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Questions about Waves, Circuits, and Magnetism

  1. Oct 1, 2011 #1
    Hi all.

    I am a student taking A levels, I was hoping the forumites here could help me answer some doubts that I have regarding the concepts above.

    1) Waves

    It was mentioned that the distances between resonances are half a wavelength.

    For example, in a water tube with a sound source above the water, the first point of resonance is x. The next point of resonance is x + λ/2. How is this expression derived?

    2) Circuits

    Why there no potential drop when no current flows? Quantitatively this holds true, but how you understand it qualitatively?

    3) Circuits

    In the most simplified sense, how you do understand Kirchkoff's Laws of Circuits and what are its implications on the circuit equations?

    4) Circuits

    In a galvanometer circuit (one where a rod with resistance is used to measure the emf on an unknown source), why doesn't current flow between the primary and the secondary circuit when their potentials are similar?

    5) Thermal Physics

    How exactly do you use a thermocouple to measure temperature, and what will be its implications on this equation: [PLAIN]http://www.s-cool.co.uk/a-level/assets/learn_its/alevel/physics/temperature-and-thermal-properties/introduction-to-temperature/eqn.jpg [Broken]

    6) Thermal Physics

    Out of curiosity, why is the triple point of water 0.01 degrees celsius, and not 0.00?

    7) Thermal Physics

    For internal energy, why don't we include macro KE / PE? By the name, I can accept this, but are there any other analogies to help me understand this exclusion?

    8) Thermal Physics

    For potential energy, wikipedia defines it as energy stored in a body or in a system due to its position in a force field or due to its configuration.

    For boiling, it was mentioned that energy has to be supplied to the system in order to raise the potential energy. I'm thinking that when the molecules are further away, they will have more tendency to attract each other, and hence potential energy will rise, similar to that of gravitational potential energy.

    9) Electromagnetism

    For motional EMI using a rod / solenoid, E = Blv. Why do you say that the rod / solenoid behaves like an emf source?

    Had an early version of my questions, but got deleted because I was timed out >:(

    Hope you could answer my questions and provide your take on the issues!

    Thank you,
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 2, 2011 #2
    for 2) and 3) i always like to think of electrical circuits as water circuits. In your mind replace all the wires with pipes, the current is the water, battery is a pump etc..

    If the water is simply being pumped around a loop of pipe there are no external forces acting on it (the only real force is the friction of the water with the pipe walls, however the overall effect of this is negligible) so as long as the pump is running the water pressure will be constant everywhere in the pipe; the same holds for the voltage in a circuit.

    Think of the voltage as the 'pushing power'; the force of the water travelling through the pipes.

    3) is also easy to understand with water circuits. Imagine a large pipe branching into 2 smaller pipes (a t-intersection is easiest to think of) lets say in 1 second 5L of water flow from the large pipe into the junction. Kirchoffs law says what flows in must flow out, so the whole 5L of water must flow out through the 2 smaller pipes (the amount flowing in each will depend on the pipe size, if they are identical then 2.5L will flow out through each, i.e it will be evenly distributed. If one smaller pipe has a radius of 1cm and the other 2cm then the ratio of water in pipe one to that of pipe 2 will be 1:2 etc)

    This is the same for electrical circuits, if 5V flows into a junction, 5V must flow out through the 2 wires. The amount of current that flows through each depends not on their size, but on their resistance.

    Hope this helps a little bit
  4. Oct 3, 2011 #3

    Philip Wood

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    (1) It's not distances between 'resonances', but between successive nodes or between successive antinodes that is lambda/2. You are dealing with standing (stationary) waves, which you may need to look up in a textbook.

    Standing waves can be regarded as the sum (learn your Principle of Superposition!) of displacements due to progressive waves of equal frequency and amplitude travelling in opposite directions. Suppose you are at an antinode (formed by constructive interference of the progressive waves). If you go a distance lambda/2 in either direction of travel, you are lambda/2 closer to one source and lambda/2 further from the other. So the path difference (You may need to look this up!) has changed by lambda, so you've got another point of constructive interference, i.e. another antinode.
    Last edited: Oct 3, 2011
  5. Oct 4, 2011 #4

    Philip Wood

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    As you push the rod through the B field, the free electrons in the rod move with it. These free electrons experience motor effect (left hand rule!) forces (Bqv) which urge them along the rod. If the rod is part of a complete circuit, the charges can flow, having work done on them by the motor effect forces. Emf = work done per unit charge on charges = BqvL/q. That's the physical picture of what's going on. (For simplicity I've assumed rod, rod's velocity and B are all mutually perpendicular.)
    Last edited: Oct 4, 2011
  6. Oct 4, 2011 #5
    dude. This is homework. You should put your homework questions here:


    ...and include some attempt at a solution. The idea is that folks at PhysicsForums don't do you homework for you.

    You can expect good help if you not just looking for a free lunch.
    Last edited by a moderator: Apr 26, 2017
  7. Oct 4, 2011 #6

    Philip Wood

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    With respect, Phrak, I think we have a mixed bag here. Quite a few questions seem to be about bits of theory which need more explanation.
  8. Oct 4, 2011 #7

    Philip Wood

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    Easier to think of it the other way round. No p.d. implies no electric field strength, so no force on free electrons, so no drift velocity, so no current. I've used various technical terms, which you may have to look up. [Free electrons need a 'forward' force to keep them drifting through the wire, to counteract resistive forces due to collisions.]
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