1. Exam 2 is Tuesday Oct. 27 5:30-7 pm, Birge 145
Students w / scheduled academic conflict please stay after class TODAY to arrange alternate time.
Covers: all material since exam 2 Book sections: Chap 27, 29, 30 (not 30.7), 31, ,
Bring: Calculator
One (double-sided) 8 1/2 x 11 note sheet
Exam review: Thursday, Oct. 22, in class
Tue. Oct. 20, 2009
Physics 208 Lecture 14

2. Last time… RC circuits Magnetic fields Magnetic forces
Tue. Oct. 20, 2009
Physics 208 Lecture 14

3. Magnetic force on electric charges
Effect of uniform B-field on charged particle
Charged particle not moving
no effect
Charged particle is moving:
Force is perpendicular to
Magnetic Field
Particle Velocity
Depends on sign of charge
Tue. Oct. 20, 2009
Physics 208 Lecture 14

4. Magnetic force on charged particle
Magnitude of force is proportional to
Charge of particle, q
Speed of particle, v
Strength of magnet field, B
sin(), =angle between and
Direction of force is perpendicular to both and
vector ‘cross product’
Tue. Oct. 20, 2009
Physics 208 Lecture 14

5. FB on a Charge Moving in a Magnetic Field, Formula
FB = q v x B
FB is the magnetic force
q is the charge
v is the velocity of the moving charge
B is the magnetic field
SI unit of magnetic field: tesla (T)
CGS unit: gauss (G): 1 T = 104 G (Earth surface 0.5 G)
Tue. Oct. 20, 2009
Physics 208 Lecture 14

6. Quick Quiz
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
Which particle experiences the greatest magnetic force? B 1 2 3 1 2 3
All same
F = q v B sin(q)
Tue. Oct. 20, 2009
Physics 208 Lecture 14

7. Quick Quiz
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
The force on all the particles is in the same direction. B 1 2 3
True
False
All forces are into page
Magnitude F = q v B sin(q)
Tue. Oct. 20, 2009
Physics 208 Lecture 14

8. Force on moving charged particle
intermediate
Tue. Oct. 20, 2009
Physics 208 Lecture 14

9. Field direction notation
Field direction indicated by vector
In-plane : draw vector
Not in-plane :
into-page
out-of-page
Tue. Oct. 20, 2009
Physics 208 Lecture 14

10. More force examples Force from B-field on moving charged particle
x x x x x x v B q
® ® ® ® ® v B q
­ ­ ­ ­ ­ ­ ­ ­ v B q F F
F = 0
Units: 1 T (tesla) = 1 N / Am
1G (gauss) = 10-4 T
Tue. Oct. 20, 2009
Physics 208 Lecture 14

11. Magnetic Force on a Current
Force on each charge
Force on length of wire
Force on straight section of wire, length L N
Current
Magnetic force S
Magnetic field
Tue. Oct. 20, 2009
Physics 208 Lecture 14

12. ‘Homopolar’ motor Why does the motor turn?
Electric Coulomb force pushes on magnet
Magnetic field pushes on current from battery
Current from battery heats the magnet
Current through wire produces magnetic field
Tue. Oct. 20, 2009
Physics 208 Lecture 14

13. Electric current produces magnetic field
Current (flow of electric charges ) in wire produces magnetic field.
That magnetic field aligns compass needle
Current
Magnetic field
Tue. Oct. 20, 2009
Physics 208 Lecture 14

14. Law of Biot-Savart
B out of page
Each element of current produces a contribution to the magnetic field. r  I ds dI dB r
= permeability of free space
r = distance from current element
Tue. Oct. 20, 2009
Physics 208 Lecture 14

15. Magnetic field from long straight wire: Directiony
What direction is the magnetic field from an infinitely-long straight wire? x I
Tue. Oct. 20, 2009
Physics 208 Lecture 14

16. Magnetic field from a current
Iron filings align with magnetic field lines
Magnetic field loops around the current.
Tue. Oct. 20, 2009
Physics 208 Lecture 14

17. Forces between currents
Which of these pairs of currents will attract each other? A
A & C B
A & B
Do demo of current carrying wires forcing each other apart. A B C
Tue. Oct. 28, 2008
Physics 208, Lecture 17

18. Force between current-carrying wires
Attractive for parallel currents.
Repulsive for antiparallel currents
Tue. Oct. 28, 2008
Physics 208, Lecture 17

19. Current dependence
How does the magnitude of the B-field change if the current is doubled? y
Is halved
Quadruples
Stays same
Doubles
Is quartered x I
Tue. Oct. 20, 2009
Physics 208 Lecture 14

20. Distance dependence
How does the magnitude of the B-field at 2 compare to that at 1? y 2
B2=B1
B2=2B1
B2=B1/2
B2=4B1
B2=B1/4 1 x I
Tue. Oct. 20, 2009
Physics 208 Lecture 14

21. Why? Biot-Savart says Why B(r) 1/r instead of 1/r2 ?
Small contribution from this current element.
~ independent of r
Large contribution from this current element.
Decreases as 1/r2 I
Tue. Oct. 20, 2009
Physics 208 Lecture 14

22. Long straight wire All current elements produce B out of page
Add them all up: r a  x
Tue. Oct. 20, 2009
Physics 208 Lecture 14

23. Field from a circular loop
Each current element produce dB
All contributions add as vectors
Along axis, all components cancel except for x-comp
Tue. Oct. 20, 2009
Physics 208 Lecture 14

24. Magnetic field from loopBz
Which of these graphs best represents the magnetic field on the axis of the loop? A. z Bz B. z z x y Bz C. z Bz D. z
Tue. Oct. 20, 2009
Physics 208 Lecture 14