electric field lines for the system?

(Total 1 mark)

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A charged spherical conductor has a radius r. An electric field of strength E exists at the surfacedue to the charge.

What is the potential of the spherical conductor?

A r2 E

B r E2

(Total 1 mark)

Four positive charges are fixed at the corners of a square as shown.

The total potential at the centre of the square, a distance d from each charge, is

Three of the charges have a charge of +Q

What is the magnitude of the fourth charge?

C √2Q

(Total 1 mark)

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Two parallel metal plates of separation a carry equal and opposite charges.10

Which graph best represents how the electric field strength E varies with the distance x in thespace between the two plates?

(Total 1 mark)

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AQA A2 Physics Electric Fields - Exam QA

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