Magnetism

• define magnetic field


  a magnetic field is a region where magnetic materials and other magnets experience a force
• what does the arrangement of magnetic field lines show?


  the more tightly packed the magnetic field lines the strong the magnetic field
• what is a permanent magnet?

  A magnet that produces its own magnetic field all the time e.g nickel, cobalt, certain steels
• what is an induced magnet?

  an induced magnet loses its magnetism as soon as it is taken out of the magnetic field e.g iron
• how can you use a permanent magnet to determine another?

  the same end of a material that becomes an induced magnet will be attracted to both poles of a permanent magnet. whereas the same end of a permanent magnet will be attached to one pole of another permanent magnetic and repelled by the other pole
• how can a compass provide evidence that the earth has a magnetic feild?

  wherever we stand on the earth a compass always points to magnetic field (north pole of compass is attracted to magnetic north therefore magnetic north is a south pole)
• explain the relationship between magnetic field strength and distance from a current carrying wire

  as the distance from a current carrying wire increases magnetic field strength decreases. This is because magnetic field lines get further apart.
• how to prove if a solenoid has this shape magnetic feild?

  place a compass around and within it. compasses will point in direction of the magnetic field. Also, its magnetic force is uniform.
• what is the rule for labelling poles of a solenoid?

  if current circulates anticlockwise = north pole

  if current circulates clockwise = south pole
• how to make an electromagnet and what to remember

  solenoid + iron core = electromagnet

  (iron core is always magnetized so that it produces a magnetic field in the same direction as the magnetic field produced by the current in the coil.)
• Explain why iron rods in magnets move apart when the switch is shut

  Rods have become induced magnets due to the magnetic field produced by the current in the coil. The rods have the same polarity at each end and like poles repel. Therefore iron rods repel
• What 3 ways can we use electromagnets?

  Sorting magnetic and non-magnetic materials in a scrap yard

  Relay

  Doorbells
• Describe how/ why we use electromagnets in Sorting magnetic and non-magnetic materials in a scrap yard

  Steel is used as it's a magnetic material, so steel cans become magnetised and are attracted to the electromagnetic. Aluminium is not a magnetic metal so isn't attracted
• Describe how/why we use electromagnets in relay

  In some circuits with high potential difference supplies it is dangerous to use a normal switch due to risk of electrocution. A relay circuit always allows a switch in a much safer circuit to operate a switch in a more dangerous circuit.
• Door bells
• What is the motor effect?

  When we put the wire carrying the current between. 2 magnetic poles we have 2 magnetic fields (produced by current in wire and permanent magnet) the two magnetic fields interact producing a force on the current carrying wire
• Explain why a current carrying wire experiences a force when placed at a non-zero angle to a magnetic field

  The magnetic field produced by the current in the wire interacts with the magnetic field produced by the permanent magnet producing a force on the wire.See an expert-written answer!We have an expert-written solution to this problem!
• Give four ways to increase the size of a force on a wire

  . Increasing the current in wire by increasing potential difference across I= V/R.

  . use a stronger permanent magnet to increase magnetic field.

  . Increase length of wire to increase magnetic field.

  . Make angle between current and magnetic field closer to 90°.
• Define magnetic flux density

  Magnetic flux density is the name for the strength of a magnetic field. It is a vector quantity and the unit is Tesla.
• What is the equation for the motor effect?

  Force = magnetic flux density x current x length
• Explain why balance reading goes up when current flows through the rod

  When current flows through the rod, the magnet exerts an upwards force on the rod therefore according to Newton's law, the rod will accept an equal magnitude downwards on the magnet. This will cause balance reading to increase.
• Explain why a DC motor needs a split ring communicator and brushes

  Every time the coil passes the verse composition the second half of the split ring communicator swaps from one brush to the other this reverse the current in the coil which reverses the force on either side of the coil so it keeps rotating
• Explain why DC motor can rotate continuously

  The magnetic field produced by the current in the coil and the magnetic field produced by the permanent magnet interact producing a force on either side of the coil current flows in opposite directions on either side of the coil so the force is on either side of the coil act in opposite directions These forces produce moments in the same direction causing the coil to rotate as the coil passes the vertical position the 2nd half of the split ring communicator swap from one brush to the other. This causes current in the coil to reverse which reverses the force on either side of the coil keeping it rotating
• Explain the job of the two slip rings in an AC motor

  Each side of the coil is attracted to a slip ring which rotates with the coil and keeps an electrical contact with the supply via a graphite brush
• Explain how the loudspeaker converts current in an electrical circuit into sound waves

  The current in the coil produces a magnetic field this interacts with the magnetic field produced by the permanent magnet producing a force on the coil. The direction of the force on the coil can be found using Flemings left hand rule. Alternating current flows in the coil therefore the direction of force on the coil alternates too. This causes the coil to oscillate this causes the paper cone to oscillate producing sound waves.
• What is the generator effect?

  When potential difference is induced across the end of a coil that is within a magnetic field it becomes a source of potential difference therefore when a potential difference is induced across a coil which is parts of a complete circuit an induced current will flow
• What is the model explanation for coil moving in a magnetic field?

  When the coil moves through a magnetic field, the coil cuts magnetic field lines this causes potential difference to be induced across its ends. if the coil is part of a complete circuit an induced current will flow
• What is the model explanation for when coil is stationary and the magnet is moving?

  When a magnet moves near a stationary coil the magnetic field in the coil changes this causes potential difference to be induced across its end if the coil is part of a complete circuit and induced current will flow
• The magnetic field produced by _ in the coil will always ______

  The magnetic field produced by current in the coil will always oppose the change that causes it
• The magnetic field produced by current in the coil will always oppose the change that causes it - what does this mean?

  Means that the magnetic field produced by the induced current will always cause a magnetic force to act on either the moving magnet or the moving coil in the opposite direction to its motion
• What is the function of a moving coil microphone?

  To use the generator to convert the pressure variations in sound waves into variations in current in electrical circuits
• How does a moving coil microphone work?

  The sound waves cause the diaphragm to oscillate. The coil is connected to the diaphragm so the diaphragm causes the coil to oscillate within the magnetic field, as the coils cuts magnetic field lines a potential difference in induced across its ends and an induced current flows in the electric circuit. Because the coils cuts magnetic is oscillating the induced potential difference and current alternates ( at the same frequency as the sound waves ) changing direction every time the coil changes direction.