Electricity and Magnetism Quiz True or False

1. Which of the following colors of visible light is the most energetic? -> Red
2. Two 4 m long wires run parallel to one another, separated by a distance of 5 cm. Both pass a current of 15 A through them, in the same direction. What force does each wire exert on the other? Will that force try to pull the wires together or push them apart? -> 60 mN, pulling together
3. Many inductors in common circuits are wound in the shape of a torus, a coil that curls around in a circle.

In a toroidal coil with N loops of wire, what is the magnetic field at some distance r within the coils when a current of I enters the inductor? ->
4. How fast, and in what direction, must an electron move in a 5 nT magnetic field directed into the screen in order to cancel out the effects of gravity? -> 0.11 m/s to the left
5. A positive charge moves to the right in a magnetic field directed out of the screen. What shape will its trajectory describe? -> Line, downwards
6. In the circuit below, a 50 cm rod is placed over a U-shaped wire and moves to the right at a constant speed of 0.5 m/s. When the loop and rod are placed in a constant magnetic field of 300 mT into the screen, a current flows around the wire. What is the magnitude of this current if the rod has a resistance of 10 Ω? Does it flow clockwise or counterclockwise?
-> 15 mA, clockwise
7. Which of the following diagrams most accurately represents the magnetic field lines between the north end of one bar magnet and the south end of another?
-> E
8. In a DC electric motor, a loop of wire (the rotor) sits between two permanent magnets (the stator) that create a magnetic field across the loop.


When current flows through the two sides of the loop perpendicular to the field, they feel equal and opposite forces that turn the motor. For a 10 V motor with an internal resistance of 20 Ω and a 2 cm square rotor, how strong must the field created by the magnets be to generate a force of 6 mN to move the wires? -> 30 T
9. A coil of wire made of a stack of 5 cm square loops moves at a speed of 0.8 m/s to the left, entering a 0.5 T magnetic field directed out of the screen. The coil has a resistance of 12 Ω. How many loops would the coil need to have a current of at least 500 mA flowing through it as it enters the magnetic field? -> 1
10. . A more advanced tool for solving magnetism questions in the Biot-Savart Law, which yields the magnetic field created a distance r away from a point charge q1 moving at speed v1: .If a second charge q2 moves through the electric and magnetic fields created by q1, at what speed v2 would q2 have to move in order to feel equal electric and magnetic forces? Express your answer in terms of v1 and c, the speed of light.
(Hint #1: First express k in terms of the permittivity of free space.)
(Hint #2: There is one speed and one speed only that q2 travels at where this can be true.) ->

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