moving electric charges ---- such as, current!
TRANSCRIPT
What creates magnetic fields?
What creates magnetic fields?
Moving electric charges ---- such as, current!
Currents Create B fields - Ampere’s Law
r = distance from wire
m0 = magnetic constant
B
0I2r
Magnitude:
Lines of B
rB Current I OUT
Direction: RHR 2
Thumb in direction of current, fingers curl around current indicating direction of magnetic field
B decreases as 1r
m0 = 4p 10-7 Tm/A
x x x x
x x x x
x x x x
x x x x
x x x x
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I
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.
1. What is the direction of the magnetic field at point A?
a) Into the pageb) Out of the pagec) Upd) down
2. What is the direction of the magnetic field at point B?
a) Into the pageb) Out of the pagec) Upd) down
3. What is the shape of the magnetic field lines?
a) circles b) spirals c) radially outward
I
A
B
4. Where is the magnetic field stronger?
a) point Ab) point Bc) it’s the same at A and B
v
•B
B
0I2r
same
v and B are normal in both cases: sinθ= 1
A long straight wire is carrying current from left to right. Near the wire is a charge q (-) with velocity v
I
va)
r
q•
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)
q•
r
b)
FF
F has different directions
same F = qvB
Two long wires carry opposite currents I
What is the direction of the magnetic field above, and midway between the two wires carrying current?
●
Adding Magnetic Fields
1) Left 2) Right 3) Up 4) Down 5) Zero
B
xI I
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
B
another I up
F
Conclusion: Currents in same direction attract!
xF
I up
B
another I down
FConclusion: Currents in opposite direction repel!
xF
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
Let’s try another …
X
B
B
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?
1) Left 2) Right 3) Up
4) Down 5) NoneZero
I I I
What is the direction of the force on the left wire, due to the two
others?
I I I
1) Left 2) Right 3) Up
4) Down 5) None 5) Zero
What is the direction of the force on the middle wire, due to the two
others?
1) Left 2) Right 3) Up
4) Down 5) None 5) Zero
I 2I 3I
What is the direction of the force on the middle wire, due to the two
others?
I I I
1) Left 2) Right 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?
1) Left 2) Right 3) Up
4) Down 5) None) Zero
I P
I
What is the direction of the force on the left, due to the two others?
I I I
1) Left 2) Right 3) Up
4) Down 5) None 5) Zero
X
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• What is the equation for induced magnetic field?
• Opposite currents … repel• Currents in the same direction ... attract
B
0I2r
SolenoidsA solenoid consists of several current loops stacked together.
Used to create a strong, uniform magnetic field-- electromagnets-- starter for car engine
B=μ0nIn = number of windings per unit length,I = current in windings B 0 outside windings