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# Ampere’s Law Physics 102 Professor Lee Carkner Lecture 19.

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Ampere’s Law Physics 102 Professor Lee Carkner Lecture 19

PAL #18 Magnetic Force  How long would it take an electron to complete one circular orbit around a 1 G magnetic field?   Distance around circle is circumference:  Can find velocity from equation:  v = rqB/m  t = (2  r)/(rqB/m) =  t = [(2)(  )(9.1X10 -31 )]/[(1.6X10 -19 )(1X10 -4 )]  t =

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   We can express Ampere’s law as:  B  L =  0 I

B Field for a Wire  Use Ampere’s law for a circle of radius r around the wire   Since B is the same everywhere around the circle  B 2  r =  0 I  Magnetic field a distance r from a long straight wire with current I

Today’s PAL  Consider two parallel metal rails 1 cm apart and each carrying 100000 A of current in opposite directions  What is the direction and magnitude of B at a point halfway in-between them?  What is the direction and magnitude of the force on a conductor also carrying 100000 A that connects the two rails? I I

Force on Two Parallel Wires   The B field then will exert a force on the other wire F = BIL =  0 IIL/2  d  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  Exam #2  Same format as Exam #1  Covers chapters 16-20  Practice problems posted on WebAssign  Will not count for grade  Reading and homework for Monday on webpage

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