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Physics 1161: Lecture 9, Slide 1
Currents and Magnetism
• Today’s lecture will cover Textbook
Sections 22-4 – 22-
Physics 1161: Lecture 09 Exam
II
Physics 1161: Lecture 9, Slide 2
Force of B-field on Current
• Force on 1 moving charge:
- F = q v B sin(θ)
- Out of the page (RHR)
• Force on many moving charges:
- F = (q/t)(vt)B sin(θ) = I L B sin(θ)
– Out of the page!
v
L = vt
B
++ + + I = q/t
Physics 1161: Lecture 9, Slide 3
A B
D C
B
I
force is zero out of the page into the page
Preflight 9.1, 9.
A rectangular loop of wire is carrying current as shown. There is a uniform magnetic field parallel to the sides A-B and C-D.
What is the direction of the force on section A-B of the wire?
What is the direction of the force on section B-C of the wire?
force is zero out of the page into the page
Physics 1161: Lecture 9, Slide 4
Net force on loop is _______.
Look from here
The net torque is __________!
Torque on Current Loop in B field
A B
D C
B
I
X
F
F
A (^) B
D C
F
F
The loop will ___________
Preflights 9.3, 9.
Physics 1161: Lecture 9, Slide 5
Torque on loop is ττττ = 2 x (L/2) F sin(φφφφ) =
Force on sections B-C and A-D: F =
(length x width = area) LW = A!
∴ Torque is ττττ =
W
L
A B
D C
B
I
X
F
F
Torque on Current Loop in B field
A (^) B
D C
F
F
φ
Physics 1161: Lecture 9, Slide 6
Torque tries to line up the normal with B! (when normal lines up with B, φφφφ=0, so ττττ=0! )
Even if the loop is not rectangular, as long as it is flat:
ττττ = I A B sinφφφφ....
(area of loop)
Magnitude:
ττττ = I A B sinφφφφ
Direction:
N
of loops
A
B
C
D B
normal
φ
F
F
Torque on Current Loop
between normal and B
Physics 1161: Lecture 9, Slide 10
Two long wires carry opposite current
What is the direction of the magnetic field above, and midway between the two wires carrying current – at the point marked “X”?
x
ACT: Adding Magnetic Fields
- Left 2) Right 3) Up 4) Down 5) Zero
x
Physics 1161: Lecture 9, Slide 11
Force between current-carrying wires
I towards us
B
Conclusion: Currents in same direction ____________!
I towards us B ×××× Another I away from us
Conclusion: Currents in opposite direction ____________!
Note: this is different from the Coulomb force between like or unlike charges.
Physics 1161: Lecture 9, Slide 12
Comparison:
Electric Field vs. Magnetic Field
Electric Magnetic
Source Charges Moving Charges
Acts on Charges Moving Charges
Force F = Eq F = q v B sin(θ)
Direction Parallel E Perpendicular to v,B
Field Lines
Opposites Charges Attract Currents Repel
Physics 1161: Lecture 9, Slide 13
B Field Inside Solenoids
Magnitude of Field anywhere inside of solenoid : B=μμμμ 0 n I
Right-Hand Rule gives Direction: Thumb - along I Fingers – curl into interior of solenoid Palm – gives B
n is the number of turns of wire/meter on solenoid. μμμμ 0000 = 4ππππ 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!
Physics 1161: Lecture 9, Slide 14
What is the direction of the magnetic field produced by these solenoids?
Preflight 9.
(1) to the Right (2) to the Left
ACT: B Field Inside Solenoids
What is the net force between the two solenoids?
(1) Attractive (2) Zero (3) Repulsive
Physics 1161: Lecture 9, Slide 15
See you Monday!
• Read Ch. 22-4 – 22-
• Happy Birthday Abraham Lincoln!
