Learning Forum

[Charlotte Blair]

Why is there negative sign in Faraday’s induced emf equation?

[rtk]

"Faraday’s laws of electromagnetic induction states that when a coil of wire is placed in a changing magnetic field (B) an EMF (E) is induced in the coil and the induced EMF is proportional to the rate of change of flux linkage with the coil (dφ/dt).

E = N(- dφ/dt)

Meaning of the minus sign in Faraday’s law is explained by Lenz’s law which says if the B field changes, the voltage induced in the coil will try to drive a current that produces a B-field which opposes the change(in primary B field)."

[Charlotte Blair]

Thanks, Could you please elaborate more on what does the negative sign signify?

[rtk]

Hi - The minus sign in Faraday’s induced emf equation signifies that when decreasing the B (negative dΦ/dt) causes positive voltage, which boosts B in the coil and when increasing the B (positive dΦ/dt) causes negative voltage, which reduces B in the coil. If the induced voltage aids the change in B-field, that would induce more voltage, further changing the B-field, which induces more voltage, further changing the B field causing a never-ending loop of infinite current and B-field. Simply to say the negative sign signifies that field and energy will remain finite.

[Charlotte Blair]

    Thanks for clarifying.

[MdShamimAhmed]

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I need to use some verification sample in ANSYS to calculate generated voltage based on Faraday laws. Which verification sample  did I select for ANSYS?

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[chad.capote]

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This PDF - Understanding and using the minus sign in Faraday's law, explains everything you need - the negative sign in faraday's law, Lenz's law, and application of the negative sign in Faraday's law. Additionally, you can also read Engineering Electromagnetics by William Hayt - this would give you an excellent insight into the world of electromagnetics and its application in major science.

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