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When the loop is the figure shown below will be rotated in clockwise direction the the slip ring connected to the red segment of the loop will give us the positive voltage and the slip ring connected to the black segment of the loop will give us negative voltage according to the Flemings right hand rule.

What if the loop is rotated in counter clockwise direction ? Will the slip rings provide us the same polarities which they were giving when the loop was moving clockwise ?

Plus

The voltage from the loop is given by the equation.

V(t) = Vmaximum * sin (theta)

Where theta is the positive angle the red segment makes in the clockwise direction or in counter clockwise direction from this position ? enter image description here

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  • 1
    \$\begingroup\$ Hint: What is the difference between -90 degrees and +270 degrees? \$\endgroup\$
    – MrGerber
    Commented Mar 2, 2018 at 13:28
  • \$\begingroup\$ in ac circuit theory the angles greater than 180 degree for clockwise direction are represented by their corresponding negative angles and vice virca. so what your point ? @MrGerber \$\endgroup\$
    – Alex
    Commented Mar 2, 2018 at 17:17
  • \$\begingroup\$ So how could the generator know which direction it is rotating? \$\endgroup\$
    – MrGerber
    Commented Mar 2, 2018 at 18:36
  • \$\begingroup\$ i dont know @MrGerber kindly explain! \$\endgroup\$
    – Alex
    Commented Mar 7, 2018 at 6:11

1 Answer 1

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When the loop is the figure shown below will be rotated in clockwise direction the the slip ring connected to the red segment of the loop will give us the positive voltage and the slip ring connected to the black segment of the loop will give us negative voltage according to the Flemings right hand rule.

With the loop in that position, the induced voltage is zero so NO, there is no voltage generated at this instant or position. Reason: the induced voltage relies on rate of change of flux and with the coil vertical the flux cutting the coil may be at a maximum but the rate of change of flux is zero: -

enter image description here

Improved picture source here.

So, using the picture above, (and considering the case from position 1 to position 3), the voltage increases to a positive peak due to the rate of change of flux (that the coil is cutting) increasing.

Conductor DC is indicating the flow of current should the generator output be loaded. Due to Fleming's right hand rule conductor BA also has current flowing the same way hence the terminal voltage on the closer slip ring is positive.

If you reverse direction it is the same as swapping the magnet poles and the waveform will be inverted. Flemings right hand (generator) rule: -

enter image description here

Strictly speaking this rule applies to the current direction but given that the current direction is dictated by the polarity of the induced voltage, it matter little. What ever way you try and rotate your right hand, if you reverse either the direction of travel or the magnetic field, current reverses and, by inference, induced voltage polarity reverses.

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  • \$\begingroup\$ Andy, the currents in the top picture look wrong for a generator, or are the resistors at the back? \$\endgroup\$
    – Chu
    Commented Mar 2, 2018 at 17:53
  • \$\begingroup\$ @Chu do'h. Looks like they've used the same picture for currents at 270 degrees and 90 degrees and forgotten to reverse + and -. Thank you for a good spot. I shall try and find a better picture. \$\endgroup\$
    – Andy aka
    Commented Mar 2, 2018 at 17:59
  • \$\begingroup\$ @Chu changed and hopefully clearer! \$\endgroup\$
    – Andy aka
    Commented Mar 2, 2018 at 18:21
  • \$\begingroup\$ @Andyaka if the conductor CD moves from the position 1 to 7 then the direction of the motion is still downward as in the case when the conductor moves from the position 1 to 3. so how the voltage induced is reversed according to the right hand rule when in both cases direction of motion of conductor CD and the direction of magnetic field is same. kindly explain that to me. \$\endgroup\$
    – Alex
    Commented Mar 7, 2018 at 6:27
  • \$\begingroup\$ @Alex when going from 1 to 7 the motion is from left to right cutting across the magnetic field hence this does produce the same voltage as when going from 1 to 3. When going from 7 to 1 the motion is right to left and current flow is opposite that when going left to right. \$\endgroup\$
    – Andy aka
    Commented Mar 7, 2018 at 8:50

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