# Chapter 30 Serway & Beichner. Force between two current carrying wires.

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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 See Ex. 30.1

Fig 30-7, p.931 See Ex. 30.3 @ z = 0z >> R

Ch. 3010

Fig 30-7b, p.931

Fig 30-17, p.938

Fig 30-12, p.935 for r > R for r < R Application of Ampère’s Law

Fig 30-13, p.936

Fig 30-19, p.939 Field inside Solenoid

start 9/13/04

Fig P30-20, p.940 Magnetic Flux  B =  BdA = BAcos 

Ampère’s Law One More Time Ampere’s law states that the line integral of B. ds around any closed loop equals  o I where I is the total steady current passing through any surface bounded by the closed loop.

Apply Ampere’s Law to red loop for a wire with a constant current I Now introduce a capacitor to interrupt the the circuit. What’s wrong? Use a power supply that will keep current constant as the cap is charged: +Q/-Q on left/right plate. Now Apply Ampere’s Law again.

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:

If the power supply can keep the current constant, the cap. will be charged: +Q/-Q on left/right plate. This establishes an E-field between the two plates.  E =  EdA = Q/  o Electric flux will change in time corresponding to an effective current called the Displacement Current

Consider to different surfaces

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 ElectronProtonneutron -9281.41-0.966 Magnetic Moments  10 -26 J/T

Polarization Generated Field points in the opposite direction

Table 30-2, p.948

Paramagnetism Generated field adds to applied field Atomic currents

Diamagnetism Generated field opposes applied field

Magnetic Domains

The Earth as a big magnet

l =0.4 m i = 10 A

Fig P30-6, p.957

Ch. 3037

Ch. 3038

Ch. 3039

Ch. 3040

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