Download Magnetism I. Magnetic Forces Magnetism and electrostatic attraction

Magnetism
I. Magnetic Forces
Magnetism and electrostatic attraction are not the same, but they are related. Magnetism is caused
by the movement of electrons. In all atoms, electrons are moving around the nucleus in areas of probability
called orbitals. Electrons are also “spinning.” In most atoms electrons spinning in one direction are
balanced by electrons spinning in the opposite direction. In a few types of atoms, such as iron, cobalt and
nickel there are electrons spinning in the same direction together. This causes these elements to be
magnetic.
Magnetic forces are like electrostatic forces in that they can either repel or attract. Magnets have 2
poles. Opposite poles attract and like poles repel.
The two poles of a magnet are simply called north-seeking and south-seeking (N and S, for short).
This is because the earth acts like a big magnet with the geographic North Pole serving as a magnetic South
Pole. Someday, earth’s magnetic field may reverse its polarity. This has occurred several times in
geological history and it is one important line of evidence for the theory of Plate Tectonics.
Like electrostatic forces, magnetic forces obey the inverse-square principle, i.e. the strength of the
force decreases with the square of the distance.
Unlike electric charges, the north and south poles of a magnet can not be isolated. If you cut a
magnet in half, you get two smaller magnets, each with a N pole and a S pole.
Magnetic fields can be represented by lines, just like an electric field. The closer the lines are
together the greater the field strength. The direction of the field lines is from North to South.
In a sample of magnetic matter such as iron, each atom acts as a little magnet and exerts a force on
the other atoms around it, causing clusters of atoms in a crystal of iron to be oriented in the same way.
These clusters are called magnetic domains. When a sample becomes further magnetized by being brought
near a stronger magnet, the domains will also realign with each other producing an even stronger magnet.
II. Electricity and Magnetism
Because moving charges cause a magnetic field, electric current produces a
magnetic field when it travels through a wire. By coiling wire up and sending current through it you can
produce an electromagnet. Electromagnetism is the basis for the function of motors, generators and all
types of sound and recording equipment and countless other forms of technology.
To determine the direction of a magnetic field we have two “right hand rules.”
#1. For current going through a straight wire, if you grasp the wire with your right hand, so that your
thumb points in the direction of conventional current (+ to -) the direction of your fingers will be the
direction of the magnetic field around the wire. See following figure.
#2. To determine the polarity of an electromagnet, you wrap your hand around the metal bar that is
wrapped in wire, so that your fingers go in the same direction as conventional current. Your thumb will
point to the N pole of the magnet.
III. Measuring Magnetic Force
Electric current (moving electrons) produces magnetism. Conversely, magnetism can cause
moving charged particles to be deflected from their path. The force that causes the deflection depends on
both the amount of the charge(q) and the strength of the magnetic field (B), and it is strongest when the
path of the moving charge is perpendicular to the magnetic field lines.
The SI unit for magnetic field strength is the Tesla, which is one Newton/ Amp*meter. The
symbol for magnetic field strength is “B”. The formulas for calculating magnetic force are
F== qvB
F=force, q=charge and v=velocity
Or
F=ILB I=current, L=length of wire
We determine the direction of a magnetic force with our 3rd right hand rule. You hold your right
hand with the thumb up and the index finger pointing like a gun. The index finger should be pointing in the
same direction as the magnetic field lines and the thumb should be pointing in the same direction as the
motion of the charge. The direction that your palm is facing is the direction of the magnetic force.
Electric current can be detected when a magnetic field causes there to be force exerted on some
indicating device, like a needle or a spring. The first such device was called a Galvanometer. Its named
after its inventor Luigi Galvani who also invented the electrochemical cell (aka batteries). The multimeters
used in lab are modern day galvanometers.
In motors a magnetic force causes wires of a loop to rotate. The magnetic force is produced by
current passing through a magnetic field. The rotating loop can be used to power other devices by
attaching belts, chains, and other simple machines.
IV. Electromagnetic Induction
Michael Faraday and Joseph Henry are both given credit for discovering that passing a magnet
through a coil of wire can produce a voltage (and therefore an electric current) in a wire. This is called
electromagnetic induction. Before this discovery, the only source of usable electric energy was batteries.
The voltage produced depends on 1) the speed of the magnet’s motion, 2)the magnetic field
strength, 3) the area of the loops and 4) the number of coils in the wire. This is Faraday’s Law. The
current produced also depends on the resistance of the material in the coil.
A simple generator like the one we used the lab works by turning a loop of conducting wire in a
magnetic field. As the loop turns,the magnitude and direction of the voltage and current change. This is
therefore an alternating current. In large scale power plants the crank is replaced by turbines that are turned
by water, wind, or steam.
Transformers are devices that allow voltage in a current carrying wire to be increased or
decreased. When a current carrying wire is coiled around a core and placed near another similar coil, the
first coil (primary) can cause a there to be a voltage in the 2 nd coil (secondary). The number of turns in the
coils determines whether the voltage is increased or decreased.
Vibrating charges cause oscillating electric and magnetic fields. Electromagnetic waves are these
vibrating fields. All kinds of EM radiation are the same thing but the effects of the waves are different
based on frequency and wavelength of the waves.