1\. **Electricity and Magnetism** seem different, but electricity can
create magnetism, and magnetism can create electricity. Describe how
this is done. Give examples illustrating how this is used for practical
applications.

**Grading Criteria:**

In addition to the general style points (same as for the first midterm
exam), we hoped to see the following subject matter. Particularly for
the first two points, how clearly you explained them had a significant
impact on your grade.

1)  Electricity creates magnetism through currents. Moving charges in
    general create a magnetic field, and currents through wires are many
    moving charges. The particular configuration of a solenoid, or
    cylindrical coil of wire, creates a nearly uniform magnetic field
    along the axis of the cylinder (you didn\'t need to know the
    specifics of the field). Often a piece of magnetic metal (such as
    iron) is place inside the coil; this amplifies the magnetic field.

2)  Magnetism can cause electricity by moving a wire through a magnetic
    field. Equivalently, by moving a magnet past a wire, or changing a
    magnetic field near a loop of wire. The current in the wire can then
    be used to power our homes or for other purposes.

3)  Electric generators use point 2 to generate electricity. Dynamos are
    particular generators where the current generated is used to power
    the electromagnet producing the magnetic field which generates the
    current. The Earth\'s core is a great example of a dynamo.

4)  The transformer is an important and illustrative example of how
    electricity can create magnetism and vice versa, and is critical to
    power distribution.

5)  Good examples of electromagnets are car door locks and the magnets
    used in junk yards to move cars.

6)  Any other examples were good too.

**Examples of excellent essays**

The following are two good essays for question #1, on electricity and
magnetism. The first has fewer practical examples, but great exposition.
The second has a nice selection of examples.

**by Alina Schlyapochnik:**

Protons and electrons have electric charges; the positively charged
protons attract the negatively (or opposite) charged electrons, but both
repel particles with the same charge as their own. Magnetism arises from
the flow of electrically charged aprticles, mainly electrons. The
magnetism of permanent magnets, such as iron, is created by the spin of
electrons. All electrons spin, and when these moving charges align
themselves within the iron, it becomes magnetized. Electric current in
wires is also used to create mangetism. These electromagnets create
magnetism by sending electricity through a metal wire coiled around a
cylindrical object (occasionally a natural magnet).

Magnetism can also createelectricity because, just as electrical current
creates magnetism, magnetism creates current. In electric generators, a
metal wire is moved through a magnetic field to generate the flow of
electrons in the wire. Dynamos work by using electrmagnets to create a
magnetic field. The electricity generated by moving a wire through the
field is then used to power the electromagnet, illustrating how
electricity creates magnetism and vice-versa. Electrical current can
also be generated by a moving electrical field instead of a moving wire,
or a magnetic field with alternating force )attraction and repulsion).
In transformers, alternating current in a coil of wire (primary) is used
to create an alternating magnetic field, which then generates the flow
of electricity in a nearby coil (secondary). The reciprocity between
electricity and magnetism in transformers proves bery important in the
modern world. Since we use AC current that transports electricity at
very high voltages, we need transformers to convert the electricity to a
safer voltage before it enters our homes. Transformers are able to do
this by retaining the electricity\'s power, while decreasing its voltage
and increasing its current, depending on the number of coils in the
secondary wire.

**by Jessica Lee:**

Electricity and magnetism seem different, but electic current creates
magnetism and changing magnetic fields create current. The two concepts
are very much related and have led to several practical applications
that we use in the world today.

One of the must useful applications is an electromagnet. By placing a
solenoid around a piece of metal and running a current through it, the
metal becomes a magnet. Turn the current off, and the magnetism goes
away. If you reverse the current, the direction of magnetism chagnes
(North becomes South). These properties allow electromagnets to
lock/unlock car doors, pick up cars through induced magnetism, and even
run cars as in the case of an electic motor.

Another useful application of electricity and magnetism is the electric
generator. Electric generators work by moving a wire past a magnetic
field created by a permanent magnet to form a current. For example in a
steam power plant, steam runs through a turbine which then pushes a ire
through a magnetic field to create electricity. In a dynamo, an
electromagnet replaces the permanent magnet, and the electricity created
is actually the current it uses to run the generator itself. Without
this, we would not be able to power our homes!

The electricity created in plants reach homes with the aid of another
electric and magnetic mechanism: a transformer. In order to avoid a loss
of energy to resistance, electricity carried opn powerlines must be high
voltage. However, homes use low voltage/high current electricity. A
transformer is made with two telsa coils \[sic\], each with a different
number of loops. As current runs through the primary coil, the secondary
coil receives current too, even if the two are not touching. The energy
is transferred through the magnetic fields! Because the coils have a
different amount of loops, the current and magnetism differe, one has
high voltage/low current and the other has low voltage/high current,
suitable for transferring electricity from plants into homes.

These 3 examples illustrate how magnetism and electricity are
intertwined. Because electricity can create magnetism, and magnetism can
create electricity, we are able to use these properties in everyday
life.