Electromagnetism

Electromagnetism

1. The Motor Effect

   • Electromagnetism
     • When an electric current flow through a wire it produces a magnetic field around it; this is only present whilst the current flows
       • This is used in electromagnets, a wire is wrapped around a soft iron core and an electric current is passed through it
         • The electromagnet acts like a bar magnet except it can be turned on and off
           • Electromagnets are useful for picking metal up (iron, steel etc.) so they are used in some cranes; had a normal magnet been used, the crane would be able to pick up an object but not drop it
   • The Motor Effect
     • A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles; when an electric current flows through the coil, it experiences a force and moves
       • This is called the motor effect
         • This size of the force is greatest when the wire is perpendicular to the magnetic field of the permanent magnetic field; if it is parallel it will not feel any force
           • Fleming's left hand rule can be used to predict which way the force will act
             • The thumb points in the direction of motion
             • The first finger points in the direction of field (north to south)
             • The second finger points in the direction of current
   • Electric Motors
     • Electric motors use the motor effect
       • The direction of the current must be reversed every half turn otherwise the coil stops; this is done using a split ring
         • The size of the force is increased by increasing by the current or by increasing the strength of the magnetic field

2. Transformers

   • Electromagnetic Induction
     • If an electrical conductor sych as a wire cuts through a magnetic field, a potential different is induced across the ends of the conductor; if the conductor is part of a complete circuit an electric current will flow
       • You can do this by moving in a magnetic field or moving magnet in a coil of wire
         • Potential difference can be increased by:
           • Moving the magnet or wire faster
           • Using a stronger magnet
           • Increasing the number of turns, or loops in a coil
           • Increasing the area of the coil
   • Transfomers
     • A transfer changes the potential difference of (a.c.) electricity; a step up transformer increases the p.d. whilst a step down transformer decreases it
       • A transformer consists of a soft iron core with two coils of insulated wire wrapped separately around it; each coil has a different number of turns
         • The primary coil is connected to an a.c. supply which acts like an electromagnet, whilst the secondary coil is where an alternating potential difference is induced
           • How a transformer works:
             • An alternating current passes through the primary coil
               • The alternating current produces a magnetic field that continuously changes; the strength of the magnetic field is increased by the iron core
                 • The secondary coil cuts through the changing magnetic field, inducing an alternating potential difference across the ends of the coil
                   • An alternating current flows if a circuit is connected to the second coil
           • Step up transformers have more coils in the second coil and vice versa
             • V(p)/V(s) = n(p)/n(s)
               • Where V(p) is the voltage in the primary coil, V(s) is the voltage in the second coil, n(p) is the number of turns in the primary coil and n(s) is number of turns in the secondary coil
             • Transformers are almost 100% efficient therefore power in equals power out
               • Vp x Ip= Vs x Is
   • Switch Mode Transformers
     • Switch mode transformers are a type of transformer that operate at higher frequencies than traditional transformers; they usually operate at 50KHz to 200KHz
       • If they work at higher frequencies they can be made smaller and lighter than traditional transformers that work from a 50Hz mains supply
         • This makes them more useful in mobile chargers and power supplies
           • They use very little power when they're switched on but no load is applied