Section6_Magnetic_Fields_Coils.notebook 1 April 07, 2014 Section 6: Magnetic Field around a Coil Conductor (a Solenoid) The picture below presents a review of the magnetic field around a straight piece of wire with the current travelling from left to right. Now, bend the wire so that the back end is on the bottom with the current going into the screen (x)and the front end is on the top with the current coming out of the screen (C). Using the left‐hand rule, you can verify that the magnetic field is in the direction shown. Note the region where the magnetic field lines are in the same direction. Now, imagine that you are wrapping the wire around a cardboard centre like the one found in the centre of a roll of toilet paper. Wrap it so that it follows the path of the last picture‐‐that is, with the current direction away from you on the bottom of the tube, but towards you on the top of the tube. The result will be something like the next picture.
Transcript
Section6_Magnetic_Fields_Coils.notebook
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April 07, 2014
Section 6: Magnetic Field around a Coil Conductor (a Solenoid)
The picture below presents a review of the magnetic field around a straight piece of wire with the current travelling from left to right.
Now, bend the wire so that the back end is on the bottom with the current going into the screen (x)and the front end is on the top with the current coming out of the screen (C). Using the left‐hand rule, you can verify that the magnetic field is in the direction shown. Note the region where the magnetic field lines are in the same direction.
Now, imagine that you are wrapping the wire around a cardboard centre like the one found in the centre of a roll of toilet paper. Wrap it so that it follows the path of the last picture‐‐that is, with the current direction away from you on the bottom of the tube,but towards you on the top of the tube. The result will be something like the next picture.
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Use your imagination again, and visualize what you would "see" if the coil were sawed down the middle:
Then, discard the half of the coil nearest you; you will be left with something like the picture below. On the top of the coil the current is coming out of the page, and on the bottom of the coil the current is going into the page. The magnetic field is shown for each turn of wire.
Look more closely at the way the fields of each turn add together. Inside the coil all the little fields add up to the left. On the outside of the coil, all the fields add up to the right. The end result can pictured as follows:
If the coil is long and has many turns it is called a solenoid. The magnetic field of a coil (or solenoid) is very similar to the field around a bar magnet. A current‐carrying coil is called and electromagnet.
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Left Hand Rule (# 2) for Coils
1. Grasp the coil in your left hand.2. Your fingers should be pointed in the direction the
electrons flow around the wire in the coil.3. Your thumb will point in the direction of the magnetic
field (The North Pole).
Examples:
1. In each diagram, mark the direction of electron flow, the direction of the field lines at the end of each coil and the North and South pole.
2. Each empty circle represents the conductor of a coil. Determine which circles should have dots and which should have x’s.
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Factors that Determine the Strength of an Electromagnet (or Coil)
An electromagnet is more useful than a bar magnet because it has a magnetic field that can be turned off and on and its strength can be altered.
To increase the strength of the magnetic field in a coil, we must increase the magnetic field lines or bring them closer together.
The strength of an electromagnet depends on 4 factors:
1 Current in the coilThe greater the current flow, the greater the field strength. In an air coil core, the magnetic field is directly proportional to the current flow.
2 Number of turn in the coilThe greater the number of turn (loops) the greater the field strength. Magnetic field strength is directly proportional to the number of turns in the coil if the current is constant.
3 Size of the coilThe smaller the diameter of the coil, the stronger the magnetic field.
4 Type of material in the coil’s centerThe more ferromagnetic the material within the coil, the greater the magnet’s strength. The factor by which a core material increases the magnetic field strength is call the material’s magnetic permeability (μ).
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The magnetic field strength varies directly with the permeability of its core material (μ).The best ferromagnetic core is soft iron because not only do the domains line up when the current is flowing, they also go back to their random state when the current is shut off. This is an ideal control condition. For example, if magnetic material (say, car wrecks) is being moved from one location to another by a crane, the load can be deposited simply by shutting down the power to the electromagnetic.
The relative magnetic permeability of a substance is a ratio of the strength of the magnet when the core is made of that substance to the strength when there is no core at all (a vacuum). Ferromagnetic substances have a high permeability. You can find the magnetic permeability of substances in Table 15.3 on p. 637 of your textbook. However, here is a cautionary note: the permeability coefficients in the table are not absolute values, but rather appear to be relative permeabilities. Notice that μ for a vacuum and oxygen (and you can assume also for air) is taken to be 1. For iron, μ is given as 6100, which we should interpret to mean that if an iron core is place inside a coil, the magnetic field strength of the coil will increase by a factor of 6100.
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You should be able to explain how the following work.
In your textbook: on pp. 638639do #2 b, e, #3, #4.
