Ohms law calculator magic triangle ohm's law pie chart mathematical electric voltage electric current amperes power formulas resistance Ohm volts amperes calculate E V = I R - P = V I specific electrical resistance conductivity resistivity relation relationship - sengpielaudio
 
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Ohm's Law
Calculator and Formulas
 
Resistance (ohms), current (amps), and voltage (volts)
 
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
 
Electrical voltage = current times resistance "VIR"

Input:
current I    
 ×
amperes      

resistance R

ohms

Output:
voltage V

volts

 
 
voltage V   
  
volts      
resistance R

ohms
current I

amperes
 
 
voltage V   
  
volts      
current I

amperes
resistance R

ohms
 
= reset.

Formulas:          V = I R          I = V / R          R = V / I

The mathematical 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.
First Version of the formula: V = I × R

If there is a voltage V across a resistor R, a current I flows through it. I can be calculated.
Second Version of the formula: I = V / R

If a current I flows through a resistor, and there is a voltage V across the resistor. R can be calculated.
Third Version of the formula: R = V / I

All of these variations of Ohm's Law are mathematically equal to one another.

 Name    Formula sign   Unit   Symbol  
 voltage  V or E volt V
 current  I  ampere (amp)   A
 resistance  R ohm Ω
 power  P watt W
 
What is the formula for electrical current?
When the current is constant:
I = Δ Q / Δ t
I is the current in amps (A)
Δ Q is the electric charge in coulombs (C),
that flows at time duration of Δ t in seconds (s).
 
Voltage V = current I × resistance R
 
Power P = voltage V × current
I

In electrical conductors, in which the current and voltage are proportional
to each other, ohm's law apply: U ~ I or UI = const.

 
Constantan wires or other metal wires held at a constant temperature meet well ohm's law.
 
"UI = R = const." ist not the law of ohm. It is the definition of the resistance.
Thereafter, in every point, even with a bent curve, the resistance value can be calculated.

 
For many electrical components such as diodes ohm's law does not apply.

The magic triangle  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.

The symbol I or J = Latin: influare, international ampere, and R = resistance. V = voltage or
electric potential difference, also called voltage drop, or E = electro motive force (EMF = voltage).

If the unit of power P = I × V and of voltage V = I · R is needed,
look for Red Power Dot "The Big Power Formulas":
Calculations: power (watt), voltage, current, resistance

Some persons think that Georg Simon Ohm calculated the "specific resistance".
Therefore they think that only the following can be the true ohm's law.

     Value of the Resistance  
 
Formel
 
 R  = Resistance Ω
 ρ  = Electrical resistivity   Ω·m 
 l  = Cable length m
 A  = Cross-sectional area m2

Electrical conductivity (conductance) σ (sigma) = 1/ρ
Specific electrical resistance (resistivity) ρ (rho) = 1/σ

 Electrical
 conductor 
 Electrical conductivity
 Electrical conductance 
 Electrical resistivity 
 Specific resistance
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.

 
Electrical conductivity σ  
 S · m / mm²
 ↔  Specific elec. resistance ρ  
 Ohm mm² / m
σ = 1 / ρ   ρ = 1 / σ
 
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.

Resistance R = ρ × (l / A) or R = l / (σ × A)

For all conductors the specific resistivity changes with the temperature. In a limited temperature range it is approximately linear:
Temperature dependence
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 we 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:
Formula calculation diameter
r = radius of the wire
d = diameter of the wire

Calculation diameter d from cross section A (slice plane):
Formel Berechnung Durchmesser aus Querschnitt
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 •

Electric voltage V = I × R      (Ohm's law VIR)
Electrical voltage = amperage × resistance
Please enter two values, the third value will be calculated.

 Electric voltage V  volts Magic triangle volt
Amperage I  amps 
Resistance R  ohms
Electric power P = V × I      (Power law PIV)
Electric power = amperage × voltage
Please enter two values, the third value will be calculated.

 Electric Power P  watts Magic triangle volt
Voltage V  volts
Amperage I  amps 
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.

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