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Explanation of Moving Electrical Charges Create Magnetic Field by Ron Kurtus - Succeed in Understanding Physics. Key words: iron shavings filings, right-hand rule, lines of force, compass, physical science, School for Champions. Copyright © Restrictions

Moving Electrical Charges Create Magnetic Field

by Ron Kurtus (revised 23 March 2012)

When an electrical charge is moving or an electric current passes through a wire, a circular magnetic field is created.

This can be seen with iron shavings or filings on a card that will align in the magnetic field when a current is passed through a nearby wire. Using standard conventions for the direction of electric current and magnetic lines of force, the direction of the magnetic can be determined by what is called the right-hand rule for magnetic field. The direction of the magnetic field can be demonstrated by using compasses.

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Magnetic field around wire

When a charged particle—such as an electron, proton or ion—is in motion, magnetic lines of force rotate around the particle. Since electrical current moving through a wire consists of electrons in motion, there is a magnetic field around the wire.

This field can be demonstrated by placing fine iron filings or shavings on a car and sticking a wire through the middle of the card. When a DC electrical current is passed through the wire, the iron filings align to show the magnetic field.

Iron filings show magnetic field around electric wire

Iron filings show magnetic field around electric wire

Conventions for electricity and magnetism

Certain conventions are used to determine the direction of the magnetic field with respect to the direction of the current.

Direction of electrical current

Although electrons have a negative (−) electrical charge and move toward the positive (+) terminal in a wire, the convention is that electrical current moves from positive (+) to negative (−). It is an unfortunate choice that was made years ago, but you need to be aware of the convention.

Direction of magnetic field

The convention for a magnetic field is that the lines of force move from north (N) to south (S). We don't know if they move in that direction or not, but it is just a convention that everyone follows.

Right-hand rule for magnetic field

The right-hand rule for determining the direction of the magnetic field is that if you wrapped your right hand around a wire with your thumb pointing in the direction of the electrical current, then your fingers would be in the direction of the magnetic field.

Right-hand rule for electric wire

Right-hand rule for electric wire

Verification with a compass

You can verify the direction of the magnetic field by placing one or more compasses on a card and observing their direction.

Compasses show direction of magnetic field

Compasses show direction of magnetic field

Note that the current must be DC (direct current), such as from a battery. Otherwise with AC, the direction of the current and magnetic field will alternate 50 or 60 times a second.

Summary

When electric current passes through a wire, a circular magnetic field is created. Iron filings on a card can demonstrate the magnetic field when current is passed through a nearby wire. Using standard conventions for the direction of electric current and magnetic lines of force, the direction of the magnetic can be determined by what is called the right-hand rule. The direction of the magnetic field can be demonstrated by using compasses.


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