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Math Help - Physics help needed about magnetic induction

  1. #1
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    Physics help needed about magnetic induction

    Since I posted this topic I need URGENT help and additional explanations approximately within 12 hours.

    Please look at the attachment and read my problem.

    What is the induction at infinity ?
    I told teacher it to be 0 but she disagreed.
    Using the formula for calculating induction and substituting big distances into the formula gives me also 0.
    I had a solenoid and its lenght was 0,222 meters. The diameter of the solenoid was 120 mm.
    My goal was to measure the magnetic induction of the solenoid at solenoid axis. The current that runs through solenoid is a sinusoidal AC.
    The coil was used to measure magnetic induction. At the axis
    the voltmeter shows me that at the center of solenoid the voltage is about 0,40 Voltage. Moving away from the point at center reduces the voltages. In other words about 12 cm away from the center shows me that the voltage is 0,18 V. Why does moving away from the center point at solenoid reduces voltages ?
    The formula for calculating magnetic induction at certain distances from the center point was:
    induction = (l/2 - x)/sqare root((l-2x)^2+D^2) + (l/2 + x)/sqare root((l+2x)^2+D^2) where l is length of solenoid and D is diameter of solenoid.
    The condition l >> D is not satisfied.

    I was told that magentic field was homogenous. Why does the voltages reduces if magnetic field is homogenous ?
    Attached Thumbnails Attached Thumbnails Physics help needed about magnetic induction-induction.jpg  
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  2. #2
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    Quote Originally Posted by totalnewbie View Post
    Since I posted this topic I need URGENT help and additional explanations approximately within 12 hours.

    Please look at the attachment and read my problem.

    What is the induction at infinity ?
    I told teacher it to be 0 but she disagreed.
    Using the formula for calculating induction and substituting big distances into the formula gives me also 0.
    I had a solenoid and its lenght was 0,222 meters. The diameter of the solenoid was 120 mm.
    My goal was to measure the magnetic induction of the solenoid at solenoid axis. The current that runs through solenoid is a sinusoidal AC.
    The coil was used to measure magnetic induction. At the axis
    the voltmeter shows me that at the center of solenoid the voltage is about 0,40 Voltage. Moving away from the point at center reduces the voltages. In other words about 12 cm away from the center shows me that the voltage is 0,18 V. Why does moving away from the center point at solenoid reduces voltages ?
    The formula for calculating magnetic induction at certain distances from the center point was:
    induction = (l/2 - x)/sqare root((l-2x)^2+D^2) + (l/2 + x)/sqare root((l+2x)^2+D^2) where l is length of solenoid and D is diameter of solenoid.
    The condition l >> D is not satisfied.

    I was told that magentic field was homogenous. Why does the voltages reduces if magnetic field is homogenous ?
    Of course the induction at infinity is 0 T since your solenoid (apparently) doesn't extend to infinity. But something tells me that that isn't the question that's really being asked. Could the question really be what is the induction if we have the condition l >> D? (In other words, l is going to infinity, not x.)

    I'm confused on a number of points, the most confusing being you are measuring the voltage of what at the center of the solenoid?? There should be nothing conducting electricity at the center of the solenoid.

    A homogeneous field is one that is the same in all directions, in this case indicating a radial symmetry along the axis of the solenoid. The field can, and does, change along this axis.

    -Dan
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  3. #3
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    Let me put my problem into other words.
    Let's discuss what happens to solenoid when current runs through.
    In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. As for solenoid I was using to do my experiments in order to calculate the magnetic field I want to ask the following.
    Because l << D is not satisfieid I suppose that the current which flows through loop of wire is the same everywhere where the current passes.
    If we look at this solenoid, which is 20 cm lenght. Is it right to say that choosing one point on this solenoid's wrap produces different induction current if we go away from the point we consider the situation ?
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by totalnewbie View Post
    Let me put my problem into other words.
    Let's discuss what happens to solenoid when current runs through.
    In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. As for solenoid I was using to do my experiments in order to calculate the magnetic field I want to ask the following.
    Because l << D is not satisfieid I suppose that the current which flows through loop of wire is the same everywhere where the current passes.
    If we look at this solenoid, which is 20 cm lenght. Is it right to say that choosing one point on this solenoid's wrap produces different induction current if we go away from the point we consider the situation ?
    The current in the loop of wire will be the same everywhere in the wire regardless of the physical situation. (Though it will, of course, vary in time if you are using an AC source.)

    In the original question you were asking about the magnetic induction and in the reply you are talking about the induction current. A solenoid will produce a magnetic induction when there is a current in the wire. This has the usual inverse square relationship with distance from the source so it falls off to 0 T at infinity. If an AC current is run through the solenoid there will be an induction current induced in the wire, which will generally oppose the direction of the applied current. That is to say the amplitude of the current will be reduced from the source current. This can't "go to infinity" because it is restricted to the wire, so I can't make any sense of the question if you are referring to an induction current.

    Again, I am rather confused about what you are asking about.

    -Dan
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