In my physics class I have this problem that shows two lightbulbs, one $60\,\text{W}$ and one $100\,\text{W}$ in series, connected to a $120\,\text{V}$ battery.
The problems are:
Which bulb is brighter? (A: $60\,\text{W}$)
Calculate the power dissipated by the $60\,\text{W}$ bulb. (A: $23.4\,\text{W}$)
Calculate the power dissipated by the $100\,\text{W}$ bulb. (A: $14.1\,\text{W}$)
Why is the power dissipated not simply the wattages of the bulbs? I followed one solution online where you first find $R$ for both using $P = V^2/R$ and then use $I = V/R$ to get a current of $0.3125\,\text{A}$. The power dissipated is then calculated using $P = I^2R$ and you get the above answers. However, doesn't that assume the voltage drop between the lightbulbs is $120\,\text{V}$ in both cases, and isn't that wrong?
I tried getting it another way where I said $P_1 = IV_1$, $P_2 = IV_2$, and $V_1+V_2=120\,\text{V}$. I solved the voltage drop on the $60\,\text{W}$ lightbulb to be $45\,\text{V}$ and $75\,\text{V}$ on the $100\,\text{W}$ one. Then, current is solved to be $4/3 \,\text{A}$ which lets us solve the resistance for each one as $33.75\,\Omega$ and $56.25\,\Omega$. Then using the formula $P = V^2/R$, the original wattages are found as the answer. Why is it right to assume $120\,\text{V}$ for both bulbs?