# 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