12.5 The Motor Principle When English physicist Michael Faraday saw that an electric current in a wire caused a compass needle to move, he was curious if the reverse could be true. Could a magnetic field cause a current-carrying conductor to move? Supporting a bar magnet in a pool of conducting liquid mercury, Faraday suspended a copper wire, allowing it to make contact with the pool. The wire and the mercury were connected to a battery to complete the circuit. Once connected, the wire rotated around the bar magnet: The first electric motor.

12.5 The Motor Principle The copper wire in Faraday’s experiment was able to rotate around the permanent bar magnet because the magnetic field in the copper wire interacted with the magnetic field of the bar magnet. Where two interacting magnetic fields are pointing in the same direction, there is a repulsion force; when the interacting fields are pointing in the opposite direction, there is an attraction force. Here the fields interacting cause a downward force.

12.5 Right Hand Rule for the Motor Principle Motor Principle A current-carrying conductor that cuts across an external magnetic field experiences a magnetic force perpendicular to both the external magnetic field and the direction of electric current. Right Hand Rule for a the Motor Principle (RHR #3): If the fingers of your open right hand point in the direction of the external magnetic field, and your right thumb points in the direction of the conventional current, then your palm faces in the direction of the magnetic force on the conductor.

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12.6 The Direct Current Motor A simple DC motor uses an electric current in a looped conductor, which generates a magnetic field that interacts with an external magnetic field to cause rotation. How do you make this motion continuous? Scientists wanted to find a way to temporarily interrupt the current flow, to change its direction, thus changing the direction of the magnetic force produced One simple, but ingenious idea was to use a split-ring commutator. Brushes, made out of conducting bristles, make contact with the commutator, but still allow rotation.

12.6 DC Motor – Step 1 A conventional current is directed from the +ve terminal through the left brush, into the purple end of the split- ring commutator. The charges flow into the left end of the loop and exit from the right side into the pink end of the commutator, through the right brush, back to the -ve terminal. Using RHR #3 for the motor principle, the magnetic force produced acts downward on the left side of the loop, and upwards on the right side of the loop. This starts the counter-clockwise rotation.

12.6 DC Motor – Step 2 The motor continues to rotate counter- clockwise as in Step 1. The current is directed into the purple end of the split-ring commutator. The charges continue to flow into the left end of the loop and exit from the right side into the pink end of the commutator, through the right brush. The forces continue to act in the same direction, as per RHR#3.

12.6 DC Motor – Step 3 The wire loop has now rotated to the split. At this point, the circuit is now open; there is no current flow, and no more magnetic fields being produced by the loop of wire. The loop however, continues to rotate forward due to its inertia.

12.6 DC Motor – Step 4 The current is now directed through the left brush, however into the pink end of the split-ring commutator. The charges continue to flow into the left end of the loop and exit from the right side into the purple end of the commutator, through the right brush, back to the -ve terminal. Using RHR #3, the magnetic force produced continues to act downward on the left side of the loop, and upwards on the right side of the loop. The counter-clockwise rotation continues. Every half turn the loop makes, the current will be interrupted, continuously changing which end of the commutator it enters, keeping the loop turning.

12.6 Improving the Design A motor with one loop will not be very strong. In order to improve the strength of the magnetic field in the loop, you can; Increase the number of loops, increase the current, and include a soft-iron core Increasing the current however is not a desirable choice because it produces more thermal energy as a side effect. Designers typically focus on more loops and include a soft iron core called an armature. In order to maintain a high constant speed of rotation, several coils are put into the motor and several splits are included in the commutator.

12.6 Armature DC Motor – Step 1 A current is directed from the +ve terminal through the lower brush, into split-ring B. The charges flow around the coil, upwards on the front side, and exit from split-ring A, through the upper brush and back to the -ve terminal. Instead of using RHR #3 for the motor principle, we use RHR #2 for coiled conductors. The north pole produced on the left side of the armature repels the north pole of the external magnet. This starts the clockwise rotation.

12.6 Armature DC Motor – Step 2 The current is still directed into split-ring B and the clockwise rotation continues. At this point of the rotation, the north pole of the armature is now attracted to the south pole of the external magnet on the right side; the south pole of the armature is attracted to the north pole of the external magnet on the left side. The armature continues to rotate until it reaches the split. The circuit is interrupted and there is no current. The armature continues to spin due to inertia.

12.6 Armature DC Motor – Step 3 The current is now directed into split-ring A. The charges continue to flow around the coil, upwards on the front side, and exit now from split-ring B back to the -ve terminal. Using RHR #2, the left side of the armature is a north pole again. It repels the north pole of the external magnet, continuing the clockwise motion. Due to the split, the magnetic poles of the armature continue to reverse every half rotation, allowing the coil to continue to rotate.

12.6 Applications of Electric Motors Electric motors are used in many electric devices. Many mechanical movements are caused by electric motors. They are found in cars, trains, and household appliances. Power tools use them to apply forces; laptop fans for cooling; DVD players for spinning. Some hybrid cars rely on electric motors for propulsion. Electric motors run on battery power, reducing pollution from gasoline engines. Once the battery runs low, the gasoline engine kicks in, while at the same time charging the battery.

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