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: d = 2 r Can find velocity from equation: r = mv/qB t = (2 r)/(rqB/m) = 2 m/qB t = [(2)( )(9.1X10 -31 )]/[(1.6X10 -19 )(1X10 -4 )] t =
Consider a charged particle in a circular orbit in a magnetic field. If the charge on the particle is doubled, what happens to the radius of the orbit? A)¼ the original B)½ the original C)the radius stays the same D)2 times the original E)4 times the original
The force on a current-carrying wire in a magnetic field is strongest when, A)the current is parallel to the field lines B)the current is at a 30 degree angle to the field lines C)the current is at a 45 degree angle to the field lines D)the current is at a 60 degree angle to the field lines E)the current is perpendicular to the field lines
Consider a vector that stands straight out from the face of a loop of wire that carries a current. The magnetic torque on the loop will be greatest when, A)the vector is aligned with the magnetic field B)the vector is at a 30 degree angle to the magnetic field C)the vector is at a 45 degree angle to the magnetic field D)the vector is at a 60 degree angle to the magnetic field E)the vector is perpendicular to the magnetic field
Currents and Magnetism We saw how magnetic fields produce a force on a moving charged particle A single moving particle produces only a small, fleeting field 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 10 -7 T m /A and is called the permeability of free space B L = 0 I
B Field for a Wire Use Ampere’s law for a circle of radius r around the wire B L = 0 I or B L = 0 I However, L around the whole circle is equal to the circumference = 2 r, so: B = 0 I/2 r 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 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 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