Force on a current-carrying conductor
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
The Motor Effect When a current-carrying conductor is placed in a magnetic field , it experiences a force . This is called the motor effect . Conditions for Maximum Force The force is maximum when the current direction is perpendicular to the magnetic field direction.
Demonstrating the Motor Effect Experiment: Wire Between Magnets Place a wire between the poles of a horseshoe magnet Pass current through the wire Observe: wire moves (experiences a force) Reverse the current → force direction reverses Reverse the magnet poles → force direction reverses FIG 4.5.17: Motor effect demonstration Horseshoe magnet with N pole on left, S pole on right.
Wire passing vertically through the field (horizontal field lines from N to S). Current flowing upward. Force on wire shown as horizontal arrow (perpendicular to both current and field). Fleming's Left-Hand Rule Use your left hand to find the direction of force, magnetic field, and current.
FIG 4.5.18: Fleming's Left-Hand Rule Left hand shown with: thumb pointing up (Force), first finger pointing forward (Field, N to S), second finger pointing right (Current, + to −). All three fingers perpendicular to each other.
FBI Memory Aid F irst finger = F ield (N to S) se Cond finger = C urrent (+ to −) Thu m b = M otion (force) Force on Charged Particles Moving charged particles in a magnetic field also experience a force.
A beam of electrons passing through a magnetic field is deflected The direction follows the left-hand rule (but remember electrons are negative, so their "conventional current" is opposite to their motion) Positive charges : force direction from left-hand rule directly Negative charges : force direction opposite to left-hand rule prediction FIG 4.5.19: Electron beam deflection Electron beam entering a magnetic field region (field into page).
Electrons moving right are deflected downward in a curved path. The force is perpendicular to velocity, causing circular motion. Applications Cathode ray tubes: Deflect electron beams in old TVs Particle accelerators: Bend particle paths Mass spectrometers: Separate particles by mass Factors Affecting the Force The force on a current-carrying conductor increases with: Stronger magnetic field Larger current Longer length of wire in the field
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