P15 - Electromagnetism

• Magnetic compass


  A tiny magnetic needle pivoted at its centre

  
• Because of the Earth's magnetic field, one end of the compass always points north, and the other end always points south

  
• North-seeking pole (N-pole)


  The end of a plotting compass or a bar magnet that points north
• South-seeking pole (S-pole)


  The other end of a plotting compass or a bar magnet
• Induced magnetism


  Magnetism created in an unmagnetised magnetic material when the material is placed in the magnetic field

  
• Permanent magnets


  Made of steel because magnetised steel does not lose its magnetism easily
• Plotting a magnetic field

  1. Mark a dot near the north pole of a bar magnet

  2. Place the tail of the compass needle above the dot, and mark a second dot at the tip of the needle

  3. Repeat the procedure with the tail over the new dot each time until the compass reaches the S-pole of the magnet

  4. Draw a line through the dots and mark the direction from the N-pole to the S-pole

  5. Repeat the procedure for further lines

  
• Like poles repel, and unlike poles attract
• The magnetic field lines of a bar magnet curve around from the north pole of the bar magnet to the south pole

  
• Induced magnetism is magnetism created in an unmagnetised magnetic material when the material is placed in the magnetic field

  
• Steel is used instead of iron to make permanent magnets because steel does not lose its magnetism easily, but iron does
• Pattern of the magnetic field around a straight wire carrying a current

  • The lines of force caused by a straight current-carrying wire are a series of concentric circles centred on the wire in a plane that is perpendicular to the wire

  
• Uniform magnetic field


  A magnetic field in which the magnetic field lines are parallel
• Electromagnet


  A solenoid in which the insulated wire is wrapped around an iron bar (the core)
• Reversing the direction of the current


  Reverses the direction of the magnetic field
• Increasing the current


  Increases the strength of the magnetic field
• Magnetic field inside a solenoid

  • The magnetic field is strong and uniform, with the field lines parallel to the axis of the solenoid

  
• Magnetic field outside a solenoid

  • The magnetic field lines bend around from one end of the solenoid to the other end, like the field of a bar magnet

  
• When the current is switched off, the iron bar of an electromagnet loses most of its magnetism
• Iron, not steel, is used for the core of an electromagnet
• Magnetic field in and around a solenoid


  • When there is a current in the solenoid

  
• Showing a plotting compass at one end of the solenoid


  Indicate the direction that the plotting compass points
• Describing and explaining the effect on the plotting compass

  Of reducing the current in the solenoid

  
• The magnetic field lines around a wire are circles centred on the wire in a plane perpendicular to the wire

  
• The magnetic field lines in a solenoid are parallel to its axis and are all in the same direction

  
• A uniform magnetic field is one in which the magnetic field lines are parallel
• Increasing the current


  Makes the magnetic field stronger
• Reversing the direction of the current


  Reverses the magnetic field lines
• Electromagnet


  A solenoid that has an iron core. It consists of an insulated wire wrapped around an iron bar
• Electromagnets


  • Used in scrapyard cranes, circuit breakers, electric bells, and relays
• How a circuit breaker works

  1. Contains an electromagnet in series with a switch

  2. Too much current passes through the coil

  3. The iron core of the electromagnet is magnetised

  4. The switch is attracted to the core of the electromagnet

  5. The current is cut off

  6. The circuit breaker switch is opened

  
• How an electric bell works

  1. When connected to a battery, the iron armature is pulled on to the electromagnet

  2. This opens the make-and-break switch

  3. The electromagnet is switched off

  4. The armature springs back and the make-and-break switch closes again

  5. The whole cycle repeats itself
• How a relay works

  1. A small current through the coil of the electromagnet magnetises the iron core

  2. The iron armature is pulled onto the electromagnet

  3. This closes the switch gap and switches the machine on

  
• The purpose of a circuit breaker is to cut off the current when it gets too high
• The buzzer vibrates when connected to a battery because the armature is pulled on to the electromagnet, opening the make-and-break switch, and then springing back to close the switch again, repeating the cycle
• The buzzer vibrates at a higher frequency than the electric bell because it has no striker or bell to slow down the vibrations
• Investigating the strength of an electromagnet

  1. Attach a known weight to an iron bar held by the electromagnet

  2. Measure the smallest current needed to hold the load

  3. Increase the weight and repeat the measurement for different known weights

  4. Repeat the measurements with more turns on the electromagnet

  5. Plot the measurements on a graph of current against weight and use the graph to draw conclusions about the factors that affect the strength of the electromagnet

  
• The force on a current-carrying wire in a magnetic field is greatest when the wire is perpendicular to the magnetic field lines, and zero when the wire is parallel to the magnetic field lines

  
• The direction of the force is always at right angles to the wire and the field lines, and the direction of the force is reversed if either the direction of the current or the magnetic field is reversed

  
• Magnetic flux density


  The measure of the strength of the magnetic field, with the symbol B and the unit tesla (T)