Download Physics 2 Lecture Notes

What produces a gravitational field?
A gravitational field exerts a force on?
Mass
Mass
What produces an electric field?
Electric charge
An electric field exerts a force on?
Electric charge
What produces a magnetic field?
Moving electric charge
A magnetic field exerts a force on?
Moving electric charge?
Direction of Magnetic Force
tail
out of
in to
page
page
Drawing vectors in
head
Direction of magnetic force is
“sideways”
 force is perpendicular
to both v and B
 use “right-hand rule”
to find direction
F = q v B sinq
ConcepTest
Magnetic Force
A positively charged beam enters into a magnetic
field region as shown. What is the direction of B?
y
1)+ y
(up)
2) – y (down)
3) + x (right)
4) + z (out of page)
5) – z (into page)
x
Radius of Circular Orbit
magnetic force:
F  qvB
centripetal accel:
v2
a
R
x x x x x x x x x x x x x x
x x x x x x x x x x x x x x
v
x x x x x x x x x x x x x x
x x x x x x x x x Fx x x x x
+q
R
Newton's 2nd Law:
F  ma


mv
R
qB
2
v
qvB  m
R
This has useful
experimental
consequences !
B
ConcepTest
Magnetic Force
x x x x x x x x x x x x
Two particles of the same
charge enter a magnetic field
with the same speed. Which
one has the bigger mass?
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
A
1)
2)
3)
4)
B
A
B
both masses are equal
impossible to tell without weighing the particles
as are ionized Helium (bare Helium nuclei)
2-protons, 2-neutrons (positively charged)
bs are simply electrons(negatively charged)
qa = -2qb
ma=7296mb
     
R=?
    
     
v= /2Vm/q /B
    
     
    
Velocity Selector
Consider a positively charged ion entering a
region where the electric and magnetic fields
are uniform and perpendicular to each other.
If the particle moves in a straight line, what is
its velocity in terms of E and B?
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
E
For the magnetic force:
direction
magnitude
up
F = qvB
For the electric force?
direction
magnitude
down
F = qE
Sum of the forces on the particle?
v=E/B
Zero (not accelerating)
 |FE| = |FB|  qE = qvB
B
Ratio of charge to mass for an electron
e– “gun”
An electron is accelerated from rest across a
potential difference and then enters a region of
uniform magnetic field, as shown at right. What is
the “charge to mass ratio”, q/m, of the electron?
What is the speed of the electron?
½ mv2 = qV
DV
e–
B
x
x
x
x
x
x
R
x
x
x
x
x
x
(Work-Energy Theorem)
What is the radius of the electron’s orbit?
R = mv / qB
Algebra: determine q/m
q / m = 2V / R2B2
(Earlier today)
(Solve second Eq for v and plug into first)
ConcepTest
I
1
2
I
3
I
I
If all wires carry the same current I, for which of the loops
above is the magnitude of the net force greatest?
A) Loop 1
B) Loop 2
C) Loop 3
D) same for all
ConcepTest
Magnetic Force
A rectangular current loop is in a uniform magnetic field. What
direction is the net force on the loop?
(a) + x
(b) + y
(c) zero
(d) – x
(e) – y
z
B
y
x
ConcepTest
If there is a DC current in the loop in the
direction shown, the loop will
A) move up
B) move down
C) rotate clockwise
D) rotate counterclockwise
E) some combination of moving
and rotating
N
S
a
b
The force on the top
segment of the
rectangular loop is
1)up.
2)down.
3)into screen.
4)out.
5)left.
6)right
7)zero.
a
b
The force on the
bottom segments of
the rectangular loop is
1)up.
2)down.
3)into screen.
4)out.
5)left.
6)right
7)zero.
a
b
The force on the
left segment of the
rectangular loop is
1)up.
2)down.
3)into screen.
4)out.
5)left.
6)right.
7)zero.
a
b
The force on the
right segment of the
rectangular loop is
1)up.
2)down.
3)into screen.
4)out.
5)left.
6)right.
7)zero.