7 - Magnetism + Electomagnetism

• On a magnet, what places are the magnetic forces the strongest?

  the poles
• What happens if two magnets are brought close together?

  they exert a non-contact force on each other
• What is the difference between 'attraction' and 'repulsion'?

  - when two LIKE poles (eg. north and north) are brought close together, they REPEL each other (move apart)

  
  

  - when two UNLIKE poles (eg. north and south) are brought together, they ATTRACT each other (move towards)

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

  - a permanent magnet produces its OWN magnetic field, eg. a bar magnet

  
  

  - an induced magnet is a material that BECOMES A MAGNET when it is placed in a magnetic field
• What is key about induced magnetism?

  - 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 the 'magnetic field'?

  - the region around a magnetic where a force acts on another magnet or on a magnetic material
• What are the 4 types of magnetic material?

  - iron

  - steel (alloy of iron)

  - cobalt

  - nickel

  
  

  (all 4 of these materials can be made into a permanent magnet or an induced magnet)
• What happens when a magnetic material is placed into a magnetic field?

  - the magnetic material will experience a FORCE OF ATTRACTION towards the magnet (material becomes an induced magnet)
• What does the strength of the magnetic field depend on?

  - the distance from the magnet (the magnetic field is strongest at the poles of the magnet)
• How can the direction of a magnetic field be found?

  - 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 pole of a magnet TO THE SOUTH pole of the magnet

  
• From magnetic fields drawn by using a compass, how do we know that the field is strongest at the poles?

  - the field lines at the poles are closest together

  
• How do we know that the Earth has a magnetic field?

  - a magnetic compass contains a small bar magnet

  - if we hold a compass away from any magnets, the needle always points in the N-S direction (the compass lines up with the direction of the Earth's magnetic field)

  - the earth's magnetic field is produced by the convection currents in the Earth's core which is made from iron and nickel
• What happens when a current flows through a conducting wire?

  - a magnetic field is produced around the wire
• What does the strength of the magnetic field around a conducting wire depend on?

  - the size of the current

  - the distance from the wire
• What is the proof that there is a magnetic field around a wire when the current is switched on?

  - deflects the direction of a compass
• What happens if we change the direction of the current ina circuit?

  - the direction of the magnetic field also changes (it is reversed)

  - the compass would deflect in the opposite direction to before
• How can we use the right hand grip rule to work out the direction of the magnetic field produced by a wire?

  1) Place your right hand so that the thumb is pointing in the direction of the conventional current.

  2) You will see that your fingers are pointing in the direction of the magnetic field.

  
• How can the shape of the wire be changed to increase the strength of the magnetic field produced by the wire?

  - to coil the wire like a slinky instead of it being straight

  - this shape is known as a SOLENOID

  
• What is the magnetic field like inside a solenoid?

  - strong

  - uniform

  (similar magnetic field shape to that of a bar magnet)

  
• How can we increase the strength of the magnetic field by adjusting the wire?

  - we can increase the number of coils of the wire
• How can we increase the strength of the magnetic field by adding to the circuit?

  - we can place an piece of iron inside the solenoid (iron core)
• What is an 'electromagnet'?

  a solenoid containing an iron core
• Why are electromagnets useful?

  - we can change the strength of the magnetic field by changing the size of the current

  - electromagnets can also be switched on or off
• Why is switching high voltage circuits on and off dangerous?

  
  

  What is a solution to this problem?

  - we can get sparking

  - there is a risk of electrocution

  
  

  => we can use a relay to turn circuits like this on and off
• What does a relay consist of?

  two circuits:

  
  

  - a low voltage circuit with an electromagnet (safe to be switched on and off)

  
  

  - two metal contacts replace the switch in the high voltage circuit (one of the contacts is connected to a spring which keeps the contacts apart)

  - there is also an iron block next to the spring
• What happens if the low voltage circuit is turned off in a relay?

  - no current flows through the electromagnet => no magnetic field => high voltage circuit is also turned off (contacts are not touching)
• How does a relay work to make it safe to turn on a high voltage circuit?

  - a current flows around the circuit => magnetic field is created around the electromagnet which attracts the iron block in the high voltage circuit => contacts touch and close to finish the circuit => high voltage circuit turns on
• How does an electromagnetic doorbell device work?

  - switch is closed when the buzzer is pressed => current can flow through the circuit => magnetic field is produced by the electromagnet

  => this attracts the iron contact => clapper hits the bell (circuit also breaks at the same time)

  
  

  - when the circuit breaks, the iron contact springs back into its orginal position => circuit is complete again => current flows around the ciruit and the process is repeated

  
• What is the 'motor effect'?

  - when a conductor carying a current is placed in a magnetic field, the magnetic producting the field and the conduct exert a force on each other
• What is the equation to calculate the size of the force in the motor effect?

  
  

  What is important about using this equation?

  magnetic flux density (T) x current (A) x length of conductor (m)

  (B, in tesla, is a measure of the strength of the magnetic field)

  
  

  - only applies to a wire which is at right angles to the magnetic field

  
• How can we use Fleming's left hand rule to determine the direction of the force in the motor effect?


  1) Place your thuMb, First finger and seCond finger so they are at right angles.

  2) Point your First finger in the direction of the magnetic Field (north to south).

  3) Point your seCond finger in the direction of the Conventional current (positve to negative).

  4) The direction that your thuMb is pointing shows you the direction of the Motion (the force).

  
• In what case will the conductor experience no force?

  - if the conductor is parallel to the magnetic field (eg. wire is pointing in the same direction as the field line), the conductor will not experience a force
• How does an electric motor rotate?

  - in a loop of wire, the current runs is opposite directions on either side of the loop

  - if the wire is placed in a magnetic field, it experiences a force on both the left and right sides (one acting upwards and one acting downwards)

  - we have a moment on both sides => loop rotates in the clockwise direction

  
• What problem occurs when the loop in the electric motor rotates 90°?

  
  

  What device can we use to solve this problem?

  - it only rotates for a short time, due to momentum, beyond this point, and then stops rotating

  
  

  => we can switch the direction of the current when the loop passes 90° by using a split-ring commutator (a split metal ring which is connected to conducting brushes that allow the electric current to pass onto the ring) which swaps the direction of the current each half turn to allow the motor to keep rotating in the same direction

  
• Why do loudspeakers and headphones use the motor effect?

  - to convert variations in current in electrical circuits to the pressure variations in ssound waves
• How do moving-coil loudspeakers and headphones work?

  - there is a cone which has a coil of wire wrapped around one end and connected to an A.C. electrical supply

  - there is also a permanent magnet which goes inside the coil of wire

  - as the current passes through the coil, it generates a magnetic field (this field interacts with the magnetic field from the permanent magnet) => these magnetic fields either attract or repel each other => produces a resultant force => cone moves

  - when the current switches direction, the direction of the force on the cone reverses => cone moves in and out, generating sound waves

  
• In a loudspeaker, how can we increase the amplitude of the vibration?

  - by increasing the size of the current => increases volume
• In a loudspeaker, how can we change the pitch of the sound?

  - by changing the frequency of the A.C. supply, we can change the frequency that the cone vibrates (eg. higher frequency => higher pritch sound)
• How is an induced potential difference made?


  - 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
• What happens to the induced potential difference if the wire stops moving?

  
  

  What happens to the induced potential difference is the wire moves downwards?


  - when the wire stops moving, the potential difference is lost

  
  

  - if the wire is moved back down through the magnetic field, the potential difference is induced again (however, the potential difference has reversed direction)