Question

Two 100w, 2oo v bulb
are connected in Series across 110v source. The net Powered delivered by the source is?​

Answer 1

hello mate!

Explanation:

Power depends on three factors :

Voltage

Current

Resistance

Of these three, the first two are liable to change according to a given circuit, but the resistance of a device is fairly constant.

Let's take a 200 volts - 100 watts bulb and calculate the resistance of its filament :

We know,

P=V2R  

or

R=V2P  

When V=200 volts and P = 100 watts,

R = (200*200)/100 = 400 ohms.

The reason why we calculated out the resistances is because R does not change with the applied voltage. It may be considered as a constant at a given temperature and pressure.

Now, let's connect the two bulbs in series :

Since now the two bulbs are in series, the total resistance offered by them is :

R' = 400 Ω + 400 Ω = 800 Ω

Let them be powered by a 200 volts DC source.

Now the current that flows through either of them is :

I=VR′  

=200800=0.25A  

Since the bulbs are in series, the same amount of current flows through each of them.

Now note that the potential across the each bulb in this case is not 200 volts. Instead, it is the voltage across the combination of bulbs.

Since the two bulbs are identical, the voltage across each bulb is :

V=IR  

V=0.25∗400  

V=100 volts

In this case, the power at which each bulb is working is given by :

P  =  V∗  I  

If we substitute V ( = I*R) from Ohm's law, we get

P=I2∗R  

P=(0.25∗0.25)∗400  

P = 25 watts

Thus, on connecting the bulbs in series, the power they consume decreases significantly.

This happened because the bulb which was originally rated at 100W at 200V has now been connected to a voltage supply of 100V.

In case of a parallel circuit :

The voltage across each bulb is 200 volts, and each of them work at their full power, 100 watts.

In fact, the power rating is just a relative term.

On appliances, you see labels like these :

Well, that means that only when this device is connected to a 230 volts supply, will it work at 2000 watts. If the source voltage is different, the power consumed will change.

On connecting to different voltage sources, the current through the appliance changes, but it's resistance remains fairly constant. And because of that, it's power changes.

I think I had this question on my first Electricity and Magnetism quiz way long ago.  

hope it helps !