Currents and Magnetism Textbook Sections 22-4 – 22-7 Physics 1161: PreLecture 13.
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Transcript of Currents and Magnetism Textbook Sections 22-4 – 22-7 Physics 1161: PreLecture 13.
Force of B-field on Current
+ v
• Force on 1 moving charge:– F = q v B sin()– Out of the page (RHR)
• Force on many moving charges:– F = (q/t)(vt)B sin()
= I L B sin()
– Out of the page!
v
L = vt
B
I = q/t+ + ++
Net force on loop is zero.
Look from here
But the net torque is not!
Torque on Current Loop in B field
A B
CD
B
I
XF
•F
A B
CD
F
F
The loop will spin in place!
Torque on loop is = 2 x (L/2) F sin() = ILWB sin()
Force on sections B-C and A-D: F = IBW
(length x width = area) LW = A !
Torque is = I A B sin()
W
L
A B
CD
B
I
XF
•F
Torque on Current Loop in B field
A B
CD
F
F
L/2 L/2
Torque tries to line up the normal with B!
(when normal lines up with B, =0, so =0! )
Even if the loop is not rectangular, as long as it is flat:
= I A B sin
(area of loop)
Magnitude:
= I A B sin
Direction:
N
# of loops
A
B
C
DB
normal
F
F
Torque on Current Loop
between normal and B
Currents Create B Fields
Lines of B
Here’s a current-carrying wire.
Current I OUT of page.
•
Right-Hand Rule, 2
Thumb: along IFingers: curl along B-field lines
r = distance from wire
0 410 7Tm/A
B
0I2r
r
Magnitude of B a distance r from (straight) wire:
B
Force between current-carrying wires
•I towards us
B•
Another I towards usF
Conclusion: Currents in same direction attract!
•
I towards us B
Another I away from us
F
Conclusion: Currents in opposite direction repel!
Note: this is different from the Coulomb force between like or unlike charges.
Comparison:Electric Field vs. Magnetic Field
Electric MagneticSource Charges Moving ChargesActs on Charges Moving ChargesForce F = Eq F = q v B sin()Direction Parallel E Perpendicular to v,B
Field Lines
Opposites Charges Attract Currents Repel
B Field Inside SolenoidsMagnitude of Field anywhere inside of solenoid : B=0 n I
Right-Hand Rule 3 gives Direction:
Fingers – curl around solenoid in direction of I
Thumb - points in direction B ==
n is the number of turns of wire/meter on solenoid.
= 4 x10-7 T m /A
(Note: N is the total number of turns, n = N / L)
Magnetic field lines look like bar magnet!
Solenoid has N and S poles!
B Field Inside SolenoidsMagnitude of Field anywhere inside of solenoid : B=0 n I
Right-Hand Rule for loop/solenoid
Fingers – curl around coil in direction of conventional (+) current
Thumb - points in direction of B along axis
n is the number of turns of wire/meter on solenoid.
= 4 x10-7 T m /A
(Note: N is the total number of turns, n = N / L)
Magnetic field lines look like bar magnet!
Solenoid has N and S poles!