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Faraday’s Law states that the induced electromotive force (or emf) in any closed circuit is equal to the negative rate of change of the magnetic flux passing through the circuit. Sounds complicated? Let’s break it down:
Induced Electromotive Force (emf): This is the voltage or ‘push’ that causes electrons to move and create an electric current. It’s like the pressure that pushes water through a hose.
Magnetic Flux: Imagine a net catching fish in a river. The more fish it catches, the greater the ‘fish flux’. Similarly, magnetic flux is a measure of the number of magnetic field lines passing through a given area.
Rate of Change: This simply means how quickly something is changing. If you’re driving a car, the rate of change of your position is your speed.
Faraday’s experiments showed that the emf induced by a change in magnetic flux depends on only a few factors. First, emf is directly proportional to the change in flux ΔΦ. Second, emf is greatest when the change in time Δt is smallest—that is, emf is inversely proportional to Δt. Finally, if a coil has N turns, an emf will be produced that is N times greater than for a single coil, so that emf is directly proportional to N.
Group Discussion: Run the simulation (Generator tab) below and answer the questions that follow.
The simulation allows you to adjust the strength of the magnet. How does the strength of the magnet affect the brightness of the bulb in the simulation according to Faraday's Law? Explain your reasoning.
The simulation allows you to change the number of loops in the coil. How does the number of loops affect the brightness of the bulb according to Faraday's Law? Explain your reasoning.
In the simulation, what happens to the brightness of the bulb if you move the magnet towards the coil versus away from the coil? How does this relate to the concept of magnetic field lines and change in magnetic flux?
Based on your observations in the simulation, how can you explain the basic principle behind how a generator works?
Think about devices in your everyday life that utilize electromagnetic induction. Can you design an experiment using the simulation to model how one of these devices works (e.g., transformer, doorbell)?
Consider a flat square coil with N = 5 loops. The coil is 20 cm on each side, and has a magnetic field of 0.3 T passing through it. The plane of the coil is perpendicular to the magnetic field: the field points out of the page.
(a) If nothing is changed, what is the induced emf?
(b) The magnetic field increases uniformly from 0.3 T to 0.8 T in 1 s. While the change is taking place, what is the induced emf in the coil?
(c) While the magnetic field is changing, the emf induced in the coil causes a current to flow. Does the current flow clockwise or counter-clockwise around the coil?
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