Magnetism

• What are the poles of a magnet?

  The poles of a magnet are the places where the magnetic forces are strongest.
• What happens when two like poles of magnets are brought close together?

  Two like poles repel each other.
• What occurs when two unlike poles of magnets are brought close together?

  Two unlike poles attract each other.
• What type of force is exhibited by the attraction and repulsion between two magnetic poles?

  It is an example of a non-contact force.
• What does a permanent magnet do?

  A permanent magnet produces its own magnetic field.
• What is an induced magnet?

  An induced magnet is a material that becomes a magnet when placed in a magnetic field.
• What happens to an induced magnet when it is removed from the magnetic field?

  It loses most or all of its magnetism quickly.
• What should students be able to describe regarding permanent magnets?

  • The attraction and repulsion between unlike and like poles for permanent magnets.
  • The difference between permanent and induced magnets.
• What is the magnetic field?

  The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material.
• What type of force is always present between a magnet and a magnetic material?

  The force is always one of attraction.
• How does the strength of the magnetic field change with distance from the magnet?

  The strength of the magnetic field decreases with distance from the magnet.
• Where is the magnetic field strongest?

  The magnetic field is strongest at the poles of the magnet.
• How is the direction of the magnetic field determined?

  The direction of the magnetic field is given by the direction of the force that would act on another north pole placed at that point.
• What is the direction of a magnetic field line?

  The direction of a magnetic field line is from the north (seeking) pole to the south (seeking) pole of the magnet.
• What does a magnetic compass contain?

  A magnetic compass contains a small bar magnet.
• What does the compass needle point towards?

  The compass needle points in the direction of the Earth's magnetic field.
• What should students be able to do regarding magnetic fields?

  • Describe how to plot the magnetic field pattern of a magnet using a compass.
  • Draw the magnetic field pattern of a bar magnet showing how strength and direction change from one point to another.
  • Explain how the behavior of a magnetic compass relates to evidence that the core of the Earth must be magnetic.
• What does a basic transformer consist of?


  A basic transformer consists of a primary coil and a secondary coil wound on an iron core.
• Why is iron used in transformers?

  Iron is used because it is easily magnetized.
• What is not required knowledge regarding transformers?


  Knowledge of laminations and eddy currents in the core is not required.
• What does the ratio of the potential differences across the primary and secondary coils depend on?

  The ratio depends on the ratio of the number of turns on each coil.
• What is the equation relating the potential differences across the primary and secondary coils of a transformer?


  \($\frac{V_p}{V_s} =$ \frac{n_p}{n_s}\)
• In a step-up transformer, what is true about the potential differences?


  In a step-up transformer, \(V_s > V_p\).
• In a step-down transformer, what is true about the potential differences?

  In a step-down transformer, \(V_s < V_p\).
• What would happen if transformers were 100% efficient?

  If transformers were 100% efficient, the electrical power output would equal the electrical power input.
• What is the equation for power output and input in transformers?


  \($V_s \times I_s =$ V_p \times I_p\)
• What do students need to recall regarding induced potential difference and current?

  Students should be able to recall the factors that affect the size and direction of the induced potential difference/induced current.
• What is the generator effect?

  The generator effect is when an electrical conductor moves relative to a magnetic field or when there is a change in the magnetic field around a conductor, inducing a potential difference.
• How does an induced current behave?

  An induced current generates a magnetic field that opposes the original change.
• What should students be able to explain regarding the generator effect?

  • Explain how the generator effect is used in an alternator to generate AC.
  • Explain how the generator effect is used in a dynamo to generate DC.
• How do microphones utilize the generator effect?

  Microphones use the generator effect to convert pressure variations in sound waves into variations in current in electrical circuits.
• What is the formula for the force on a conductor at right angles to a magnetic field carrying a current?


  \($F =$ BIl\)
• What do the symbols in the formula \(F = BIl\) represent?

  F is force in newtons, B is magnetic flux density in tesla, I is current in amperes, and l is length in meters.
• What is the basis of an electric motor?

  A coil of wire carrying a current in a magnetic field tends to rotate.
• How do loudspeakers and headphones work?

  Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to pressure variations in sound waves.
• What should students be able to explain regarding electromagnetism?

  • Describe how the magnetic effect of a current can be demonstrated.
  • Draw the magnetic field pattern for a straight wire carrying a current and for a solenoid.
  • Explain how a solenoid arrangement can increase the magnetic effect of the current.
• What does Fleming's left-hand rule represent?

  Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor, and the magnetic field.
• What factors affect the size of the force on a conductor?

  Students should be able to recall the factors that affect the size of the force on the conductor.
• What is the relationship between the current in a conductor and the magnetic field it produces?

  When a current flows through a conducting wire, a magnetic field is produced around the wire.
• How does shaping a wire into a solenoid affect the magnetic field?

  Shaping a wire into a solenoid increases the strength of the magnetic field created by a current through the wire.