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Electricity - The Motor Effect

This GCSE Physics Electricity quiz takes a look at the motor effect. When an electric current passes through a wire, it creates a magnetic field around the wire. If the magnetic field from the wire is placed in another magnetic field, they interact, causing the wire to move. This is known as the motor effect. Electric motors that work off direct current are found in many devices we use in everyday life. They are energy transfer devices that convert electrical energy into rotational kinetic energy. As with any energy transfer, it is never a perfect conversion, some energy is always wasted. In the case of an electric motor, the wasted energies are heat (from friction and the heating effect of an electric current) and sound.

The motor effect is sometimes called the catapult effect since that is the effect on a single current-carrying wire in a magnetic field.

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If either the current or the magnetic field is reversed, the wire is catapulted in the opposite direction. The reason that a motor turns is because the wire is in the form of a coil wound onto a block called an armature. As the current flows through the coil, it is flowing in opposite directions on each side. One part of the coil is being catapulted out of the magnetic field in one direction; the other side is being catapulted in the other direction so the coil is able to turn on an axle. Fleming's left hand rule is a favourite with the GCSE examiners. It is used to predict in which direction a current-carrying wire will move in a magnetic field so make sure that you revise it well.

If the two ends of the wire used to make the coil were connected directly to the electricity supply, they would soon twist together, preventing the coil from turning. To overcome this problem, a commutator is used. In a DC motor, this is a split-ring commutator. This consists of two semi-circular pieces of copper, separated by a thin strip of an insulating material. The commutator is connected to the electricity supply by two carbon brushes. These are held against the commutator using springs.

The discovery of the motor effect is usually credited to Michael Faraday, however, as is often the case in science, it was based on earlier work by others. The theory had already been developed by André Ampère and the commutator was developed a few years after Faraday's demonstration of the motor effect by the Hungarian scientist Ányos Jedlik. It took about 10 years after the invention of the commutator for useful DC motors to be built and about 50 years before the world's first commercially successful electric motor to be built.

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  1. What is produced when a current flows through a wire?
    The magnetic field can be visualised as a pattern of circular lines surrounding the wire. These show up using iron filings on a piece of card
  2. Which of the following is an application of an electromagnet?
    Electromagnets can be very powerful
  3. How can the force applied to a current carrying wire be increased?
    In large electric motors, the magnets are electromagnets
  4. Another way to increase the force is to …
    The force generated by the interaction of the two magnetic fields depends on both of the factors in questions 3 and 4
  5. An electrical conductor will not experience a force in a circuit if it is placed in what position relative to the magnetic field?
    The two magnetic fields will interact if they are at any angle other than zero degrees
  6. How can the force experienced by a current carrying wire be reversed?
    Simple really!
  7. Fleming's left hand rule can be used to work out the direction of movement (force), magnetic field and current. What does the thumb represent in this rule?
    Knowing Fleming's left hand rule can quickly gain you marks. Knowing which direction a particular force, field or current is acting is often required to answer certain questions
  8. What is used to transfer an electrical current to a loop of wire sitting in a magnetic field, in a motor?
    Commutators allow current to travel through a circuit despite not being fixed to a position in a circuit. This allows the wires that make up the coil not to become tangled when the motor is spinning
  9. Revisiting Fleming's left hand rule, the current is represented by …
    First finger = Field, SeCond finger = Current, ThuMb = Movement (direction of force)
  10. What is the direction of the magnetic field relative to the current in a current carrying wire?
    In other words, whatever direction the current is flowing, the direction of the magnetic field will be at ninety degrees to that

Author: Martin Moore

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