Module 4—Magnetic and Electric Fields in Nature and Technology
Earth‘s Magnetic Field
Earth’s magnetic field is similar to that of a bar magnet. Compare the two magnetic fields illustrated below. Notice that the magnetic south pole of Earth exists near the geographic North Pole.
This explains why the “north” magnetic pole of a compass is attracted to the geographic “North” Pole of the planet—which is really the “south” magnetic pole when you consider Earth as a giant bar magnet.
The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). © The Regents of the University of Michigan; All Rights Reserved.
Magnetic Fields and Moving Charge—The Cause of Magnetism
On April 21, 1820, Hans Christian Ørsted (1777–1851) was preparing for an evening lecture when he noticed that a nearby compass needle was deflected when an electric current from the battery he was using was switched on and off. The deflection of a compass needle near an electric current confirmed that a magnetic field is produced by moving charge, indicating a direct relationship between electricity and magnetism.
Watch and Listen
Watch the following video clip that explains the effect of an electric current on a compass needle. Describe the materials, setup, and procedure shown in the video Ørsted’s Discovery—Magnetic Fields. What did you observe?
Ørsted had discovered that an electric current passing through a wire produces a circular magnetic field that surrounds the wire.
The direction of the magnetic field surrounding a current-carrying conductor can be determined using the “left-hand rule for current-carrying conductors.” This rule states the following:
Grasp the conductor with your left hand, such that your thumb points in the direction that the electrons are flowing. Your fingers will curl around the wire in the direction that the magnetic field follows.
There is also a right-hand rule for moving positive charges:
Grasp the conductor with your right hand, such that your thumb points in the direction that the positive charges are flowing. Your fingers will curl around the wire in the direction that the magnetic field follows.
magnetic flux: the number of magnetic field lines passing through a given area perpendicular to the field
solenoid: an electromagnet that operates a mechanical device by using the magnetic field produced by a current-carrying conductor wrapped into a coil
The magnetic field around a single wire is not always strong enough for practical applications (such as electromagnets). The intensity (magnetic flux) of the magnetic field can be increased by wrapping the current-carrying conductor around a tube numerous times to create a solenoid.
In this orientation the magnetic field of each coil on the tube contributes, in an additive manner, to the intensity of the magnetic field. Notice that inside the tube, the magnetic field is directed from the south pole to the north pole and that outside the tube it completes the loop, pointing from the north to the south pole.
Compare this with the magnetic field produced by a regular bar magnet.
The direction of the magnetic field “inside” the solenoid is determined, again, with a hand rule.
Grasp the coil with your left hand, such that your fingers curl around the coil in the same direction the current flows. The extended thumb will indicate the direction of the magnetic field within the coil.
© CHEN HENG KONG/shutterstock
A common application of the solenoid is the electromagnet, a mechanical device for moving metal objects and operating remote switches. In the photo to the right notice the small copper solenoid in the clear circuit breaker just below the red switch. When there is too much current moving in the circuit attached to this breaker, the magnetic field produced by the solenoid becomes strong enough to mechanically disconnect the circuit and stop the current.
According to the behaviour of a compass near a current-carrying conductor, the cause of magnetism is related to the movement of charge. Does this mean that moving charge is the cause of Earth’s magnetic field?
Watch and Listen
Is moving charge the source of Earth’s magnetic field? Watch the video clip Earth’s Magnetic Field to find out.