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# help with physics?

im having problems with the both of these

The generator of a car idling at 875-rpm produces 12.4 V. What will the output be at a rotation speed of 1550 rpm assuming nothing else changes?

ans: _ V

i set this up like this 1550/875×12.4 but the answer im getting is wrong

When a car drives through the Earth’s magnetic field, an emf is induced in its vertical 75.0-cm-long radio antenna. If the Earth’s field (5.0 X 10-5 T) points north with a dip angle of 45°, what is the maximum emf induced in the antenna? The car’s speed is 30.0 m/s on a horizontal road.

and i just need help figure out how to set this up

thanks

### 3 Answers

- Andrew SmithLv 71 year agoFavorite Answer
"nothing else happens" is quite unrealistic.

The alternator is a parallel wound field coil. Without a regulator the volts are proportional to the square of the rotational rate.

This is because the increasing rate of rotation puts increasing volts on the field coils increasing the current through the field coils and increasing the value of B which then subsequently causes a further increase in volts.

Your question is presupposing a PERMANENT MAGNET generator which is not used in motor vehicles.

For that generator your set up is correct. It gives the correct answer.

So it isn't your problem There is something else that is wrong.

The basis is that if E = Bqv

then E2/E1 = v2/v1

E2 = E1 * (v2/v1)

=12.4 * 1550 / 875 = 22.0 V

So your figures are correct for the problem as specified.

You MIGHT have used an incorrect number of significant figures.

The second one is the product of the BLv where B is the component of the field perpendicular to the motion and to the length of the aerial.

This will only be a maximum if the car is travelling east or west. It must be zero if the car is driving North or South.

The COMPONENT of the field which is perpendicular to the aerial is 0.75 * cos(45)

Now just multiply the values.

- derframLv 71 year ago
"assuming nothing else changes"

Well there's your problem. If you are dealing with an unloaded generator, the output voltage will be proportional to rotational speed. If the generator is connected to a battery and incorporates a field regulator, then you need more information to find an answer.

- billrussell42Lv 71 year ago
(1550/875)×12.4 would be correct (22v) if the generator were not connected to a battery, which controls the voltage.

If there is a battery connected, the answer depends on info you don't have available, such as battery charge, internal resistance, temperature, etc. But it will be about 13-14 volts