ELECTRICITY FUNDAMENTALS
Prior to the discovery of the photoelectric effect, the nature of electricity had been a source of detailed investigation throughout the 19th century.
Electricity is an apparent force in nature that exists whenever there
is a net electrical charge between any two objects.
Basics of Electrostatics:
- Electrical charges are either negative (electrons) or positive
(protons)
- The unit of charge, q , is called the coloumb.
- When there are equal numbers of positive and negative charges
there is no electrical force as there is no net charge. This is the
case for a neutral atom.
- Electrical force is created when electrons are transferred from
one material to another (e.g. rubbing a wool cloth with a plastic comb).
- Electrical charge is conserved; charge is neither created nor destroyed
Properties of Electricity:
CURRENT: denoted by I
and measured in amperes. Current flows from negatively charged material to
positively charged material and is essentially the number of electrons per
second that are carried through a conductor. Current is measured in units
of amps. 1 amp = 1 coloumb/sec = 6.2 x 1018 electrons per
second!
VOLTAGE: Potential difference between
a negatively charged object and a positively charged one (like two
terminals on a battery). Potential difference is measured in units of
Volts ( V )which represents the work done per unit charge to move electrons
between the positive and negative terminals. If a potential
difference exists, then energy can be extracted.
Imagine that you have two opposite charges that you want to
separate. It takes work to separate the charge and thus
the separated charges store energy. The amount of stored energy
is given by:
E = qV where V is the voltage or electric potential of some system.
The units of voltage or Volts: 1 Volt = 1 Joule/Coulomb
If the separated charges get back together, work/energy can be
extracted from the system. If there is some pathway for the
charges to flow then we get a current. Current is denoted
by I and is in units of amperes or amps
 1 Ampere = 1 coulomb/second
RESISTANCE: Property of material
that helps prevent the flow of electrons in it. Metals are good
conductors due to low resistance. Wood is a poor conductor due
to high resistance. Resistance, R , is measured in ohms and
depends upon both the type of material and its size. Long wires have
more resistance than short wires; thin wires have more resistance than
thick wires. R is also temperature dependent.
OHM's LAW
Is there a relation between I, V, and R ? Let's
do an experiment:
In the above circuit there is a battery (V), some resistors (R), and
a light bulb that can only be activated if the right number of amps reach it.
We can control this buy putting the right resistance in the circuit
for a given battery Voltage. Simply drag some resistors into the circuit and turn on the switch. If too much current reaches the
light bulb then it is destroyed and if too little reaches the light bulb it does nothing but state at you in its unlighted state asking for more current. If you get the exact right current relative to the Battery Voltage and the total resistance, the lightbulb happily shines.
Experimental results then lead to Ohms law:
V = R * I
This is a linear relation. If you double the voltage (V) then for
the same value of R you get twice the current. If you want to keep
the current the same value after doubling V, you would have to
double the resistance (R).
Example:
- Standard US household voltage is 120 Volts.
- The heating element in your toaster has R = 15 ohms.
- What is the current flowing through your toaster?
I = V/R = 120/15 = 8 amps
Your electricity bill essentially measures the amount of current
that you use but you use this current as Power .
Power = V * I
So the toaster has a power of 120x8 =
960 Watts.
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