A LevelPhysicsOCRExam Questions6. Particles & Medical Physics6.3 Electric Fields

Electric Fields (OCR A Level Physics)

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6.3 Electric Fields

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6.3 Electric Fields

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1a6 marks

A ball coated with conducting paint has weight 0.030 N and radius 1.0 cm. The ball is suspended from an insulating thread. The distance between the pivot and the centre of the ball is 120 cm.

The ball is placed between two vertical metal plates. The separation between the plates is 8.0 cm.
The plates are connected to a 4.0 kV power supply.

a)
The ball receives a positive charge of 9.0 nC when it is made to touch the positive plate. It then repels from the positive plate and hangs in equilibrium at a displacement x from the vertical, as shown below. The diagram is not drawn to scale.

q1a-paper-3-nov-2020-ocr-a-level-physics

i)
Show that the electric force acting on the charged ball is 4.5 × 10–4 N.

[2]

ii)
Draw, on the diagram above, arrows which represent the three forces acting on the ball.

Label each arrow with the name of the force it represents.

[2]

iii)
By taking moments about the pivot, or otherwise, show that x = 1.8 cm.

[2]

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1b4 marks
b)
The ball is still positively charged.

The plates are now moved slowly towards each other whilst still connected to the 4.0 kV power supply. The plates are stopped when the separation is 5.0 cm.

Explain the effect that this has on the deflection of the ball and explain why the ball eventually starts to oscillate between the plates.

[4]

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1c2 marks
c)
When the ball oscillates between the plates, the current in the external circuit is 3.2 × 10–8 A.

A charge of 9.0 nC moves across the gap between the plates each time the ball makes one complete oscillation.

Calculate the frequency f of the oscillations of the ball.




f = ...................................... Hz [2]

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2a7 marks
a)
The diagram below shows the arrangement of the 3 protons inside the nucleus of lithium-6 (begin mathsize 16px style Li presubscript 3 presuperscript 6 end style)

q23a-paper-2-nov-2020-ocr-a-level-physics

The separation between each proton is about 1.0 × 10−15 m.

i)
Calculate the magnitude of the repulsive electric force F experienced by the proton P.






F = .......................................... N [4]

ii)
On the diagram above, draw an arrow to show the direction of the electric force F experienced by P.

[1]

iii)
Explain how protons stay within the nucleus of lithium-6.

[2]

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2b7 marks
b)
A spherical metal dome shown below is charged to a potential of −12 kV.

q23b-paper-2-nov-2020-ocr-a-level-physics

The dome is supported by a cylindrical plastic rod. The radius of the dome is 0.19 m.

i)
Show that the magnitude of the total charge Q on the dome is 2.5 × 10−7 C.

[2]

ii)
The dome discharges slowly through the plastic rod.

It takes 78 hours for the dome to completely discharge.

1
Show that the mean current I in the plastic rod is about 9 × 10−13 A.

[2]

2
The average potential difference across the plastic rod during discharge is 6000 V.
The rod has cross-sectional area 1.1 × 10−4 m2 and length 0.38 m.

Calculate the resistivity ρ of the plastic.





ρ = ...................................... Ω m [3]

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3a6 marks
a)
Fig. 24 shows two horizontal metal plates in a vacuum.

q24a-paper-2-nov-2021-ocr-a-level-physics

Fig. 24

The diagram is not drawn to scale.
Electrons travelling horizontally enter the space between the charged plates and are deflected vertically.

The potential difference between the plates is 4000 V.
The distance between the plates is 0.08 m.
The initial speed of the electrons is 6.0 × 107 m s–1.
The vertical deflection of the electrons at the far end of the plates is x.

i)
Show that the vertical acceleration a of an electron between the plates is 8.8 × 1015 m s–2.

[3]

ii)
The length of each plate is 0.12 m.

Show that the time t taken by the electron to travel this length is 2.0 × 10–9 s.

