The Electromagnetic Field

Once basic properties of magnets were known, early experimenters sought new ways, other than using natural magnets, of producing a magnetic field. In 1820, the Danish physicist Hans Christian Oersted was the first to discover the existence of a magnetic field around a current-carrying wire. The interaction of the wire's field with an external magnetic field produced a force on the wire, and this eventually led to the invention of the electric motor.

Current Produces a “Curling” Magnetic Field

Oersted placed a number of small compasses on a horizontal plane near a vertical wire, through which a strong electric current was flowing. The compasses tended to align at tangents to circles around the wire, either clockwise or anti-clockwise depending on the current direction. The circular direction can be determined by applying the right-hand screw rule (Fig. 1).

External Magnetic Field Affects Current

Electric current is observed to respond to an external magnetic field, B. If a wire is allowed to hang freely near a magnet and the current is turned on, the wire moves – there is a magnetic force acting on it. Moreover, the force is a maximum if the current is perpendicular to the field and drops to zero if parallel to the field.

• The direction of the magnetic force is determined by applying the right-hand force rule (Fig. 2)
• The magnitude of the magnetic force depends on the field strength B, the current I, the length of wire L, and the angle theta between the wire and the direction of B. That is, F = B i L sin (theta)

Magnetic Field Unit - The Tesla

The units of B can be determined by making B the subject in the above formula, i.e. B = F/iL. This provides a natural way to measure the strength of a field in terms of the force (Newtons) on a current (Amps) in a wire of length (metres). The unit becomes the Newton per Amp metre, but the unit has been given its own name in honour of Nikola Tesla.

1 Tesla = 1 Newton per Amp metre (or, 1 T = 1 N/Am)

As a worked example, to gain a feel for size and units of quantities, determine the magnetic force per metre due to the earth’s magnetic field acting on a suspended power line that runs East-West and carries a current of 100A.

Solution: As the line is running East-West, the current and magnetic field are at right angles to each other. Assuming the Earth’s magnetic field is .00005 T, the magnitude of the force on 1 m of the power line is F = .00005 x 100 x 1 = .005 N per metre.

Magnetic Force Drives the Electric Motor

The operating principle of all electric motors is simple. An electric current flows in a wire loop placed in an external field and experiences a magnetic force (F = BiL), which creates movement of the loop (coil). Normally, several coils and many turns are used and the external magnetic field is provided either by a permanent magnet or an electromagnet. When the loop is aligned with the magnetic field, motion stops. At this point the current direction in the loop is reversed and the process is repeated over, so that continual mechanical motion is created.

The reader may be interested in more details on this topic, or to learn about Electromagnetic Induction.

Magnetic Field

Fig 1. The Right Hand Screw Rule

Fig 2. The Right Hand Force Rule

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