Basic example of electricity (C) Carson Dunlop Associates Definition of Ohm's Law
Explanation & measurement of electrical resistance

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Plain language definition of Ohms & Ohm's Law of electrical resistance:

What are Ohms and how are ohms or resistance measured & used in understanding electrical circuits, electrical resistance, and heat?

We also give a little of the history of the development of Ohm's law as well as links to supporting research.

This article series gives definitions of amps, volts, watts, resistance, current, ohms, electrical phases. We include basic formulas relating amps, volts, resistance, watts, and we explain what these electrical terms mean in practical applications such as for building or appliance electrical power, electrical wiring, and basic troubleshooting.

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What is the definition of Electrical Resistance & what is Ohm's Law?

Illustration of electrical resistance (C) Carson Dunlop Associates

Electrical Resistance is illustrated at left courtesy of Carson Dunlop Associates.

Electrical resistance is measured in Ohms Ω and is related to Watts and Volts by the simple equations we show here:

Watts = Volts 2 / Ohms

Current (Amps) = Potential (Volts) / Resistance (Ohms)

Electrical resistance can be thought of as how easily electricity flows through a material. Where resistance is high more effort is needed. A smaller-diameter electrical wire has more resistance to electrical flow than a larger-diameter wire.

A reason that the light bulb filament has high resistance is that it is very small in diameter.

Beginning with Thomas Edison, researchers discovered that if resistance in a wire is high enough the wire will get hot enough to glow (produce light) or even to start a fire (which is why the inside of an incandescent light bulb is a vacuum - to deny oxygen and thus protect the filament from simply burning up).

Ohm's Law, written about as early as 1791 and first formally published by Georg Simon Ohm (1789-1854) in 1827, states very simply the relationship between electrical current (Amps), electrical voltage (Volts) and the resistance of movement of electrical energy in a metallic conductor (Ohms).

I = V / R 

tells us that the current (Amps) through a conductor (wire) between two points on a circuit is proportional to the potential difference (Voltage drop) across the two points and that the current (Amps) between the same two points is inversely proportional to the resistance between them (Ohms or Ω).

We can re-write Georg Ohm's law to describe each of amps, volts, or resistance in terms of the other parameters, as shown below.

I = the current, measured in Amps; I = V / R

and using simple algebra to re-write the Ohm's Law equation,

V = the difference in potential between the same two points, measured in Volts;

V = I x R

R = Ω or the resistance in the conductor or circuit between the same two points, measured in Ohms;

R = V / I 



What is the Relationship Between Resistivity and Conductivity?


Resistivity or electrical resistance describes how strongly a material opposes or "resists" the flow of electric current. Low resistance means easy flow. High resistance means more-limited flow of electricity.

Symbols for resistivity are the greek letter ρ (rho) or the SI unit of electrical resistivity the ohm-metre (Ω . m)

The resistance between two contacts at two points of an electrical circuit is expressed in ohms or Ω.

Resistivity is the reciprocal of conductivity.

Technically, resistivity or electrical resistance is a measure of the scattering of electrons, where when more electrons are scattered the resistance is higher and can be written as

σ = ne2ℓ / mevrms


σ = electrical conductivity [S/m]
n = density of free electrons [e/m3]
e = charge of an electron (1.60 × 10−19 C)
me = mass of an electron (9.11 × 10−31 kg)
vrms = root-mean-square speed of electrons [m/s]
ℓ = mean free path length [m]

Conductivity Electrical conductivity is a measure of the degree to which a material will conduct electricity.

Similarly, heat conductivity is the rate with which heat passes through a material, or the amount of heat that flows per unit of time through a unit area with a temperature gradient of one degree per unit of distance.

Conductivity can be calculated as the ratio of current density in the material to the electric field that causes the current flow.

The conductivity between two contacts at two points of an electrical circuit is expressed by the greek letter σ (sigma) as you'll see making a cameo appearance in Glenn Elert's homely formula above. Some texts use kappa κ or gamma γ or in SI units (S/m) (Siemens per metre) to express conductivity.

Conductivity is most-easily expressed as the reciprocal of resistivity.

Technically, conductivity in metals is a statistical/thermodynamic quantity. But that's not going to help us undestand how the little electric heater works in a room thermostat heat anticipator.

Research & History of Ohms Law & Electrical Resistance


Continue reading at JOULES HEATING LAW that explains resistive heating or ohmic heating, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see HEAT ANTICIPATOR OPERATION that explains the principles of Ohms law and Jules Heating Law applied to a tiny electric heater in a room thermostat.

Or see this

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