1. Lecture 19: MON 02 MAR Magnetic fields Ch.28.6-7 Physics 2102
Jonathan Dowling
Aurora Borealis
Lecture 19: MON 02 MAR
Magnetic fields Ch
“I’ll be back….

2. Second Exam Review:
6-7PM WED 04 MAR
Nicholson 130
Second Exam (Chapters 24–28):
6–7PM THU 05 MAR
Lockett 6

3. Circular Motion: v F r B into blackboard.
Since magnetic force is perpendicular to motion, the movement of charges is circular.
B into blackboard. v F r
In general, path is
a helix (component of v parallel to field is unchanged).

4. . . Radius of Circlcular Orbit Angular Frequency: Independent of v
electron
Radius of Circlcular Orbit . C r
Angular Frequency:
Independent of v
Period of Orbit:
Independent of v
Orbital Frequency:
Independent of v

5. Example
Two charged ions A and B traveling with a constant velocity v enter a box in which there is a uniform magnetic field directed out of the page. The subsequent paths are as shown. What can you conclude? v A B
(a) Both ions are negatively charged.
(b) Ion A has a larger mass than B.
(c) Ion A has a larger charge than B.
(d) None of the above.
Same charge q, speed v, and same B for both masses.
So: ion with larger mass/charge ratio (m/q) moves in circle of larger radius. But that’s all we know! Don’t know m or q separately.

6. Examples of Circular Motion in Magnetic Fields
Aurora borealis
(northern lights)
Synchrotron
Fermilab,
Batavia, IL (1km)
Suppose you wish to accelerate charged
particles as fast as you can.
Linear accelerator (long).

7. Magnetic Force on a Wire.L
Note: If wire is not straight,
compute force on differential elements and integrate:

8. 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,
and this would be true no
matter what the shape of
the central segment!. L R

9. 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!

10. 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:

11. 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. Destroy NYC in about 10 minutes.