Faraday’s Experiments and Electromagnetic Induction
Faraday’s Experiences
The English physicist and chemist M. Faraday experimentally obtained an electrical current from magnetism in 1831. These are his stories:
- First experience: moving a magnet inside a coil. If approaching the magnet to a coil, an induced current occurs during the movement of the magnet. The direction of the induced current is reversed if the magnet moves away. With a fixed coil and magnet, no induced current is observed. The same results are obtained if we keep the magnet fixed and move the coil.
- Second experience: opening and closing an electrical circuit. If we connect the switch, an electric current is induced in the second coil. The currents in the coils circulate in opposite directions. When disconnecting the switch, an electric current is induced in the second coil in the previous direction. Current is induced in the second coil as the current in the first coil increases or decreases, but not while it remains constant. This shows that the induction of electric current in a circuit is due to changing magnetic fields.
Action of Electric and Magnetic Fields on a Charge
An electric charge moving in the presence of a magnetic field is subjected to a force. This force is described using Lorentz’s Law.
- If the charge is at rest, it experiences no magnetic force.
- If the charge is in motion, it is subjected to a deflecting force perpendicular to the velocity and magnetic field.
Lenz’s and Faraday’s Laws
Lenz’s Law: The direction of the induced current opposes the cause that produces it.
Faraday’s Law: The electromotive force (emf) induced in a circuit is equal to the rate of change of the magnetic flux through the circuit, with a changed sign.
The applications of electromagnetic induction include electric generators (alternator and generator), inductance, mutual induction, and the production of electrical energy.
AC Generator and Power Plants
To generate power, a loop needs to rotate within a magnetic field. An electric generator transforms a particular form of energy into electrical energy. The AC generator is called an alternator.
Power plants differ in the fuel they use and how they make the loop twist. In hydroelectric plants, water moves the shaft with its coils. In thermal power plants, water is evaporated to create steam that moves turbine blades. Other plants use similar systems, except for solar power, which uses photovoltaic cells to generate electricity when illuminated by the Sun.
Light and Sound
Sound waves are longitudinal waves that need a medium to propagate. Light is composed of electromagnetic waves that require no material medium to propagate.
The Human Eye
Our optical system consists of the cornea and lens. The lens acts as a converging lens, and the image is formed on the retina.
Two common defects of our optical system are:
- Myopia: The person cannot see distant objects clearly. The far point is at a finite distance. The image is formed in front of the retina. Myopia can be corrected using diverging lenses.
- Hyperopia: The person cannot see nearby objects clearly. The near point is farther than normal (approximately 25cm). The image is formed behind the retina. Hyperopia can be corrected using converging lenses.
Nature of Light
Newton defended the corpuscular theory, which mechanically treated all known phenomena related to the propagation of light, like reflection and refraction.
Huygens advocated the wave theory, stating that light consists of the propagation of a wave disturbance of the environment, believing they were longitudinal waves.
In the twentieth century, the photoelectric effect was discovered, reigniting the debate on the nature of light. The photoelectric effect demonstrates the corpuscular nature of light, where energy is exchanged through packets called photons.
The Magnifying Glass
The operation of a lens is represented in the ray diagram. The object is placed at the focal point so that the image is formed at infinity. The eye forms the final image, and the size on the retina is larger than that formed without the magnifier.
The Camera
The operation of a camera is similar to the human eye; the target is the lens, and the film is the retina. The focus of objects is achieved by adjusting the distance between the lens and the film. The amount of light entering the camera is regulated by the exposure time and aperture of the diaphragm.
Polarization (Light and Sound)
A beam of light is linearly polarized if the electric field oscillations always occur in the same direction.
Limit Angle and Total Reflection
The critical angle is the angle of incidence corresponding to a refractive angle of 90 degrees.