PhotoDiode

What is a photo-diode?

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.

The common, traditional solar cell used to generate electric solar power is a large area photodiode. Photodiodes are similar to regular semiconductor diodes except that are exposed (to detect UV or X-rays or visual light to reach the sensitive part of the device so that the electron excitation can take place from valence band to the conduction band, thereby developing required potential difference.

How photo-diodes are operated?

A photo-diode is designed to operate in reverse bias When photo diode is directly biased it acts like normal diode. But in reverse biased mode current through the diode depends on the brightness and you can use this relation between brightness and current for something you want your circuit to do.

Why photo-diodes are preferably used in reverse bias mode?

A photo diode is a diode and it do act as a barrier in reverse bias as in the case of diode, but in case of photo-diode as the light falls on the reverse biased PN junction there forms a discharge that discharge leads to formation of new electrons and holes, as we have electrons and holes in the reverse bias PN junction and due to the external voltage applied across a the diode leads to the reverse break down (i.e Zenner breakdown)and hence the (minority dimonated) current flows. The number of electrons and holes in reverse bias junction depends upon the discharge which in turn depends upon the light intensity and with the increase intensity of light the current through the diode in reverse biase increases. Thus in reverse biased mode current through the diode depends on the brightness and you can use this relation between brightness and current for something you want your circuit to do.

In reverse biased mode the width of the depletion layer increases thereby reduces the junction capacitance and hence causes the faster response times for the photodiode. The photocurrent is linearly proportional to the illuminance.

Electromagnetic Induction

Description of the phenomenon

A rapidly changing magnetic field induces electric currents to flow in a closed circuit.

In the diagram above, a bar magnet is dropped vertically through a coil linked to a centre-zero galvanometer.

A graph of coil EMF against time shows that:

When the first pole(S) falls through the coil EMF increases to a level then decreases.

When the middle of the magnet falls throught the coil, the EMF is at a minimum. No lines of force are being cut by the coil.

Maximum EMF is obtained when the second pole(N) falls through the coil. This is when the rate of cutting lines of force is highest, because the magnet is falling faster. As a result of the velocity being greater the period of high EMF is shorter.

Note that because the field direction is reversed when the poles drop through the coil, the induced current direction is also reversed. So the EMF is reversed (EMF is directly proportional to current).

Faraday's Law

Consider different sized coils when the same magnet is introduced into the body of each coil with the same velocity.

It is found that,

So induced EMF E is directly proportional to number of turnsN,

Now consider just one coil and in turn introduce three magnets. The magnets are of different strengths and are introduced into the coil at the same velocity.

By measuring the maximum EMF and flux for each magnet, it is found that,

So induced EMF E is directly proportional to flux φ,

Faraday's Law simply states:

The induced EMF in a closed circuit is directly proportional to the flux linkage.

Flux linkage Nφ is the product of flux φ and the number of turns N on a coil.

We have seen that,

Therefore,

Lenz's Law

The direction of the induced EMF is such that the induced current opposes the change producing it.

So when a magnetic south pole is moved towards a coil in a circuit, the face of the coil presents a south pole. The induced current is opposing the change that produced it by trying to prevent the south pole from entering the coil (by repelling it).

Similarly, when a south pole is pulled from a coil in a circuit, the face of the coil presents a north pole. The induced current is opposing the change that produced it by trying to prevent the south pole from leaving the coil (by attracting it).

Note:

1.) How the current direction is changed by the magnet direction.

2.) On each coil face, how a line drawn between the ends of arrows(in grey) makes an 'N' and an 'S' , giving the polarity of the coils.

Neumann's Equation

This combines the proportionalities in Faraday's Law with the direction of the induced current from Lenz's Law.

As a result of the consistency of units used (SI), there is no need for a constant of proportionality.

The minus sign is from Lenz's law, indicating the opposing nature of induced EMF and rate of flux linkage cutting.

The equation can be amended to include the rate of flux cutting dφ/dt by taking the number of coils N out of the differential.

Flemming's Right Hand Rule

The rule describes the resulting directional motion of the induced current for a conductor moving at right angles to the field direction.

The three quantities FIELD, CURRENT AND MOTION are mutually at right angles to each other.

using the right hand, position the first finger, second finger and thumb to form the x,y,z axes. The highlighted letters within the words help you remember the three quantities

First finger - Field direction

seCond finger - Current direction

thuMb - Motion produced

EMF induced in a metal rod

For an induced EMF E to be produced across the length L of a metal rod, the magnetic field B, the velocity v and the major axis of the rod must all be mutually at right angles to each other.

The derivation of E = BLv :

Consider a metal rod of length L.

If the rod is travelling at a velocity v at right angles to its length then the area A swept out per second is given by:

The total flux φ threading through this area per second is the product of the area A and the flux density B.

Substituting for the area A,

In this case, since the total flux φ refers to 1 second, we can write :

where dφ/dt is the rate of flux cutting.

Hence,

By definition, EMF (E) is equal to the rate of flux cutting,

Therefore,

Astronomical Telescope

Diagram of Astronomical Telescope in Normal Adjustment

Disadvantages of a refracting telescope:

Lenses suffer from colour distortion – this means that when white light passes through the lens it is split into the colours of the spectrum. Because violet light refracts more than red light it is brought to a focus closer to the lens than the red light – this makes the image coloured and blurred. This effect is called chromatic aberration.

Advantages of a reflecting telescope:

Mirrors do not suffer from the colour defects of chromatic aberration.

For these reasons all the really large telescopes in the world today are reflectors.