Posted: July 7, 2013 in PHYICS

The discovery of photoelectric effect
The wave nature of light suggests that energy is emitted or absorbed continuously as electromagnetic waves…but it cannot explain the phenomenon like photoelectric effect, thermo-ionic emission etc. In 1901, Max Planck gave a theory called Planck’s quantum theory about the nature of radiation. According to this theory
Energy is emitted in bundles but not continuously as wave. Each bundle, called quanta carries a certain amount of energy
In 1888 Hallwach discovered that when an insulated zinc plate charged negatively was exposed to a beam of UV light, it lost it negative charges. When the zinc plate was charged positively and exposed to the beam of UV light..there was no loss of charges.
In 1902, Philipp Eduard Anton von Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light. This appeared to be at odds with James Clerk Maxwell’s wave theory of light, which was thought to predict that the electron energy would be proportional to the intensity of the radiation. In 1905, Albert Einstein solved this apparent paradox by describing light as composed of discrete quanta, now called photons, rather than continuous waves. Based upon Max Planck’s theory of black-body radiation, Einstein theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck’s constant. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This discovery led to the quantum revolution in physics and earned Einstein the Nobel Prize in Physics in 1921.

Lenz’s Law in accordance to the conservation of energy
Main concept of Lenz’s Law
The emf is induced in a conductor when the magnetic flux linked to it changes. Lenz’s law states that “The current flows due to induced emf in such a way that it always opposes the change causing it”. Hence, mechanical energy is required to generate electrical energy. It is an example of conservation of energy.
Let us consider a solenoid connected with a galvanometer and a bar magnet:
When the N-pole of the bar magnet is brought to the solenoid then the magnetic flux linked to the solenoid increases. Now, current flows in such a direction that the induced emf opposes the motion of magnet. So, N- pole is formed in solenoid towards the magnet due to which motion of magnet is opposed by induced emf.

Then the N-pole of magnet is taken away from solenoid the magnetic flux linked to the conductor decreases. Due to this current flows in such a direction that the S-pole is formed towards the magnet so that the motion of magnet is opposed by the induced emf. Herr, mechanical energy is needed to keep the motion of magnet continuous. So, electrical energy is generated on the expense of mechanical energy.

In this way the electrical energy is generated in the conductor on the expense of mechanical energy. So Lenz’s Law is example of conservation of energy.

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