Current in a Magnetic Field – Learning Outcomes 1

Force on a Current-Carrying Conductor  Recall a current-carrying conductor has a magnetic field around it.  This field will interact with other nearby magnetic fields.  This results in a force on the conductor.  The size and direction of the force depends on the nature of the two fields.  The force is always:  perpendicular to the current,  perpendicular to the magnetic field. 2

To Demonstrate the Force on a Current- Carrying Conductor in a Magnetic Field 1.Set up a strip of tin foil between the north and south poles of a magnet (or two magnets). 2.Note no deflection on the foil. 3.Pass a current through the foil. 4.The foil will move towards one of the poles, depending on which way the current flows. 5.The foil must be experiencing a force due to the external magnetic field. 3

Uses of this Force  This force is the basis for a number of electronic devices:  Electric motor  Moving coil loudspeaker  Various moving coil meters (voltmeter, ammeter etc.)  The jist is these devices respond to the size of the current being passed through them.  More detail on these in the applied electricity option (we probably won’t do this). 4

Fleming’s Left-Hand Rule  If the thumb, index finger, and middle finger of the left hand are held at right angles to each other; with the index finger pointing in the direction of the magnetic field and middle finger pointing in the direction of the current, the thumb will point in the direction of the force. 5

Fleming’s Left-Hand Rule  e.g. the diagram shows a current-carrying wire in a magnetic field. In which direction is the force on the wire? 6

Magnetic Flux Density 7

 The unit of magnetic flux density is the tesla, T.  The magnetic flux density at a point is 1 tesla if a conductor of length 1 m carrying a current of 1 A experiences a force of 1 N when placed perpendicular to the field. 8

Force on a Current-Carrying Conductor 9

 e.g. A straight piece of wire of length 3 m carrying a current of 2 A experiences a force of 12 N when placed perpendicular to a uniform magnetic field. Calculate the value of the magnetic flux density.  e.g. A straight piece of wire carrying a current of 4 A is placed at an angle of 30 o to a magnetic field of flux density 2 T. The wire is 2 m long. What is the magnitude and direction of the force on the wire? 10

Force on a Current-Carrying Conductor  e.g. The loop shown is free to rotate about its axis. i.Find the magnitude and direction of the force acting on each part of the loop. ii.Find the moment of the force about the axis. iii.What happens to the moment as the loop rotates? 11

D.C. Motors  Consider a loop that is free to rotate in a magnetic field.  What is the direction of force on each part of the loop?  What is the effect on the loop? 12

D.C. Motors 13

D.C. Motors 14

Force Between Wires  The force experienced by these wires is: A.both left B.both right C.attractive D.repulsive 15

Force Between Wires  The force experienced by these wires is: A.both left B.both right C.attractive D.repulsive 16

The Ampere 17

Force on a Charge in a Magnetic Field  Just as current-carrying wires generate a magnetic field and are affected by external fields, charges moving without a wire will also exhibit this.  e.g. cathode rays, thermionic emissions, photoelectric emissions, ions in electrolyte solutions.  We will look at each of these devices in turn in other sections of the course. 18

Force on a Charge in a Magnetic Field  What direction is the force on this stream of electrons? 19

Force on a Charge in a Magnetic Field 20

Force on a Charge in a Magnetic Field 21

Force on a Charge in a Magnetic Field  Consider an electron moving with velocity v in a magnetic field.  In what direction is the force on the electron? 22

Force on Charge in a Magnetic Field 23