* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electromagnetic Induction In earlier work, we learned that an electric
Earthing system wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetic nanoparticles wikipedia , lookup
Maxwell's equations wikipedia , lookup
Magnetic monopole wikipedia , lookup
Wireless power transfer wikipedia , lookup
Alternating current wikipedia , lookup
Magnetic field wikipedia , lookup
Electromagnetism wikipedia , lookup
Multiferroics wikipedia , lookup
Electricity wikipedia , lookup
Magnetochemistry wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
History of electrochemistry wikipedia , lookup
Magnetoreception wikipedia , lookup
Hall effect wikipedia , lookup
Superconductivity wikipedia , lookup
Electric current wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
Lorentz force wikipedia , lookup
Induction heater wikipedia , lookup
Electric machine wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Magnetic core wikipedia , lookup
Scanning SQUID microscope wikipedia , lookup
Force between magnets wikipedia , lookup
Electromagnet wikipedia , lookup
History of geomagnetism wikipedia , lookup
Eddy current wikipedia , lookup
Superconducting magnet wikipedia , lookup
Electromagnetic Induction In earlier work, we learned that an electric current produces a magnetic field, and that a magnetic field exerts a force on an electric current or a moving electric charge. These discoveries were made in the 1820's. Scientists began to wonder that if electric currents produce a magnetic field, then would it be possible for a magnetic field to produce an electric current. Ten years later, the American Joseph Henry (1797-1878) and the Englishman Michael Faraday (1791-1867) independently found that this was possible. It was actually Henry who made the discovery first, but Faraday published his results earlier and investigated the subject in more detail. In attempting to produce an electric current from a magnetic field, Faraday used an apparatus similar to the one shown below. A coil of wire at X was connected to a battery. The current that flowed through X produced a magnetic field inside the coil. The iron core intensifies this magnetic field. Faraday hoped that by using a strong enough battery, the steady current in X would produce a great enough magnetic field to produce a current in the second coil at Y. The second circuit, Y, contained a galvanometer to detect any current but it did not contain a battery. When Faraday used steady currents from the battery, the galvanometer would not register any current. Faraday did finally see a current in the galvanometer in circuit Y at the moment he closed the switch in circuit X. Furthermore, the galvanometer deflected strongly in the opposite direction when he opened the switch. Thus a steady current in X did not produce a current in Y. A current in Y was produced only when the current in X was starting or stopping. Faraday concluded that although a steady magnetic field does not produce a current, a changing magnetic field could produce a current. The current in the coil in circuit Y is called an induced current because it is brought about (or "induced") by a changing magnetic field. Since a source of emf, like the battery in Faraday's experiment, is always needed to produce a current, the coil behaves as if it were a source of emf. This emf is known as the induced emf. We can summarize the effects above by stating that an induced emf or an induced current is produced by a changing magnetic field. In general, we can define electromagnetic induction as the production of an induced current or emf in a conducting circuit brought about by a changing magnetic field. Inducing EMF and the North Pole of a Magnet Faraday did further experiments on electromagnetic induction. These experiments illustrated other ways to induce emf in a circuit. In the left diagram above, the north pole of a magnet is moving upwards towards the coil of wire that is connected to a galvanometer. This induces a current to flow in the coil and the needle of the galvanometer deflects to the right. This occurs because the strength of the magnetic field is changing as the magnet approaches the coil. In the right diagram above, the north pole of the magnet is moving downwards away from the coil of wire. The induced current flows in the opposite direction, and the needle of the galvanometer moves to the left. Notice in both diagrams that the magnetic field lines of the magnet point away from the north pole of the magnet. This magnetic field direction does not change as the magnet is moved but the strength of the field does change. Suppose that in the left diagram, the magnet was held steady and the coil moved downwards. In that case, the current would move in the same direction as before and the needle would deflect to the right. In the right diagram, a similar effect would be observed. If the magnet was held stationary and the coil was moved away from it, then the current in the coil would be the same as it was before and the needle would deflect to the left. It does not matter whether the coil or the magnet moves to produce an induced current or emf in the coil. Inducing EMF and the South Pole of a Magnet Similar effects can be observed if the bar magnet is turned around and the south pole is used. Notice that the direction of the magnetic field lines is towards the south pole of the magnet. This is opposite to the direction for the north pole of the magnet. So when the south pole of the magnet is moved upwards, the direction of the current is opposite to what it was when the north pole of the magnet moved upwards. The right diagram shows that when the south pole of the magnet moves away from the coil, the current is once again opposite to what it was when the north pole of the magnet moved downwards. As was the case with the north pole of the magnet facing upwards, the same effects in current direction can be achieved if the coil moves towards the magnet instead of the magnet moving towards the coil. What is important is that relative motion occurs. Induced EMF and the Area or Orientation of a Coil Another way to induce an emf in a coil of wire is to change the area of the coil. For example, assume that a magnetic field is directed into the paper. A coil of wire with a galvanometer attached is initially held in this field. The diagram to the right shows that the ends of the coil have been stretched so that the area of the coil facing the magnetic field is less. This causes a current to flow in the coil, and this is registered on the galvanometer. Keep in mind that as long as the area is changing, an induced current and emf exist; they vanish when the area is no longer changing. If the distorted coil is returned to its original circular shape, thereby increasing the area, an oppositely directed current is generated while the area is changing and the galvanometer needle will move in the opposite direction. An induced emf is also generated when a coil of constant area is rotated in a constant magnetic field and the orientation of the coil changes with respect to the field. When the rotation stops, the emf, and hence the current, vanishes. In the above examples, both an emf and a current are induced in the coil because the coil is part of a complete closed circuit. If the circuit were open, as in the case of an open switch, there would not be an induced current. However, there would still exist an induced emf whether the current exists or not. Thus changing the magnetic field, changing the area of the coil, and changing the orientation of the coil are all methods that can be used to create an induced emf. 1. Descirbe three ways in which a current or emf can be induced. 2. What is electromagnetic induction?