E5D06 Magnetic Field Orientation

E5D06: In what direction is the magnetic field oriented about a conductor in relation to the direction of electron flow?

A.  In the same direction as the current
B.  In a direction opposite to the current
C.  In all directions; omnidirectional
D.  In a direction determined by the left-hand rule

First, let’s establish a fundamental principle of electromagnetism upon which this question depends:  A flowing electric current creates a magnetic field. So, when you apply a voltage to a wire and compel current to flow through the wire, a magnetic field is generated around the wire as the current flows. The magnetic field lines will curl about the wire in a circular manner, or in a cylindrical pattern if you consider a straight length of wire through which the current flows.

magnetic field about a wire

A flowing current generates a magnetic field in a circular pattern, as shown.

This question asks about the orientation of the magnetic field lines in the circular pattern about the wire. By convention, magnetic field lines of force point to the “north” magnetic direction. With a circular field orientation like this, “north” is simply one direction about the wire, and “south” will be the opposite direction. If you position a conventional magnetic compass anywhere within the magnetic field near the wire its needle will point in the northerly orientation of the circular magnetic field lines.

So, how can you determine which direction around the wire is north? It depends on your current convention. A brief historical tangent…

When Ben Franklin first began experimenting with electricity ‘round about 1752 the concept of electrons was not clearly understood. Franklin chose the convention of a positively charged particle that theoretically flowed from positive to negative direction when a voltage was applied. The subsequent physical reality of electrons was discovered by J.J. Thomson over 100 years later, illuminating the fact that electrons flow from negative to positive voltage direction, but conventional current flow of supposed positively charged particles was already well established and

Ben Franklin xkcd Comic

Courtesy xkcd Comics.

works just fine.  So, we now have both “conventional current” with “holes” that are the gaps into which electrons fit to neutralize electric charge, and we have “electron current” which considers the flow of the tiny negatively charged particles. The convention used in science or engineering is really quite arbitrary, but it matters when we want to determine the orientation of the magnetic field generated about a flow of current!

Back to the future now…

If you are considering electron current flow, as stated in question E5D06, you can conveniently use your left hand to

Left Hand Rule

The left hand rule for electron current flow.

Right hand rule for conventional current flow.

Right hand rule for conventional current flow.

determine the north direction of the magnetic field. Simply point your left thumb in the direction of the flow of electrons and your left hand fingers will curl in the northerly arc of the generated magnetic field. This is the “left hand rule,” and a straight-fingered version of this can also be applied to electric motors and electric generators to establish relationships among current direction, magnetic field direction, and resultant forces that will occur with their interaction (See Fleming’s Left Hand Rule for motors).

Conversely, if you are Benjamin Franklin, you should instead use the right hand rule to establish the magnetic field orientation about a current flow. If the conventional current flow is used (positive flow), point the right hand thumb in the conventional current direction and the right hand fingers will curl in the direction of the magnetic field’s northerly orientation.

The answer to Extra Class question E5D06, In what direction is the magnetic field oriented about a conductor in relation to the direction of electron flow? is “D. In a direction determined by the left-hand rule.”

 Related Questions (2012-2016 question pool):  E5D05, E5D07