## Tuesday, 23 September 2014

### Electric Fields Questions

Charge on an electron = -1·6×10-19C
Mass of an electron = 9·1×10-31kg
Question 1
 a) Define i) electric field strength ii) electric potential at a point in a field. b) Calculate the magnitude of the electric field strength at a point 5×10-8m from an electron. c) Calculate the magnitude of the electric potential at a point 5×10-8m from an electron.
Question 2
 Sketch graphs (with distance on the horizontal axis) of the following a) field strength against distance from a point charge b) potential against distance from a point charge c) field strength against distance from the centre of a charged metal sphere of radius R (in this case the distance axis should go from zero to a value much greater than R) d) potential against distance from the centre of a charged metal sphere of radius R (again the distance axis should go from zero to a value much greater than R) For the next two graphs, refer to the diagram below e) field strength at point p against x f) potential at point p against x.
Question 3
 Calculate the amount of electrical potential energy gained by an electron when it is moved from infinity to a point 0·5cm from another electron.
Question 4
 The diagram below shows two small isolated metal spheres having charges, Q1 = -1·5×10-12C and Q2 = +2·0×10-12C a) Calculate the magnitude of the potential at point p1. b) Find the magnitude and direction of the field strength at point p2. Assume that the spheres are in a vacuum (or air).
Question 5
 Calculate the speed of an electron which has "fallen" freely through a p.d. of 500V.
Question 6
 Calculate the strength of the electric field needed to accelerate alpha particles (of mass 6·68×10-27kg and charge 3·2×10-19C) from rest to a speed of 2·5×105ms-1 in a distance of 5mm.
Question 7
Two positive changes are 15cm apart as shown below. On the line joining the centres of the two charges there exists a point at which the resultant field strength is zero (a “neutral point”).
a) Explain why this “neutral point" exists.
b) Calculate the distance, r, of this point from the charge Q1.
Question 8
A small positively charged ball is released from the position shown in the diagram below. The mass of the ball is m = 0.5g and it carries a charge of magnitude Q = 4.9 µC. The two metal plates are charged and the potential difference between them is V = 5V. Find the direction in which the ball will move when released. (The apparatus is on the earth.)
Question 9
 A beam of electrons enters the uniform electric field between two parallel charged plates, as shown in the diagram below. The fluorescent screen is placed very close to the end of the plates.The voltage across the plates is 1·15V and the electrons enter the field with a velocity of 106ms-1, parallel to the plates. a) At what distance from O will the electrons hit the screen? b) Calculate the magnitude and direction of the velocity of the electrons at the instant when they hit the screen. c) Calculate the kinetic energy possessed by one electron at the instant when it hits the screen. Answer in Joules then convert to electron-volts.