7.5 - Magnetic Fields

• When a magnetic field is perpendicular to a current carrying wire, does the wire feel a force?

  Yes, the magnitude of the force is F = BIlSee an expert-written answer!We have an expert-written solution to this problem!
• Fleming's left hand rule for motors represents what properties on what fingers?

  Thumb - Thrust/Force

  First Finger - Field (magnetic)

  Second Finger - Current
• What is magnetic flux density (B)?

  Flux density measured in Tesla (T) or Webers/meters² (Wb/m²), is the flux per metre².
• A charged particle moving through a field feels a force when it is travelling along the field lines or perpendicular to them?

  Perpendicular to the field
• Is the force applied to the particles applied perpendicular to the particles motion or in one direction?

  Perpendicular to its motion, causing it to move in a circular motion
• Which fields do cyclotrons use?

  Electric and Magnetic
• How does a Cyclotron work and what's the electric and magnetic fields purpose in a cyclotron?

  - A cyclotron is made up of 2 semicircular electrodes called "Dees" with a magnetic field applied perpendicular to the Dees and an alternating pd applied between the dees

  - Each dee is a metal electrode with opposite charges, this creates an electric field in the gap between the two dees. This is what accelerates particles.

  - The magnetic field causes the particles to move in a circular motion, which allows it to gain speed whilst minimising space. As they speed up the radius of their motion increases, until it breaks free tangential to one of the dees.
• What is magnetic flux (quantitative)?

  Φ = BA

  Flux = Flux Density x Area
• What is flux linkage?

  NΦ = The number of turns cutting the flux at one time.
• What is the flux linkage of a rectangular coil rotating through a magnetic field?

  NΦ = BANcosΘ
• What is Faraday's Law?

  The induced emf is directly proportional to the rate of change of flux linkage
• What is Lenz's Law?

  The induced e.m.f is always in such a direction as to oppose the change that caused it.

  
• What happens when you move a straight conductor through a magnetic field?

  The electrons experience a force pushing them to one end of the conductor creating an emf across the conductor. The rod obeys Faraday's Law, it is changing flux as it moves through the field, hence an EMF is induced
• What would be the emf produced when rotating a coil at a constant rate in a magnetic field?

  
• Describe how one would use an oscilloscope

  Oscilloscopes are used to displace AC waves, the x axis is called the time base and shows how long it takes the wave to move 1 division and the y axis shows how much PD is needed to move the wave up 1 division. Using this we find the peak voltage, time period and frequency.
• How does a transformer work?


  A primary coil wrapped around an iron core with an alternating p.d. creates an alternating magnetic field, this magnetic field induces an EMF in a secondary coil also wrapped around the core. This creates a current in the secondary coil.
• What kind of current is produced by a transformer and why?

  An alternating current.

  An emf is induced by a changing magnetic field, hence the emf induced is alternating, producing an alternating current
• Why are transformers used?

  By changing the number of coils, the transformers can be used to increase the voltage and reduce current when transporting power, with minimal energy loses. The voltage is then dropped again locally to ensure safe usage in households.
• In a step-up transformer, does the secondary or primary coil have more coils?

  The secondary coil - step-up transformers increase the voltage, hence more coils need to be in the secondary coil for a larger pd
• What is an eddy current?

  As the primary coils magnetic field induces emf in the secondary coil, it also induces emf and hence mini currents within the iron core.These are known as eddy currents.
• Why are eddy currents a problem?

  By Lenz's law the emf created and its field opposes that of the primary coil. This causes energy loss via resistive heating of the iron core by the eddy currents, which reduces efficiency.
• How can you reduce eddy current losses?

  Use a laminated iron core.

  Thin sheets of iron with an electrical insulator inbetween, which reduces the eddy currents circuit.