[1]

iii)
Calculate the vertical deflection x of the electron.



x = ..................................................... m [2]

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3b2 marks
b)
The arrangement shown in Fig. 24 is now used to investigate positrons emitted from a radioactive source.
The speed of the positrons is also 6.0 × 107 m s–1.

The initial path of the positrons is the same as that of the electrons in Fig. 24.

On Fig. 24, sketch the path of the positrons between the plates.

[2]

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3c3 marks
c)
Beta-minus particles (electrons) emitted from a radioactive source have a range of speeds.

Describe and explain how a uniform magnetic field can be applied in the space between the charged plates to select beta-minus particles with a specific speed. No calculations are required.

[3]

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4a2 marks
a)
Fig. 20.1 shows a positively charged metal sphere and a negatively charged metal plate.

q20a-paper-2-june-2017-ocr-a-level-physics

Fig. 20.1

On Fig. 20.1, draw a minimum of five electric field lines to show the field pattern between the sphere and the plate.

[2]

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4b1 mark
b)
Define electric potential at a point in space.

[1]

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4c5 marks
c)
A metal sphere is given a positive charge by connecting its surface briefly to the positive terminal of a power supply. The electric potential at the surface of the sphere is + 5.0 kV. The sphere has radius 1.5 cm.

i)
Show that the charge Q on the surface of the sphere is 8.3 × 10−9 C.

[2]

ii)
Fig. 20.2 shows the charged sphere from (i) suspended from a nylon thread and placed between two oppositely charged vertical plates.

q20c-ii-paper-2-june-2017-ocr-a-level-physics

Fig. 20.2 (not to scale)

The weight of the sphere is 1.7 × 10−2 N. The string makes an angle of 4.0° with the vertical.

1.
Show that the electric force on the charged sphere is 1.2 × 10−3 N.

[1]

2.
Calculate the uniform electric field strength E between the parallel plates.




E = ................................... N C−1 [2]

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1a4 marks
a)
Fig. 22.1 shows two horizontal metal plates in a vacuum.

q22a-paper-2-june-2019-ocr-a-level-physics

Fig. 22.1

The plates are connected to a power supply. The potential difference V  between the plates is constant. The magnitude of the charge on each plate is Q. The separation between the plates is d.

Fig. 22.2 shows the variation with d of the charge Q on the positive plate.

q22a-2-paper-2-june-2019-ocr-a-level-physics

Fig. 22.2

i)
Use Fig. 22.2 to propose and carry out a test to show that Q is inversely proportional to d.

Test proposed:

Working:

[2]

ii)
Use capacitor equations to show that Q is inversely proportional to d.

[2]

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1b4 marks
b)
Fig. 22.3 shows a negatively charged oil drop between two oppositely charged horizontal plates in a vacuum.

   q22b-paper-2-june-2019-ocr-a-level-physics

Fig. 22.3

The plates are fixed and connected to a variable power supply. The weight of the oil drop is 1.8 × 10–14 N.

i)
The power supply is adjusted so that the potential difference between the plates is 200V when the oil drop becomes stationary.

State the magnitude of the vertical electric force FE acting on the charged oil drop.

FE = ...................................... N [1]

ii)
The potential difference between the plates is now increased to 600 V.
The oil drop accelerates upwards.

Calculate the acceleration a of the oil drop.

a = .................................... ms–2   [3]

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1c6 marks
c)
Fig. 22.4 shows an arrangement used by a student to investigate the forces experienced by a small length of charged gold foil placed in a uniform electric field.

q22c-paper-2-june-2019-ocr-a-level-physics

Fig. 22.4

The two vertical metal plates are connected to a high-voltage supply.

The foil is given a positive charge by briefly touching it to the positive plate.

The angle θ made with the vertical by the foil in the electric field is given by the expression tan space theta equals fraction numerator q E over denominator W end fraction

where q is the charge on the foil, E is the electric field strength between the plates and W is the weight of the foil.

The angle θ can be determined by taking photographs with the camera of a mobile phone.

Describe how the student can safely conduct an experiment to investigate the relationship between θ and E.
Identify any variables that must be controlled.

[6]

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