SPM Physics: AC Generator

The different between AC & DC generator is:
-it consists of 2 slip rings instead of commutator
-slip rings allows current in outer circuit to flow in different direction

For better undertanding, watch how it moves in the video below..


The output induce current varies at different angle of the coil rotation. Observe the graph below.

• The current is zero when the coil is vertical. This is because the coil is parallel with magnetic field results in no magnetic field being cut. Thus no current is induced.

• The current is maximum when the coil is horizontal. At this point maximum rate of change or rate of cutting of the magnetic fields occur.

• You also must be able to differentiate the output graph of AC & DC generator. Click image below to see the diffrence.

Videos on Lenz's 's Law

I've explained a bit on Lenz's law in previous post. Some students having problem to apply tis law..
So please clarify it b4 its too late..go & ask yr teacher or tutor..

In SPM we use this law to find the direction of induced current in the solenoid.
However lenz's law is not only about finding induced current. The following video demonstration might be usefull to describe the whole concept of lenz's law.Enjoy watching..

SPM: Induced Current

ok.lets continue our discussion on Electromagnetic induction..
In SPM syllabus, its important 4 u to know how to determine the direction of induced current in the following cases:
i. direction of induced current in a straight wire..
u can use Flemming's right hand rule to identify the induced current's direction.
BUT remember!!!!!! we use RIGHT hand for this..NEVER use your left hand!!! Left hand is used to find the FORCE in electromagnet. We use RIGHT hand to find the CURRENT direction..and its only valid for induction. How to use this rule?? look at the diagram below

One student asked me..Sir..i'm confused ..whats the difference between left & right hand rule???
OK, to answer that question.. please notice the following comparison carefully,

Electromagnet	Electromagnetic Induction
- Use FLEMMING LEFT HAND	- Use FLEMMING RIGHT HAND
- current is needed	- Motion/force is needed
-FORCE is produced	-INDUCE CURRENT is produced
- process of making a temporary magnet	- process to generate induced EMF to generate induce current
- concept used to build D.C motor	- used to build Generator

ii.direction of induced current in solenoid
for this case u really need to understand the whole concept carefully..student always get confuse when using this law...
Ok..to determine the current direction in solenoid..we use LENZ's law. What is LENZ's law???

Lenz's law states that...
the direction of induce current is such that the change producing it will beopposed.
*******please understand this law & try to memorize it*********


Lets now apply what actually this law trying to tell us. Observe the following figure 1 & figure 2.

In figure 1:
- the NORTH pole of the bar magnet is moving into the coil,

According to Lenz's law..
- the induced current produced will flow in such a way to oppose the approaching bar magnet
- thus, the flowing induce current produces North pole on the end of coil as to repel the
approaching magnet.


In figure 2:
the NORTH pole of the bar magnet is pulled out from the coil,

According to Lenz's law..
- the induced current produced will flow in such a way that it won't allow the bar magnet leaving
the coil
- thus, the flowing induce current produces SOUTH pole on the end of coil as to prevent the
magnet from leaving.

Electromagnetic Induction

Today i wanna go back to where we've left our previous discussion on electromagnet . The last sub-topic we've discussed was about d.c motor principle.

The next discussion will be subtopic 3.3 which is one of my favorate.. its about electromagnetic induction. I like this one bcoz its like doing an illusion .Just imagine without a battery or solar power supply can we still light up a bulb?..

Usually to light up the bulb we need continuous supply of current, which we get it from batteries or solar energy. But in this topic, we dont need those batteries..all we need is a magnet.


YEah..magnet alone can produce current..but HOW??
This process is what we call magnetic induction..
By bringing the magnet closer or moving it into the coils of wire, u can produce current which is known as Induced Current...Meanwhile, the emf produced is known as induced emf.
U can actually ligt up your torchlight without any batteries....watch this


So in SPM, u must be able to
i. describe electromagentic induction... HOW??

When we move a magnet into a coil of wire,there is change in magnetic field. In other wrords, the magnetic fields are being cut which cause an induced e.m.f (electromotive force) to be produced.
An induced current flows through the wire and this phenomenon is called electromagnetic induction

To understand this better..u can watch these videos..

look!.. what happen to the ammeter reading when a magnet is brought close to the coil..




u can also download this note to get a clearer picture on electromagnetic induction.
The same concept is applied in bicycle's dynamo . The bulb lights up when u start to cycle..no batteries are needed..all u need is to continue cycling.
This is the mysterious world of magnet..it generates current when the magnetic fields being cut.
I'll discuss more about this on next post.