Suppose 5 lamps were placed in series. Each lamp is identical and rated at 20 watts. They are all attached by a 100 volt power supply in series. NOW FIND a) the current, b)the voltage, and c) the new resistance of each lamp in series.

Question

Suppose 5 lamps were placed in series. Each lamp is identical and rated at 20 watts. They are all attached by a 100 volt power supply in series. NOW FIND a) the current, b)the voltage, and c) the new resistance          of each lamp in series.

noelgreco · Answer

First, calculate the current.  Remember thaty the current through each bulb has to be the same.

$$\Sigma Power=Voltage \times Current$$

anonymous · Answer

Okay I have 5(20)=V(I) 100= 100 x Current when I solve for current I get 1 ampere

whpalmer4 · Answer

Yes, 1 ampere is the correct value for the current. 

The power dissipated in each lamp will be $$P=I^2R$$We know I and we know P, so we can solve for R:
$$R = P/I^2 = 20 \text W/(1 \text A)^2 = 20 \text{ ohms}$$

Armed with the current and resistance, we can find the voltage across an individual lamp.  $$V=IR = 1\text A * 20 \text{ ohms} = 20 \text V$$
If we sum up the voltage drop across 5 resistors, 5*20 V = 100 V, which is the voltage applied across the string.  If we didn't get that result, we would have an error somewhere.