Purpose: Demonstrations of the conceptual ingredients for generating the Earth's magnetic field
Supplies: DC Motor, Ammeter, Electromagnet, DC power supply or battery
Background and Demonstration:
The Earth's magnetic field cannot be generated by a permanent magnet in the Core, because although the Core is predominantly iron, the temperature is well above the Curie temperature for iron. Therefore, the Earth's magnetic field must be generated by electromagnetic means. A system that uses electromagnetic induction to convert mechanical energy (motion) into a magnetic field is called a dynamo. Conceptually, it consists of two components (or ingredients), each of which can be demonstrated.
The first ingredient is the concept of a DC generator. The principle involved is that an electrical conductor in motion within a magnetic field generates a current. To illustrate this concept, we use a toy DC motor. A motor consists of a coil of wires which rotate within the field of a permanent magnet. As current is applied to the coil of wires (connect the DC power supply to the motor), an electromagnetic field is generated which is repelled or attracted by the permanent magnet, causing the coil of wires to turn on its axis (the motor axis spins). A generator acts in exactly the reverse of this. If we turn the coil of wires (connect the motor leads to the ammeter and rotate the axis of the motor by hand), recalling that they are within the field of the permanent magnet, we see that a current is generated. The faster we turn the axis, the more current is generated. If we turn the motor in the other direction, we generate a current in the opposite direction (the meter should indicate positive current one way and negative current the other way). Nearly all power plants make use of this principle in generating electricity; coal, oil, natural gas, nuclear fission (etc.) are burned to make steam, which turns a turbine, attached to which is a large coil of wires within the field of a permanent magnet.
The second ingredient is the electromagnet. This principle is that current flowing in a loop generates a magnetic field perpendicular to the loop. To demonstrate this, supply power to an electromagnet, which consists of many coils of wire wrapped around a ferromagnetic core (this core enhances the field, but is not necessary in principle). When current is applied, the electromagnet develops a magnetic field just like a permanent magnet. [You can pick up paper clips or, with a really big electromagnet, a waste basket or metal lab stool.] As soon as the current is turned off, the electromagnet looses its field (except for a weak remnant field in the core). Using a compass or a permanent magnet, you can show that if the direction of current is reversed (by reversing the poles on the battery or power supply), the direction of the electromagnet field is reversed (the compass rotates, or where the permanent magnet had been attracted, it is now repelled).
Putting the two together, we see that if an electrical conductor is in motion within a magnetic field, a current will be generated in the conductor, and if that current flows around in a loop, it will, in turn, generate a magnetic field. If the current loop can be oriented in the proper direction, the generated magnetic field will reinforce the original magnetic field and the process will continue even if the original magnetic field stops. This is the idea of the self-exciting dynamo. It is not a perpetual motion machine, since it requires mechanical energy (motion), but once started, it will continue to generate a magnetic field.
The Outer Core of the Earth consists primarily of molten iron, which is a good electrical conductor. Because it is a liquid, it is not rigidly connected to the overlying mantle, and need not rotate at the same rate as the outer portions of the Earth. This differential rotation, as well as convection in the Outer Core, ensure that the conductor is in motion. The tricky part is how to get the resulting currents to generate a magnetic field like the dipole field observed on the Earth's surface. The details are quite complex, and are the subject of continuing study.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)