Electromagnetism

Electromagnetism

1. Magnetic Fields

   • Any iron or steel object can be magnetized or demagnetized
   • The space around a magnet is called the magnetic field. The earth has a magnetic field which is most concentrated at the magnetic north and south pole.
   • The north seeking pole of a magnet, is the end that points north when the magnet is freely suspended. The other end is the south seeking pole
   • A plotting compass is a tiny magnet that lines up with the earths magnetic field and its tip points to the earths magnetic north pole.
   • The two lines of force of a magnetic field are the lines along which a plotting compass would point.

2. Electromagnets

   • When a current flows through a wire, a magnetic field is produced around that wire. The magnetic field lines are circles around the wire.
   • The circles are centered on the wire and are in a plane perpendicular to the wire.
   • Increasing the current makes the magnetic field stronger.
   • The corkscrew rule gives the direction of the field lines
   • Reversing the direction of the current reverses the direction of the magnetic field.
   • An electromagnet is made by wrapping insulated wire around a piece of iron, called a core. When a current flows through the wire the iron becomes strongly magnetized. When the current is switched off the iron loses its magnetism. This temporary magnetism makes electromagnets very useful.
   • Electromagnets are used in devices such as scrapyard cranes, circuit breakers, electric bells and relays. A relay is an electrically operated switch.

3. The motor effect

   • When we place a wire carrying an electric current in a magnetic field, it may experience a force. This is called the motor effect.
   • The force is at its maximum if the wire is at an angle of 90 degrees to the magnetic field, and zero if the wire is parallel to the magnetic field.
   • FLEMINGS LEFT HAND RULE is used to determine the ddirection of the force. The thumb and first two fingers are at right angles to each other.
     • The first finger represents the magnetic field. Pointing north - south.
     • The second finger represents the current, pointing positive - negative
     • The thumb represents the direction of the force.
   • The size of the force can be increased by either increasing the strength of the magnetic field, or by increasing size of the current.
   • The direction of the force on the wire is reversed if either the direction of the current or the direction of the magnetic field is reversed.

4. The Generator effect

   • The generator effect is the effect of an inducing a potential difference using a magnetic field.
   • The faster a wire cuts across the lines of a magnetic field, the greater the induced potential difference. When a direct current electromagnet is used it needs to be switched on of off to induce a potential difference.
   • When a wire cuts the lines of a magnetic field, a potential difference is induced across the ends of a wire.
   • A simple AC generator is made up of a coil that spins in a uniform magnetic field.

5. Transformers

   • A transformer works only on AC current because a changing magnetic field is necessary to induce AC into the secondary coil.
   • A transformer has an iron core unless it is a switch mode transformer which has a ferrite core.
   • A switch mode transforrmer is lighter and smaller than ordinary transformers. It operates at high frequency.

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