Winter wk 4 – Thus.27.Jan.05
• Ch.28: Magnetic fields
– How to make a magnetic field
– Magnetic forces and fields
– Charge/mass ratio
– Charges circulate around B fields
– Magnetic force on current-carrying wire
• Solar applications
Energy Systems, EJZ
How to make a magnetic field
Recall your observations with Science Kit
CURRENTS cause magnetic fields:
Straight wire
Solenoid
Permanent magnets are due to domains with
aligned electron spin
Electromagnetic forces and fields
Force on a charge in an electric field
is parallel to the E field
F E  qE
Magnetic force is PERPENDICULAR to B field
FB  qv  B
Magnetic force is perpendicular to MOTION of q
Practice with magnetic forces
p. 756 Q1
Crossed E and B fields
Lorentz Force
F  qE  qv  B
Electric and magnetic forces can be considered
independently
1. How can we align E and B so a moving charge
is not deflected?
2. Solve F=0 for the charge’s speed v.
Practice: #6, 7
Thomson’s experiment
1. Charges are deflected by E field.
2. Charge motion is deflected by B field.
First, how do B fields deflect moving charges?
Deflection of charges by E
See Sample Problem 22-5, p.593
2
Deflection of the charge by E is
Recall that F=0 when v=E/B=
Solve for m/q=
q EL
y
2
2mv
Deflection of MOVING charge by B
Consider a charge q moving with speed v
perpendicular to a magnetic field B.
Sketch the direction of the magnetic force.
Cyclotron radius
Radius of orbit of charge around B line
depends on what?
F = ma
F = qvB
a = v/r2
Combine:
Solve for r
Practice: #16, 79, 84
Charge/mass ratio in playroom
1. Electron is accelerated through potential
Potential energy  Kinetic energy
qV = ½ mv2
2. Electron is deflected in the magnetic field
r=mv/|q|B
3. Measure V, B, and r.
Solve for q/m=
Magnetic force on current
F=qvxB, current I = q/t, speed v=L/t
Find q(I,L,v)
Find F on I
Practice:#33 p.759
Solar applications
Storms from the Sun:
p.13: If a CME travels at 1 million miles per hour, how
long does it take to reach Earth?
p.16: The 2 May 1994 event dumped 4600 GW-hr of
electricity into Earth’s upper atmosphere. How much
energy is that in Joules?
p.16: If the Earth’s mean magnetic field is B0=0.5 Gauss,
and one Tesla=104 Gauss, by what percent does 2000
nanoTesla change Earth’s field?
p.54: For the CME of 1 Sept 1859: calculate its speed v,
if it took 18 hours to reach Earth.
more Solar applications
Storms from the Sun:
p.77: If Rsun = 100 REarth, then find the ratio of their
volumes, Vsun/VEarth
p.77: If m=5 millions tons of mass is converted to energy
(E=mc2) each second, calculate the power (P) produced
by the Sun.
p.82: If the Sun’s mass is M=2x1030 kg, and it keeps
losing dm/dt = 5 million tons per second, how long (T)
can the Sun last?
p.83: If the solar wind pours I=1 million amps into Earths
magnetosphere, how much charge (Q) is that per day?
Extra solar applications
p.13: Calculate vthermal from Tsolar wind. Compare to vflow.
p.16: Derive the altitude for a geosynchronous orbit
p.77: If the Sun’s core temperature is about T=107K,
calculate the thermal speed vth of protons in the core.