Magnetism and Electromagnetism #2

Magnetism and Electromagnetism #2

1. Electric Motors

   • A Simple D.C. Electric Motor
     • 4 Factors which speed it up
       • 1. More current
       • 2. More turns on the coil
       • 3. Stronger magnetic field
       • 4. A soft iron core in the coil
   • 1. The diagram shows the forces acting on the two side arms of the coil
     • 2. These forces are just the usual forces which act on any current in a magnetic field
       • 3. Because the coil is on a spindle and the forces act one up and one down, it rotates
         • 4. The split-ring commutator is a way of swapping the contacts every half turn to keep the motor rotating in the same direction
           • 5. The direction of the motor can be reversed either by wapping the polarity of the d.c. supply or swapping the magnetic poles over
             • 6. The speed can be increased by ADDING MORE TURNS, INCREASING THE CURRENT, INCREASING THE STRENGTH OF THE MAGNETIC FIELD or by ADDING A SOFT IRON CORE

2. Relative motion between a magnet and a conductor induces a current

3. Loudspeakers

   • A.C. electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone
   • The coil is surrounded by a permanent magnet, so the a.c. signals cause a force on the coil and make it move back and forth
   • These movements make the cone vibrate and this creates sounds

4. Electromagnetic Induction

   • Electromagnetic Induction: The creation of a voltage in a wire which is experiencing a change in magnetic field
   • Dynamo Effect
     • Using an electromagnetic induction to generate electricity using energy from kinetic energy stores is called the dynamo effect
     • There are 2 different situations where you get EM induction
       • 1. An electrical conductor moves through a magnetic field
       • 2. The magnetic field through an electrical conductor changes
     • You can test this by connecting an ammeter to a conductor and moving the conductor through a magnetic field
       • The ammeter will show magnitude and direction of the induced current
     • If the direction of the movement is reversed, the induced voltage/current will be reversed
       • To get a bigger voltage, increase:
         • 1. the strength of the magnet
         • 2. Number of turns on the coil
         • 3. speed of movement
   • A.C. generators
     • 1. Generators rotate a coil in a magnetic field
       • 2. Their construction is like a motor - as the coil spins, a current is induced in the coil. This current changes direction every half turn
         • 3. Instead of a split-ring commutator, a.c. generators have slip rings and brushes so the contacts don't swap every half turn
           • 4. This means they produce a.c. voltage, as shown by CRO displays. Faster revolutions produce not only more peaks but higher overall voltage too
             • 5. Power stations use a.c. generators to produce electricity - they get the energy needed to turn the coil or magnetic field in different ways

5. Transformers

   • Transformers change the voltage
     • 1. Transformers change the size of the voltage of an alternating current
       • 2. They all have 2 coils, the primary and the secondary, joined with an iron core
         • 3. When an alternating voltage is applied across the primary coil, the magnetically soft core magnetises and demagnetises quickly
           • 4. This induces an alternating voltage in the secondary coil
             • 5. The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils
   • Step-up transformers
     • Increase the voltage
     • They have more turns on the secondary coil than than the primary coil
     • Boosts the voltage before it is transmitted
   • Step-down transformers
     • Decrease the voltage
     • They have more turns on the primary coil than the secondary
     • Used at the end of the journey to reduce the voltage so its more useful and safer to use
   • Input (primary) voltage/Output (secondary) voltage = Number of turns on primary/Number of turns on Secondary
     • Vp/Vs = Np/Ns
       • OR
         • Vs/Vp = Ns/Np
   • Efficiency
     • Power = voltage x Current
     • VpIp = VsIs
       • Input power = Output power
   • The voltage produced by power stations is too low to be transmitted efficiently
     • The lower the voltage, the higher the current for a given amount of power, and current causes wires to heat up