Magnetic field in 3D

It is important that you have a 3D view of magnetic fields that you have learnt.

Electromagnetic induction

Here is the link to a video on how soft iron core helps to increase the induced emf in the secondary coil

Particle Art

Bubble chamber shows the path made by particles. The particles are not known. Need further investigation. Spiraling ones shows the particle losing speed....
Identify the start and end of the path. Have fun.

Particle tracks in cloud chamber

cloud chamber and B field applied Alpha tracks. The alpha particles have high ionising power and approx the same energy.
In a cloud chamber or bubble chamber , tracks of alpha, beta and gamma can be seen as the ions formed along its tracks can be made visible by producing condensation of vapour just like the trails of an aeroplane in the sky. The ionisation power of gamma is weak and therefore tracks from gamma are difficult to see.
The particles curve is opposite direction due to formation of opposite charges subjected to magnetic field. The tracks need proper identificaiton to know what produces it.

PN junction

The P type has majority holes and N type majoritiy electrons. Bring the two side by side , the electrons at the edge touch the P type and fall into the hole . When that happens the positive ions are found where the electrons left ,similarly negative ions are formed where the holes are filled. This is because initially p and n are neutral eventhough they have holes and electrons.

This region where where join is call a junction and depleted of charge carrier and therefore referred to as depletion region. The width of this region is limited. This diffusion process will stop when internal electric field is set up (dir from +ions to -ve ions) that prevent electrons and holes from entering this region. Draw the dir of E field , you find the force on holes and electrons is away from this region. Try electron , force is right (E is to the left in depletion region)so it stays in the n type. For holes , the ssme explanation.

If an external E field is applied , using a battery with positive terminal to the p type , the direction of E ext is the right. This E ext will cancel the E int (overcome) and electrons will move into the depletion region thereby narrowing or removing the depletion region . The flow of electrons and holes through the junction means that the pn junction is now forward biased allowing current to flow. Hence conduction is possible and resistance low.

If an external E field is applied , using battery with positive terminal to the ntype, the dir of E ext is the the left .This E ext will increase the depletion region as the electrons in the n type is forced to the right away from the junction . It does the same to the holes therby widening the depletion region. No flow of charge is possible as the depletion region does not have any mobile charge carriers. Hence , the diode is connected in reverse biased and resistance is very high.

SUmmary : forward biased-battery positive terminal to p type-depletion region narrows (dissapear) , resistance low.
reverse biased- battery positive terminal to n type - depletion region widens , resistance high.

Depletion region - no mobile charge carrier - ions set up internal E field that prevents further diffusion of mobile carriers into this region.

google : hyperphysics
See other older entries at semiconductor (13)

Heavy damping

Heavy damping is welcomed.

Damping to a different degree

You learn the technique of damping the guitar strings.

Testing for resonance

Three different systems have different natural frequency, tested by varying the driving frequency. When the amplitude is at its maximum the driving frequency has approached its natural frequency.

Critical damping

When displaced , the object returns to it's equilibrium position in the shortest time i.e T/4

Resonance

The driving frequency : earthquake has a frequency. The natural frequency : buildings have natural frequency When the driving frequency approaches the natural frequency, resonance take place. The building shakes at large amplitude.
The driving frequency : walking motion exerts a periodic force side ways The natural frequency : The bridge has a natural frequency and it is also not suffiently damped. Hence resonance can take place.

Forced oscillation

Forced oscillation: The swing has a natural frequency. When her Dad drives the swing at a frequency equal to the natural frequency , the swing reaches maximum amplitude.
A child can learn to drive the swing herself if she learns to drive at the natural frequency.

Circular motion

Resultant force that commonly provides for circular motion :
1. Tension in string
2. Friction
3. Normal reaction
4. Weight (Gravitational force)
5. Lift force

Banked curve

To turn round a curve on a horizontal surface is limited by the frictional force. To increase the ease of turning round a curve , banked curve has a component N sin@ which is the accelerating force. Banked tracks are used for trains, cars , waterslide and velodromes(bicycle).

Circular motion

Circular motion is an extension of dynamics. A resultant force acts to sustain the circular motion. If a train were to circle the track. It would be constantly accelerating towards the centre of the circle. If a pendulum bob were to be hung from the ceiling ,it will be tilted at an angle. This will be similar to the tilting when the train is accelerating/decelerating forward but in a perpendicular plane .

Forces and thermal CA

1. Summation of torque = 0. Do not use other different definition.
Summation of force = 0 , not restricted to upward and downward force only. The resultant force at B is not needed , only component form required, hence to the left and upwards direction required. Many gave strange angles.
2. The mistake of not converting T to kelvin
3. (iii)The change of state and rise in temperature of water would be detected if proper diagram is included.
(iv)The mass of ice that has been converted to water can easily be missed out even by good students.The part is a continuation of (iii)

Forces: Cool applications in U-tube

http://www.youtube.com/watch?v=3XlNsGQK8oo

Forces: Some common applications

Forces -some fancy applications

Vector subtraction

Vector subtraction is needed if you want to find change in a vector, be it velocity, momentum, etc
eg. change in velocity = final velocity - initial velocity
change in momentum = final momentum - initial momentum
But subtraction is same as addition of a negative vector
eg C = B - A = B + (-A)
to do subtraction , just add B to A (in the opp direction)
But be careful as A-B is not the same as B-A. Hence final vector - initial vector has to be carefully done. Start with final vector and continue with the initial vector in the opposite direction.View diagrams to confirm your understanding.

Resistance thermometer

Looks rather neat . The resistance wire is at the tip of the needle like probe.