Practice Paper for Electric Fields: A - Level Physics

 

A Level Physics

Electric Fields - practice paper

Mass of electron: 9 x 10-31 kg | Charge of electron: 1.6 x 10-19

Mass of proton: 1.7 x 10-27 |  Permittivity of air: 8.854 x 10-12

1. 
   

1. State Coulomb’ Law.

2. Sketch a graph of the force against the reciprocal of the square of the distance between the two charges.

3. What’s the gradient in the graph of part b?

4. Two, small metal spheres of the same size are given +12 µC and - 4 µC charges and placed 6 cm apart. Calculate the electrostatic force between them.

5. The two spheres in part d, were then brought into contact and then immediately placed at the previous positions. What would be the new electrostatic force between them and the nature of the force?

2) 

1. Define the electric field strength.

2. Two charges, +12 µC and +4 µC are placed 6 cm apart. Find the electric field strength at the midpoint between the two charges.

3. Hence find the force on +5 C charge placed at the midpoint.

4. Find the position of the neutral point from the smaller charge.

5. If the sign of the smaller charge was reversed, where would be the new position of the neutral point?

3) 

A charged metal ball was placed near a metal plate that is connected to the Earth as shown by the diagram below:

1. What would happen to the electrons in the metal plate?

2. What would happen to the protons in the metal plate?

3. Name the phenomenon that could describe what you notice in part a and part b?

4. Draw electric field lines between the metal sphere and the plate.

5. Explain why two field lines cannot cross each other, as you would draw in part d).

6. The metal ball was first placed near the metal plate and then slowly moved away from the latter. How would the nature of the electric field change with the distance?

4)

‘Action at points’ is a well known phenomenon in electrostatics: that means,when a pointed conductor is given a charge, it tends to accumulate at the tip.During a thunderstorm, the negative charges at the bottom of a dark cloud induce the opposite charges on the ground or objects on the ground.

1. Why is it dangerous to hold pointed objects like knives, pairs of scissors or even needles during a thunderstorm?

2. It is a well-known fact that coconut trees are always prone to lightning in the tropics. Explain this.

3. Comment on the old wives’ tale, ‘A thunder does not strike the same place twice,’ at least, in the short term, after a certain place is being struck by a thunder.

4. The space between a thunder cloud, the ground and the space in between is often described as a charged capacitor. If wind blows the cloud up, what will happen to the energy stored in the natural capacitor? Will it go up or down? Comment on the answer.

5) 

The lengths of a triangular lamina, ABC,  are AB = 5 cm, BC = 12 cm and AC = 13 cm. Two charges, +6 µC and -8 µC are placed at the vertices, A and C respectively. 

1. Find the electric field strength at B and its direction.

2. Find the force on +2 C, placed at B.

3. An extra charge, q, is added to the negative charge so that the force on + 2 C acts at 45° to the side BC. Calculate the value of q.

The above charges were completely removed from the lamina and two positive charges are placed at A and C. The ratio of the magnitude of the charges is 1 : k. 

4. If a point, at a quarter of the side AC from A,  is a neutral point, find the value of k.

6) 

Define electric potential due to a point charge. 

Two charges, + 12 µC and -3 µC are placed 12 cm apart.

1. Find the electric potential at the midpoint between the two charges.

2. Find the work done in moving -6 µC from a place very far from the midpoint.

The point, S, is between the two original charges. The work done in moving  -6 µC from a very far place to the point S, turns out to be zero. c. Find the distance between S and + 12 µC.

d. Sketch a graph to show how electric potential between the two charges changes with the distance. Clearly mark the point S, on your graph.

7) 

1. Define a uniform electric field.

The electric field between two charged parallel plates is assumed to be a uniform electric field.

2. Show that the electric field strength is equal to the potential gradient between the plates.

An electron enters the field along mid-axis between the plates. It bends before emerging from the space between the plates.

3. Show that the path taken by the electron within the electric field is parabolic.

4. Prove that the electron has, in fact, gained energy due to the motion through the electric field.

5. How does the phenomenon in part d) play a key role in particle accelerators?

6. If an electric field plays such a significant role in particle accelerators, what is the need of magnetic fields in them?

8) 

The distance between two horizontal charged plates is 20 mm with the upper being negative and the other positive. The potential difference is maintained at 60 V and an electron enters the space between the two plates horizontally at 2 x 108 m/s.

1. Find the electric field strength between the two plates.

2. Find the force on the electron due to the electric field.

3. Find the acceleration of the electron. State your assumptions.

4. If the length of a plate is 60 mm, find the time taken by the electron to be within the space between the plates.

5. Calculate the emerging speed of the electron from the electric field.

6. Hence calculate the energy gained by the electron due to its motion through the electric field.

9)

Two charged plates are placed horizontally as shown below. An electron enters the electric field between the two through a hole of the bottom plate at 2.0 x 10-8 m/s.

1. Find the kinetic energy of the electron.

2. Find the minimum value of V that could stop the electron from reaching the top plate. Write down the assumptions made in the calculations.

A charged oil drop of radius 1 mm stays in balance between plates. The density of oil is 800 kg/m3.

3. Find the number of protons attached to the oil drop, stating your assumption.

4. If the polarity of the plates were reversed, what would be the initial acceleration of the oil drop?

10) 

1. Define an equipotential surface.

An electric field line is emerging from an equipotential surface at an angle Θ to the surface.

2. Show that the electric field strength at any point on the surface is zero.

3. Hence show that Θ = 90°.

4. Can the surface of a  charged, hollow metal sphere be an equipotential surface? Explain.

5. If the above is true, can there be an electric field inside the sphere? Explain.