By Valentina Tobos | Laurentiu Tobos

Updated on Sep 27, 2011

Review these concepts at: Magnetism Study Guide

**Practice Questions**

- In Figure 15.3, find the direction of the magnetic force acting on the negative charge.
- Under what conditions can the force on a charge in a magnetic field be zero?
- A laboratory superconductive magnet can achieve a field of 9 T. How does the force on an electron moving at the same speed in a superconductive magnet compare to the one in a permanent magnet (field of 10 gauss)?
- In a laboratory experiment, a proton accelerates linearly (on the
*x*-axis direction) and then passes a magnetic field oriented along*z*direction. Neglecting all other forces, in what direction is the net force acting, and what effect will it have on the initial direction of motion of the proton? - At what angle will a 3 · 10
^{6}m/s moving proton enter an accelerating magnetic field of 8,000 gauss if the net force is required to be 3 · 10^{–13}N? Consider only the magnetic force to be acting on the charge. - Two electrons enter a magnetic field of 0.5 T with speeds 3 · 10
^{6}m/s and 6 · 10^{6}m/s. The first electron is oriented perpendicular to the field, but the second one is not. What is the angle between the velocity of the second electron and the field if they both experience the same force?

- Consider a wire conducting a current
*I*in a magnetic field. What is the relative orientation of the current with respect to the field so that the magnetic force on the wire is maximum? - Consider a wire conducting a current
*I*in a magnetic field. What is the relative orientation of the current with respect to the field so that the magnetic force on the wire is zero?

- The magnetic field of Earth is about 10 gauss. Convert that into tesla.
- Consider a negative charge moving in a downward direction and in close proximity to an electric current passing through a wire such as the one in Figure 15.13. Find the direction of the magnetic force on the charge.
- Show in a diagram the interaction between two parallel wires in which electrical currents flow in opposite direction.
- If two parallel current-carrying wires move apart to twice the initial distance, what happens to the initial magnetic force between the two currents.
- Find the maximum magnetic force per unit of length on a wire conducting a current of 10 A in the presence of a field of 300 G.
- A number of 10
^{20}electrons pass through the area of a conductor during a time period of 1 μs. Find the magnetic field at a distance of 30 cm from the wire. - If a straight wire was to create a magnetic field of 1 T at a distance of 10 cm, what would the current through the wire have to be?

**Answers**

*F*= 0 since the angle between the field and the speed is zero.- When the field and the velocity are either in the same or opposite directions
*Fs/F*= 9_{magnet}*T*/10 gauss ~ 10^{4}- The force is in the –
*y*direction and the trajectory will be curved as shown in the figure. - 12°
- 30°
- The two have to make a 90° angle.
- The two have to make a 0° angle.
- 10
^{–3}*T* - The force will be oriented toward the wire.
- The wires will repel each other since the fields are in the same direction (as
*N*and*N*would do). - The force between the wires is inverse proportional with the distance between, so the force would be half when the two wires are at twice the distance.
- F/L = 0.3 N/m
- 11 · 10
^{– 6}T - 0.5 · 10
^{6}A

From Physics Success in 20 Minutes A Day. Copyright © 2006 by LearningExpress, LLC. All Rights Reserved.

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