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 wireSolenoid

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

Magnetic force is PERPENDICULAR to B field

Magnetic force is perpendicular to MOTION of q

EF qE����������������������������

BF qv B ������������������������������������������

Practice with magnetic forces

p. 756 Q1

Crossed E and B fields

Lorentz Force Electric and magnetic forces can be considered

independently1. 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

F qE qv B ��������������������������������������������������������

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

Deflection of the charge by E is

Recall that F=0 when v=E/B=

Solve for m/q=

2

22

q ELy

mv

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 = maF = 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/tFind 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.