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User:Inconspicuum/Physics (A Level)/Magnetic Fields Questions

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Flux

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  1. A circular steel core has a cross-sectional area of 9 cm2, and a length of 0.5m. If the permeability of steel is 875 μNA-2, what is the permeance of that core?
  2. A coil of insulated wire is wrapped 60 times around the top of the core, and a 9A direct current is put through the coil. How much flux is induced?
  3. Assuming that all the flux goes through the core, what is the flux density at any point in the core?
  4. Draw a diagram showing the lines of flux within the core.

Induction

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  1. What is the flux linkage of a 30cm coil of 0.5mm thick wire with a flux perpendicular to it of 10Wb?
  2. If the above coil is crushed steadily over a period of 2s, what emf is maintained?
  3. The flux in a flux circuit varies according to the equation Φ = sin ωt. What is the equation for the emf induced?
  4. Using a constant k, what is the equation for a current which could induce the flux in the flux circuit above?
  5. Draw a graph of the flux, flux linkage, emf and current as deduced in the previous two questions.

Force

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  1. What force is exerted by a 1T magnetic field on an electron (of charge -1.6 x 10-19C) moving at 5% of the speed of light (3 x 108 ms-1)?
  2. What force is exerted by a 5mT magnetic field on a 20cm wire with resistance 1μΩ attached to a 9V battery?
  3. The following diagram shows a positive charge moving through a magnetic field. Draw an arrow representing the direction of the force on the charge.

 

  1. The following diagram shows a wire in a magnetic field. Draw an arrow representing the direction of the force on the wire.

 

Transformers

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  1. A step-down transformer has 300 coils on one coil, and 50 coils on the other. If 30 kV AC is put in, what voltage comes out?
  2. A step-up transformer has 200 coils on one coil, and 980 coils on the other. If 25 kV AC comes out, what voltage was put in?
  3. An ideal transformer transforms a 50A current into a 1A current. It has 40 coils on the primary coil. How many coils are in the secondary coil?
  4. Transformers tend to vibrate. Why is this? What effect does this have on the efficiency of the transformer?
  5. Air does have some permeability. What effect does this have on the efficiency of the transformer? Why?

Motors

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  1. How could you adapt the simple DC motor to use AC?
  2. Why does a three-phase motor have a constant angular velocity?
  3. What is the difference between a split-ring and a slip-ring commutator?
  4. How could the angular velocity of a three-phase motor be increased?
  5. A squirrel-cage motor relies on eddy currents running along the rotor to function. However, if eddy currents run across the rotor, then the force on the rotor is reduced. How may these eddy currents be reduced without reducing the desired eddy currents?

Generators

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  1. Draw diagrams of an alternating current, the 'direct current' produced by a DC generator, and this current once it has been smoothed with a capacitor.
  2. What is the phase difference (in radians) between the voltages produced by a three-phase generator?
  3. According to Faraday's law, what three things will increase the amplitude of the emf created by a generator?
  4. If an albatross touched two power cables carrying AC in phase, what would happen?
  5. What would happen if the two cables carried three-phase power?
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