^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.
Posts
^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.
^Circular ring
Circular ring
Magnetic field on the axis of a circular ring
To find field at C use r = R & r = x for far away point.
Magnetic field is maximum at centre of ring, it is
*Average speed
*Average speed
Speed between two instants is called average. Mathematically it is measured as the total distance covered per unit time spent. It is scalar & always positive.
i.e. Average speed
- For a given time interval average velocity is single valued while average speed can have values depending upon path followed.
- Average velocity & average speed are equal only for motion along a straight line with no reversal.
- The average speed can never be – ve or zero while average velocity can be.
- If a particle travels distances x1 , x2 & x3 with speeds v1 , v2 & v3 during time intervals t1, t2 & t3 respectively, then its average speed is
If x1 = x2 = x3, then this relation becomes harmonic mean of individual speeds. 
Also if t1 = t2 = t3, then the average speed is be given by Arithmetic mean of individual speeds. 
^Point object
^Point object
An object is said to be a point object if it covers distance much greater than its size.
^Two cells in parallel
Two cells in parallel
I = I1 + I2
^A long horizontal fixed wire carries a current of 100 A. Directly above and parallel to it is another wire carrying a current of 20 A and weighing 0.04 N/m.
^A long horizontal fixed wire carries a current of 100 A. Directly above and parallel to it is another wire carrying a current of 20 A and weighing 0.04 N/m. The separation between the two wires so that the upper wire is just supported by the magnetic repulsion is
