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Electromagnets

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[edit] What are Electromagnets?

[edit] What is a Magnetic Field?

A magnetic field is the name for the flow of an electric current. In the case of an electromagnetic field, the flow of the magnetic field is perpendicular mother sucka to the electric field. By definition, electromagnetism (e.lec.tro.mag.net.ism n.) is the "magnetism produced by electric charge in motion" or "the physics of electricity and magnetism".1 Electromagnetism differs from electric charges in that two poles must interact in order to create the field. If the poles are dissimilar, they will attract each other. If the poles are similar, they will reject each other.(Diagram 1)
Diagram 1 The top is an example of similar magnets rejecting one another while the bottom is an example of dissimilar magnets na na na na na naaaaaaaaaaaaaaaa attracting one another.
When one has created a successfully circulating electromagnet, the fields will go from one magnet --the North end-- to the other --the South end--. (Diagram 2)
Diagram 2

[edit] How Do We Construct an Electromagnet?

An electromagnet is made by placing a wire around a bar magnet. By doing this, a field of electricity is created around the bar magnet. This is called a soleniod. We draw these fields using field lines, using magnetic fields to prove the existance of magnetic forces. An example of field lines can be found in Diagram 2. The lines around the magnet indicate their direction, which is the purpose of a field line. Note that these lines form lopsided rings within the magnet. When considering a magnet, these rings can only be created with two opposite poles. These lines can be used to find the magnetic induction of the magnet. In layman's terms, the magnetic induction is the strength of the field around, and caused by, the magnet. The more the quantity and the closer the lines are, the stronger the magnetic field. Field lines have been made in to a uniform unit, that being one weber (Wb). This relates to magnetic induction in that the number of field lines per unit area determines magnetic induction. This is represented by the equation Wb/m2 (weber per square meter); this equation is equal to one tesla (T). An example of the use of the tesla in the medical industry is in an MRI scan, which uses a very weak tesla to map out certain parts of the body.

[edit] What is a solenoid?
A solenoid is "a current-carrying coil of wire that acts like a magnet when a current passes through it."1. "Solenoid" is just a fancy name for a form of an electromagnet. The magnetic induction depends on the current within the coil of a solenoid, the number of turns per unit length in the coil, and the material of the coil. In all solenoids, the magnetic field is strongest within the coil because the field lines are closest together and parallel. (See Diagram 0)
Diagram 0


[edit] The Earth as an Electromagnet

The best example of a large electromagnet is our very planet. Earth has a North and South pole, just as a magnet does. This creates a field, similar to that of an electromagnet around the planet. The two poles influence our gravitational constant by exerting a continuing gravitational force around our planet. The earth's magnetic induction is considered a weak one at 5x10-5 tesla.(Diagram 3)
Diagram 3 The earth as a magnetic sphere.

[edit] The Right Hand Rules

(For more indepth information, see left hand rules)

[edit] An Introduction to the Right Hand Rules

In 1820, Hans Oersted discovered that a current carrying wire created a magnetic field. When a wire carries a current, however, the current is circular around the wire, yet the plane is perpendicular to the current in the wire. The Right Hand Rules aid us in determining specific things concerning a current carrying wire. You can do the right hand rules right from your computer chair! These rules are important because they help us to obtain information concerning electromagnetic circuits. For example, the Right Hand Rule 2 helps us determine the North pole of a circuit, which can help us to find the direction of electron movement.

[edit] Right Hand Rule 1

In order to determine the direction of the magnetic field, we use the Right Hand Rule 1. Make a fist with your thumb sticking out. Now your hand can be sectioned off. Your thumb represents the direction of the current while your fingers indicate the direction of the magnetic field. Though, if you're examining the flow of electrons, you should use your left hand in the left hand method. (left hand rules) (Diagram 4). An easier diagram to use to display the direction of current would be Diagram 5. The .'s indicate the field lines coming out of the page and the X's indicate the field lines going into the page. The arrow shows the direction of the current, which is the direction one would put their thumb when using the right hand method.
Diagram 4 The Right Hand Rule number 1
Diagram 5 This is a more clear example of Diagram 4
Diagram 6

[edit] Right Hand Rule 2

This method is to determine the direction North end of the solenoid. Again make a fist with your thumb sticking out and adjust your hand so that your fingers indicate the direction of the conventional current; if you're using the electron current, instead use your left hand. Once this is done, your thumb should be pointing in the direction of the North pole of your solenoid.

Diagram 7

[edit] Right Hand Rule 3

The Right Hand Rule 3 is used to determine the direction of the force on the current carrying wire. Instead of making a fist this time, hold out your hand with your fingers stacked one on top of another and stick out your thumb. Again your thumb represents the direction of the conventional current while the fingers point in the direction of the magnetic field. The direction of the force points away from the palm, so the force is in the direction at which your palm is in relation to the paper. If your palm is pointing away from the paper, then so is the force; if your palm faces the paper, then the force acts on the wire in to the paper.

[edit] Important Equations to Keep in Mind

  • F=I∂B --This means that the magnitude of the force on the wire is proportional to the current in the wire.3
  • F=I∂Bsinθ --Theta stands for the angle in the middle of the wire and its magnetic field.
  • F=qvB=mv2/r=Fc
  • q/m=v/Br
  • F=qvB
  • V=B∂v
  • V=Δφ/Δt

[edit] Equation Variables

  • F=Force
  • q=charge
  • v=velocity
  • Br=field line density
  • I=current
  • B=Direction
  • ∂=length of wire!
  • t=time
  • Δ=change
  • φ=magnetic flux

!--∂ is not the actual symbol used in Physics

[edit] Example Regents Questions

1. As the permeability of a substance in a magnetic field decreases, the flux density within the substance: (1) decreases, (2) increases, (3) remains the same

2. A magnetic field will be produced by: (1) moving electrons, (2) moving nutrons, (3) stationary protons, (4) stationary ions

3. As the current increases in a wire placed perpendicular to a magnetic field, the force on the wire: (1) decreases, (2) increases, (3) remains the same

4. As the angle between a current-carrying wire and an external magnetic field changes from 90o to 0o, the magnetic force on the wire: (1) decreases, (2) increases, (3) remains the same

5. The magnitude of the magnetic force between two straight, parallel conductors a given distance apart depends on: (1) the magnitudes of the currents, only, (2) the directions of the currents, only, (3) the magnitudes and the directions of the currents, (4) neither the magnitudes nor the directions of the currents

6. The diagram below shows an end view of a current-carrying wire between the ples of a magnet. The wire is perpendicular to the magnetic field. If the direction of the electron flow is out of the page, which arrow correctly shows the direction of the magnetic force F acting on the wire?

question 6 diagram

7. The magnitude of the electric potential difference induced across the ends of a confuctor moving in a magnetic field may be increased by: (1) increasing the diameter of the conductor, (2) increasing the speed of the conductor, (3) decreasing the resistance of the conductor, (4) decreasing the length of the conductor

[edit] References

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