Magnetic Field Generated by Current in Straight Wire (1)

Magnetic Field Generated by Current in Straight Wire (1). Consider a field point P that is a distance R from the axis of the wire. • dB = µ0. 4π. Idx r2 sinφ = µ0. 4π.
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Magnetic Field Generated by Current in Straight Wire (1) Consider a field point P that is a distance R from the axis of the wire. µ0 Idx µ0 Idx sin φ = cos θ 4π r2 4π r2 R R r2 dx = = 2 2 = • x = R tan θ ⇒ dθ cos2 θ R /r R • dB =

µ0 I µ0 I r2 dθ cos θ = cos θdθ • dB = 4π r2 R 4π R Z θ2 µ0 I • B= cos θdθ 4π R θ1 µ0 I (sin θ2 − sin θ1 ) = 4π R • Length of wire: L = R(tan θ2 − tan θ1 )

Wire of infinite length: θ1 = −90◦ , θ2 = 90◦ ⇒ B =

µ0 I 2πR

26/3/2008

[tsl216 – 3/16]

Magnetic Field Generated by Current in Straight Wire (2) Consider a current I in a straight wire of infinite length. • The magnetic field lines are concentric circles in planes prependicular to the wire. • The magnitude of the magnetic field at distance R µ0 I . from the center of the wire is B = 2πR • The magnetic field strength is proportional to the current I and inversely proportional to the distance R from the center of the wire. • The magnetic field vector is tangential to the circular field lines and directed according to the right-hand rule.

26/3/2008

[tsl217 – 4/16]

Magnetic Field Generated by Current in Straight Wire (3) ~ in the limit R → 0. Consider the magnetic field B • B=

µ0 I (sin θ2 − sin θ1 ) 4π R

• sin θ1 = √

a a2 + R 2

= q

2a

1 1+

R2 a2

1 R2 ≃1− 2 a2

1

1 R2 ≃1− 2 4a2

• sin θ2 = √

4a2 + R2

µ0 I • B≃ 4π R

„ « 1 R2 1 R2 −1+ 1− 2 4a2 2 a2

= q 1+

µ0 I 3R R→0 −→ 0 = 4π 8a2

R2 4a2

a B

R

a

θ2

a

θ1 I 26/3/2008

[tsl380 – 5/16]