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Physics Notes For SSC CGL / CHSL Exam 2019-20 on : Electric Motors

Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by a magnetic field on an electric current. They transform electrical energy into mechanical energy.

Principle:

It is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand rule and whose magnitude is given by

Force, F = B I l newton

Where B is the magnetic field in weber/m2.

I is the current in amperes and

l is the length of the coil in meter.

The force, current and the magnetic field are all in different directions.

How an electric motor works

The link between electricity, magnetism, and movement was originally discovered in 1820 by French physicist André-Marie Ampère (1775–1867) and it’s the basic science behind an electric motor. But if we want to turn this amazing scientific discovery into a more practical bit of technology to power our electric mowers and toothbrushes, we’ve got to take it a little bit further. The inventors who did that were Englishmen Michael Faraday (1791–1867) and William Sturgeon (1783–1850) and American Joseph Henry (1797–1878). Here’s how they arrived at their brilliant invention.

Parts of a Electric Motor

    • Insulated Copper wire: A rectangular coil of wire ABCD
    • Magnet Poles: A magnet as placed above ie North Pole and South Pole. This creates a magnetic field as shown above.
  • Split Rings: Two disjoint C-shaped rings P and Q. It acts as a commutator (which can reverse the direction of current)
  • Axle: The split rings are placed on the axle which can rotate freely.
  • Brushes: The outside of the split rings are connected to conducting brushes X and Y.
  • Source Battery: To source current.

Working

  • When the current begins to flow, current flows through brush X, then A to B, B to C, C to D and then to brush Y and into the battery.
  • Now applying Fleming’s Left Hand Rule to wire AB, Current is along AB, Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
  • Now applying Fleming’s Left Hand Rule to wire CD, Current is along CD, Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
  • The rectangular coil begins to move in the anti-clockwise direction
  • Note that during anti-clockwise motion, the split rings and axle also move, whereas the brushes don’t move.
  • After half a rotation, Wire CD and Split ring Q moves to the left.  Wire AB and Split ring P moves to right. Brushes X and Y donot move.
  • Now applying Fleming’s Left Hand Rule to wire CD, Current is along DC. (Battery -> Split ring Q -> DC , Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
  • Now applying Fleming’s Left Hand Rule to wire AB, Current is along BA. (Battery -> Split ring Q -> DC –> CB -> BA –> Split ring P) , Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
  • So, again the coil rotates in the anti-clockwise direction.
  • The reversal of current in the coil results in the continous rotation of the coil. The reversal of current is achieved by the commutator rings

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