Resistance
• RESISTANCE is the
opposition to electric
current in a conducting
wire or material. A large
wire can carry more
electrons than a small
wire. It has less
resistance to the flow,
because of its larger
size.
• Resistance depends on
size and material.
Current
•
•
•
Current is the rate of charge movement
A current exists whenever there is a net
movement of electric charge through a
medium. To define current more
precisely, suppose positive charges are
moving through a wire. The current is
the rate at which these charges move
through the cross section of the wire. If
∆Q is the amount of charge that passes
through this area in a time interval, ∆t,
then the current, I, is the ratio of the
amount of charge to the time interval.
Current can be direct or alternating
There are two different types of current:
direct current (dc) and alternating
current (ac). In direct current, charges
move in only one direction. In alternating
current, the motion of charges
continuously changes in the forward and
reverse directions. In alternating current,
the terminals of the source of potential
difference are constantly changing sign.
Hence, there is no net motion of the
charge carriers in alternating current;
they simply vibrate back and forth.
Ohms Law
OHM’S LAW
One of the fundamental laws of electrical
circuits was derived from experimentation
done by George Simon Ohm, the
German scientist and philosopher, during
the 19th century. To honor the
achievements of Mr. Ohm, the standard
unit of measurement for resistance is
called the OHM. It is frequently
represented by the Greek letter “omega”
as . If you see on a diagram 1000 c , it
means 1000 ohms. In electronic circuits
Using Ohms Law
Series Circuits
• SERIES CIRCUITS
• If a circuit is so arranged that all
of the current flowing in the circuit
will pass through all components,
these components are connected
in SERIES.
• As current passes through a
resistor, a certain amount of
energy is used and a certain
amount of pressure or voltage is
lost. The voltage loss across
each resistor may be calculated
by Ohm’s Law, using formula,
E(V) = I x R,
Series Circuits….
Kirchhoff’s Laws
• Since these resistors are
connected in series, all the
current flowing in the circuit
must pass through each
resistor. Therefore, current
is the same in all parts of
the circuit.
• 1. The sum of the voltage
drops around a series circuit
will equal the source voltage.
• 2. The current is the same
when measured at any point
in the series circuit.
•
Parallel
Circuits
Current
When several components are
When equal resistors are connected in
parallel, the total resistance of the parallel
network is equal to any one resistor divided
by the number of resistors in the network.
connected to the same voltage
source, the components are
connected in parallel or “side
by side.” Multiple paths for
current flow are provided by a
parallel circuit, because each
resistor constitutes a path of its
own.
The applied voltage is the same for each
resistor, because each is connected across
the same voltage source.
Therefore, the currents are, using Ohm’s
The total current flowing through the
network would be the sum of the individual
branch currents or:
.2 + .2 + .2 = .6 amps
Multiple resistors with
different values.
• The conductance of a
circuit is equal to the sum
of the conductances of
the branch circuits.
• Sum the 1/R of the
resistances and then
recipicate the answer.
Parallel Circuits
Resistance…
Parallel Circuits…
Summarizing
• The voltage across all branches of a parallel
network is the same.
• The total current is equal to the sum of the
individual branch currents.
• The total resistance of any parallel circuit must
always be less than the value of’ any resistor in
the network..
• The branch of the circuit containing the greatest
resistance conducts the least current.