1. Lecture 25: WED 21 OCT Magnetic Fields III
Physics 2113
Jonathan Dowling
Aurora Borealis
Lecture 25: WED 21 OCT Magnetic Fields III
“They are not supposed to exist….”

2. Magnetic Force on a Wire.L

3. Magnetic Force on a WireL
Note: If wire is not straight,
compute force on differential elements and integrate: i

4. . i
(28-12)

6. A portion of a loop of wire passes between the poles of a magnet as shown. We are viewing the circuit from above. When the switch is closed and a current passes through the circuit, what is the movement, if any, of the wire between the poles of the magnet?
a) The wire moves toward
the north pole of the magnet.
b) The wire moves toward the
south pole of the magnet.
c) The wire moves upward
(toward us).
d) The wire moves downward (away from us into board).
e) The wire doesn’t move. i

7. Example Wire with current i. Magnetic field out of page.
What is net force on wire?
By symmetry, F2 will only
have a vertical component,
Notice that the force is the same as that for a straight wire of length R,
and this would be true no
matter what the shape of
the central segment!. L R

8. i

9. SP28-06 i
ICPP:
Find Direction of B

10. Example 4: The Rail Gun
rails
Conducting projectile of length 2cm, mass 10g carries constant current 100A between two rails.
Magnetic field B = 100T points outward.
Assuming the projectile starts from rest at t = 0, what is its speed after a time t = 1s? B I L
projectile
Force on projectile: F= iLB (from F = iL x B)
Acceleration: a = F/m = iLB/m (from F = ma)
v = at = iLBt/m (from v = v0 + at)
= (100A)(0.02m)(100T)(1s)/(0.01kg) = 2000m/s
= 4,473mph = MACH 8!

11. Rail guns in the “Eraser” movie
"Rail guns are hyper-velocity weapons that shoot aluminum or clay rounds at just below the speed of light. In our film, we've taken existing stealth technology one step further and given them an X-ray scope sighting system," notes director Russell. "These guns represent a whole new technology in weaponry that is still in its infancy, though a large-scale version exists in limited numbers on
battleships and tanks. They have incredible range. They can pierce three-foot thick cement walls and then knock a canary off a tin can with absolute accuracy. In our film, one contractor has finally developed an assault-sized rail gun. We researched this quite a bit, and the technology is really just around the corner, which is one of the exciting parts of the story."
Warner Bros., production notes, 1996.
Also: INSULTINGLY STUPID MOVIE PHYSICS:

12. Rail guns in Transformers II

14. Rail guns in Reality!

17. Electromagnetic Slingshot
These Devices Can
Launch 1000kg Projectiles
At Mach 100 at a Rate of
1000 Projectiles Per Second.
Using KE = 1/2mv2
This corresponds to an output about 1012 Watts = TeraWatt.
Uses: Put Supplies on Mars 80 days after launch.
Current time to Mars on Ordinary Rocket? years.

18. Torque on a Current Loop:
Principle behind electric motors.
Rectangular coil: A=ab, current = i
Net force on current loop = 0
But: Net torque is NOT zero!
For a coil with N turns,
τ = N I A B sinθ,
where A is the area of coil

19. Magnetic Dipole Moment
We just showed: τ = NiABsinθ
Right hand rule:
curl fingers in direction of current;
thumb points along μ
Define: magnetic dipole moment m
N = number of turns in coil
A = area of coil.
As in the case of electric dipoles, magnetic dipoles tend to align with the magnetic field.

20. Electric vs. Magnetic Dipoles-Q q QE +Q
p=Qa

21. Magnetic Dipole Moment
We just showed: t = NiABsinq
Right hand rule:
curl fingers in direction of current;
thumb points along μ
Define: magnetic dipole moment m
N = number of turns in coil
A=area of coil.
As in the case of electric dipoles, magnetic dipoles tend to align with the magnetic field.

22. 1 and 3 are “downhill”.
2 and 4 are “uphill”.
U1 = U4 > U2 = U3
τ is biggest when B is at right angles to μ

24. ICPP