Electromagnetism
This unit corresponds to pages 295 - 347 in the text. The guided notes are not
working for many of you, so I am switching to more of a review sheet for this.
This does NOT mean that you needn’t take notes.
What is electromagnetism?
Electromagnetism is the branch of science that studies charged particles (ions).
In other words, it is the study of electrons and their effects.
We will begin with static electricity. Static means that the charges are not
moving. There are three ways to build up a charge on an object. For that charge
to stay, the object has to not conduct the charge away from the newly charged
object. The ways to build a charge are:
Friction -- by rubbing two objects with different pulls on electrons together,
one will pull the electrons off the other and a charge will build up.
Conduction -- by touching a charged object, electrons will flow from that
object and a charge will build up.
Induction -- by bringing a charged object near another object that
conducts, the like charges will be repelled and the opposite charges will be
attracted and charge will be built up in the new object.
An object becomes charged when its atoms gain (-) or lose (+) electrons
(never protons!). Charged objects exert a force on other charged objects (like
charges repel / opposites attract). This force is called an “electric force”. The
size of an electric charge depends on both the amount of each charge, and the
distance between them. The greater and closer the charges, the greater the
electric force. An “electric field” is a region around the charged object in which an
electric force is exerted on another.
A magnet is simply any solid whose atomic charges are lined to to yield a
net positive on one side of the object and a net negative on the other side. Iron,
Nickel, and Cobalt are the three that tend to line up easiest and become
magnets. We can make magnets by rubbing one of those metals with a magnet
(and thereby align its atomic charges), or by running a current around one of
those metals (making an electromagnet). When you move a magnet inside a coil
of wires you make electricity.
An electrical conductor is any material (usually metal) whose electrons are
free to move and conduct the charge. An electrical insulator in any material
which does not conduct electricity well. We must have insulators around
conductors to prevent flow.
Electrical energy (i.e. from an outlet) is the energy of electric charges.
There are three terms to know when studying flowing electricity. They are
voltage, current, and resistance.
Voltage is the driving force that moves the electrons. The bigger the
Voltage, the more push (energy) there is on the electrons. Voltage is measured
in volts (V). Charges will flow so long as there is a voltage between two points
on a wire. Batteries typically range from 1.5V to 12V in cars. Outlets have 120V.
Current is the flow of these charges. The greater the current, the more
charges pass a point each second. Current is measured in amperes (amps--A or
I). There are two types of currents: direct and alternating. DC (like from a
battery) has all the charges go in only one direction. AC (like from an outlet) is
where the charges shift their directions (120 times every second in the USA).
Resistance is the opposition to the flow of current (like friction). It is
expressed in ohms (Ω or R) and is a product of the object’s material, thickness,
length, and temperature. The higher the Ω, the lower the current. Good
conductors have a low resistance. Thickness, shortness, and coolness decrease
resistance. Resistance causes heat in the wires.
Current, voltage and resistance are related by Ohm’s law: R = V / I or V = I
x R or I = V / R. As resistance goes up, current goes down and vise versa.
“Cells” convert chemical or radiant energy into electrical energy. Batteries
are made of one or more cells. Two electrodes are placed in an electrolyte
(substance that allows charges to flow). One electrode reacts to gains electrons.
The other reacts to lose them. If the two electrodes are attached by a wire, then
the charges flow. Photocells convert light energy into electrical. Several of them
together make up a solar panel. These work because of silicon whose electrons
flow when excited with light.
The rate at which electric energy is changed into other forms of energy is
called electric power and is measured using Watts (W or P). This is calculated
using P = V x I. The more power a light uses, the brighter it burns.
An electric circuit is a complete, closed path through which electric
charges flow. The three main parts of a circuit are the energy source, the load
(radio, light, fan, etc.), and the wires. Loads change the electric energy into other
forms (thermal, light, sound, motion, etc.) and provide resistance. A switch is
often used to open and close a circuit by completing or breaking the loop.
If you are going to put more than one load on the same circuit, you can put
them in series or parallel. A series circuit is one in which all the parts are
connected in a single loop. All of the loads share the same current meaning that
if you added another light, all the bulbs would get equally dimmer and if one blew
out, all would go out. This is practical for burglar alarms. Parallel circuits have
loads connected side by side. This means that they all share the same voltage,
but not the same current.
Circuit failure occurs when the current is made to increase which makes
the wires heat up to the point where a fire could start. This usually occurs when
something (water, broken wires, etc.) causes the flow to skip past some loads.
This drops the resistance and therefore increases current. Another cause is
overloading a circuit.
Circuit failure can be prevented with fuses and breakers. A fuse is a thin
strip of metal which will “burn out” when too much current is drawn. A breaker is
a switch that is tripped open if the current is too high. It is “tripped” by a metal
bar which expands as it heats up.
Safety: make sure the wire’s insulation is not worn. Do not overload a
circuit. Do not use while wet or near water. Never put anything but a plug into an
outlet.
If you enjoyed this unit, here are some optional suggestions for you:
Keep playing around with circuits. Build something?
Find out more about electronics. What is a capacitor? What is a
transducer?
Research lightning. Teach us something about this natural bolt of
electricity.