Consider the circuit shown above. A metal bar rests on top of two wires such tha
ID: 1776752 • Letter: C
Question
Consider the circuit shown above. A metal bar rests on top of two wires such that there is a good electrical contact between the ends of the bar and the wire. The wires are connected to a battery and switch as shown. The whole circuit is in a constant magnetic field Bext out of the page. You can assume the total resistance in the circuit, independent of the position of the bar, is a constant R and any friction between the bar and the wires is negligible
G) How would changing the resistance, R, in the circuit change the operation of this circuit and the movement of the bar?
H) (Thought question) This is the basic operation of a “Rail Gun”, an electro-magnetic projectile launcher. Real operation of these launchers does not use an external magnetic field, as described here. Explain how this system would operate without an external field.
Bext (out)Explanation / Answer
G) As the current flows from the positive teminal of battery, it encounters a resistance R and some of the current is then dissipated by resistance, now current moves through wires in opposite direction. Now we can understand from ohm's law, V = I*R and R = V/I .
It means for a given voltage source, if we increase the resistance, current will decrease. If current is decreasing, the field generated will be weaker and metal bar will move slightly.
Therefore, movement of metal bar depends on how much current is passing through wires and this current ultimately depends on R.
H) In rail guns , we don't need an external magnetic field because the magnetic field is set up internally by the opposite current flowing through two parallel wires. Suppose, the current in the upper wire flows to left and in lower wire, flows to right, then a magnetic field will set up in the wire. which will be adding up at the center of wire and will exert a force on the metal bar. This force will act to the right. Obviously, current will also flow through the metal bar due to connection with wires.