Lights wired in series

1. May 24, 2005

senorbrown

:surprised I need some help with a formula, I am constructing some aids for a class room demonstration on basic electricity. I am using lights to demonstrate series and parallel cicuits, also trying to explain some of the basic laws. My question, If I hook three 25 watt bulbs in series with a supply voltage of 123 VAC the displayed wattage (under power) is about 12 watts (I am using a meter). I would like to explain using an appropriate formula but I don't know how.

2. May 24, 2005

dextercioby

Where do you measure that Wattage...(in which point of the circuit)?VAC is that continuous or alternative current...?

Daniel.

3. May 31, 2005

senorbrown

I am using a watt meter that allows me to select either Volts, Amps, Watts, or KWHR. It is AC.

4. May 31, 2005

whozum

Hes asking between which nodes did you connect the multimeter

5. May 31, 2005

Meir Achuz

If the resistance did not change with temperature (But, it does.) due to the current through it, then the resistance of the three bulbs in series would be 3 times the resistance of one bulb. The power (given by P=V^2/R) would be
25/3=8 Watts. You read 12 Watts (>8), because there is 1/3 the current through each bulb. They stay cool and have a smaller resistance than a hot bulb.

6. May 31, 2005

whozum

The bulbs are in series, they all take the same current.

$$P_{res} = IV = \frac{V^2}{R} = I^2R$$

7. Jun 1, 2005

senorbrown

Thank you, that is very clear