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Chapter 10
Electrostatic force is the force between electrical charges. It is even stronger
than gravity.
The Law of Charges says that like charges repel (push apart); unlike charges
attract (pull together) – two positive ends of a magnet will push apart and a
positive + negative end of a magnet will pull together
Electrons with weak affinity (sticking strength) for their atoms can be removed
through friction, such as rubbing. When this happens the electrons become
charged (filled) with electricity. When you touch something, the difference
between you and the other object results in a shock (discharge/going out of
electricity).
When electrons move from one object to another, the objects get an uneven
amount of protons and electrons. This is called a net force. A positive net
force means there are more protons than electrons. A negative net force
means there are more electrons than protons. Net force is also called electrical
charge. It is measured in the coulomb, abbreviated as C.
Electrical insulators make it hard for electrical charge to leave something.
These include wool and glass.
Electrical conductors help to move electrical charge. These include metals.
An electrical conductor cannot keep electrical charge unless it is covered by an
electrical insulator. This is why wires are covered with rubber.
Some materials are semiconductors – they allow electric charge to move only
under certain conditions – example: silicon.
Lightening rods are electrical conductors that are used to make sure that the
lightening discharges into the metal instead of into a house.
Electrical fields around an electromagnetic object create lines of force radiating
outward from the object. Lines of force help us to understand the direction and
strength of the force, as well as the charge of the objects. See page 229 for
picture.
Electrical induction happens when a neutral object gets a charge because you
put a charged object near the neutral object. Water is a neutral object, when you
run a comb through your hair, you charge the comb with friction. The comb can
then bend the water without touching it. You have done electrical induction to the
water.
Molecules will shift their electrons around in the matter to make a positive end
and a negative end. These magnetically charged ends are called dipoles.
Electroscopes are used to detect (find) and measure charges.
The Layden jar was the first thing created to store electricity. It was a jar lined
with lead that could have wires attached to the top to remove or refill the jar with
electricity. This was the world’s first battery. See page 234 for picture.
Electrostatic energy is energy made from friction. It is used in copiers and
filters to constantly recharge small parts with electricity.
Electrical current is continually moving electrical charges. It is used to power
most things today.
Cathode-Ray Tubes are used in television to make pictures using continuously
moving electrons.
Electrical current will flow from a point of higher electrical potential energy to a
point of lower electrical potential energy. Electrical potential energy is the
source for all charges of electricity.
Electrical current is the flow of electrons. Conventional current is the flow of
positive charges through a conductor. Positive charges and negative charges
flow in opposite directions.
Direct current (DC) is current that is flowing in one direction through a
conductor. Most electronic devices are powered by direct current.
An electrical circuit is the path of electricity. The electrical load is powered by
the electricity. A light bulb is an electrical load. Electrical circuits must be closed,
with no holes in them, because electrical current has a hard time moving through
the air. A switch allows you to control when electricity is moving through the
electrical circuit. See page 237 for pictures.
Electrical circuits need electrons to keep electricity moving. Electrons can be
gotten from several places.
 Electrochemical cells are also called batteries. They allow stored
electrons to be removed from them to power a circuit.
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Electrical generators use engines to change magnetic energy into
electrical energy by removing the electrons moved by magnetic energy.
Photovoltaic cells are batteries that store electricity and can give that
electricity to a circuit so long as they are powered by light.
We can measure the strength of an electrical current by measuring the difference
in electrical potential between two points, such as the north and south poles of a
battery. This difference in electrical potential energy is measured using the volt
(V).
Math Formula: V = J/C
The volt is equal to the amount of joules needed to move 1 coulomb from pole to
pole.
1V = 1J/1C
2V = 2J/1C
Many volts together are called a voltage. Voltages are measured using a
voltmeter.
Electrical current is measured in ampere (A). It can also be called the amp. 1
amp is equal to 1 coulomb of charge moving past a point in the circuit every 1
second.
Math formula: A = C/s (s = second)
1A = 1C/1s
Electrical resistance means that every part of a circuit inhibits (slows) the
electrical current in some way. The moving and bouncing around of the
electrons can decrease the electric energy while increasing the kinetic energy. In
order to spare the electric energy, some circuits use a resistor. A resistor
decreases the amount of energy in a certain part of a circuit so that there is not
an increase in kinetic energy. This saves the electrical energy.
Ohm’s Law says that if the resistance in a circuit remains constant, the size of
the voltage source and the current measured were directly proportional (go up or
down together). If the voltage is constant, the resistance and the measured
current were inversely proportional (one goes up while one goes down).
These two observations can be written mathematically as I = V/R
Measured current = voltage over resistance
This is usually rearranged for easier use as V = IR.
Resistance is measured in ohms. Ohms is written as Ω. Ohms means that it
takes 1 volt of difference in charge to draw 1 amp of current through a resistor of
1 ohms.
Math: R = V/I
1 ohms = 1V/1A
Measuring ohms is done with an ohmmeter.
The faster a circuit uses energy, the more power it is said to have. Electrical
power is often measured in Watts (W) or kilowatts (kW). Remember the power
math: Energy = power times time interval. (Power = energy over time interval)
Electrical energy is billed to customers in kilowatt-hours (kWh), which means
that you used a machine that uses x amount of energy for x amount of hours.
If you want to measure electrical power in DC circuits (direct circuits), use
power = voltage times current (P = V times I)
A series circuit has many loads all connected to each other. Any energy that
goes through load one must also go through all the other loads.
A parallel circuit has different loads on different branches of the circuit, so the
loads are unrelated to each other.
See pictures of loads on page 243 and 244
A short circuit happens when energy goes from one wire to another wire
because the wires are touching. This is dangerous because the wire may
overheat and start a fire. To prevent problems with short circuits or too much
power going through wires, most buildings have overcurrent protection.
Overcurrent protection can shut down the electricity if there is too much power
in a circuit. An example of this is a circuit breaker, which can shut down one
individual circuit if there is too high an amp going through the circuit. There are
even arc-fault-circuit-interrupters (AFCIs) that can shut down a circuit before
there is a problem. These are intelligent circuits that act before there is a
problem.
Fuses are a glass and metal device that will melt if there is too much amp in a
circuit. These work after there is already a problem, and have a higher risk of fire
compared to AFCIs.
Ground-fault circuit interrupters (GFCIs) protect against shock when plugging
a machine into an electrical circuit (usually in a wall). They are in most homes
today.