Magnetism & electromagnetism

• The poles of a magnet are the places where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.
• Why do poles repel?

  Two like poles repel because the field lines are going in opposite directions. They repel because they have the same polarity on each end.
• A permanent magnet produces its own magnetic field. This will always be magnetic.
• An induced magnet is a material that becomes a magnet when it is placed in a magnetic field which causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly. Induced magnetism always causes a force of attraction.
• What is a magnetic field?

  The region around a magnet where a non-contact force acts on another magnet or on a magnetic material.
• What are the types of magnetic material?

  Iron

  Cobalt

  Steel

  Nickel
• The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet because the field lines are closer.
• Field lines are always drawn pointing to the South away from the north point at all points.

  
• How can we prove the Earth has a magnetic core?

  A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle aligns itself with the Earth's field lines points in the direction of the Earth’s magnetic field.
• Explain how to plot a magnetic field line using a compass.

  1. Take a bar magnet and place the compass near the north pole of the magnet.
  2. Draw a dot/cross at the north pole of the magnet.
  3. Move the compass so the south pole of the compass is touching the dot. Draw another cross at the north pole.
  4. Repeat this process until you have a magnetic field line going clockwise from the North pole to the South pole.
• When a current flows through a conducting wire a magnetic field is produced around the wire.
• The strength of the magnetic field depends on the current through the wire and the distance from the wire.

  A greater current means a stronger magnetic field.

  A greater distance from the wire means a weaker field.

  Changing the direction of the current changes the direction of the magnetic field.
• Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.
• How does a solenoid pattern increase magnetic strength?

  Coiling the wire causes the field to form a larger magnetic field in the same direction as the current.
• What happens if you change the direction of current?

  Direction of magnetic field changes.
• What factors affect the strength of a solenoid?

  Size of current/ size of potential difference

  Length of wire

  Cross sectional area

  Number of turns

  Strength of magnet
• What is an electromagnet?

  A solenoid with an iron core.
• Adding an iron core increases the strength of the magnetic field of a solenoid.
• What is the motor effect?

  When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. The magnetic field created by the current interacts with the permanent magnetic field of the magnet.
• How does an electric motor generate an alternating current?

  As the coil moves through the magnetic field, potential difference is induced through the coil. This creates a complete circuit. Every half turn, the potential difference reverses direction so every half turn the current also changes direction.
• What does a slip ring do?

  Provides contact between the coil and brushes.
• How does rotation occur in an electric motor.

  When a conductor carries a current, it generates a magnetic field. When a magnet and a wire interact, they will exert a force as the magnetic fields interact. One side of the coil is pushed by the force of repulsion and the other side is pulled by the force of attraction so it begins to rotate. When current changes direction, the direction of force also changes - causing the object to turn around.

  
• Using Fleming's left hand rule, which direction is the field pointing?

  Right

  
• Fill in the blanks.

  A) Force

  B) Current

  C) Field

  
• What direction is the force acting on this wire?

  Upwards

  
• What factors affect the size of the force on a conductor?

  Current of wire

  Strength of magnetic field
• Describe how to plot the magnetic field pattern of a bar magnet.

  Place a compass near the north pole of the magnet and mark the direction that the compass points.

  Move the compass around the bar magnet to the other pole marking at intervals the direction the compass points.

  Join the points up and add an arrow pointing from the north pole to the south pole.
• When there is a current in the coil, the coil rotates continuously. Explain why.

  The current creates a magnetic field.

  The two magnetic fields interact with each other to generate a force which causes the coil to rotate.

  Every half turn, the two halves of the split-ring commutator swap from one carbon brush to another.

  The commutator reverses the current in the coil which keeps the current in the same direction as the magnetic field. (keeps the forces in the same direction)

  
• There is a straight wire passing through a piece of card. Describe how you could show that a magnetic field has been produced around a wire.

  Sprinkle iron fillings onto the card and gently tap the card to reveal the pattern of the magnetic field.
• Suggest two changes to the electric motor, that would make the coil spin in the opposite direction.

  Reverse the polarity of the cell

  Reverse the polarity of the magnet (change the direction of the magnet)
• In what circumstance will no force act on a conductor carrying an electric current and in a magnetic field?

  If the conductor is placed at a right angle and is parallel to the magnetic field.
• Explain why the coil rotates when there is a current in the coil.

  Current in the wire causes a magnetic field. Current is in opposite directions in each side of the coil so forces act in opposite directions of the coil. The current reverses every half turn.