CHAPTER 1: ELECTROSTATICS

Transcription

1 CHAPTER 1: ELECTROSTATICS PSPM II 2005/2006 NO A 2 μc charge lies on the straight line between a 3 μc charge and a 1 μc charge. The separation between the 3 μc and 1 μc is 4 cm. (a) Draw the position of the three charges and show the forces acting on the 2 μc charge. (b) Calculate the distance of the 3 μc from 2 μc where net force on 2 μc is zero. [Ans: x = m] PSPM II 2006/2007 NO A charged particle of C enters the region between two parallel plates as shown in. Calculate (a) The electric field strength in the region between the plates. [Ans: E = V m 1 ] (b) The magnitude of the electric force that acts on the particle. [Ans: F = N] PSPM II 2006/2007 NO. 10(A) 10. (a) FIGURE 5 Three point charges, Q 1 = +15 μc, Q 2 = 25 μc and Q 3 = 35 μc are arranged as in FIGURE 5. PCH 1

2 (i) Copy FIGURE 5 and draw the direction of the electrostatic forces on Q 2. (ii) Calculate the magnitude and direction of the resultant force on Q 2. [Ans: F 2 = 9520 N; θ = above x-axis.] [7 marks] PSPM II 2007/2008 NO shows an electron with horizontal velocity v entering a uniform electric field E between two charged parallel plates. (a) Copy and label the sign of the charge on each plate. (b) Sketch the subsequent path of the electron in the electric field. (c) Why does the electron follow such a path? PSPM II 2007/2008 NO. 10(A),(B) 10. (a) FIGURE 2 FIGURE 2 shows two charges q 1 and q 2 separated 4 cm apart. At position P, (i) Sketch the electric fields due to q 1 and q 2. (ii) Determine the resultant electric field. [Ans: E P = V m 1 ; Φ EP = 108 ] (iii) Calculate the electrical potential. [Ans: V = V] [9 marks] (b) Determine work done in moving a charge along an equipotential line. Explain your answer. [Ans: W = 0.] PCH 2

3 PSPM II 2008/2009 NO FIGURE 5 FIGURE 5 represents the equipotential surfaces and electric field lines near a positive point charge Q. (a) From FIGURE 5, give three relationships between the equipotential surfaces and the electric field lines. (b) Calculate the work done by the electric field on a 3 μc test charge that is displaced from point (i) A to B. [Ans: W AB = J ] (ii) A to C. [Ans: W AC = 0 J] (c) If charge Q is 5 nc, (i) Calculate the distance between the point charge Q and A. [Ans: r = m] (ii) Calculate the electric field at A. [Ans: E = 2000 N C 1 ] (iii) And a new point charge of 5 nc is placed near Q, sketch where this new charge could be placed such that the potential at A is zero. Explain your answer. [7 marks] (d) If the point charge Q is replaced by a negative point charge of equal magnitude, describe the changes in the (i) Equipotential surfaces. (ii) Electric field lines. PSPM II 2009/2010 NO (a) The charges and coordinates of two point charges, Q 1 and Q 2, are given in TABLE 2. PCH 3

4 TABLE 2 Point Charge Charge Coordinate (cm) Q 1 10 μc (0,0) Q 2 5 μc (2,0) (b) (i) Calculate the coordinate of a third charge +Q 3 for it to be in equilibrium. Explain your chosen coordinate. [Ans: x = cm; (6. 83, 0) cm] (ii) Would the coordinate be the same as in part (i) if the third charge is Q 3? Explain your answer. [Ans: Same coordinate] [8 marks] FIGURE 3 FIGURE 3 shows two flat parallel metal with plate separation d connected to batteries. Sketch a labeled graph of electric field E against plate separation d for the metal plates. (c) FIGURE 4 FIGURE 4 shows two 20 μc charges at point P and S respectively. The distance between P and S is 10 cm. A point M is midway between PS. Calculate the work needed to move a 2 μc test charge from M to R, 2 cm away from S. [Ans: W = 8. 2 J] [5 marks] PSPM II 2010/2011 NO (a) Four electrons are released onto a thin aluminium disc of diameter 10 mm. If the disc is initially neutral, (i) determine the electric field strength at the center of the disc. [Ans: E center = 0] (ii) calculate the electric potential at the center of the disc. [Ans: V = V] (iii) calculate the energy stored in the disc. [Ans: U = J] [8 marks] PCH 4

5 (b) FIGURE 4 FIGURE 4 shows a graph of potential V against distance r. The horizontal lines are equipotential lines. Calculate the (i) Potential difference between points P and Q. [Ans: V = 2000 V] (ii) Work needed to bring a point charge C from P to Q. [Ans: W = 2. 4 J] (c) A point charge C with kinetic energy 8.5 MeV approaches a nitrogen nucleus C. Calculate the closest possible distance of approach between the point charge and the nucleus. [Ans: r = m] PSPM II 2011/2012 NO (a) State Coulomb s law. [1 mark] (b) An amount of charge is transferred from a neutral plastic bead to another identical neutral bead located 15 cm away. The force of attraction between the beads is N. How many electrons were transferred from the first bead to the second? [Ans: n = electrons] (c) PCH 5

6 Two point charges q 1 = μc and q 2 = 5.00 μc are placed at the two corners of a triangle of sides 0.30 m, 0.40 m, and 0.50 m as shown in. P is the third corner of the triangle. Calculate (i) the magnitude of the electric field at P. [Ans: E R = N C 1 ] (ii) (iii) the electric potential at P. [Ans: V P = 0 V] the work needed to bring a test charge from infinity to P. [Ans: W = 0 J] [10 marks] PSPM II 2012/2013 NO. 1(A),(B) 1. (a) (i) Define the electric potential V at a point P in an electric field. (ii) An isolated charge Q = C is placed in a region and it creates an electric field around it. Calculate the work done to move a point charge q = C from point S to point P which is located at 60 cm and 30 cm respectively from charge Q. [Ans: W SP = J] (b) shows two point charges q and +2q placed at points B and C respectively. If q = C, calculate (i) The electric field at the point D. [Ans: E D = N C 1 to the left (OR negative value)] (ii) The potential energy of all charges when the point charge C is placed at A. [Ans: U D = 0 J] [6 marks] PSPM II 2013/2014 NO. 1(C) 1. (c) FIGURE 2 PCH 6

7 FIGURE 2 shows a charge Q at the vertex of an equilateral triangle with sides 1 mm. If 138 J of work is done in bringing a 4.8 μc point charge from infinity to position M, (i) Determine the magnitude and type of charge Q. [Ans: Q = C] (ii) Calculate the electric field at position N. [Ans: E N = N C 1 ] [7 marks] PSPM II 2014/2015 NO. 1(A),(B) 1. (a) (i) What is meant by electric field strength at a point in an electric field? (ii) Define time constant in a capacitive circuit during discharging. (b) shows a charged ball floating vertically above another charged ball at an equilibrium distance d apart in a test tube. (i) Sketch the forces acting on the floating ball. (ii) What is the type of charge on the balls? (iii) If the charge on each ball is tripled, determine the new equilibrium distance between the balls in terms of d. [Ans: d new = 3d] [5 marks] PCH 7