A motor driving a fan and a lamp are connected in series. If the fan is removed, the lamp dims. Why?
My nephew has one of those electronics kits consisting of some electronic components and a project book. The book suggests circuits to create using the kit and makes some obvious observation such as "When you closed the switch, the current flowed, lighting the lamp."
One project consists of a battery, a switch, a lamp and a motor which drives an axle on the end of which can be placed a fan. The project instructions call for the circuit to be closed with the fan in place and with the motor without the fan. In the latter case, the lamp is dimmer than in the former. The book provides no explanation, so my nephew turned to me. I haven't got a clue.
I apologize for the confusingly phrased question. By "fan" I meant the fan blade assembly. The motor stayed connected in series with the lamp. The circuit is a DC circuit as in all (most?) such electronic kits.
The motor and fan were connected in series in this instance primarily as a demonstration of the effect described.
Thank you all for your explanations. The current drop explanation satisfied my nephew. I'm going to have to digest the EMF back force concept before I try explaining that one to him.
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When the fan is removed, the fan motor runs with no load except for a little bearing and brush friction and a little windage caused by the motor's rotor moving a little air. Since there is practically no load, the motor draws very little current. The current through the lamp in series is reduced because the motor is not drawing any current to drive the fan.
When the motor has no load, it goes to full speed and generates a higher "back EMF," an electromotive force or voltage that opposes the battery voltage and causes less current to flow.
The lamp is acting as an ammeter to display the amount of current that is flowing in the motor. This only works because the lamp, motor and battery voltage have been carefully selected so that the lamp doesn't interfere with the motor too much but still gives a noticeable indication. An actual ammeter would display the current with almost no influence on the motor.
The fan and lamp are NOT in series, or else when you removed the fan (disconnected it) the lamp would go out. When you break a series circuit, no current can flow. Zero.
You need to describe better what the circuit is before and after you "remove the fan".
My only guess is that the motor speed is limited by the fan. When the fan is removed the speed of the motor increases such that can have a larger voltage drop.
I am not sure that makes sense. There might be an inductive issue here, but I can't see it.
My guess is that the motor without the fan is running under no load conditions which means less current. Less current means dimmer lamp.
My question is why anyone put a lamp in series with a motor. It appears to have no function (except possibly as a ballast resistor).
I assume this is a DC motor we're talking about.
When the rotor spins without a load, a very high counter-EMF is developed in the rotor windings due to Lenz's law.
http://en.wikipedia.org/wiki/Lenz%27s_law
This works to counter the incoming voltage and reduce the current through the winding.
When the motor is LOADED, the counter-EMF developed in the motor is reduced, so the motor is able to draw more current.
When the motor draws more current due to a mechanical load (such as a fan blade or some other mechanical resistance), a series lamp will illuminate with greater brightness.
Do you mean the blade of the fan? If so, you are removing some of the load from the fan, so it can spin drawing less power so the current goes down dimming the light. Current through both the fan and the light must be the same in a series circuit.
yaktur is correct. Think of it this way, grab the spindle of the motor and stop it from turning. Now you will basically have a short circuit between the positive and negative terminal through the windings of the motor (open up the motor and you will see its just coils of wire and magnets). The result is that lots of current will flow and the lamp will be at it's brightest (btw, I wouldn't recommend doing this for long as it may over-heat the motor).
So the only thing that limits the current in normal operation is the speed of the motor – it inductively throttles the current as it spins, reducing the current the faster and freer it turns.