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Re: induced emf again



Ludwik Kowalski wrote:

1) We have copper wire whose diameter is 5 mm.
The length of the U-shaped rail will be 5+5+5=15 cm;
the effective length of the cross bar will be 5 cm.

OK.

This means that the total R will be 0.0173 ohms.

Maybe. You need to worry about additional resistance
at the point where the slider meets the rail. I
suggest MEASURING the resistance... or doing an experiment
where the resistance is unimportant (see below).

2) Suppose B=0.5 T (a neodymium magnet) and that
dt=0.25 second. With the d(AREA)=25 cm^2 the
emf will be 0.01 volts. The current will be 1.15 A.

3) The voltage across the bar will 25% of the total

Again, if the contact resistance can be kept small.

or 2.5 mV; its polarity can be measured with the
oscilloscope.

OK. That should be easy.

Note that the voltage on the cross
bar would be negligible is q 100 ohm galvanometer
was inserted into the loop.

So don't do that. If you want to measure the current
without disturbing the circuit, measure the voltage drop
across the "4th leg" i.e. the stationary piece that joins
the two rails, i.e. the leg that isn't the slider and isn't
either rail. Use Ohm's law to infer the current. This
is a standard way to measure current. Just as you want
voltmeters to have a large input impedance, you want
ammeters to have a low input impedance, and this scheme
certainly achieves that.

On the other hand, I don't have a clear idea of what
the point of this experiment is. The Subject: line
says something about EMF. EMF is a voltage, not a
current, so why are we using ammeters (galvanometers)
at all????? Why not just insert a high-impedance
voltmeter in series with the 4th leg? That gives a
direct, uncomplicated measurement of the EMF. The
result is insensitive to the resistance of the
rails, slider, et cetera.

If this uncomplicated experiment doesn't answer the
question, please explain in more detail what the
question is.