^Maxwell equations
^Maxwell equations
Following five equations give a complete description of all em– interactions and are called Maxwell eq:
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^ γ-rays
^γ – rays
(a) Produced in nuclear reactions and are also emitted by radioactive nuclei (such as Co 60 & Cs 137).
(b) Properties: Effect on photographic plate, fluorescence, ionization, diffraction, high penetrating power & can cause serious damage if absorbed by living tissues.
(c) Used
- in radio therapy to treat certain cancers and tumors
- to produce nuclear reactions
- to study the atomic nuclei.
^Ultraviolet light
^Ultraviolet light
(a) Produced by special lamps like mercury and iron are lamps and by very hot bodies like sun.
(b) Properties: Effect on photographic plate, fluorescence, ionization, highly energetic, tanning of the human skin.
(c) Used
- To destroy bacteria and for sterilizing the surgical instruments
- In the detection of forged documents finger prints
- In burglar alarms etc as they can cause photoelectric effect.
- For studying fluorescence
- In the study of molecular structure and arrangement of electrons
- To prevent the food stuff.
^Visible light
^Visible light
- Produced by hot bodies like sun, electric bulbs and tubes etc. as a result of electronic transitions. Our eyes are sensitive to visible light which helps us to collect the information about our surrounding.
- Properties: Reflection, refraction, interference, diffraction, polarization, photoelectric effect, photographic action, sensation of sight.
^Infra red waves
^Infra red waves
(a) Also called heat waves & are produced by hot bodies and molecules.
(b) Properties: Heating effect, reflection, refraction, diffraction & propagation through fog.
(c) Used
- In taking photographs during the conditions of fog, smoke etc. as these waves are scattered less than visible rays and hence can travel longer distances through atmosphere under fog, smoke etc.
- In weather forecasting
- In checking the purity of chemicals
- In the study of molecular structures
- For producing dehydrated fruits
- In green houses to keep the plants warm
- To provide electrical energy to satellites using solar cells.
- To treat muscular strain.
- In solar water heaters and cookers.
^Magnetic screening
Magnetic screening
Magnetic field inside a ferromagnetic substance e.g. soft iron is zero. Thus any object can be saved from magnetic field by placing it inside a ferromagnetic cavity. This is called magnetic screening or shielding.
^Ceramics
Ceramics
Or ferrites (usually brittle) are made by heat & pressure treatment from the powders of BaO & FeO. Used to coat tapes for tape recorders & as computer memory stores.
^Magnetic keepers
Magnetic keepers
The bar magnets are stored in pairs with their opposite poles together & a soft iron piece across them called magnetic keepers and used to make the circuit continuous, so that no free pole is left to cause demagnetize) in order to retain the magnetism. As otherwise they may demagnetize each other. Also it protects the magnets from external demagnetizing fields.
^Electromagnets
Electromagnets
Core of electromagnets are made of ferromagnetic materials which have high permeability and low retentivity.
Soft iron is a suitable material for electromagnets. On placing a soft iron rod in a solenoid and passing a current, we increase the magnetism of the solenoid by a thousand fold. When we switch off the solenoid current, the magnetism is effectively switched off since the soft iron core has a low retentivity. Electromagnets have temporary magnetism & are used in making electromagnets, cores of transformers, motors & generators etc.. e.g. Soft iron, Stalloy
^Solenoid
Solenoid
Mag. field on the axis of a solenoid is
If solenoid is very long, loops are tight, then for a point situated on the axis & well inside, B = μ0 nI & for a point near end on the axis, 
Field pattern of a long solenoid & a bar magnet are similar, with the difference that magnetic field is maximum at the end of poles where as for a long solenoid it is at centre.
