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Carrier Drift, Current and Cross-Section

Balance electric current with carrier density, charge magnitude, cross-sectional area and signed drift using all captured units and inverse relationships.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A 1 A current through 1 mm² with 8.5×10²⁸ carriers/m³ and charge magnitude one elementary charge corresponds to a drift-speed magnitude of about 7.3431×10⁻⁵ m/s.

I = n|q|Av; n = displayed coefficient × 10²⁸ per selected volume.

Worked example

Enter these known values and leave the other values blank.

Signed conventional current I
1 A
Conductor cross-sectional area A
1 mm²
Carrier number density n
8.5 ×10²⁸ carriers/ m³
Carrier charge magnitude |q|
1 e
Signed drift reference v
0.00007343 m/s

Assumptions and limitations

  • This single-carrier uniform-current model relates cross-sectional mean drift and current. Number density, area and carrier-charge magnitude must be positive; signed current and drift reference may be negative or zero.
  • The density entry is a coefficient with the retained ×10²⁸ carriers per selected volume prefix. Enter 8.5 with m³ selected for 8.5×10²⁸ carriers/m³. Inverse volume conversion preserves the actual physical density.
  • Charge is a positive magnitude, not a signed electron charge. The computed signed v follows the conventional-current reference for positive carriers. Electrons move in the opposite physical vector direction; when using electron density and |e|, interpret |v| as their speed and reverse direction relative to current.
  • The retained editable charge default is one elementary charge. Unit changes preserve that physical magnitude; enter a different positive value when the carrier charge differs. The elementary-charge definition is exact in SI.
  • This model does not calculate thermal/random carrier speed, signal propagation, resistance, scattering time or multiple-species current. All values refer to the same cross-section and operating condition.
  • At zero current and zero drift, the product is consistent for any positive area, density and charge. Those unknown factors cannot be recovered uniquely from the zero pair; division inverses remain blank. Clear the intended unknown before solving an inverse.

Common questions

Does drift speed equal the signal speed?

No. It is the mean carrier drift associated with current. Electrical signal propagation and random thermal motion are different quantities.

What does the density multiplier mean?

With m³ selected, a coefficient of 8.5 means 8.5×10²⁸ carriers per cubic meter. Other volume choices preserve density using reciprocal-volume conversion.

Which direction do electrons move?

Electron drift is opposite conventional current. This positive-charge-magnitude relation gives their speed magnitude; its signed reference follows positive carriers.

References

The retained drift relation is I=n|q|Av with the density control scaled by 10^28 and equation charge base in nanocoulombs. Four dividing inverses require positive area, carrier-charge magnitude and density, and nonzero drift where it is a denominator. Zero current and zero drift remain valid forward states; they do not uniquely determine area, charge or density.

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