Question 1

Why do electrons drift opposite to the electric field?

Accepted Answer

An electron's force and acceleration are opposite to the electric field because the electron carries negative charge; acceleration is along the electric-field force acting on it. Their random thermal velocities cancel on average, leaving only a small net drift opposite E. Conventional current uses the positive-charge direction and therefore points along E.

Question 2

What does Ohm's law state?

Accepted Answer

For a conductor under fixed physical conditions, the potential difference across it is directly proportional to the current, so V = IR. Temperature, material, length and area also affect the operating resistance, so these conditions must remain controlled when comparing ratios.

Question 3

How does a wire's length and area affect resistance?

Accepted Answer

For a uniform conductor, R = rho L/A. Doubling L doubles R, doubling the conducting area halves R, and changing resistivity changes both the material and temperature conditions represented by rho.

Question 4

Why is copper's resistivity given at a temperature?

Accepted Answer

Copper has a positive temperature coefficient, so its resistivity rises as temperature rises. Its tabulated coefficient is about 0.00393 per deg C at 0 deg C, and its resistivity at 20 deg C is approximately 1.68 x 10^-8 ohm.m.

Question 5

Is electron drift speed the same as signal speed in a wire?

Accepted Answer

No. Typical electron drift velocity in a wire is about 10^-4 m/s or less. Changes in the electrical signal propagate much faster because they involve the coordinated response of the existing charge distribution, not electrons travelling from one end to the other at that speed.

Ohm's Law, Resistivity & Drift Velocity — Inside a 3D Conductor

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