BASICS OF ELECTRIC
CIRCUITS
CONDUCTORS AND INSULATORS
• Materials that have loosely bound electrons are
considered conductors because they conduct electricity
easily(mostly metals)
• Silver, copper, gold, aluminum, iron etc.
• Materials that have tightly bound electrons are
considered insulators because they conduct electricity
very poorly
• Glass, rubber, plastic, air, pure water etc.
ELECTRON FLOW
• In order for electrons to move, they must have somewhere
to go
• can’t have a buildup of electrons since electrons repel each other
• Analogous to marbles in a tube
• Overall effect is an almost instantaneous
• electricity flow happens at the speed of light while electron flow
is much slower
ELECTRICAL CIRCUITS
• In order to create a continuous flow of electrons, the path
must also be continuous
• ANY break in the circuit stops the flow of electrons
VOLTAGE
• A continuous path is not enough to make electrons move
• Need a ‘push’ to initiate flow
• This push is called VOLTAGE (measured in volts)
• The higher the voltage, the harder the ‘push’
• Similar to pumping water into a
water tower
• Higher tower = higher pressure
GENERATING VOLTAGE
• Voltage is usually generated in 3 ways
• Chemical reaction (batteries)
• Radiant energy (solar cells)
• Electromagnetism (generators)
MEASURING VOLTAGE
• Voltage measurements are always relative measurements
• Often referred to as potential difference
• No ‘absolute zero’ voltage
• Voltage is measure ACROSS a source or load
POLARITY
• Voltage sources always have 2 poles
• One is negative (excess electrons)
• One is positive (deficit of electrons)
• Electrons flow from negative to positive
ELECTRON FLOW VS CURRENT
• When Benjamin Franklin was experimenting with
electricity, he did not know about electrons and so
assumed that is was positive charge that flowed
• Charge flows from positive to negative
• Backwards to actual electron flow
• ALL ELECTRONICS ASSUMES CONVENTIONAL CURRENT
DC VS AC
• In a circuit with a battery this current always flows in the
same direction
• This is called direct current (DC)
•
Most household power is generated by electromagnetic
generators that constantly alternate the flow of electrons
• This is called alternating current (AC)
MEASURING CURRENT
• The number of electrons that actually move past a given
point is known as CURRENT (measured in amperes or
amps)
• Current is measured THROUGH a given point
RESISTANCE
• Resistance is the measure of opposition to electric charge
• Resistance is provided in a circuit by a LOAD which lowers the
voltage
SPECIAL CASES
• An open circuit (broken path) has infinite resistance
• A short circuit has no resistance (produces dangerous
amounts of heat)
SERIES AND PARALLEL
• Series circuit (only one path)
• Parallel circuit (multiple paths)
OHM’S LAW
• Ohm’s law provides the relationship between voltage,
current and resistance given by:
• Giving us 3 equations:
• V=IR
• I=V/R
• R=V/I
OHM’S LAW PROBLEMS
• An alarm clock draws 0.5 A of current when connected to
a 120 volt circuit. Calculate its resistance.
• How much voltage would be necessary to generate 10
amps of current in a circuit that has 5 ohms of resistance?
OHM’S LAW PROBLEMS
• An electric heater works by passing a current of 100 A
though a coiled metal wire, making it red hot. If the
resistance of the wire is 1.1 ohms, what voltage must be
applied to it?