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Physics test - all u shld noe abt it

Electromagnetic Induction

Several things u haf to noe, including laws, rules, formulas, fixed way of explaining induction

Laws:
1. Faraday's Law

Faraday's Law of Electromagnetic Induction states that the magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage.

This means that when a magnet is pushed into a coil connected to a galvanometer, the speed the magnet is pushed in will result in the different rate of change of magnetic flux linkage, and the faster the magnet is pushed in, the greater the rate of change of magnetic flux linkage, resulting in a bigger magnitude of induced current ( shown on the galvanometer when the needle point more to one side)

2. Lenz's Law

Lenz's Law states that induced current will flow in a such a way as to oppose the change of producing it.

This means, when u move a magnet into a coil connected to a galvanometer, the induced current produced will be one that will have a pushing effect on the magnet to prevent it from coming in, when the magnet is leaving the coil, the coil will produce a induced current
pulling it into the coil. (sounds like humans, 拥有对方时不懂得珍惜她，失去的时候才开始后悔)

Rule:

Fleming's Right Hand Rule*

Thumb represents the motion
First finger represents the field (magnetic field)
Second finger represents the induced currenty

*right hand rule is only used when there is induced current, which means an AC current produced, not a DC current, when dealing with DC current questions, use the left hand rule.
The right hand grip rule can also be used in this case to determine the polarity of magnets.

AC generator:

components:
1. Slip rings (connected together one)
2.Carbon brush (2)
3. Magnets (2)
4. Copper wires (forming a loop, each end connected to one of the slip rings)
4. Wires (connected to carbon brushes)

AC generator consists of a coil rotating between poles of a permanent magnet

When the coil rotates the magnetic flux through the coil changes and an e.m.f. is induced in the coil
The ends of the coil are connected to the external circuit by a pair of slip rings in contact with carbon brushes
The slip rings ensure that when the coil rotates, the same end of the coil is constantly in contact with the same carbon brush
As the coil rotates, an Alternating e.m.f. is induced. The direction of the induced current changes every half rotation of the coil

Negative e.m.f is produced when the sides of the copper wire coil is opposite to its starting position. (this is also possible if magnets' polarity is switched)

Factors affecting the magnitude of induced e.m.f:

number of turns of coil
strength of magnets
speeds of rotation of coil
type of core used (most preferably soft iron)

Transformer

Equation:

$\frac{V_{S}}{V_{P}} = \frac{I_{P}}{I_{S}} = \frac{N_{S}}{N_{P}}$

(This equation is onli applicable when the efficiency of the transformer is 100% but can be used to determine efficiency of a transformer)

V is voltage
I is current
N is number turns in coil

s represents the secondary coil
p represents the primary coil

Step-up transformer: more coils in the secondary coil than primary
Step-down transformer: more coils in the primary coil than secondary coil

A transformer can never be 100% efficient (unless u count in ur dream) thus we need to improve its efficiency thr:

use thick copper wires in both primary and secondary coils --- reduce heating effects on wire
iron core is laminated (sandwiching of layers together) --- large resistance between laminations help to reduce the size of eddy currents (induced currents which generates heat) and reduce heating effects of core
soft iron iron core used --- lesser energy is lost in magnetizing and demagnetizing the core
design of core, forming a loop --- reduce flux leakage/improve flux linkage

Power calculations:

P=IV

P l o s s = I

R To reduce power loss, I is reduced because if I is smaller, V will be larger and R will be smaller thus reducing power loss thr heating. Turning effects of Forces

When doing the questions:

$Always remember to look which pivot to look at which pivot to calculate from.$

$Take into consideration the weight of the thing where u obtain ur pivot$

$Standard way of writing:$ $Taking moments about the pivot xxx$
Sum of upward forces = Sum of downward forces
$Sum of Anticlockwise moments = Sum of Clockwise moments$

Forces are calculated by: position of force from pivot x force exerted (in newtons using W=mg)

When solving questions which involves a rod held by a wire stay and the angle between the rod and the wall is 90 degress,

use maths formulas such as sin, cos, tan, sin rule, cos rule, to find the tension in wire.

the use of each is dependent on question type, u haf to figure out dat urself as my notes was taken away and never returned by some fellow 4D mates.

Sample question:
A boy lying in a V shaped seat seat discovered that he can balance in when his hands are directly upward, and can rock himself forward and backwards simply by moving his arms forward and backwards respectively.
Answer:
When the boy moves his arms backwards, his centre of gravity will shift to the right of the pivot, which generates a clockwise moment about the pivot, causing the boy to rock backwards
When he brings his arms forward, the centre of gravity shifts to the left side, generating an anti-clockwise moment, causing him to move forward.
When his arms are directly above the pivot, the centre of gravity will be at the pivot, generating no moments, thus he is able to balance.

Resolution of vectors

This topic is pure maths, or art because u apply all the math theories and artistic talents when dealing with vectors questions.

First, you must noe the *Pythagorean Theorem ---

a²+b² =c²

*This is used when a right angel is available.

Second, Sine rule

when solving for sides, use --- a/sinA = b/sinB = c/sinC
when solving for angles, use --- SinA/a = SinB/b = SinC/c

Third, Cosine rule

Just apply this forumlar: c²=b²+a²- 2ba cosC (a, b, c can be replaced by naming the angels as u wish)

Cosine rule is applied when sin rule cannot help u get ur answer, and sin rule is used when both toa cah soh and Pythagoras theorem cannot be used.

Things to note when doing vector drawing questions:

Scale: big and clear
Labeling: every available angle and magnitude and direction must be named
Parallelogram: used when doing drawing
Triangle: used when doing calculation questions

Technique of doing drawing questions:

Select scale
draw out forces and angle
draw dotted lines for the other sides of the Parallelogram
Resultant force is the line cutting through real forces and fake forces
measure specific angle or resultant force
conclusion written as: Resultant force is xxxN, XXX degrees from xxx

When doing calculation questions:

moving forces and rearrange them to form a triangle
find out unknown angle using the law (angles in a triangle adds up to 180degrees)
use either cos rule or sin rule to find specific magnitude or degree
if a right angle is present, use Pythagorean Theorem
conclusion written as: Resultant force is xxxN, XXX degrees from xxx

If angle between two forces is unknown, use compass and set a fixed length (one of the magnitude of force) and move along an extended resultant force (draw extra long for R)
If anyone is unclear can ask me show him on Monday for one such question.

Thats about all for physics test, hope u guys can practice more and do well in the test, though Mrs seck screwed me (and mani) in the chem test, an A1 will definitely pull us back to 1. smthing msg! GD LUCK MUGGING =D

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