Physics

• Electromagnet


  A magnet that is created by an electric current flowing through a coil of wire
• Electromagnet


  • Can be turned on and off
  • Strength can be increased or decreased by adjusting the current
• How an electric bell works

  1. Switch is pressed, turning on the electromagnet

  2. Iron armature is attracted to the electromagnet, making the hammer strike the gong

  3. Circuit is broken, electromagnet turns off

  4. Armature springs back, circuit is complete again and the cycle repeats
• When a current-carrying wire is placed in a magnetic field


  It experiences a force
• Fleming's left-hand rule


  Determines the direction of the force on a current-carrying wire in a magnetic field
• Magnetic flux density


  A measure of the strength of a magnetic field
• The force on a current-carrying wire in a magnetic field is proportional to the magnetic flux density, the current, and the length of the wire
• Circuit breaker


  • A switch that is in series with an electromagnet
  • If the current becomes too large, the electromagnet becomes strong enough to pull the switch into the open position, turning the current off
• In a simple electric motor, a current-carrying coil of wire in a magnetic field will tend to rotate
• Loudspeakers and headphones


  Use the motor effect to convert changes in current in a coil of wire to changes in pressure that create sound waves
• Simple electric motor


  • Rectangular coil of wire that can rotate between the poles of a magnet
• Using Fleming's Left-hand Rule

  1. For the wire near the N-pole of the magnet

  2. For the wire near the S-pole of the magnet
• The forces will make the coil turn clockwise
• Split-ring commutator


  Reverses the current round the coil every half turn of the coil, ensuring that whichever side of the coil is nearest the N-pole, it always has current flowing out towards us and so the wire keeps turning clockwise
• When the coil turns through 90°

  The current stops flowing because the gaps in the commutator break the circuit
• When the brushes make contact again


  The commutator and the coil have turned over so that the wire which is now nearest the N-pole has current coming out towards us
• The force on the wire is upwards and so the coil still turns clockwise
• Generator effect


  If a wire is moved to "cut" across lines of flux, then a current is induced in the wire (if there is a complete circuit)
• If the wire moves along the magnetic field lines, there is no current. If the wire is not moving there is no current
• If a magnet is moved into a coil of wire, which is part of a complete circuit

  A current is induced in the wire
• If the magnet is held stationary inside the coil


  There is no induced current
• If the magnet is moved out of the coil

  The direction of the current is reversed
• If the other pole of the magnet is moved into the coil

  The direction of the current is also reversed
• As the magnet is moved in and a current induced, there is a force which opposes the movement of the magnet
• Simple a.c. generator


  Consists of a magnet that rotates near a coil of wire
• As the magnet turns

  The magnetic field through the coil changes, inducing a p.d.
• When the magnet has turned through half a turn


  The direction of the magnetic field through the coil reverses, and the p.d. reverses
• If the magnet continues to turn in the same direction then the p.d. will keep on reversing every half turn producing an alternating current
• Microphones


  Generate current from sound waves
• How microphones work


  Sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet, causing the coil to move back and forth in the magnetic field and inducing an alternating current
• Alternators


  Generate alternating current (a.c.)
• Dynamos


  Generate direct current (d.c.)
• Dynamos use a split-ring commutator to keep the current flowing in the same direction
• Oscilloscopes


  Show how the p.d. generated in the coil changes over time
• Alternating p.d.


  The p.d. keeps alternating, i.e. the positive and negative keep swapping, and so the current keeps changing direction
• Direct p.d.


  The p.d. doesn't alternate and so the current always flows in the same direction
• Nebula


  A collection of dust, gas and rock
• Formation of a protostar

  1. Gravity makes the gas and dust spiral in together

  2. Gravitational attraction increases density of protostar

  3. Particles collide more frequently, increasing temperature

  4. Hydrogen nuclei begin fusion to form helium
• Protostar


  A star in the process of formation
• Main Sequence


  A long stable period for a star where nuclear fusion provides outward pressure balancing gravity