Download What creates magnetic fields?

Moving electric charges ---- such as, current!
Currents Create B fields - Ampere’s Law
Magnitude:
m0I
B
2r
Direction: RHR 2
Thumb in direction
of current, fingers
curl around current
indicating direction
of magnetic field
r = distance from wire
m0 = magnetic constant
m 0 = 4
 10-7 Tm/A
B
1
B decreases as r
r
Current I OUT
Lines of B
When indicating direction of B
by crosses and dots we always
draw it like this.
Note that the magnetic field
lines form circles around the
wire.
Circles close to the wire have
a stronger filed than those far
away.
I
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x
x
x
x
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x
x
x
x
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x
x
x
x
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x
x
x
x
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x
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x
x
1. What is the direction of the
magnetic field at point A?
a)
b)
c)
d)
Into the page
Out of the page
Up
down
A
2. What is the direction of the
magnetic field at point B?
a)
b)
c)
d)
Into the page
Out of the page
Up
down
3. What is the shape of the
magnetic field lines?
a) circles
b) spirals
c) radially outward
I
B
4. Where is the magnetic field
stronger?
a) point A
b) point B
c) it’s the same at A and B
q
v
•
a)
F
B•
v
q
•
b)
r
F
r
I
A long straight wire is carrying current from left to right. Near
the wire is a charge q (-) with velocity v
Compare magnetic force on q in (a) vs. (b)
a) has the larger force
b) has the larger force
c) force is the same for (a) and (b)
same B 
m0I
2r
v and B are normal in both cases: sinθ= 1
same
F = qvB
F has different directions
Adding Magnetic Fields
Two long wires carry opposite currents I
B
●
I
x
I
What is the direction of the magnetic field above, and midway
between the two wires carrying current?
1) Left 2) Right
3) Up
4) Down 5) Zero
Force between wires carrying current
Current-carrying wires create magnetic fields
Magnetic fields exert a force on current-carrying wires
Current carrying wires exert forces on each other!
Force between wires carrying current
I up
another I up
F
B F
x
B
First, use RHR #2 to find the
direction of the magnetic field on
each wire.
Then, use RHR #1 to find the force
on each wire
Conclusion: Currents in
same direction attract!
Let’s try another …
another I down
I up
X
F
B
Bx
F
Conclusion: Currents in
opposite direction repel!
What is the direction of the force on the top
wire, due to the two below?
1) Left 2) Right
3) Up
4) Down 5) Zero
What is the direction of the force
on the middle wire, due to the two
others?
I
1) Left
I
2) Right
I
3) Up
4) Down 5) NoneZero
What is the direction of the force
on the left wire, due to the two
others?
I
1) Left
I
2) Right
I
3) Up
4) Down 5) None 5) Zero
What is the direction of the force
on the middle wire, due to the two
others?
I
1) Left
2I
2) Right
3I
3) Up
4) Down 5) None 5)
Zero
What is the direction of the force
on the middle wire, due to the two
others?
I
1) Left
I
2) Right
I
3) Up
4) Down 5) None5) Zero
What is the direction of the
magnetic field on a point P in the
middle of two wires?
I
I
P
What is the direction of the force
on the left, due to the two others?
I
I
I
X
1) Left
2) Right
3) Up
4) Down 5) None) Zero
1) Left
2) Right
3) Up
4) Down 5) None 5) Zero
Quick Review
• How do you determine the direction of a magnetic field
induced by a current?
• How does magnetic field vary with distance from a wire?
• What is the equation for induced magnetic field?
• Opposite currents …
• Currents in the same direction ...
Quick Review
• How do you determine the direction of a magnetic field
induced by a current? RHR #2 – Thumb goes in direction
of current, fingers curve in direction of mag. field
• How does magnetic field vary with distance from a wire?
Magnetic field decreases with distance
m0I
• What is the equation for induced magnetic field?
B
2r
• Opposite currents … repel
• Currents in the same direction ... attract
Solenoids
A solenoid consists of
several current loops
stacked together.
Used to create a strong,
uniform magnetic field
-- electromagnets
-- starter for car engine
B=μ0nI
n = number of windings
per unit length,
I = current in windings
B  0 outside windings