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Unformatted text preview: Solution for Practice Test 8 for Test 3 Solution to Practice Test Problem 8.1(Field of Stationary Charge) Problem: A positive charge is fixed at the origin. What is the direction of the magnetic field at a point P at +1cmˆy ? Select One of the Following: (a) + x (b)- x (c) + y (d)- y (e) + z (f)- z (g-Answer) Zero. Solution Zero. Only moving charge produces field. Total Points for Problem: 2 Points Solution to Practice Test Problem 8.2(Current Induced in Conducting Ring) Problem: The figure to the right shows a magnetic field whose strength is increasing with time and whose direction is into the page. What is the direction of the induced current in the con- ducting ring (shown as a dark circle)? Select One of the Following: (a) Clockwise (b) Out of the page. (c) Into the page. (d-Answer) Counterclockwise. (e) There is no induced current. Magnetic Field Increasing in Strength Solution To oppose the increasing flux into the page, a magnetic field out of the page will be created by the induced current. Using the right hand rule for a wire, this gives the direction of induced current flow as counterclockwise. Total Points for Problem: 2 Points 1 Solution to Practice Test Problem 8.3(Force on Telephone Wire) Problem: The telephone poles through my backyard are about 25m apart. Assume the power wires carry about 1000A and that they run East-West. In Arkansas the magnitude of the Earth’s magnetic field is 4 × 10- 5 T . What is the total magnetic force on the power line between two telephone poles? Select One of the Following: (a) 25N . (b) . 04N . (c) 10N . (d-Answer) 1N . (e) There is no magnetic force. Solution The force on a current carrying wire is vector F = I vector ℓ × vector B = IℓB since the wire is perpendicular to the field, therefore F = IℓB = (1000A)(25m)(4 × 10- 5 T) = 1N Total Points for Problem: 2 Points Solution to Practice Test Problem 8.4(Why Does Wire Deflect?) Problem: If lab, you performed an experiment on something I called a “magnetic pith ball” where an aluminum rod was suspended by copper wire so it hung in one of the big lab magnets. When a current was run through the circuit, the rod deflected. This happened because of which of the following physical laws? Select One of the Following: (a) Gauss’ Law. (b) Ampere’s Law. (c) Faraday’s Law. (d-Answer) The Lorentz force. (e) Heisenburg Uncertainty Principle. Solution The Lorentz Force. Total Points for Problem: 2 Points Solution to Practice Test Problem 8.5(Radius of Electron Orbit) Problem: The electrons in cosmic rays enter the atmosphere at about the speed of light, 3 × 10 8 m s . The Earth’s magnetic field is 4 × 10- 5 T . An electron has charge- 1 . 6 × 10- 19 C and mass 9 . 11 × 10- 31 kg . Neglecting relativistic corrections, what is the radius of the orbit of the electrons?...
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