What is Resistance ?

Resistance is the opposition to flow or movement.

Resistance in a a mechanical system could be friction, wind, gravity.
Resistance can be opposition to a policy, or to an invasion (e.g. French Resistance).
Resistance in an electrical circuit is the limiting or opposition to current flow. It is measured in Ohms.

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Electrical Resistance

The technical definition is "Electrical resistance is a ratio of the degree to which an object opposes an electric current through it, measured in ohms."
In simplfied terms, electrical resistance is broadly equivalent to friction in a mechanical system. If you applied a voltage to a circuit with zero resistance then you would get an infinite current. In reality all circuits have some resistance which limits the current.
We can predict the current in a given circuit using Ohms law:
V = I x R
Where:
V is Voltage (measured in Volts)
I is current (measured in Amps)
R is resistance (measured in Ohms)
You will often see this in the form E = I x R. It is exactly the same thing, just a different convention where "E" is used for voltage instead of "V"

Ohms law is:

V=I/R
V=Voltage (volts)
I=Current (amps)
R=Resistance (Ohms)
Resistance in an AC circuit is called Impedance (Z)
Electrical resistance is the amount that an object opposes electrical current. All things being equal (temperature, purity, etc.), resistance is a function of both its physical geometry and the resistivity of the material it is made from. Impurities in materials and fluctuation of temperature can effect the amount of resistance a material will exhibit.

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Ohm’s Law Resistance and Resistors

Resistance is the difficulty applied by the conductor to the current flowing through it. Each material has different resistance. We show resistance with R and unit of it is ohm (Ω).

1 Ω=resistance of the conductor when 1 A current flows under the 1 V potential difference. Resistance is represented with the following picture in circuits;

resistance

Rheostat

Rheostat is a kind of device used to vary existing resistance. It is shown in the circuits as;

rheostat

Resistance of the conductor depends on;

· Types of the material or electrical resistivity of the material. It is shown with Greek letter ρ. Resistance of the material is linearly proportional to electrical resistivity.

· Length of the material (l). Resistance is linearly proportional to the length of the conductor.

· Cross sectional are of the conductor. Resistance is inversely proportional to the cross sectional area.

· Temperature. Temperature shows different effects with respect to the type of material.

We write resistance formula with the explanation given above as;

resistance

Ohm’s Law

Ohm’s law gives the relation between voltage, current and resistance. According to Ohm, current in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor. We can summarize this explanation with following formula;

ohm's law

If we take resistance of the conductor constant, then potential difference and current of the system changes linearly as shown in the following graph;

ohm's law graph

Example: Potential difference vs. Current graph of a conductor is given below, Find the behavior of resistance in intervals I, II and III.

resistance example

Ohm’s law states that V=I.R

In the first interval, since the potential difference and current are linearly increases then resistance of the system becomes constant.

In the second interval, potential difference is constant however, current increases. This can be possible by decreasing in the amount of resistance.

In the third interval, current is constant, however, potential difference increases. This can be possible by increasing in the amount of resistance.