# Light bulbs in parallel and series

Q: You have a light bulb in series with a battery to make a complete circuit. Then you have another circuit with two light bulbs in parallel connected to a battery. The lightbulbs in all the circuits are identical and the batteries are the same in the two circuits. Before working with these circuits, you are asked to make some predictions: (a.) What is the brightness of the bulbs in circuit 2 (the light bulbs in parallel) relative to each other and to the bulb in circuit 1? (b.) If one of the bulbs is removed from circuit 2, how will the brightness of the other bulb be affected? Does it matter which bulb is removed?

I think that (a.) the light bulb in series with the battery is brighter than the two in parallel and they are as bright as each other.
(b.) If one is removed, the bulb will become as bright as the bulb in circuit 1; and that it does not matter which is removed.
Am I right in thinking this? And can someone explain this more to me?

Tom Mattson
Staff Emeritus
Gold Member

FYI, I've moved this thread over to Introductory Physics.

Am I right in thinking this?

You sure are!

And can someone explain this more to me?

Yes, but rather than type it all out I am going to ask you some leading questions. You don't have to answer them of course, but if you do then you will be well on your way towards expressing your thoughts in the same way that a physicist would.

Let the voltage of the battery in each circuit be $V$ and let the resistance of each bulb be $R$ and the current drawn from the battery be $I$. The answers to all the questions I am going to ask should be put in terms of these three symbols.

1.) In the first circuit, what is the voltage $V_L$ across the lightbulb?

2.) In the first circuit, what is the current $I$ drawn by the lighbulb?

3.) In the first circuit, what is the power $P_L$ drawn by the lightbulb? You should have an equation for power.

4.) In circuit 2, what is the voltage $V_L$ across each lightbulb? (Hint: The two voltages are the same. Can you explain why?)

5.) In circuit 2, what is the current $I_L$ drawn by each lightbulb? (Hint: The two currents are the same. Can you explain why?)

6.) In circuit 2, what is the power $P_L$ drawn by each battery? (Hint: If you get 4 and 5, then it should be clear that the two values of the power are the same).

4.) In circuit 2, what is the voltage $V_L$ across each lightbulb? (Hint: The two voltages are the same. Can you explain why?)

5.) In circuit 2, what is the current $I_L$ drawn by each lightbulb? (Hint: The two currents are the same. Can you explain why?)

6.) In circuit 2, what is the power $P_L$ drawn by each battery? (Hint: If you get 4 and 5, then it should be clear that the two values of the power are the same).

But in circuit two, if the voltage and current drawn by each lightbulb is the same, wouldn't the total voltage and current in the circuit be twice that of the first path, since there are two lightbulbs?