12+-+Electromagnetism

**Electromagnetism** **Electromagnets** An electromagnet starts with a battery (or some other source of power) and a wire. What a battery produces is **electrons**. If you look at a battery, say at a normal D-cell from a flashlight, you can see that there are two ends, one marked plus (+) and the other marked minus (-). Electrons collect at the **negative** end of the battery, and, if you let them, they will gladly flow to the **positive** end. The way you "let them" flow is with a wire. If you attach a wire directly between the positive and negative terminals of a D-cell, three things will happen:  You can even see the magnetic field set up around a wire carrying a current by using iron filings.
 * 1) **Electrons will flow** from the negative side of the battery to the positive side as fast as they can.
 * 2) **The battery will drain** fairly quickly (in a matter of several minutes). For that reason, it is generally not a good idea to connect the two terminals of a battery to one another directly. Normally, you connect some kind of **load** in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
 * 3) **A small magnetic field is generated** in the wire. It is this small magnetic field that is the basis of an electromagnet.

The magnetic field produced by a current in a wire is not very strong, we can increase the strength of the magnetic field by using **coils of wire**. The strength from the coils can be further increased by adding a "core" made out of a material which can be magnetised, such as iron. These coils of wire are called **solenoids**. As you can see each cross section of wire sets up a field and these combine to form a field, highly concentrated in the center.

**Magnetic Force** If we put two magnets near to each other, their magnetic fields will interact. Interact means that the magnets will experience forces on them as like poles will repel and unlike poles attract. It follows then that a wire in a field of a permanent magnet will experience a force when current flows through it. The magnetic field generated around the wire will interact with the field around the magnet. The two fields will produce a force.

**The Force on a Current-Carrying Conductor in a Magnetic Field** <span style="color: #000000; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">A current-carrying conductor in a magnetic field will experience a force in a specific direction if the current and the field are perpendicular. The size of that force is given by F = BIL. If the direction of the current is "off" by a certain angle then we can use F = BIL sin(theta)**.**

<span style="color: #000000; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">A moving coil loudspeaker is an application of the force acting on a current-carrying conductor in a magnetic field. When a varying electrical signal is sent to the coil, the coil is pushed in and out. This makes the cone vibrate, creating sound waves.

<span style="color: #000000; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">[|An applet similar to the demonstration in class] on the force on a current-carrying wire in a magnetic field.

<span style="color: #e24646; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 400%; text-align: center;">**Induction**

<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">We know that a current will produce a magnetic field. <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Interestingly the converse is also true: A magnetic field will induce a current in a wire.

<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">This current flows because something is producing an electric field that forces the charges around the wire. (It cannot be the magnetic force since the charges are not initially moving). This "something" is called an **electromotive force**, or **emf**, even though it is not a force. Instead, emf is like the voltage provided by a battery. A changing magnetic field through a coil of wire therefore must //induce// an emf in the coil which in turn causes current to flow.

<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">The law describing induced emf is named after the British scientist Michael Faraday(Although Joseph Henry, an American from the Albany area, discovered that changing magnetic fields induced current before Faraday did).

<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Briefly stated, Faraday's law says that a changing magnetic field produces an electric field. If charges are free to move, the electric field will cause an emf and a current. For example, if a loop of wire is placed in a magnetic field so that the field passes through the loop, a change in the magnetic field will induce a current in the loop of wire.

<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">There are a number of ways of changing the magnetic field. We will focus on making the field weaker/stronger by moving a magnet away from/toward the loop of wire (solenoid). (Just in case you're curious, other ways of changing the magnetic field are:

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<span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Summary in Fact Form:
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">If free to rotate, permanent magnets point approximately north-south.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Like poles repel, unlike poles attract.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Permanent magnets attract some things (like iron and steel) but not others (like wood or glass).
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Magnetic forces act at a distance, and they can act through nonmagnetic barriers (if not too thick).
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Things attracted to a permanent magnet become temporary magnets themselves.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">A coil of wire with an electric current flowing through it becomes a magnet.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Putting iron inside a current-carrying coil greatly increases the strength of the electromagnet.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">Changing magnetic fields induce electric currents in copper and other conductors.
 * <span style="color: #0a0b0a; display: block; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 120%; text-align: left;">A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion.