(455) HL Lenz' law [IB Physics HL]

Faraday's Law and Lenz's Law

  • Induced EMF: The induced electromotive force (EMF) behaves according to Lenz's Law, which states that it will act in opposition to the motion causing it.

  • Faraday's Law: Indicates that the induced EMF is proportional to the rate of change of magnetic flux linkage; a key part of the equation includes a negative sign indicating opposition to change.

Current Direction Examples

Example 1: Magnet Approaching Coil

  • Setup: A stationary coil of wire around metal with a magnet approaching (north pole).

    • When the north moves towards the coil, it will temporarily induce a north in the coil (opposite).

    • Hand Rule: Use Hand Rule #2, thumb points in the direction of the induced magnetic field, fingers indicate current direction.

    • Result: Current flows to the right (towards the observer).

Example 2: Magnet Moving Away from Coil

  • Setup: Magnet (north) moving away from the coil.

    • Induces a magnetic field with a temporary south in the coil to oppose the motion of the north leaving.

    • Hand Rule: Thumb points to the left (opposing motion).

    • Result: Current flows to the left.

Importance of Induced EMF

  • Generating Electricity: Fundamental to generating electricity in various applications, such as:

    • Hydroelectric power (water turning a wheel with magnets).

    • Nuclear power (nuclear energy creates steam to turn turbines with magnets).

    • Wind energy (wind turns blades connected to magnets).

    • Bicycle lights powered by magnets through the motion of the wheel (no battery needed).

Alternating Current (AC) Explanation

  • AC is generated due to alternating motion of magnets, causing the current to switch direction as magnets approach and recede.

  • Induces an EMF that constantly reverses direction, which is the principle behind AC electricity.

Additional Example: Metal Rod in Magnetic Field

  • Setup: A metal rod on wires, free to slide left in a magnetic field going into the page.

    • To oppose the leftward motion of the rod, the induced current will flow down.

    • Hand Rule #3: Palm faces right to oppose motion; fingertips point into the page (magnetic field).

    • Result: Current flows down to complete the circuit, maintaining opposition to the rod’s movement.