Ampere’s Law Physics 102 Professor Lee Carkner Lecture 18

Currents and Magnetism   It is also true that moving charged particles produce magnetic fields   Serious magnetic fields are produced by currents  What is the magnitude and direction of these fields?

Magnetic Field from a Current in a Wire   Needle deflected tangentially to the wire cross section   How can we find the direction and magnitude of the B field for any situation?

Right Hand Rule Revisited  Grasp the wire with your thumb in the direction of the current and your curled fingers indicate the direction of the field 

Ampere’s Law  To find the magnitude of the B field we use Ampere’s law   The sum of the product of  L and B around the entire path is equal to  0 I  Where  0 = 4  X T m /A and is called the permeability of free space   B  L =  0 I

B Field for a Wire   B  L =  0 I or B   L =  0 I  Since B is the same everywhere around the circle  B 2  r =  0 I B =  0 I/2  r  Magnetic field a distance r from a long straight wire with current I

Force on Two Parallel Wires   The B field then will exert a force on the other wire B =  0 I/2  d  For two wires of equal length but different currents: F =  0 I 1 I 2 L/2  d

Magnetic Field: Loop   Can apply the right hand rule all the way around   Loop acts as a bar magnet

Magnetic Field: Solenoid  What happens if you stack several loops up?   You produce a solenoid   Field inside the solenoid is strong and uniform (far from the ends)

Electromagnet  We can write an expression for the solenoid magnetic field: B =  0 nI   If you put a piece of iron in the center you get an electromagnet 

Next Time  Read:  Homework: CH 21 P: 1, 2 CH 20 P: 38, 50