5H40.45 • Forces on Current in Wires: Rail Gun
Photo Show the Demonstration Used by the University of Texas at Austin.
Concept Conveyed:
- This demo consists of a power supply and a circuit of 3' aluminum rails connected by a free-to-roll aluminum rod projectile over an array of strong magnets.
- Over the magnets, the standing field generates a Lorentz force on the current in the projectile to cause it to accelerate noticeably. Ask the students to predict which way the projectile will move using the right hand rule.
Instructions:
- When you are ready to use the apparatus, place the projectile on the rails over the magnets as perpendicular to them as possible so that it rolls straight.
- Predict the expected direction of the Lorentz force on the projectile by assuming an "upwards" pointing magnetic field and noting the direction of the current from the power supply.
- Turn on the power supply, allow it to be current-limited at 5-10A by setting a small voltage of about 2.5V and raising the current slowly.
- If the projectile does not move, using the acrylic wand, apply a small nudge to initiate motion in the opposite direction as the anticipated force so that the projectile reverses direction.
Safety:
- Reduce both the voltage and the current on the power supply to zero and turn off the supply when not in use.
- Wear insulated gloves and use the acrylic wand to move the projectile.
- Stand on the provided insulated mat.
Demo Staff Instructions:
- Place the magnet strip at the center, between the rails of the apparatus.
- Plug banana cables into the ends of the rails and into corresponding power supply outputs AT THE BACK OF THE SUPPLY. Try not to cross the wires, and use black for negative for ease of circuit analysis.
- Provide gloves, acrylic wand, and insulated mat.
Image depicting the insulating mat.
Last updated on October 8, 2025