Topic 7- Magnetism and Electromagnetism

• What direction does the magnetic field flow in?

  North into South
• What is a permanent magnet?

  A magnet that can produce its own magnetic field.
• What is an induced magnet?

  A material that becomes magnetized when placed in a magnetic field.
• What is the right-hand thumb rule?

  -point thumb in the direction of the current and curl fingers

  -the direction the fingers curl in is the direction of the magnetic field
• What is a solenoid?

  A coil of wire that produces a magnetic field when an electric current passes through it.
• How do you increase the strength of a solenoid?

  Increase the number of turns on the coil, increase the current, add a metal core
• What can the metal core be made out of in a solenoid?

  Iron, nickel, cobalt
• What does each finger represent on Flemming's left hand rule?

  Thumb: Direction of force experienced by the wire Index finger: Direction of magnetic field Middle finger: Direction of current
• What is the motor effect?

  The effect that causes a wire carrying a current to move when placed in a magnetic field.
• What is the magnetic flux density?

  The magnetic flux density is the amount of magnetic field passing through a given area- shows the strength
• What is the current?

  Flow of electric charge.
• What is the equation for the force experienced by a wire in a magnetic field?

  F= BIL

  force= magnetic flux density * current * length
• What are some uses for electromagnets?

  Electric motors, separating metals, speakers, and magnetic locks.
• What is a transducer?

  A device that converts one form of energy into another.
• What do loudspeakers and microphones use to convert variations in current to sound waves?

  The motor effect.
• How does a loudspeaker work?

  uses the motor effect

  -AC current in wire

  -changes polarity of magnetic field around wire

  -interacts with magnetic field of the magnet

  -causes movement and vibrations in the cone, creating sound waves
• How does a microphone work?

  -persons voice causes the membrane to vibrate

  -causes wire to move in the magnetic field

  -induces potential difference in wire, creating an AC current

  -sound waves are converted into electrical signals
• What is the generator effect?

  Where moving a wire in a magnetic field induces potential difference
• What is a generator?

  A device that converts kinetic energy into electrical energy.
• What do compasses show?


  The directions of magnetic fields
• What does a moving charge create?


  A magnetic field
• What is an electromagnet?


  A magnet powered by electricity.
• What are electromagnets used for?


  -used in cranes to attract and pick up things made from magnetic materials (in scrap yards)

  -used within other circuits to act as switches
• What is the generator effect?


  The induction of a potential difference in a wire which is moving relative to a magnetic field, or is experiencing a change in a magnetic field
• How do you create the generator effect?


  By moving a magnet in a coil of wire or moving a wire in a magnetic field
• What happens when you flip a magnet in a magnetic field?

  The poles will switch, which in turn reverses the direction of the potential difference or current
• What does an induced current always do?

  Opposes the charge that caused it
• How do you change the size of an induced potential difference?


  -increase the speed of movement

  -increase the strength of magnetic field
• What do alternators do?


  Rotate a coil in a magnetic field to produce an alternating potential difference
• What do dynamos do?

  Generate electricity- a direct current produced using split-ring commutator
• How do alternators work?


  -construction is like a motor

  -as the coil or magnet spins, a current is induced in the coil

  -current changes direction every half-turn

  -have slip rings and brushes so the contacts don't swap every half turn
• What do oscilloscopes show?


  Show how potential difference generated in a coil changes over time
• What is the shape of an alternating current on an oscilloscope graph?

  Goes up and down across the horizontal axis
• What is the shape of a direct current on an oscilloscope graph?


  Goes up and down but always stays above the graph (as it's always positive)
• How do microphones generate current from sound waves?


  -sound waves hit flexible diaphragm that is attached to a coil of wire, wrapped around a magnet

  -causes coil of wire to move in the magnetic field which generates a current

  -the movement of the coil depends on the properties of the sound wave (louder sound moves diaphragm further)

  -microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit
• What do transformers do?

  Change the size of potential difference of an alternating current
• What coils of wire do transformers have?


  Primary and secondary coils, joined with an iron core
• What do step-up transformers do?


  Increase the potential difference- more turns of wire on the secondary coil
• What do step-down transformers do?


  Decrease the potential difference- more turns of wire on the primary coil
• What is the efficiency of a transformer?

  Almost 100% efficient