# electromagnetic induction

Joined Jan 19, 2004
220
hello
this question has been teasing me 4 a long time

faraday's law states that when mag flux thru a coil changes it induces an emf in it..
the magnitude of which is equal to the rate of change of flux

now for an 'n' turn coil e= - n.d(phi)/dt -----this expression is given in all textbooks

the question is:-
is it possible that emf can be INDUCED(i emphasize INDUCED)by only changing the number of turns of the coil thereby changing flux linkage of the coil.

more over can emf be induced in open (or not necessarily closed) circuits???

thank u

#### n9xv

Joined Jan 18, 2005
329
Hi,

For a given ( di / it ) ratio, you can increase/decrease the induced emf by adding/subtracting respectively, turns in the coil. Remember the basic variables when considering emf induction:

- how fast the current is changing in a given period of time (di/dt) ratio.

- number of turns on the coil.

- permiability of the core on which the coil is wound.

- dc resistance of the wire/coil.

- ac resistance (inductive reactance) of the coil.

and yes, you can induce "out of this world" amounts of emf in an open coil. Typically, this high voltage will not produce any useful current though. An example of this is when you pulse/touch a simple 9-volt battery across a small 10-mH coil. I've actually tried this, and you can generate as much as 4000-volts across that little coil! But you cant do much with that voltage because it produces no usefull current.

A fascinating idea concerning emf is that, if you could build a coil or array of coils big enough, you could rotate them in accordance with the earths magentic feild and generate you own power! Obviously, thats not as simple as it sounds but there are curious minds out there working on it!

n9xv

Joined Jan 19, 2004
220
ok...so changing the number of turns alone cannot induce emf...
mag field induction/area of loop necessily has to change so as to induce emf

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Think of gasoline engines. Their spark coils are open-circuited until the voltage jumps the sparkplug gap. That's 12 volts getting jumped up to around 30 kV by a collapsing magnetic field.

#### n9xv

Joined Jan 18, 2005
329
The amount of induced emf is proportional to the number of turns. More/less turns would result in more/less emf accross the coil. X number of turns/volt or volts/turn.

#### XMR

Joined Jan 21, 2005
3