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This video is about induced EMF in a coil that is turning in a magnetic field.

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We'll discuss two different cases.

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First, when the coil is rotating clockwise or anticlockwise, in the plane of the paper or screen,

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and second, when the coil is flipped.

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You can picture the first case like this.

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The direction of the magnetic field is into the page or screen.

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It doesn't matter whether we rotate the coil clockwise or anticlockwise.

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As long as it remains in the plane of the paper or screen,

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the number of field lines that cross the coil is unchanged.

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As a result, magnetic flux through the coil is also unchanged.

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The formula that expresses Faraday's law tells us

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that induced EMF is a result of changing magnetic flux.

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Since there is no change in magnetic flux, no EMF is induced in the coil.

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The second case is when the coil is flipped.

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If it is not fully clear what flipping means here,

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picture the coil being rotated by 180 degrees about this axis.

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After the coil is flipped, it ends up in the position that you see on the right-hand side.

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Let's consider this situation from the coil's perspective.

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What practically happens is that the direction of the magnetic field through the coil is reversed.

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So we take the magnetic flux through the coil to be phi before it is flipped,

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then the flux through the coil after it is flipped is negative phi.

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The change in flux through the coil is final flux minus initial flux,

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so negative phi minus phi, which gives us negative 2 phi.

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Let's apply Faraday's law.

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Since the change in flux is negative 2 phi,

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we get that the induced EMF is equal to negative N

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times negative 2 phi divided by delta T.

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Simplifying we get 2 times N times phi divided by delta T for the induced EMF in the coil.

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Therefore, when we flip the coil in the field, there is an induced EMF in the coil.

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This wraps up our discussion of induced EMF in a coil that is turning in a magnetic field

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and concludes the final video of subtopic 11.1.

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In the first video of subtopic 11.2, we'll start learning about alternating current generators.