Magnetic Fields 4.0 / 5 PhysicsMagnetic fieldsA2/A-levelAQA Created by: LZ95Created on: 07-04-14 15:04 What is a line of force in a magnetic field? It is a line along which a free North Pole would move in the field. 1 of 26 What is the motor effect? When a force is experienced due to a magnetic field on a current-carrying wire that is placed at a non-zero angle to the lines of force of the field. 2 of 26 When is the magnetic force the greatest? When the wire is at right angles to the magnetic field. 3 of 26 When is the magnetic force zero? When the wire is parallel to the magnetic field. 4 of 26 What is the magnetic force proportional to? The current (I) and the length of the wire (L) 5 of 26 What is the unit for magnetic flux density (B)? Tesla (T) 6 of 26 How would you find the direction of the magnetic force? Using Fleming's left hand rule. 7 of 26 When there is a rectangular coil in a magnetic field, what is experienced by each long side if they are vertical? A horizontal force F=BILn where n = number of turns of wire 8 of 26 What is significant about this force? It acts in opposite directions at right angles to the field lines. 9 of 26 What do the pair of forces act as? A couple 10 of 26 What is the torque of the couple? Torque = Fd and as d=wcos(x) the torque = Fwcos(x) which can be written as BIAcos(x) where A is the coil area (Lw) 11 of 26 What happens when (x) is 90 degrees? The torque = 0 as cos(90) = 0 12 of 26 In which direction does current flow? Opposite to the direction of the electrons 13 of 26 When is there no magnetic force acting on a particle? When it moves parallel to field lines and when it is stationary. 14 of 26 Why is there no work done by the magnetic field on a charged particle? The force always acts at right angles to the direction of the particle and as W = Fscos(x), no work would be done as cos(90) = 0 15 of 26 What does the magnetic field do to each of the following: direction of motion, speed and kinetic energy. Direction of motion = changed by the magnetic force, speed = unchanged, kinetic energy = unchanged 16 of 26 Why does a charged particle move in a circular path when a magnetic force acts on it? The magnetic force acts in a perpendicular direction to the velocity of the charged particle. This causes centripetal acceleration. 17 of 26 If r = the radius of the circular orbit of a charged particle, what would cause 'r' to decrease? Increase in magnetic flux density (B), decrease in velocity (v) and a larger specific charge (Q/m) 18 of 26 In which direction is the magnetic field applied in a cyclotron? Perpendicular to the plane of the dees. 19 of 26 What is applied between the dees? A high-frequency alternating voltage. 20 of 26 A proton is directed into a dee and it emerges due to the magnetic field. What happens when the proton emerges from one dee? The alternating voltage reverses so the proton is accelerated into the other dee. 21 of 26 When does the proton exit the cyclotron? When the radius of its path is the same as the radius of one dee, 22 of 26 Why does the time taken to move round a semi-circular path not depend on particle speed. r = mv/BQ, the time taken to complete one semi circle = (pi)r/v which can be written as m(pi)/BQ, so speed does not have an effect 23 of 26 What happens to atoms in a mass spectrometer? They are ionised. 24 of 26 The ions are directed in a narrow beam at the same velocity. How do the conductors of the experiment produce ions with the same velocity? They pass the ions through a velocity selector. 25 of 26 Which ions are undeflected by the velocity selector? Ones that experience equal and opposite forces due to the electric and magnetic force so as BQv=QV/d their speed will be v=V/Bd 26 of 26

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