P7 - Magnetism

• What are the poles of a magnet?

  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.
• What is the difference between a permanent and induced magnet?

  A permanent magnet produces its own magnetic field. An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.
• What is a magnetic field?

  The region around a magnet where a force acts on another magnet or on a magnetic material is called the magnetic field. The force between a magnet and a magnetic material is always one of attraction. The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north(seeking) pole of a magnet to the south(seeking) pole of the magnet.
• Strength of magnetic fields?

  The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.
• What are the magnetic materials?

  Iron, steel, cobalt, and nickel
• How is the behaviour of a magnetic compass evidence that the core of the earth must be magnetic?

  A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.
• How to plot the magnetic field pattern of a magnet using a compass?

  1. Place the plotting compass near the magnet on a piece of paper.
  2. Mark the direction the compass needle points.
  3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.
  4. Join the points to show the field lines.
• Electromagnetism

  • 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.
  • 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. The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid. 
• What is an electromagnet?

  A solenoid with an iron core
• 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 Left Hand Rule

  Fleming’s Left Hand Rule is a rule used to find the direction of force/motion of the conductor in an electric motor when the magnetic field direction and the current direction are known. It states that if we arrange our thumb, forefinger and middle finger of the left hand perpendicular to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the current.

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

  Current, length of conductor, magnetic field strength
• State the equation to calculate force for a conductor at right angles to a magnetic field and carrying a current

  Force = magnetic flux density x current x length

  F = B I l
• Electric motors

  A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
• How does the force on a conductor in a magnetic field cause the rotation of the coil in an electric motor?

  • current in the left-hand side of the coil causes a downward force, and current in the right hand side of the coil causes an upward force;
  • the coil rotates anticlockwise because of the forces are in opposite directions;
  • each side of the coil is now near the opposite magnetic pole.

  
• How can you reverse the direction of rotation in an electric motor?

  • reversing the direction of the current OR
  • reversing the direction of the magnetic field (changing over the north and south poles).
• How can you increase the speed of rotation in an electric motor?

  • increasing the size of the current;
  • using a stronger magnet;
  • increasing the number of turns of wire in the coil;
  • reducing friction between the coil and the axel it rotates on.
• Loudspeakers

  Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
• How do moving coil loudspeakers and headphones work?

  • Using an electromagnet to produce a magnetic field.
  • Applying audio voltages across the plates.
  • Causing variations in the magnetic field produced by the electromagnet.
  • Moving a cone back and forth.
  • Creating pressure variations in the air and forming sound waves.
• Induced potential

  If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
• What factors affect the size of the induced potential difference / induced current?

  1. Magnetic field strength
  2. Number of turns in the coil
  3. Area of the coil
  4. Speed of the relative motion between the coil and the magnetic field
• What factors affect the direction of the induced potential difference / current?

  1. Magnetic field strength
  2. Area of the loop
  3. Angle between the magnetic field and the loop's normal
  4. Number of turns in the loop
• What is the generator effect

   Used in an alternator to generate ac and in a dynamo to generate dc.
• How is the generator effect used to generate alternate current in an alternator?

  A coil in-between a magnet rotates due to the magnetic field, and as the coil rotates, a current is induced across it. Every half rotation the current changes direction, hence producing an alternating current and potential difference. As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction, producing a current that is constantly changing

  
• How is the generator effect used to generate direct current in a dynamo?

  In a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction.

  
• Dynamo output on a graph

  The output of a rotating dynamo can be shown on a potential difference-time graph. The graph shows a sine curve that stays in the same direction all the time. The maximum potential difference or current can be increased by:

  • increasing the rate of rotation
  • increasing the strength of the magnetic field
  • increasing the number of turns on the coil

  
• Alternator output on a graph

  The output of an alternator as it rotates can be represented on a potential difference-time graph with potential difference (voltage) on the vertical axis and time on the horizontal axis.

  The graph shows an alternating sine curve. The maximum potential difference or current can be increased by:

  • increasing the rate of rotation
  • increasing the strength of the magnetic field
  • increasing the number of turns on the coil
• Microphones

  Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
• How does a microphone work?

  In a moving-coil microphone:

  1. pressure variations in sound waves cause the flexible diaphragm to vibrate
  2. the vibrations of the diaphragm cause vibrations in the coil
  3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
  4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
  5. the changing size and direction of the induced current matches the vibrations of the coil
  6. the electrical signals generated match the pressure variations in the sound waves

  
• Transformers

  • A basic transformer consists of a primary coil and a secondary coil wound on an iron core.
  • Iron is used as it is easily magnetised.
  • The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns.
• How is the effect of alternating current in one coil inducing a current in another used in transformers?

  When a transformer is working:

  1. a primary voltage drives an alternating current through the primary coil
  2. the primary coil current produces a magnetic field, which changes as the current changes
  3. the iron core increases the strength of the magnetic field
  4. the changing magnetic field induces a changing potential difference in the secondary coil
  5. the induced potential difference produces an alternating current in the external circuit

  
• Electrical Power

  Power = Voltage x Current

  Assuming that a transformer is 100 per cent efficient, the following equation can be used to calculate the power output from the transformer:

  potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil