Ampere`s law
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Transcript Ampere`s law
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 = (2pr)/(rqB/m) =
t = [(2)(p)(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 DL and B around
the entire path is equal to m0I
We can express Ampere’s law as:
S BDL = m0I
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 2pr = m0I
Magnetic field a distance r from a long straight wire with
current I
Today’s PAL
I
I
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?
Force on Two Parallel Wires
The B field then will exert a force on the
other wire
F = BIL = m0IIL/2pd
F = m0I1I2L/2pd
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 = m0nI
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