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D is the Answer

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Physics  =  Total No of Teachers  - Maths   + Both  =  20 - 12 + 4 = 12

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Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday’s law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

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Mutual inductance:

If two coils of wire are brought into close proximity with each other so the magnetic field from one links with the other, a voltage will be generated in the second coil as a result. This is called mutual inductance: when voltage impressed upon one coil induces a voltage in another.

Wattless current:

In an a.c. circuit, there are two components to impedance. There is a resistive component and a reactive component. The reactive component is the algebraic sum of the total inductive and capacitive reactances. Both the resistive and reactive components impede the flow of a.c. current. The resistive component converts power into heat that is wasted. The reactive component stores power in a magnetic field (for inductors) and an electric field (for capacitors). When the power source is turned off, the magnetic and electric fields collapse and return the stored energy back to the circuit in the form of an induced current. So no energy is wasted in the reactive components. Therefore since power is measured in watts, the current flowing through reactive components is considered to be "wattless current".

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The Davisson-Germer experiment demonstrated the wave nature of the electron, confirming the earlier hypothesis of deBroglie. Putting wave-particle duality on a firm experimental footing, it represented a major step forward in the development of quantum mechanics. The Bragg lawfor diffraction had been applied to x-ray diffraction, but this was the first application to particle waves.

Davisson and Germer designed and built a vacuum apparatus for the purpose of measuring the energies of electrons scattered from a metal surface. Electrons from a heated filament were accelerated by a voltage and allowed to strike the surface of nickel metal.

The electron beam was directed at the nickel target, which could be rotated to observe angular dependence of the scattered electrons. Their electron detector (called a Faraday box) was mounted on an arc so that it could be rotated to observe electrons at different angles. It was a great surprise to them to find that at certain angles there was a peak in the intensity of the scattered electron beam. This peak indicated wave behavior for the electrons, and could be interpreted by the Bragg law to give values for the lattice spacing in the nickel crystal.

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In 1924, Luis deBroglie (Nobel Prize in Physics in 1929) proposed that a wave function is associated with all particles.  Where this wave function has nonzero amplitude, we are likely to find the particle.  The standard interpretation is that the intensity of the wave function of a particle at any point is proportional to the probability of finding the particle at that point.  The wavelengths of the harmonic waves used to build the wave function let us calculate the most likely momentum of the particle and the uncertainty in the momentum.  The wave function for a material particle is often called a matter wave.

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Light is the only way your eyes perceive things. They can detect when light hits an object, and they can detect when an object emits light. When scientists talk about light, they have the option to relate to light as a wave or as a collection of particles. Originally, the ancient Greeks thought light worked as a stream of individual units; they observed how light moved in straight lines and could bounce off a mirror in the same way that a ball could bounce off a wall. Even though no individual particles were ever actually seen, this particle theory was acceptable to many people.

Meanwhile, the light wave theory was developed by Christian Huygens in the late 1600s. His theory was furthered by Thomas Young in 1807 when he observed light passing through a narrow slit and then spreading out on the other side. He also realized that one beam of light could interfere with another beam of light. Young's experiments wouldn't have had the same results if light worked as a stream of particles. Instead of remaining in a directed beam of light, the light waves spread outward whenever they can.

Even though both theories are accepted, the light wave is the most common way that light is described, since our eyes take in light as waves and not particles. Still, the particle theory of light got a boost from Albert Einstein in 1905. He observed the photoelectric effect in which ultraviolet light forces a surface to release electrons when the light hits. Einstein explained the reaction by defining light as a stream of photons, or energy packets. Modern physicists have concluded that both the particle theory and the wave theory are simplified explanations for a very complicated phenomenon.

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Whenever, we apply a potential difference i.e. voltage across a resistor of a closed electric circuit, current starts flowing through it. The current (I) is directly proportional to the applied voltage (V), provided temperature and all other factors remain constant.

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In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field. Displacement current has the same units as electric current, and it is a source of the magnetic field just as actual current is. However it is not an electric current of moving charges, but a time-varying electric field. In physical materials (as opposed to vacuum), there is also a contribution from the slight motion of charges bound in atoms, called dielectric polarization.

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The average velocity attained by some particle such as an electron due to the influence of an electric field is termed as the drift velocity. The movement or the motion of the particles is assumed to be along a plane and hence the motion can also be referred to as the axial drift velocity.   

The concept of drift velocity can also be understood by studying or noticing the random motion of free electrons moving around in the conductor. These free electrons keep on moving inside the conductor in a haphazard way with random velocities. At times, a conductor may be subjected to an electric field and as a result of it, some kind of electrical force is exerted on the randomly moving electrons but in the direction of the field. This field forces the electrons to switch towards high potential while maintaining their randomness of motion. Scientifically, we can say that the electrons will drift towards higher potentials with their random motions. Hence, we have seen that every electron will have certain net velocity with which it will move towards the higher potential end of the conductor. This net velocity is termed as the drift velocity of the electrons. Since the electrons are said to be moving with drift velocity, even the current which results due to this drift movement of electrons in an electrically charged conductor is called as the drift current. In fact, every current flowing through the conductor is termed as the drift current.

When the charged particles move around in a conductor, their motion is not in a straight line because of their collision with other particles in the conductor. Therefore, the average speed of the particle in the conductor is taken into consideration. This is termed as the drift velocity.