Chapter 30

Serway & Beichner

Force between two current carrying wires

Electric current The Ampere

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce a force between them equal to 2 × 10-7 Newton per meter of length.

Fig 30-8, p.932

Fig 30-9c, p.933

Fig 30-1, p.927

Fig 30-3, p.929

120

0

coscos4

sin4

2

1

aI

daIB

See Ex. 30.1

Fig 30-7, p.931

See Ex. 30.3

2/322

20

axis2 Rz

IRB

R

I2

0 @ z = 0

3

20

2zIR

z >> R

Ch. 30 10

Fig 30-17, p.938

Fig 30-12, p.935

rIB

2

0

rRIB 2

0

2

for r > R

for r < R

Application of Ampère’s Law

Fig 30-19, p.939

nI

IlNB

0

0

Field inside Solenoid

start

9/13/04

Fig P30-20, p.940

Magnetic Flux

B = B•dA = BAcos

Ampère’s Law One More Time

Ampere’s law states that the line integral of B . ds around any closed loop equals oI where I is the total steady current passing through any surface bounded by the closed loop.

Surface

oIldB

Now introduce a capacitor to interrupt the the circuit. If our power supply is strong enough to keep I constant, the gray surface will give B = 0!

What’s wrong?

Assume that I is constant.

Apply Ampère’s Law to either the,white or gray surfaces, both of which are bounded by the red loop.This leads to:

rIB

2

0

If the power supply can keepthe current constant, the cap.will be charged: +Q/-Q onleft/right plate. This establishesan E-field between the two plates.

E = E•dA = Q/o

Electric flux will change in time corresponding to an effective current called the Displacement Current

dtdQ

dtd

o

Consider to different surfaces

Slide 39 Fig 30-26, p.943

Fig 30-27, p.945

Orbital Motion of the Electron in an Atom

Fig 30-28, p.946

Magnetic Moments due to Spin of electron, neutron and proton

Table 30-1, p.946

Electron Proton neutron

-928 1.41 -0.966

Magnetic Moments 10-26 J/T

Polarization

Generated Field points in the opposite direction

Table 30-2, p.948

Paramagnetism

0M

Generated field adds to applied field

Atomic currents

Diamagnetism0M Generated field opposes applied field

Magnetic Domains

The Earth as a big magnet