6.3 - Electromagnetism

• Magnetic field 


  A region of space where charged particles are subject to a magnetic force. The force is caused by the interaction between two magnetic fields
• Magnetic field line

  The path from north to south when placed in a magnetic field
• How is the strength of a magnetic field represented

  How close the lines are

  Closer lines = stronger field
• Magnetic flux density

  Force per unit current per unit length on a current carrying conductor placed in a magnetic field perpendicular to the field lines
• Right hand thumb rule

  
• Solenoid


  A cylindrical coil of wire acting as a magnet when carrying electric current

  
• Motor effect 


  When a current carrying conductor is placed in a magnetic field it experiences a force. This force is perpendicular to the flow of current and the field lines causing it to be pushed out of the field
• Flemings left hand rule

  Thumb - force

  First - field

  Second- current

  
• Magnetic flux

  The product of the magnetic flux density and the area perpendicular to the field lines
• Magnetic flux linkage

  The magnetic flux of an entire coil of wire - product of the magnetic flux and the number of turns on the coil
• Len'z law

  Lenz’s law states that the induced emf is generated in a direction so that it opposes the change that produced it.
• Len'z law

  Lenz’s law states that the induced emf is generated in a direction so that it opposes the change that produced it.
• Lenz law in terms of energy 


  Follows the principle of conservation of energy. If induced emf was in a direction that aided the change which caused it, it would be creating electrical energy from nowhere
• Faradays law

  The induced emf in a circuit is proportional to the rate of change of flux linkage throughout the circuit
• What is a search coil

  A flat coil of insulated wire connected to an ammeter (galvanometer). It can be used to determine magnetic flux density from the current induced in the coil when it is moved out of a magnetic field
• A.C generator structure

  Rectangular coil which spins in a uniform magnetic field The flux linkage in the coil will be continuously changing, inducing an alternating current in the coil

  
• How do emf and flux linkage change vary in an AC generator

  Emf is maximum when coil is parallel to the field lines

  Flux linkage is maximum when coil is perpendicular to field lines

  
• How can the peak emf of an A.C generator be increased?

  Increase speed of rotation Increase magnetic flux density of the field Increase the cross sectional area of the coil Increase the number of turns on the coil
• Structure of a transformer

  Two coils wrapped around the two sides of a laminated iron core. In a step-up transformer, the secondary coil has more turns In a step-down transformer, the primary coil has more turns

  
• How does a transformer work?

  An alternating current is run through the primary coil, inducing an alternating magnetic field in the iron core. This will induce an alternating emf in the secondary coil
• Why are transformers used in the national grid


  In the national grid step-up transformers are used to increase the voltage (and decrease current) pefore the electricity travels long distances. This is to reduce the energy lost as heat due to the resistance in the wires.
• Describe an experiment to investigate the relationship V1/V2 = N1/N2


  Vary the number of turns on each of the coils and measure the peak potential difference of each one every time. Then use the values to prove the relationship between potential difference and number of turns. It is better to use an oscilloscope than a voltmeter for an alternating p.d since it is easy to read peak value
• The Cyclotron:

  When a particle reaches the gap in

  the accelerator between the two

  magnetic fields, the alternating

  supply makes the second field an opposite charge to the particle. This causes acceleration across the gap

  into the next field.

  Continuously happening until the particle reaches the speed required to leave the cyclotron.

  F=BQr=mv

  v is proportional to r
• Why do charged particles move in a circular orbit in a magnetic field?

  Force is perpendicular to the velocity of the particle, so they end up being forced into a circular orbit. The particles undergo centripetal acceleration with the centripetal force being the magnetic force
• Why do charged particles move in a circular orbit in a magnetic field?

  Force is perpendicular to the velocity of the particle, so they end up being forced into a circular orbit. The particles undergo centripetal acceleration with the centripetal force being the magnetic force
• Experiment to measure magnetic flux density

  Place horseshoe magnet on c digital balance and zero it Connect wire to DC supply and ammeter Align wire so the force acts upwards , meaning there will be a downwards force on the magnet Measure the length of wire in the field Record mass on balance Plot graph of current against mass. mg = BIL, r L and g are known
• How a transformer works (workbook)

  Changing (AC) current produces changing flux

  Changing the magnetic flux through the iron core

  Emf induced from flux linkage in 2nd coil
• Efficieny equations for transformers


  Ns/Np=u x Vs/Vp

  Where u is Efficiency
• Eddy current 


  Current that switches back on itself

  
• How can the efficiency of a transformer be increased

  Low resistance wires - less ohmic heating (larger area for current)

  High permeable of the core means more flux linkage reaches secondary coil,

  Laminated core (with insulation between layers) reduces eddy current of magnetic field so more flux linkage in second coil