The Motor Effect
Übersicht
Stop reading for a moment and count the electric motors within ten metres of you. The fan in a laptop. The buzz in a phone. The pump in a fridge, the drum of a washing machine, the passenger window of a car, the disc that spins a hard drive, the wipers, the fuel pump. A family car carries more than thirty and almost nobody could name five. Every one runs on a single idea small enough to fit in a sentence: put a current through a wire that is already sitting in a magnetic field, and something shoves the wire sideways.
This lesson is about that shove. You will find out where it comes from, why it acts sideways to both the current and the field rather than along either, and why the magnet gets shoved just as hard the other way. You will learn to read a force direction straight off an arrangement with nothing but your own left hand, a skill the specification names outright. Then you will bend the wire into a loop and watch a push turn into a spin, which is the whole of an electric motor in one step.
Ziele
- A current carrying conductor has a magnetic field around the wire. When a current carrying conductor is placed in a magnetic field so that it cuts lines of magnetic force, the magnet and conductor exert a force on each other. This is called the motor effect. The conductor will not experience a force if it is parallel to the magnetic field.
- The size of the force can be increased by: increasing the strength of the magnetic field; increasing the size of the current; increasing the length of the conductor in the magnetic field.
- The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed. Students should be able to identify the direction of the force using Flemings left-hand rule.
- A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
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Unterrichtsbewertung
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- In Fleming's left-hand rule, what does the first finger point along? A. The direction of the force on the conductor B. The direction of the magnetic field C. The direction of the conventional current D. The direction of the flow of electrons Answer: B
- A straight wire lies between the poles of a magnet, parallel to the magnetic field lines. A large current is passed through the wire. What force acts on the wire? A. No force at all B. A force at right angles to the field C. A force along the direction of the current D. A force twice the size of the force when the wire is at right angles to the field Answer: A
- Which change would NOT increase the size of the force on a current-carrying wire in a magnetic field? A. Using a stronger magnetic field B. Increasing the current in the wire C. Increasing the length of the wire that lies inside the field D. Turning the wire so that it lies along the magnetic field lines Answer: D
- The current in a wire in a magnetic field is reversed, and the magnet is also turned round so that its poles trade places. What happens to the direction of the force on the wire? A. It is reversed B. It is unchanged C. It becomes zero D. It turns through 90 degrees Answer: B
- Why does a rectangular coil carrying a current in a magnetic field tend to rotate? A. Its two long sides carry current in opposite directions, so they are pushed in opposite directions on opposite sides of the axle B. Its two long sides carry current in the same direction, so they are pushed the same way C. The magnet is attracted to the coil D. A potential difference is induced across the ends of the coil Answer: A
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Lehrbuchabschnitt
Magnetic Force on a Current-Carrying Conductor
Because charges ordinarily cannot escape a conductor, the magnetic force on charges moving in a conductor is transmitted to the conductor itself. We can derive an expression for the magnetic force on a current by taking a sum of the magnetic forces on individual charges. (The forces add because they
"College Physics: Magnetic Force on a Current-Carrying Conductor" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/22-7-magnetic-force-on-a-current-carrying-conductor. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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Lehrbuchabschnitt
Torque on a Current Loop: Motors and Meters
Motors are the most common application of magnetic force on current-carrying wires. Motors have loops of wire in a magnetic field. When current is passed through the loops, the magnetic field exerts torque on the loops, which rotates a shaft. Electrical energy is converted to mechanical work in the
"College Physics: Torque on a Current Loop: Motors and Meters" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/22-8-torque-on-a-current-loop-motors-and-meters. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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