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Physics Question of the Week

Each week, we highlight one of our many demonstrations by asking a physics question relevant to the topic designed to be illustrated. This not only challenges audiences to carefully examine their previously held views on physics, but also provides suggestions for teachers looking to use these demonstrations in their own classrooms.

Be sure to check back each week for the solutions to the previous weeks' questions and for newest question of the week.

  • Question of the Week Archive
  • Question of the Week Summer Girls Special Part 4 with Answer
  • Question of the Week Summer Girls Special Part 3 with Answer
  • Question of the Week Summer Girls Special Part 2 with Answer
  • Question of the Week Summer Girls Special Part 1 with Answer

Missed one of our older Question of the Weeks? Click here to access our archive.

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It's about time, so we are finally off the topic of sound and on to other important issues.

The photograph below shows a compass, with the red end of the compass pointing toward the north, inside a coil. Notice that Dan is about to connect a wire to the second contact of the coil so that about ten amperes of current will flow in the coil. The direction of the (positive) current in the coil is shown by the arrow on the coil. If it matters, the coil contains 10 turns of wire, and is about 7cm in radius.

q329

When Dan connects the coil, it might cause the needle to deflect in a different direction, if the field is strong. On the other hand, if the magnetic field of the coil is a different type of field from that of the earth, perhaps nothing will happen. Or maybe the needle will just deflect slightly from where it is with no current in the coil.

When Dan connects the coil, what will happen?

  • (a) The compass needle will point in the upward direction in the picture.
  • (b) The compass needle will point in the downward direction in the picture.
  • (c) The compass needle will point to the left in the picture.
  • (d) The compass needle will point to the right in the picture.
  • (e) The compass needle will remain where it is after the coil is connected.
  • (f) The compass needle will deflect only slightly from where it is before the coil is connected (Specify which direction.)

Enough information has been given above so that a simple calculation can be made, if it is actually appropriate.

After July 18th, 2014, click Read More for the answer.

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Everyone knows that when you heat something up it expands. Well, almost everything, except water around the freezing point and a few other weird substances. One example of something that expands when heated is the brass plate shown in the photograph below.

 i1-11

The question this week involves the hole.

When the brass plate is heated up by the gas burner shown in the photograph, the hole will:

  • (a) get bigger.
  • (b) get smaller.
  • (c) remain the same size.

 Click Read More for the asnwer.

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Shown in the photograph below is an open tube about 50 cm long. If you blow at an angle into one end of the tube you can make a sort of musical tone.

 q259

Click your mouse on the photograph above to hear the fundamental frequency of the tube on an mpeg video.

Shown in the photographs below are (top to bottom) a closed tube the same length as the original tube, an open tube with half the length of the original tube, and a closed tube with half the length of the original tube.

(a) tube a: 
(b) tube b: 
(c) tube c: 

What is the relationship of fundamental frequency of each of the three tubes above to the frequency of the original tube?

Case (a): Relative to the original tube, the frequency of tube (a) is:

  • (a) higher in frequency by a factor of two.
  • (b) lower in frequency by a factor of two.
  • (c) the same frequency.

Case (b): Relative to the original tube, the frequency of tube (b) is:

  • (a) one octave higher.
  • (b) one octave lower.
  • (c) the same frequency.

Case (c): Relative to the original tube, the frequency of tube (c) is:

  • (a) one octave lower.
  • (b) two octaves lower.
  • (c) the same frequency.

Click Read More for the answer

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A candle flame is positioned immediately in front of a loudspeaker, as seen in the photograph below. This view is "along the axis" of the loudspeaker.

 h1-02

An audio oscillator/amplifier connected to the loudspeaker is turned on at the resonant frequency of the loudspeaker, about 36 Hz, so that the loudspeaker produces a large sound wave. The input to the speaker is several watts of power, and its efficiency is over ten percent at its resonant frequency, so it is probably putting out nearly one watt of audio.

The question this week involves what the candle flame will do, if anything, when the audio input is turned up to a high level. In particular, will the flame move, and if so, how?

When the loudspeaker is operated at a large power level at its resonant frequency, the candle flame will:

  • (a) oscillate in the direction along the axis of the sound wave as it leaves the speaker, forming a long bright line when viewed across the speaker axis (from left to right in the photograph above).
  • (b) oscillate in the direction perpendicular to the axis of the sound wave as it leaves the speaker, forming a long bright line when viewed looking toward the speaker along its axis (the view in the picture above).
  • (c) oscillate in both directions, forming a large bright region when viewed from any direction.
  • (d) not move at all, because of the nature of the sound wave.

Click Read More for the answer

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