INTRODUCTION TO OHM`S LAW
OHM`S LAW ORIGINS
Ohm's Law is made from 3 mathematical equations that represent the relationship between electric voltage, current and resistance.
What is voltage?
Voltage is a representation of the
electric potential energy per unit charge. If a unit of electrical charge were
placed in a location, the voltage indicates the potential energy of it at that
point. In other words, it is a measurement of the energy contained within an
electric field, or an electric circuit, at a given point.
What is resistance?
Resistance is a term that describes
the forces that oppose the flow of electron current in a conductor. All
materials naturally contain some resistance to the flow of electron current. We
have not found a way to make conductors that do not have some resistance.
What is the current?
Electrical current is a measure of the
amount of electrical charge transferred per unit time. It represents the flow
of electrons through a conductive material.
OHM'S LAW
1. Ohm's Law deals with the
relationship between voltage and current in an ideal conductor. This
relationship states that:
“The
potential difference (voltage) across an ideal conductor is proportional to the
current through it.
“
The
constant of proportionality is called the "resistance", R.
Ohm's Law is
given by:
V = I R
Where V is the
potential difference between two points which include a resistance R. I
is the current flowing through the resistance. For biological work, it is often
preferable to use the conductance, g = 1/R; in this form Ohm's Law is:
I = g V
2. Material that
obeys Ohm's Law is called "ohmic" or "linear" because
the potential difference across it varies linearly with the current.
3. Ohm's Law can be used to solve simple
circuits. A complete circuit is one which is a closed loop. It contains at
least one source of voltage (thus providing an increase of potential energy and
at least one potential drop i.e., a place where potential energy decreases. The
sum of the voltages around a complete circuit is zero.
4. An increase of
potential energy in a circuit causes a charge to move from a lower to a higher
potential (i.e. voltage). Note the difference between potential energy and
potential.
Because of the
electrostatic force, which tries to move a positive charge from a higher to a
lower potential, there must be another 'force' to move charge from a lower
potential to a higher inside the battery. This so-called force is called the electromotive
force, or emf. The SI unit for the emf is a volt (and thus this is
not really a force, despite its name). We will use a script E, the symbol E,
to represent the emf.
A decrease of
potential energy can occur by various means. For example, heat lost in a circuit
due to some electrical resistance could be one source of energy drop.
Because energy is conserved, the potential
difference across an emf must be equal to the potential difference across the
rest of the circuit. That is, Ohm's Law will be satisfied:
E = I R
SUMMARY:
Statement:
The current passing through a
conductor is directly proportional to the potential difference applied across
the terminals of the conductor provided the temperature and other physical
condition of the conductor remains unchanged
Mathematical expression:
V α I
V=
(constant) I
V=IR
Graphical representation:
Limitations:
- Ohm's law is applicable when resistance remains constant.
- Ohm's law is applicable when temperature of conductor remains
unchanged.
- Ohm's law is applicable to metallic conductors only.
