Magnetic Fields

1. Magnetic Fields SACE Stage 2 Physics

2. N S Strong field Weak field Pictorial Representation of Magnetic Fields Magnetic fields can be seen by sprinkling iron filings around a permanent magnet. The direction of the magnetic field is the direction of the force on the north pole of a compass needle. (i.e. a bar magnet) placed in the field.

3. N S N N Neutral Point (Zero field) Pictorial Representation of Magnetic Fields Different arrangements of permanent bar magnets produce the following fields. (Magnetic field is also inside the bar magnet. Magnetic field lines form closed loops)

4. North Pole North Geomagnetic Pole S N South Geomagnetic Pole South Pole The Earth’s Magnetic Field

5. I Top View Field Due To A Current Carrying Wire The magnetic field surrounding a current carrying wire are found to be concentric circles. the circles are drawn further apart as the distance from the wire increases because the magnetic field is becoming weaker. Side View Current (I) is directed out of the page.

6. Lines of magnetic force Field Due To A Current Carrying Wire Deflection of a compass needle near a current carrying wire showing the presence and direction of a magnetic field. Right hand curl rule showing the direction of the magnetic field. N S N S

7. Field Due To A Current Carrying Loop of Wire and a Coil Single loop of current carrying wire.

8. Field Due To A Current Carrying Loop of Wire and a Coil A loosely wound coil of wire.

9. X X X X X X X X X X Field Due To A Current Carrying Loop of Wire and a Coil For a tightly wound solenoid. Use the right hand curl rule to find the direction of the magnetic field A solenoid of finite length. The right end from which lines of B emerge, behaves like the north pole of a compass needle. The left end behaves like the South end. I

10. Notes on Magnetic Fields (B) 1. The cause of all magnetic fields is electric currents. (ie moving electric charge(s)). Static charges do not cause magnetism. Permanent magnets result from electron currents in the magnet.

11. N N S N S N S Notes on Magnetic Fields (B) 2. magnetic fields result from closed loops. [Electric fields begin and end on electric charges] A consequence of this is that it is impossible to isolate a magnetic pole. North-South poles always occur in pairs. Eg. Breaking a permanent bar magnet Creates two magnets with North and South poles being created at the breakage point

12. I (Electric Current) S N F The Motor Effect Investigations show that magnetic fields not only are caused by electric currents, but exert forces on them as well.

13. S N Current In Force The Motor Effect Right Hand Palm Rule: Fingers - Field Thumb - Current Palm - Force

14. F Position 1 Position 2 no force  S N   B B F F Position 3 Position 4 no force N  S S S N    N B B F The Motor Effect

15. Magnetic Field Strength Defined as the force per unit current element. Current (I) x length (l) is known as a current element. Units Hence units are NA-1m-1 or Tesla (T). Earth’s magnetic field strength = 50 T and a bar magnet = 100 mT

16. Magnetic Field Strength Re-arranging Yields, Only applies when field is at 90o to current otherwise use,

17. Small current element = Ix Dl NO force on these wires parallel to the magnetic field lines Force perpendicular to and directed out of the plane of the page Magnetic Field Strength Consider the section of current carrying wire under force of 10N (out of page). I=1Amp L=10cm By definition, Direction can be found using the right hand curl rule.

18. Direction Dependence on Force 30o

19. Direction Dependence on Force

20. Example Calculate the magnitude of the force on a current carrying wire in a magnetic field of B = 0.1 T, length of Dl = 10 cm and passing a currentof I = 5.0 A. (The length of wire is at 90o to the magnetic field.)

21. Example Calculate the magnitude of the force on a current carrying wire in a magnetic field of B = 0.1 T, length of Dl = 10 cm and passing a currentof I = 5.0 A. (The length of wire is at 90o to the magnetic field.)

22. Moving Coil Loud Speaker • Has three parts, • A large cylindrical permanent magnet at the rear of the speaker. The outside of the magnet is of one polarity while the part of the magnet that is inside is of the opposite polarity. • Wrapped around the central pole is a coil of wire, suspended in such a way that it is free to move back and forth along this central pole piece. The coil is attached to the cone. • The cone of the loudspeaker, which is attatched to the moving coil. As the coil vibrates back and forth it pulls and pushes the cone with it.This vibration generates the sound.

23. Moving Coil Loud Speaker Consider an alternating potential difference, as depicted in the graph below, Lets assume that the current is going into the page on the top half of the coil and out of the page in the lower half of the coil. Due to the right hand rule there will be a net force to the right on the coil which, inturn moves the cone to the right. Reversing the current in the coil will change the direction of the force causing the cone to move to the left.

24. Moving Coil Loud Speaker If there is an alternating current of 300Hz in the coil, then the speaker will vibrate in and out of the magnet 300 times every second and the sound heard will be at 300Hz. The movement of the cone represents the current in the coil.