I.
Electrical circuits
a. Electrical current is the flow of charge, either electrons or ions, through
matter
i. electrons have a negative charge, in materials that are able to
conduct electricity, electrons are free to flow
ii. ions are molecules with a charge, ions are able to pass a charge
through a material
b. Law of Electric Charges
i. Electric force - all charged objects exert an electric force on each
other
ii. Opposite charges are attracted
iii. Like charges repel
iv. Electric field - space around a charge which exerts a force
1. Any spinning or moving electron has an electric field
2. Electric fields may surround any material
c. Voltage
i. Voltage is electric push, measure of how much electrical energy a
source can provide.
ii. Potential difference – the difference in electric potential (voltage)
between two points. Free charge flows when there is a difference,
and will continue until both points reach a common potential
iii. Push needed to move an electron from one place to another
iv. Measured in volts
v. Source of Electricity (Battery)
1. Supplies potential energy to a circuit
2. Sets the voltage for the circuit
d. Current
i. Current is the rate of flow of charge
ii. Amperes – flow electrical charge past a point per second
iii. Often shortened to amps
iv. Symbol for ampere is A
v. In equations, the symbol for current is I
e.
Resistance
i. Resistance is the opposition to the flow of electrons by the atoms of
the material
ii. Measured in ohms
iii. Ohms – the SI unit of electric resistance. One ohm is the resistance
of a device that draws a current of one ampere when a voltage of one
volt is impressed across it.
II.
f. Electric circuit
i. There must be a complete path for electricity to follow and a voltage
difference
ii. Circuit is a closed conducting pathway for electrons to flow through
iii. Simple circuits must have
1. a source of electricity “battery”
2. conductor “wire”
3. load “light bulb or motor” what the electrons do work on
iv. There are two types of circuits
1. Series circuits - a circuit that has only a single path (one loop)
for current to flow through all of its parts.
a. Total voltage is divided, equal to the sum of all the
voltage
b. Total resistance is equal to the sum of all the resistors
c. Current stays the same
2. Parallel circuits - requires more than one path for current flow
in order to reach all of the circuit parts.
a. Current is divided, total current is equal to the sum of
all the currents
b. Voltage stays the same
c. The combined resistance is less than the smallest
resistance in the circuit
Electrical Symbols
Source
Voltmeter
Bulb
Ammeter
Motor
Resistance
Switch on
Fuse
Switch off
Ohm’s Law
a. Ohm’s Law – relationship between current, voltage, and resistance
b. Current equals voltage divided by resistance
i. I = V/R
ii. R = V/I
iii. V = I x R
iv. Problems
1. What is the resistance of an electric frying pan that draws 12
amperes of current when connected to a 120-volt circuit?
2. How much current is drawn by a lamp that has a resistance of
100 ohms when a voltage of 50 volts is impressed across it?
3. If the resistance of your body were 100,000 ohms, what would
be the current in your body when you touched the terminals of
a 12-volt battery?