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Flashcards in Magnetism Deck (22):

What are magnetic field lines?

• Lines of force, which can be used to show the size and direction of magnetic fields.


What direction to magnetic field lines point?

North to South.


What is a magnetic field line?

A line which is used to show the soze and direction of magnetic fields. (also line of force)


How would one create a uniform field between two magnets?

Put the North and South poles of two permanent bar magnets near each other.


What are soft and hard magnetic materials?

• If a magnetic material loses its induced magnetism quickly, it is soft.

• If a magnetic material keeps its induced magnetism, it is hard.

• Iron is a soft magnetic material, whereas steel is hard.


How do you induce magnetism?

• Bring a magnet near a magnetic material and its field.

• The material acts as a magnet - induced magnetism.

• The closer the magnet and magnetic material, the stronger the induced magnetism will be.


What would the magnetic field pattern be for a bar magnet?

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What would the magnetic field look like between two bar magnets with similar poles together?

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What would the magnetic field pattern look like for two bar magnets with opposite poles?

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What would the magnetic field pattern look like for a straight wire?

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What would the magnetic field pattern look like for a flat circular coil?

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What would the magnetic field pattern look like for a solenoid?

  • Inside the coil, the magnetic field is strong and uniform.
  • Around the coil, the field is just like that of a bar magnet (the ends are like a north and south pole - electromagnet).

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What will a wire carrying an electric current produce?

a magnetic field


What happens when a wire carrying a current is put between magnetic poles?

The two magnetic fields affect one another and there is a force on the wire, which can cause the wire to move.


How can you increase the force on a current-carrying conductor?

Increase the strength of the magnetic field and increasing the current.


What is Fleming's Left Hand Rule?

A way to determine the direction of a resulting force when a wire is carrying a current perpendicular to the magnetic field.

  • thumb - force
  • first finger - field
  • second finger - current


How does a d.c. electric motor work?

  • In a motor, a wire is wound around a block called the armature, which is on a splindle t=so that it can rotate.
  • The current on the armature flows in opposite directions on either side, and due to the magnetic fields and currents affecting each other, the armature is pulled down and pushed up on either side (and spins).
  • After doing a half turn and becoming vertical, the part that is pushed upwards cannot go any farther up and the part that is pushed down cannot go any further down, so the armature stops spinning.
  • For the armature to continue to spin, the current flowing through the wire must change directions using a split ring commutator.


How does a split ring commutator work?

  • The commutator is made from two pieces of copper on either side of the spindle. 
  • There are two brushes made from carbon (graphite) which push lightly against the copper and conduct electricity to the armature. 
  • The brushes push against the armature when it spins and ensure that there is a good contact.

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How would you increase the speed of an electric motor turning?

  • Increase the current.
  • Add more coils.
  • Increase the strength of the magnets.


How does a loud speaker produce sound?

  • A.c. signals from an amplifier are fed to a wire coil in a speaker (wrapped around the base of a cone).
  • The coil is surrounded by a permanent magnet which causes there to be a force that makes the coil move backwards and forwards.
  • The movements make the cone vibrate and a sound is produced. 

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What experiment could you do to show the shape of a magnetic field around a bar magnet?

Have a magnet, and mark the line of a compass' direction. Move the compass to new points along the line and continue to mark it. Start a new line from a different place to show all field. 


The iron increases the strenghth of the magnetic field throughout the coil.

How does having curved magnets and a piece of iron improve a motor's performance?

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  • A stronger magnetic field increases the force so the coil would turn more quickly.
  • The magnetic field is radial (acts all the way around) so that the current stays at right angles to the magnetic field for longer and the force acts over a larger part of a cycle.