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PHY303L Solutions%207

# PHY303L Solutions%207 - homework 07 FAUSAK TAYLOR Due Mar 3...

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homework 07 – FAUSAK, TAYLOR – Due: Mar 3 2008, 4:00 am 1 Question 1, chap 29, sect 1. part 1 of 1 10 points An electron is in a uniform magnetic field B that is directed out of the plane of the page, as shown. v e B B B B When the electron is moving in the plane of the page in the direction indicated by the arrow, the force on the electron is directed 1. toward the left 2. out of the page. 3. into the page. 4. toward the bottom of the page. 5. toward the top of the page. correct 6. toward the right Explanation: The force on the electron is vector F = q vectorv × vector B = e vectorv × vector B. The direction of the force is thus hatwide F = hatwide v × hatwide B , pointing toward the top of the page , using right hand rule for hatwide v × hatwide B , and reversing the direction due to the negative charge on the electron. Question 2, chap 29, sect 1. part 1 of 1 10 points Why will a magnet attract an ordinary nail or paper clip, but not a wooden pencil? 1. A nail or paper clip has a magnet inside which a wooden pencil does not have. 2. A nail or paper clip has magnetic do- mains which a wooden pencil does not have. correct 3. A magnet can generate electromagnetic waves which can be absorbed by an nail but not by a pencil. 4. A nail or paper clip has molecular circuits which a wooden pencil does not have. Explanation: A magnet will induce the magnetic domains of a nail or paper clip into alignment. Oppo- site poles in the magnet and the iron object are then closest to each other and attraction results. A wooden pencil, on the other hand, does not have magnetic domains that will in- teract with a magnet. Question 3, chap 29, sect 1. part 1 of 1 10 points Can an electron at rest in a magnetic field be set into motion by the magnetic field? What if it were at rest in an electric field? 1. An electron at rest will be set into mo- tion in a magnetic field but not in an electric field. 2. It cannot be determined because it de- pends on the intensity of the fields, which is not provided in the problem. 3. None of these 4. An electron at rest will be set into motion in an electric field but not a magnetic field. correct 5. An electron at rest will be set into motion in both fields. 6. An electron at rest will not be set into motion in either fields.

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homework 07 – FAUSAK, TAYLOR – Due: Mar 3 2008, 4:00 am 2 Explanation: An electron has to move across lines of mag- netic field in order to feel a magnetic force, so an electron at rest in a stationary magnetic field will feel no force to set it in motion. However, an electron in an electric field will accelerate regardless of its current state of mo- tion. (A combination of magnetic and electric fields are used in particle accelerators such as cyclotrons. The electric field accelerates the charged particle in its direction, and the mag- netic field accelerates it perpendicular to its direction, causing it to follow a nearly circular path.) Question 4, chap 29, sect 1.
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