1.9- Electromagnetism

1.9- Electromagnetism

1. Magnetic Fields

   • Magnetic fields are places where magnets 'feel' a force. Magnetic fields are indicated using magnetic field lines, which show the pattern of the magnetic field. Magnetic field lines point from North poles to South poles and the closer the magnetic field lines are together, the stronger the magnetic field. Magnetic fields are produced by permanent magnets, or they can be produced by electric current flowing a wire, a coil or a solenoid.
   • A permanent magnet produces its own magnetic field.
   • A solenoid is a long coil of wire.
   • • Bar Magnent
   • • Current Carrying Wire
   • • Current Carrying Coil
   • • Solenoid

2. The Motor Effect

   • When a current passes through a wire inside a magnetic field, the wire experiences a force that can move the wire experiences a force that can move the wire- this is called the motor effect. The direction of the force on the wire depends on the direction of the current and the direction of the magnetic field- can be determined by using the fingers on your left hand, sometimes called Fleming's left-hand rule.
   • The motor effect is used to design d.c. motors and Fleming's left-hand rule can be used to determine the direction of rotation of the motor. The speed of rotation and the turning force of a d.c. motor increases with: the size of the current, the strength of the magnetic field and the number of coils.
   • The strength of magnetic field, B, is related to the density of the magnetic field lines and is measure in tesla, T. A laboratory bar magnet has a magnetic field of about 0.1 T. The equation that links the force (F) on a conductor to the strength of the magnetic field (B), the current (I) and the length of the conductor (L) is:
     • F=BIL

3. Electric Motors

   • Electric motors convert electrical energy into kinetic energy and they work due to the motor effect. They are designed to convert the force generated into a rotational movement that can drive wheels. Fleming's LHR can be used here, as they always rotate in the direction of the force.

4. Electromagnetic Induction

   • When a conducting wire moves inside a magnetic field, or a magnetic field changes around a conducting wire, an electric current is induced inside the wire- this is called electromagnetic induction. The size of the induced current depends on the rate at which the wire cuts the magnetic field lines. Simple a.c. generators work due to electromagnetic induction. The electrical output of the generator (current or voltage) increases with: the speed of rotation of the generator coil, the number of turns on the coil and the strength of the magnetic field. The direction of induced current in the generator depends on the direction of the magnetic field and the direction of rotation of the coil. This can be determined by using the fingers on your right hand (Fleming's RHR).
   • Electromagnetic Induction- is the current induced in a wire by a change in a magnetic field or by the movement of the wire in a field.

5. Simple a.c. Generator

   • Knowledge of the process of electromagnetic induction is used to design simple a.c. generators. Generators convert kinetic energy into electrical energy. Inside the generator, a coil of wire rotates inside a magnetic field, or a magnet rotates inside a coil of wire. Electromagnetic induction then induces a voltage in the coil.
     • As the coil rotates, the wire cuts across magnetic field lines between the poles of the magnet. This induces a voltage in the wire and a current flows around the coil. Two 'slip' rings connect to the external circuit and the current flows out of the generator. The size of the induced voltage depends on:
       • the rate of rotation
         • the number of turns in the coil
           • the strength of the magnetic field
   • The direction of the induced current depends on the direction of rotation of the coil and the direction of the magnetic field. One side of the wire coil moves up for half a rotation, then down for the other half. This means that the current flows in one direction, then in another. Fleming's RHR allows the motion to be determined.

6. Transformers

   • Electric current flowing through a wire causes the wire to heat up. the bigger the current, the greater the heating effect and the greater the energy lost from the wire. The national grid is a series of wires that allows electrical energy to flow around the country, connecting power stations to users. If the national grid had wires containing a high current, it would be too inefficient. Transformers can step up can step down the voltage. This is possible because the elctricty is transmitted as alternating current, which can be transformed using electromagnetic induction
   • An alternating current in the primary coil produces a changing magnetic field in the primary coil. This changing magnetic field is kept within the iron core of the transformer, linking with the secondary coil. The changing magnetic field within the secondary coil induces a voltage, which generates alternating current in the secondary coil. The output voltage depends on the number of turns on the coils

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