topic 13

• What happens if you place a current carrying wire in the presence of a magnetic field

  The wire will experience a force (the motor effect)
• What does a split ring commutator do

  Swaps the positive and negative connections every half turn of the coil, so the direction of the current swaps every half turn, so the forces acting on the coil will always be acting in the same direction, so the coil will rotate in the same direction all the time, so the coil will continue turning 360 degrees instead of stopping and reversing every 180
• How to increase the speed of rotations in an electric motor

  - increase the current

  - add more turns to the coil

  - increase the magnetic flux density- more powerful magnets
• How does the electric motor work

  When the wire is in the presence of a magnetic field, the fields interact and it causes a force to be exerted on the wire, the direction depending on the direction of current and field.

  Effectively the wire is pushed up or down

  The coil of wire means that the two sides of wire will experience forces in opposite directions because the current is flowing in opposite directions, pushing it round 180 degrees.

  Once it has been pushed round 180 degrees though, the current is now flowing in the opposite direction pushing it back the way it came meaning it gets stuck at 180 degree rotations when a split ring commutator is not being used
• What happens if you take a coil of wire and move it through a magnetic field

  An alternating potential difference will be induced in the wire (generator effect)
• Generator effect- what happens when the wire stops moving

  The potential difference disappears
• Generator effect - what happens if you change the direction of the wire moving through the field eg up to down


  The direction of the potential difference swaps each time the direction is changed
• In what circumstances would a current be generated in the generator effect

  If the coil is joined up and it is a closed circuit, the induced potential difference would generate a current because electrons can move around
• What's the key idea in electromagnetic induction

  That a wire experiences a change in magnetic field to produce an induced potential difference and therefore a current if the circuit is closed
• What happens if you instead just move the wire back and forth not through the field

  There won't be any induced potential difference or current because the wire is not experiencing a change in the magnetic field
• How do you change the size of the induced potential difference or current in the generator effect

  1. Change the strength of the magnetic field, so there are stronger magnets which means a stronger magnetic field so a larger potential difference will be induced

  2. Move the wire or magnets more quickly so the magnetic field will change faster, so the potential difference will be bigger

  3. Shape the wire into a proper coil, more turns it has, the bigger the potential difference
• What if you were to move a bar magnet in and out of a coil of wire

  The movement of the magnet will generate a current, every time you move it in or out the direction of the force and current switches
• What do generators do

  Generate electricity from rotational motion
• What's a key difference between an alternator and a dynamp

  A Dynamo just has a split ring commutator- meaning they produce direct current

  An alternator has slip rings and brushes- meaning they produce alternating current
• How do alternators work

  1. The coil of wire rotates relative to the magnets which produces a magnetic field in the coil which then induced a voltage and current in the coil

  2. The slip rings and brushes mean that the contacts don't swap every half turn (like they do in a motor or Dynamo)

  3. This means they produce an alternating potential difference and an alternating current
• How do dynamos work

  1. As the coil of wire spins relative to the magnet, a magnetic field and therefore a voltage and current is induced in the coil

  2. The split ring commutator means that the contacts swap every half turn

  3. This means they produce a direct potential difference and hence a direct current- because the current is always flowing in the same direction
• What happens to the oscillations on an oscilloscope when you increase the speed at which a coil rotates in an alternator

  The amplitude and frequency both increase
• What colours correspond to what rings in an alternator


  Red side of the wire always connects to ring a

  Orange side of the wire always connects to ring b
• When is the maximum potential difference generator effect

  When the coil is horizontal because the wire is sweeping directly through the magnetic field lines at the fastest possible rate
• How is an alternating current produced by an alternator and how does it correspond to the graph


  When the wire at first is cutting the field lines directly the potential difference is at maximum, then when the wire is parallel the potential difference will fall to 0, when it goes back round and cuts directly again, the direction of the current in each side of the wire is flipped, and the presence of multiple commutators allows an alternating current to be produced.
• How can the size of an alternating current in an alternator be increased

