1. Submitted To: Mr. Ihtisham Islam Saib
Submitted By: Salman Khan
Department: Geology
Topic: Ohm’s Law
Roll No: 3418
Subject: Geophysics
Programe: BS
Semester: 5th
Date: 30/10/2017
2. OHM’S LAW
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Ohm's law. The law stating that the direct current flowing in a conductor is directly
proportional to the potential difference between its ends. It is usually formulated as V = IR, where
V is the potential difference, or voltage, I is the current, and R is the resistance of the conductor.
Ohm's law says that in an electrical circuit, the current passing through a resistor between
two points, is related to the voltage difference between the two points, and inversely related to the
electrical resistance between the two points. ... Current is directly proportional to voltage loss
through a resistor.
This is called Ohm's law. Let's say, for example, that we have a circuit with the potential
of 1 volt, a current of 1 amp, and resistance of 1 ohm. ... Because the resistance is greater, and the
voltage is the same, this gives us a current value of 0.5 amps: So, the current is lower in the tank
with higher resistance.
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
Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference
across it varies linearly with the current.
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 drops 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 force is called the electromotive force, or emf. The SI unit for the emf is
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a volt (and thus this is not really a force, despite its name). We will use a script E, the symbol ,
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:
= I R
RESISTIVITY:
Voltage depends on current and resistance. ... Apparent Resistivity - Resistivity is
determined from Ohm's law using the potential difference (voltage) between two electrodes for a
known current. At small spacing between electrodes, apparent resistivity is close to
the resistivity of the upper layer.
The true resistivity (Rt) of a formation is its resistivity when not contaminated by drilling
fluids. It may contain formation water only (water saturation [Sw] = 100%) or formation water and
hydrocarbons (Sw < 100%).
Direct current apparent resistivity, ρa, is an Ohm's-law ratio of measured voltage V to applied
current l, multiplied by a geometric constant k which depends on the electrode array: ρa=kV/l.
Usually has units of ohm-meters.
Apparent resistivity
1. The resistivity of homogeneous, isotropic ground that would give the same voltage-current
relationship as measured. Direct current apparent resistivity, ρa, is an Ohm’s-law ratio of
measured voltage V to applied current l, multiplied by a geometric constant k which depends on
the electrode array: ρa=kV/l. Usually has units of ohm-meters.
2. With electromagnetic methods, quantities such as the moduli of the electric and magnetic field
intensities (E and H) are measured at a certain frequency or time. If the subsurface were
homogeneous and isotropic, these would yield the true resistivity via a certain equation. However,
use of the same equation for a heterogeneous subsurface yield the ‘‘apparent’’ resistivity ρa:
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Where ρHS=resistivity of a homogeneous half-space, F is a function of V=observed voltage,
and VHS=voltage for a half-space. Asymptotic definitions such as late-time and early-time
apparent resistivity is sometimes used for apparent resistivity values over a limited range of
measurement parameters. For some EM models apparent resistivity may be undefined or
multivalued.
3. The resistivity recorded by an electrical log that differs from the true resistivity of the formation
because of the presence of mud column, invaded zone, influence of adjacent beds, etc.
