Magnetic Effect of a Current

• When a current flows through a conducting wire a magnetic field is produced around the wire

  • A conducting wire is any wire that has current flowing through it
• The shape and direction of the magnetic field can be investigated using plotting compasses

  • The compasses would produce a magnetic field lines pattern that would like look the following

  
• The magnetic field is made up of concentric circles

  • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
• As the distance from the wire increases the circles get further apart

  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
• The right-hand thumb rule can be used to work out the direction of the magnetic field

  A) current

  B) direction

  C) field

  D) direction

  E) current

  
• If there is no current flowing through the conductor there will be no magnetic field
• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field

  • This means the field lines will become closer together
• When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it!

  
• • Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field

  A) magnetic field lines

  B) dot

  C) centre

  D) out

  E) cross

  F) into

  
• To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid

  • The magnetic field around the solenoid is similar to that of a bar magnet!

  
• The magnetic field inside the solenoid is strong and uniform
• One end of the solenoid behaves like the north pole of a magnet, the other side behaves like the south pole

  • To work out the polarity of each end of the solenoid it needs to be viewed from the end
  • If the current is travelling around in a clockwise direction then it is the south pole
  • If the current is travelling around in an anticlockwise direction then it is the north pole
• If the current changes direction then the north and south poles will be reversed
• If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid!

  A) N

  B) S

  C) current

  
• A solenoid can be used as an electromagnet by adding a soft iron core
• The iron core will become an induced magnet when current is flowing through the coils

  • The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall
• The magnetic field produced by the electromagnet can be switched on and off

  • When the current is flowing there will be a magnetic field produced around the electromagnet
  • When the current is switched off there will be no magnetic field produced around the electromagnet

  
• Changing the direction of the current also changes the direction of the magnetic field produced by the iron core
• The strength of the magnetic field produced around a solenoid can be increased by:

  • Increasing the size of the current which is flowing through the wire
  • Increasing the number of coils
  • Adding an iron core through the centre of the coils
• The strength of an electromagnet can be changed by:

  • Increasing the current will increase the magnetic field produced around the electromagnet
  • Decreasing the current will decrease the magnetic field produced around the electromagnet
• When trying to figure out how an electromagnetic device works:

  • Look for a coil / solenoid - this is going to act as an electromagnet
  • Look for a piece of iron - this will be attracted to the solenoid
• Electromagnets are used in a wide variety of applications, including:

  • Relay circuits (utilised in electric bells, electronic locks, scrapyard cranes etc)
  • Loudspeakers & headphones
• Electromagnets are commonly used in relay circuits

  • Relays are switches that open and close via the action of an electromagnet
• A relay circuit consists of:

  • An electrical circuit containing an electromagnet
  • A second circuit with a switch which is near to the electromagnet in the first circuit
• Relay circuit diagram

  A) no

  B) coil

  C) off

  D) open switch

  E) on

  F) coil

  G) on

  H) closed switch

  
• When a current flows through Circuit 1, a magnetic field is induced around the coil

  • The magnetic field attracts the switch, causing it to pivot and close the contacts in Circuit 2
  • This allows a current to flow in Circuit 2

  
• When no current flows through Circuit 1, the magnetic force stops

  • The electromagnet stops attracting the switch
  • The current in Circuit 2 stops flowing

  
• Scrapyard cranes utilise relay circuits to function:

  • When the electromagnet is switched on it will attract magnetic materials
  • When the electromagnet is switched off it will drop the magnetic materials
• Electric bells also utilise relay circuits to function, when the button K is pressed:

  • A current passes through the electromagnet E creating a magnetic field
  • This attracted the iron armature A, causing the hammer to strike the bell B
  • The movement of the armature breaks the circuit at T
  • This stops the current, destroying the magnetic field and so the armature returns to its previous position
  • This re-establishes the circuit, and the whole process starts again

  
• Loudspeakers and headphones convert electrical signals into sound

  • They work due to the motor effect
• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet

  A) magnet

  B) coil

  C) speaker come

  
• An alternating current passes through the coil of the loudspeaker

  • This creates a changing magnetic field around the coil
• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing
• The magnetic field produced around the coil interacts with the field from the permanent magnet
• The interacting magnetic fields will exert a force on the coil

  • The direction of the force at any instant can be determined using Fleming’s left-hand rule
• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction

  • This makes the coil oscillate
• The oscillating coil causes the speaker cone to oscillate

  • This makes the air oscillate, creating sound waves
• right hand grip rule - thumb pointing same as current and fingers the same as magnetic field
• magnetic relay - switch operated by electromagnet

  • a small switch with thin wires turn current on digitally
  • electromagnet gets the relay running as it attracts and iron amature, pivoting and closing the contacts
  • used for high currents as it’s safer

  
• Electromagnet in loudspeakers:

  • current creates electromagnetic field
  • field interacts with permanent magnet which creates forces that push the cone outward
  • current is made to flow in opposite directions
  • direction of field reverses
  • force pulls core back
  • repeatedly alternating directions make the cone vibrate in and out
  • variations in air create sound waves