Lenz's Law of Electromagnetic Induction

Lenz's Law

• Describes the direction of current induced in a conductor by a changing magnetic field
• States that the magnetic field created by the induced current opposes the initial changing magnetic field that produced it
• The direction of current flow is determined by Fleming’s right-hand rule
• Obeys Newton’s third law of motion, where every action has an equal and opposite reaction
• The induced current creates a magnetic field equal and opposite to the field that creates it, resisting changes in the magnetic field
• Follows the principle of conservation of energy, the induced current creates a magnetic field that opposes the creating field
• In the case of a magnet moving towards a coil, the magnetic flux linking to the coil increases
• This induces an electromotive force (EMF) and current in the coil which then generates its own magnetic field, according to Faraday’s law
• When the north pole of a magnet moves away from a coil, the magnetic flux decreases
• This also induces an EMF and current in the coil, creating its own magnetic field as described by Faraday's law
• The right-hand thumb rule helps determine the direction of magnetic field or current
• Point the thumb in the direction of current flow, and curled fingers indicate magnetic field direction

Explanation of Bar Magnet Thrust into Coil

• When a bar magnet is thrust into a coil, magnetic field strength increases
• The induced current creates another field in the opposite direction of the bar magnet to oppose the increase
• Induction opposes any change in flux

Using Lenz's Law:

• Sketch the situation to visualize and record directions
• Determine the magnetic field direction B
• Determine if the magnetic flux is increasing or decreasing
• Determine the induced magnetic field B direction, it opposes the change in flux by adding or subtracting from the original field
• Use RHR-2 to find the induced current I direction, which is responsible for induced magnetic field B
• The induced EMF’s direction drives a current, emerging from the positive terminal and returning to the negative terminal

Direct Current

• Direct current (DC) flows in one direction
• Solar cells and fuel cells, along with batteries, can provide direct current

Alternating Current

• Alternating current (AC) reverses direction.
• AC current from a power plant supplies electricity to homes and businesses.
• Current changes direction 60 times per second.

LC Circuit

• An LC circuit, also known as an LC filter or network, is an electrical circuit of passive components
• Consists of an inductor (L) and a capacitor (C) connected together
• It is also a resonant, tank, or tuned circuit
• A tuned circuit (LC circuit) operation is displayed
• The capacitor (C) stores energy in its electric field (E), and the inductor (L) stores energy in its magnetic field (B)
• Oscillations are slowed down, in a tuned circuit charge may oscillate thousands to billions of times
• LC circuits oscillating at their resonant frequency is able to store electrical energy
• Capacitors store energy in the electric field E between plates, depending on voltage
• Inductors store energy in the magnetic field B, depending on current
• It is achieved through the cyclic charging and discharging of the capacitor through the inductor
• An inductor connected to a charged capacitor causes voltage across the capacitor to drive a current through the inductor, creating a magnetic field
• As the charge is used up, voltage across the capacitor drops to zero
• As inductors oppose changes, the energy stored in the coil's magnetic field induces a voltage across the coil
• This induced voltage then causes the current to recharge the capacitor with a voltage of opposite polarity
• As magnetic field decreases, EMF drives energy required to charge the capacitor from the magnetic field, by Faraday's Law
• A tuned circuit behaves like a harmonic oscillator, similar to a pendulum or water sloshing in a tank
• When current from the capacitor dies out, the inductor reverses its EMF
• This EMF keeps charges flowing until the top plate holds negative charge and the bottom plate becomes positively charged
• The process then reverses, causing current to flow clockwise
• The capacitor's top plate is once again positively charged, and the bottom plate negatively charged
• This "filling and emptying" of capacitor plates occurs with a specific frequency
• LC circuits achieve charging and discharging through a cyclic manner
• Energy oscillates back and forth between capacitor and inductor until internal resistance ceases oscillations
• Energy is shared between the electric field of the capacitor and the magnetic field of the inductor, in an oscillating LC circuit