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Extended Response

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Extended Response

20.1 Magnetic Fields, Field Lines, and Force

49.

Summarize the properties of magnets.

  1. A magnet can attract metals like iron, nickel, etc., but cannot attract nonmetals like piece of plastic or wood, etc. If free to rotate, an elongated magnet will orient itself so that its north pole will face the magnetic south pole of Earth.
  2. A magnet can attract metals like iron, nickel, etc., but cannot attract nonmetals like piece of plastic or wood, etc. If free to rotate, an elongated magnet will orient itself so that its north pole will face the magnetic north pole of Earth.
  3. A magnet can attract metals like iron, nickel, etc., and nonmetals like piece of plastic or wood, etc. If free to rotate, an elongated magnet will orient itself so that its north pole will face the magnetic south pole of Earth.
  4. A magnet can attract metals like iron, nickel, etc., and nonmetals like piece of plastic or wood, etc. If free to rotate, an elongated magnet will orient itself so that its north pole will face the magnetic north pole of Earth.
50.

The magnetic field shown in the figure is formed by current flowing in two rings that intersect the page at the dots. Current flows into the page at the dots with crosses (right side) and out of the page at the dots with points (left side).

A figure of a magnet field being created by four wires. The left two wires are coming out of the page, while the right two are going into the page. The resulting magnetic field lines are close to vertical between the left two wires and the right two wires. On the left and right hand sides, the magnetic field lines are loops that center around the two nearest wires.


 Where is the field strength the greatest and in what direction do the magnetic field lines point?

  1. The magnetic field strength is greatest where the magnetic field lines are less dense; magnetic field lines points up the page.
  2. The magnetic field strength is greatest where the magnetic field lines are most dense; magnetic field lines points up the page.
  3. The magnetic field strength is greatest where the magnetic field lines are most dense; magnetic field lines points down the page.
  4. The magnetic field strength is greatest where the magnetic field lines are less dense; magnetic field lines points down the page.
51.

The forces shown below are exerted on an electron as it moves through the magnetic field. In each case, what direction does the electron move?

Three images indicating the direction of a magnetic field and a force. Image a shows a force pointing upward while the magnetic field points out of the page. Image b shows a force pointing upward with a magnetic field pointing to the right. Image c shows a force pointing to the left and a magnetic field pointing out of the page.
  1. (a) left to right, (b) out of the page, (c) upwards
  2. (a) left to right, (b) into the page, (c) downwards
  3. (a) right to left, (b) out of the page, (c) upwards
  4. (a) right to left, (b) into the page, (c) downwards

20.2 Motors, Generators, and Transformers

52 .
Explain why increasing the frequency of rotation of the coils in an electrical generator increases the output emf.
  1. The induced emf is proportional to the rate of change of magnetic flux with respect to distance.
  2. The induced emf is inversely proportional to the rate of change of magnetic flux with respect to distance.
  3. The induced emf is inversely proportional to the rate of change of magnetic flux with respect to time.
  4. The induced emf is proportional to the rate of change of magnetic flux with respect to time.
53.

Your friend tells you that power lines must carry a maximum current because P = I2R, where R is the resistance of the transmission line. What do you tell her?

  1. Ptransmitted = Itransmitted2Rwire and Plost = Itransmitted Vtransmitted, so I must be high to reduce power lost due to transmission.
  2. Plost = Itransmitted2Rwire and Plost = Itransmitted Vtransmitted, so I must be high to reduce power lost due to transmission.
  3. Ptransmitted = Itransmitted2Rwire and Plost = Itransmitted Vtransmitted, so I must be low to reduce power lost due to transmission.
  4. Plost = Itransmitted2Rwire and Plost = Itransmitted Vtransmitted, so I must be low to reduce power lost due to transmission.

20.3 Electromagnetic Induction

54.

When you insert a copper ring between the poles of two bar magnets as shown in the figure, do the magnets exert an attractive or repulsive force on the ring? Explain your reasoning.

The figure shows south pole of a bar magnet at the top of the figure and a north pole of a bar magnet at the bottom of the figure. A copper ring is approaching and almost inside the location between the two magnetic poles as shown by an arrow. The copper ring runs in and out of the page so that the maximum surface area enclosed by the loop cannot be seen in the figure.
  1. Magnets exert an attractive force, because magnetic field due to induced current is repulsed by the magnetic field of the magnets.
  2. Magnets exert an attractive force, because magnetic field due to induced current is attracted by the magnetic field of the magnets.
  3. Magnets exert a repulsive force, because magnetic field due to induced current is repulsed by the magnetic field of the magnets.
  4. Magnets exert a repulsive force, because magnetic field due to induced current is attracted by the magnetic field of the magnets.
55.

The figure shows a uniform magnetic field passing through a closed wire circuit. The wire circuit rotates at an angular frequency of about the axis shown by the dotted line in the figure.

The diagram has two rectangles that are folded at the middle at about 90 degrees. This creates an upper flap of the rectangle that points to the right and a lower flap that points straight down. The variable lowercase L labels the length of the lower flap of the rectangle. The label lowercase L over 2 is on the height of the lower flap of the rectangle. The label lowercase L over 2 is also on the width of the upper flap of the rectangle. Vector B labels four downward-pointing, diagonal arrows that each pass through the fold in the rectangle at a 45 degree angle.

What is an expression for the magnetic flux through the circuit as a function of time?

  1. expression for the magnetic flux through the circuit Φ(t) = BAcos⁡ωt
  2. expression for the magnetic flux through the circuit Φ(t)= 2 BAcosωt Φ(t)= 2 BAcosωt
  3. expression for the magnetic flux through the circuit Φ(t)= 3 BAcosωt Φ(t)= 3 BAcosωt
  4. expression for the magnetic flux through the circuit Φ(t) = 2BA cos ωt
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