Faraday's Law of Induction

When the magnetic field passing through a loop of wire changes, it creates a voltage in that loop — even with no battery attached. Change the field faster, and you get a bigger voltage; this is exactly how generators turn motion into electricity.

ε = −ΔΦ / Δt

• ε — induced emf (V): the voltage created in the loop
• Φ — magnetic flux (Wb): the amount of magnetic field passing through the loop
• t — time (s): how long the change in flux takes

The magnetic flux through a loop drops by 6 webers over 3 seconds. What voltage does this induce?

• ΔΦ = 6 Wb
• Δt = 3 s

1. ε = −ΔΦ / Δt
2. ε = −(6 Wb) / (3 s)

ε = −2 V

The flux still changes by 6 webers, but now it happens in just 2 seconds instead of 3. What is the size of the induced voltage now?

• 2 V
• 3 V
• 12 V

According to Lenz's law, the current induced by a changing flux will create its own magnetic field that…

• opposes the change in flux
• matches the change in flux
• has no effect on the flux