  By increasing the strength of the magnetic field

  By increasing the number of turns on the coil

  By increasing the area of the coil

  By increasing the rotation speed of the coil we also increase the frequency of the current
• What enables a direct current in a Dynamo

  The split ring commutator means each side of wire is always connected to the same side of the ring, the direction of the potential difference and current does not reverse when the coil rotates so a direct current is produced
• How are microphones and loudspeakers similar

  Both convert between sound waves and electrical signals, for both, the electrical signals involved are alternating currents
• What's the difference between loudspeakers and microphones

  Loudspeakers take electrical signals and use the motor effect to convert them to vibrations, which then generate sound waves, microphones detect vibrations caused by sound waves and use the generator effect to convert them into electrical signals.
• What is the structure of a loudspeaker

  A coil of wire which is wrapped around one pole of a permanent magnet and connected to a cone.

  The coil of wire is permanently attached to the cone, but the coil and cone can slide back and forth along the magnet
• How does a loudspeaker work

  Using the motor effect

  
  

  1. An alternating current flows through the wire and creates a magnetic field in rye coil

  2. The magnetic field of the coil interacts with the magnetic field of the permanent magnet

  3. The intersecting magnetic fields exert a force on the coil, causing it to move back or forth along the permanent magnet, which causes the cone to change shape

  4. As the current is alternating, it's magnetic field and the force it experiences will also alternate, this means that type coil of wire and cone rapidly move back and forth, meaning they vibrate

  5. The vibrations are so fast the cone vibrations cause pressure variations in the air- sound waves
• What happens if you change the frequency of the ac supply used for loudspeakers

  The frequency that the cone vibrates will change, higher frequency- higher pitched sound, lower frequency - lower pitched sound
• What happens if you increase the size of the current in a loudspeaker

  Increases the volume of the sound produced
• How do microphones work

  1. Sound waves hit the diaphragm

  2. Causes the diaphragm and coil of wire to move

  3. Since the wire is moving within the magnetic field of the permanent magnet, it will generate a current and potential difference

  4. The frequency and amplitude of the sound waves will determine how much the diaphragm vibrates, and therefore determine the frequency and amplitude of the current
• Broadly how is power generated

  By converting thermal energy of heat to electrical energy
• Where is most energy generated

  Power stations
• Why don't we use very high currents when running power through the national grid

  Because very high current generates lots of heat because of the resistance produced, meaning we would lose lots of energy to the surroundings that we could use
• Why do we have to increase the voltage before sending power across the country

  Because we want the power to be as high as possible, that means we either need really high voltage or really high current, voltage is better because high current loses heat and is inefficient
• What do step ups transformers do

  Increase the voltage before going across the country
• Where does the power go after step up transformer

  Wires then transmit electricity across the country using pylons, then right before it reaches its destination the voltage has to be stepped back down again (by a step down transformer)
• Why do we lower the voltage back down before it reaches towns

  High voltages are dangerous, and they would blow the appliances in our homes
• Overall how does power get to our homes

  1. Electric power is generated at a power station

  2. Step up transformers increase the voltage and decrease the current

  3. The electric power is transmitted via the national grid at low current which minimises power loss

  4. Step down transformers decrease the voltage and increase the current near our homes

  5. Electric power is wired into our homes at a voltage that is relatively safe or use
• Why do we use stem up transformers

  To minimise energy loss by making voltage high instead of current
• Broadly how do transformers work

  The primary coil is transferring the current to the secondary coil
• Steps of how a transformer works

  1. An alternating potential difference is applied across the primary coil, which causes a current to flow and generates a magnetic field around the coil

  
  

  2. Because the potential difference is alternating, the direction of the current and therefore the magnetic field will also alternate

  
  

  3. The alternating magnetic field in the coil induced an alternating magnetic field in the iron core because iron is a magnetic material

  
  

  4. The iron core's magnetic field induces a potential difference across the secondary coil which causes a current to flow