Magnetic Field Pattern


 [Figure (a)]

The magnetic field forms by straight wire are concentric circles around the wire as shown in figure (a) above. Take notes that when the direction of the current is inversed, the direction of the magnetic field line is also inversed.

The direction of the magnetic field line can be determined by the Maxwell's Screw Rule or the Right Hand Grip Rule.


[Figure (b)]

Sometime, the magnetic field pattern may be given in plan view, as shown in figure (b). In plan view, a dot in the wire shows the current coming out from the plane whereas a cross in the wire shows the current moving into the plane.

Direction of the Magnetic Field

The direction of the magnetic field formed by a current carrying straight wire can be determined by the Right Hand Grip Rule or the Maxwell Screw Rule.

Right Hand Grip Rule
Grip the wire with the right hand, with the thumb pointing along the direction of the current. The other fingers give the direction of the magnetic field around the wire. This is illustrated in


[Figure (c)]

The Maxwell's Screw Rules
The Maxwell Screw Rules sometime is also called the Maxwell's Corkscrew Rule. Imagine a right handed screw being turn so that it bores its way in the direction of the current in the wire. The direction of rotation gives the direction of the magnetic field.

Strength of the Magnetic Field

The strength of the magnetic field form by a current carrying conductor depends on the magnitude of the current. A stronger current will produce a stronger magnetic field around the wire as shown in Figure (e) below.


The strength of the field decreases out as you move further out. This is illustrated in figure (f) below. Thus, you must be very careful when you are asked the draw the magnetic field in your exam. The distance of the field lines must increase as it is further out form the wire.


Physics Animation
Applet
Magnetic Field of a Straight Current Carrying Wire - Walter Fendt

Youtube Video


Exercise
1.
Which of the pictures below shows the magnetic field that produced by a single straight current-carrying wire?
A. P
B. Q
C. R
D. S

2.

Which symbol in the figure shows that current is flowing into the screen?

A. P
B. Q
C. R
D. S

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