Electric Fields and Forces 1. A test charge is a A. charged object with a very small mass. B. charged object with a very small charge. C. point charge which has no effect on the electric field in which it is placed. D. point charge which slightly changes the electric field in which it is placed. (Total 1 mark) 2. The diagram below shows a particle with positive charge q accelerating between two conducting plates at potentials V 1 and V 2. Which of the following is the kinetic energy gained by the charge in moving between the plates? A. V 2 q B. V 1 q C. (V 1 V 2 )q D. (V 2 V 1 )q (Total 1 mark) IB Questionbank Physics 1
3. A positively charged particle follows a circular path as shown below. Which of the following electric fields could have caused the charged particle to follow the above path? (Total 1 mark) IB Questionbank Physics 2
4. The diagram shows two parallel metal plates X and Y. Plate X is at Earth potential (0 V) and the potential of plate Y is V 0. Which of the following is correct in respect of the magnitude and the direction of the electric field between the plates? Magnitude Direction A. constant X Y B. increasing Y X C. constant Y X D. increasing X Y (Total 1 mark) IB Questionbank Physics 3
5. The diagram below shows a uniform electric field of strength E. The field is in a vacuum. An electron enters the field with a velocity v in the direction shown. The electron is moving in the plane of the paper. The path followed by the electron will be A. parabolic. B. in the direction of E. C. in the direction of v. D. circular. (Total 1 mark) IB Questionbank Physics 4
6. Four point charges of magnitudes +q, +q, q, and q are held in place at the corners of a square of side r. The Coulomb constant is k. Which of the following is the electrical potential at the centre of the square O? A. 0 B. 4kq r C. 4kq 2 r D. 4kq r 2 2 (Total 1 mark) IB Questionbank Physics 5
7. This question is about gravitational and electric fields. (a) The equation for the magnitude of the gravitational field strength due to a point mass may be written as below. Y = KX 2 s The equation for the magnitude of the electric field strength can also be written in the same form. In the table identify the symbols used in the equation. Symbol Gravitational field quantity Electrical field quantity Y K X s (4) (b) The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is F E. The magnitude of the gravitational force between them is F G. FE Determine the ratio. F G............ (3) (Total 7 marks) IB Questionbank Physics 6
8. This question is about electric charge and resistance. (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.................................. (5) IB Questionbank Physics 7
(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 (i) the electric field pattern due to the two charges. (ii) a line to represent an equipotential surface. Label the line with the letter V. (1) (c) A resistor of resistance 1.5 Ω is made from copper wire of radius 0.18 mm. The resistivity of copper is 1.7 10 8 Ω m. Determine the length of copper wire used to make the resistor............. IB Questionbank Physics 8
(d) The manufacturer of the resistor in (c) guarantees that the resistance is within 10 % of 1.5 Ω, provided that the power dissipation in the resistor does not exceed 1.0 W. (i) Suggest why the resistance of the resistor might be greater than 1.65 Ω if the power dissipation in the resistor is greater than 1.0 W. (ii) Show that, for a power dissipation of 1.0 W, the current in a resistor of resistance 1.5 Ω is 0.82 A. (1) IB Questionbank Physics 9
(iii) The 1.5 Ω resistor is connected in series with a variable resistor and battery of emf 6.0 V and internal resistance 1.8 Ω. Estimate the resistance R of the variable resistor that will limit the current to 0.82 A. (3) (Total 16 marks) 9. This question is about a lightning discharge. (a) Define electric field strength.......... IB Questionbank Physics 10
(b) A thundercloud can be modelled as a negatively charged plate that is parallel to the ground. The magnitude of the charge on the plate increases due to processes in the atmosphere. Eventually a current discharges from the thundercloud to the ground. On the diagram, draw the electric field pattern between the thundercloud base and the ground. (3) (c) The current discharges when the magnitude of the electric field between the ground and the thundercloud base is 0.33 MN C 1. The thundercloud base is 750 m above the ground. (i) Calculate, just before discharge, the potential difference between the thundercloud base and the ground. (3) IB Questionbank Physics 11
(ii) The potential difference V, between the thundercloud base and the ground, is given by V = Qd Aε 0 where Q is the charge on the thundercloud base, A is the area of the base, and d is the distance between the base and the ground. The area of the base is 1.2 10 7 m 2. Calculate, just before discharge, the charge on the base of the thundercloud. (iii) Determine the energy released in the discharge. (4) (Total 14 marks) IB Questionbank Physics 12
10. This question is about motion of a charged particle in an electric field. (a) An α-particle of mass 4u and charge +2e is accelerated from rest in a vacuum through a potential difference of 2.4 kv. Show that the final speed of the α-particle is 4.8 10 5 m s 1............. (b) The α-particle is travelling in a direction parallel to and mid-way between two parallel metal plates. (not to scale) The metal plates are of length 2.4 cm and their separation is 0.80 cm. The potential difference between the plates is 600 V. The electric field is uniform in the region between the plates and is zero outside this region. (i) Calculate the magnitude of the electric field between the plates. IB Questionbank Physics 13
(ii) Show that the magnitude of the acceleration of the α-particle by the electric field is 3.6 10 12 m s 2. (c) (i) Calculate the time taken for the α-particle to travel a horizontal distance of 2.4 cm parallel to the plates. (1) (ii) Use your answers in (b)(ii) and (c)(i) to deduce whether, as the α-particle passes between the plates, it will hit one of the plates. (3) (Total 10 marks) IB Questionbank Physics 14