## Tuesday, 23 September 2014

### Electro-Magnetism Questions

Question 1
 a) Define the term "magnetic flux density". b) An electron is moving at 107ms-1 at 90° to a magnetic field. The electron follows a circular path of radius 5×10-6m. Calculate the magnitude of the flux density.
Question 2
 A beam of alpha particles passes undeflected through a region of space in which there are a magnetic field (of flux density 5×10-2T) and an electric field (of field strength 750Vm-1) at 90° to each other. Calculate the speed of the particles.
Question 3
 The charge to mass ratio of an electron is 1·76×1011Ckg-1. Calculate the time period of the circular motion of an electron in a magnetic field of flux density 0·1T.
Question 4
 A proton enters a uniform magnetic field of flux density, B = 0·1T with a velocity of 104ms-1 at 60° to the flux lines. It follows a helical path.Calculate a) the time taken to complete one revolution of the helical path b) the distance moved parallel to the flux lines during this time c) the kinetic energy possessed by the proton when it is in the magnetic field (mass of a proton = 1·67×10-27kg).
Question 5
 A very long straight wire lies along the z axis and carries a current, I which flows in the positive z direction in a vacuum (see diagram below). Two flux densities BA and BB are measured at points A (x = 0, y = 10cm, z = 0) and B (x = 0, y = 20cm, z = 0) a) In which direction do BA and BB act? b) If I = 1·5A, calculate the magnitude of BA. c) What value of current would produce a flux density at point B equal to the value you have just calculated for BA? d) Find the magnitudes and directions of the flux densities BC and BD at the points C and DC has co-ordinates x = 4cm, y = 4cm, z = 0cmD has co-ordinates x = 4cm, y = 4cm, z = 10cm
Question 6
 A piece of wire is 8cm long and is placed at an angle of 30° to a magnetic field of flux density 0·5T. Calculate the magnitude of the force acting on the wire when a current of 500mA flows through it.Name the rule used to predict the direction of the force on the wire.
Question 7
 a) Explain why a moving coil galvanometer must have control springs (or something equivalent to control springs). b) How is a moving coil galvanometer made to have a high current sensitivity? c) A moving coil galvanometer could be made without a soft iron cylinder. What would be the main disadvantage of a meter without a soft iron cylinder?
Question 8
 Two straight wires are parallel to each other and 5cm apart. Wire A-B has a current of I1 = 5A flowing through it and wire C-D has a current of I2 = 20A flowing through it (see diagram below). What is the magnitude and direction of the electro-magnetic force acting on wire A-B? 