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Lv 4
Yuki 發問於 科學及數學其他 - 科學 · 7 年前

phy magnetism


1. why are (2) (3) correct?

3. D is the answer stated. is there any better answer?

更新 2:

4. what do you mean by 'a single coil with rectification'? as seen from the picture, the coil is not always in contact with the carbon brushes? so there are times when V=0?

6. that means the answer is C?


更新 3:

4. If there are multiple coils, the voltage graph is the sum of all curves in option B

>>>how come the points where V=0 in option B can add up to become non-zero voltage in option C? thanks!!

1 個解答

  • ?
    Lv 7
    7 年前

    1. Statement 1 is wrong as the magnetic field produced by coil Y needs to oppose that on coil X, the induced current direction in Y should be opposite to that of X, i.e. anticlockwise.

    2. When the magnetic approaches the coil, the end of the coil near the magnet is [N]. Hence, induced current flows from Y to X throught the galvanometer. When the magnet is inside the coil, no induced current is produced. When the magnet is withdrawn, current flows from X to Y.

    3. Are you sure that option D is the right answer?

    4. There are mutiple coils in the armature. Hence, the resultant voltage variation is the sum of sinusoidal voltages generated by each coil.

    Option A is that given by a single coil with rectification.

    5. No, statement 2 is wrong. X is higher in potential than Y.

    6. When current passes through the coil, the side facing the magnet becomes North Pole (try to apply Right Hand Grip Rule). Thus repulsive force is produced between the magnet and the coil, making the coil moves away from the magnet.

    2014-11-02 16:41:08 補充:

    Your suppl quesions:

    1. Statement 2 is right, because of Lenz's law. Statement 3 is right, because the magnetic fields produced by X abd Y are opposite in direction. Hence, the net magnetic field seen by Z is lower in strength than that seen by Y, leading to a lower induced current.

    2014-11-02 16:44:25 補充:

    3. By Lenz's Law, the ring should be attracted to the solenoid in order to maintain the strength of the decreasing magnetic field.

    2014-11-02 16:48:14 補充:

    4. As the coil is rotating at fast speed, the time of no connection is short compared with the time that any one of the coils are connected.

    If there is only a single coil, the voltage graph is what is shown by option B....

    2014-11-02 16:50:40 補充:


    If there are multiple coils, the voltage graph is the sum of all curves in option B, with one slightly displaced laterally from the previous one.

    2014-11-02 16:55:09 補充:

    6. The coil moves away from the magnetic, i.e. along the y=direction.

    The answer is option D.

    2014-11-04 20:09:24 補充:

    Please see [意見] column.

    2014-11-04 20:11:51 補充:

    Your further question:

    4.When V=0 occurs in any one coil, the other coils have non-zero voltages. Hence when add up together, the zero voltage point vanished.