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Chapter 20: Q61P (page 861)

A neutral copper bar oriented horizontally moves upward through a region where there is a magnetic field out of the page. Which diagram (1-5) in Figure 20.123 correctly shows the distribution of charge on the bar?

Short Answer

Expert verified

The correct diagram is (3).

Step by step solution

01

Given Data

speed=vmagnetic field=BForce=F

02

Concept

The speed is perpendicular to the magnetic field.

03

Find which diagram shows the distribution of charge on the bar

Magnetic force is given by Lorentz force law. This says the direction of the magnetic force is given by the cross product of velocity and magnitude field.

Therefore, F=qv×B

Since the direction of velocity is in the upward direction and the direction of magnetic field is out of the page so by the right hand thumb rule the direction of force on the positive charge is towards right and that on the negative charge is towards left.

Hence the correct diagram is (3).

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Most popular questions from this chapter

We will consider the possibility that a free electron acted on by an electric field could gain enough energy to ionize an air molecule in a collision. (a) Consider an electron that starts from rest in a region where there is an electric field (due to some charged objects nearby) whose magnitude is nearly constant. If the electron travels a distance dand the magnitude of the electric field is E ,what isthe potential difference through which the electron travels? (Pay attention to signs: Is the electron traveling with the electric field or opposite to the electric field?) (b) What is the change in potential energy of the system in this process? (c) What is the change in the kinetic energy of the electron in this process? (d) We found the mean free path of an electron in air to be about role="math" localid="1662205184726" 5×10-7 m, and in the previous question you calculated the energy required to knock an electron out of an atom. What is the magnitude of the electric field that would be required in order for an electron to gain sufficient kinetic energy to ionize a nitrogen molecule? (e) The electric field required to cause a spark in air is observed to be about 3×106 V/mat STP. What is the ratio of the magnitude of the field you calculated in the previous part to the observed value at STP? (f) What is it reasonable to conclude about this model of how air becomes ionized? (1) Since we used accurate numbers, this is a huge discrepancy, and the model is wrong. (2) Considering the approximations we made, this is pretty good agreement, and the model may be correct.

In Figure 20.115 two long straight wires carrying a large conventional current I are connected by one-and-a-quarter turns of wire of radius R. An electron is moving to the right with speed v at the instant that it passes through the center of the arc. You apply an electric field E→at the center of the arc in such a way that the net force on the electron at this instant is zero. (You can neglect the gravitational force on the electron, which is easily shown to be negligible, and the magnetic field of the coil is much larger than the magnetic field of the Earth.)

Determine the direction and magnitude of the electric field . Be sure to explain your work fully; draw and label any vectors you use.

In the simple mass spectrometer shown in figure 20.101, positive ions are generated in the ion source. They are released traveling at very low speed, into the region between two accelerating plated between which there is potential difference ∆V . In the shaded region there is negligible magnetic field. The semicircle traces the path of one single charged positive ion of mass M, which travels through the accelerating plates into the magnetic field region, and hits the ion detector as shown. Determine the appropriate magnitude and direction of the magnetic field B→ , in terms of the known quantities shown in figure 20.101. Explain all steps in your reasoning.

You hold your magnet perpendicular to a bar of copper that is connected to a battery (Figure 20.84). Describe the directions of the components of the electric field at the center of the copper bar, both parallel to the bar and perpendicular to the bar. Explain carefully.

Explain briefly why there is a limit to how much charge can be placed on a metal sphere in the classroom. If the radius of the sphere is 15 cm what is the maximum amount of charge you can place on the sphere? (Remember that a uniform sphere of charge makes an electric field outside the sphere as though all the charge were concentrated at the center of the sphere.)
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