When an electron moves along a vertical line towards the observer, the magnetic field it produces forms clockwise circles around its path, as viewed by the observer. Since the electron carries a negative charge, its motion constitutes a conventional current in the opposite direction (away from the observer). Applying the right-hand rule to this effective current, or directly using Fleming's left-hand rule for negative charges, gives a clockwise magnetic field.
An electron moving towards the observer produces a clockwise magnetic field (as viewed from the front).
Electron has negative charge (−e), so its motion creates conventional current in the opposite direction.
Right-hand rule applied to the effective conventional current gives the magnetic field direction.
For electron moving towards observer: effective current is away from observer → B is clockwise.
For a proton moving towards observer: B is anticlockwise (positive charge, current towards observer).
The Biot-Savart law: B = (μ₀/4π) × (qv × r̂)/r²; the sign of q determines direction.
An electron has negative charge (−1.6 × 10⁻¹⁹ C).
Method 1: Right-Hand Rule with reversed current
Method 2: Direct rule for negative charges
Conclusion: Electron moving towards observer → B is clockwise.
| Particle | Charge | Motion | Effective Current | B-field Direction |
|---|---|---|---|---|
| Proton | +e | Towards observer | Towards observer | Anticlockwise |
| Electron | −e | Towards observer | Away from observer | Clockwise |
| Proton | +e | Away from observer | Away from observer | Clockwise |
| Electron | −e | Away from observer | Towards observer | Anticlockwise |
Key rule: Magnetic field direction depends on the direction of CONVENTIONAL current (= direction of positive charge flow), not electron flow.
Magnetic force on a moving charge (Lorentz force): F = qv × B
For electron: q = −e = −1.6 × 10⁻¹⁹ C
Magnetic field produced by electron: B = (μ₀/4π) × (qv × r̂)/r²
The negative charge reverses the direction of B compared to a proton moving in the same direction.
This is why:
When an electron moves towards the observer (out of the page), the magnetic field is clockwise as viewed by the observer. Since the electron carries negative charge, the effective conventional current is in the opposite direction (away from observer), and by the right-hand rule, the field curls clockwise.
For a proton (positive charge) moving towards the observer, the magnetic field is anticlockwise. For an electron (negative charge) moving in the same direction, the magnetic field is clockwise — exactly opposite — because the conventional current direction is reversed.
The right-hand rule is used, but applied to the equivalent conventional current (not the electron's motion). Since conventional current is opposite to electron flow, you reverse the electron's direction and apply the right-hand rule. Alternatively: for negative charges, reverse the result obtained by the right-hand rule for the actual direction of motion.
If an electron moves away from the observer (into the page), the effective conventional current is towards the observer (out of the page). Applying the right-hand rule, the thumb points towards you and the fingers curl anticlockwise. So the magnetic field is anticlockwise.
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