|Ohm's Law is the linear proportionality between current and voltage that occurs for most conductors
of electricity. A graph of voltage against current is a straight line. The gradient is the resistance.
Practitioners rarely speak of potential difference, when electrical voltage (drop) is meant. VIR
V = I × R I = V / R R = V / I
The formulas of Ohm's Law
Ohm's Law can be rewritten in three ways for calculating current, resistance, and voltage.
If a current I should flow through a resistor R, the voltage V can be calculated.
V = I × R
If there is a voltage V across a resistor R, a current I flows through it. I can be calculated.
I = V / R
If a current I flows through a resistor, and there is a voltage V across the resistor R can be calculated.
R = V / I
|voltage||V or E||volt||V|
| Tip: Ohm's magic triangle
The magic V I R triangle can be used to calculate all formulations of ohm's law.
Use a finger to hide the value to be calculated. The other two values then show
how to do the calculation.
If you need the unit of power P = V × I look for
The Big Power Formulas:
Calculations: power, voltage, current, resistance, and power
Some are of the opinion that Georg Simon Ohm calculated the "specific resistance".
Therefore they think that only this can be the true ohm's law.
Electrical conductivity (conductance) σ (sigma) = 1/ρ
Specific electrical resistance (resistivity) ρ (rho) = 1/σ
| Electrical conductivity
| Electrical resistivity
|silver||σ = 62||ρ = 0.0161|
|copper||σ = 58||ρ = 0.0172|
|aluminium||σ = 36||ρ = 0.0277|
|Simply enter the value to the left or the right side.
The calculator works in both directions of the ↔ sign.
|The value of the electrical conductivity (conductance) and the specific electrical resistance
(resistivity) is a temperature dependent material constant. Mostly it is given at 20 or 25°C.
|For all conductors the specific resistivity changes with the temperature. In a
temperature range it is approximately linear:
where α is the temperature coefficient, T is the temperature and T0 is any temperature,
such as T0 = 293.15 K = 20°C at which the electrical resistivity ρ (T0) is known.
Cross-sectional area - cross section - slice plane
Now there is the question:
How can you calculate the cross sectional area (slice plane) A
from the wire diameter d and vice versa?
Calculation of the cross section A (slice plane) from diameter d:
r = radius of the wire
d = diameter of the wire
Calculation diameter d from cross section A (slice plane):
Cross section A of the wire in mm2 inserted in this formula gives the diameter d in mm.
Calculation − Round cables and wires:
• Diameter to cross section and vice versa •
|Ohm's law. V = I × R, where V is the potential across a circuit element, I is the current
through it, and R is its resistance. This is not a generally applicable definition of
resistance. It is only applicable to ohmic resistors, those whose resistance R is
constant over the range of interest and V obeys a strictly linear relation to I. Materials
are said to be ohmic when V depends linearly on R. Metals are ohmic so long as one
holds their temperature constant. But changing the temperature of a metal changes R
slightly. When the current changes rapidly, as when turning on a light, or when using AC
sources, slightly non-linear and non-ohmic behavior can be observed. For non-ohmic
resistors, R is current-dependent and the definition R = dV/dI is far more useful. This is
sometimes called the dynamic resistance. Solid state devices such as thermistors are
non-ohmic and non-linear. A thermistor's resistance decreases as it warms up, so its
dynamic resistance is negative. Tunnel diodes and some electrochemical processes
have a complicated I to V curve with a negative resistance region of operation. The
dependence of resistance on current is partly due to the change in the device's
temperature with increasing current, but other subtle processes also contribute to
change in resistance in solid state devices.
Calculation: Parallel Resistance (Resistor) Calculator
Color Code Calculator for Resistors
In acoustics we use ohm's law as acoustic equivalent
|How electricity works.
Ohm's Law clearly explained.
[top of page]