Homework Help: Photoelectric device in circuit

1. Jun 30, 2009

catie1981

1. The problem statement, all variables and given/known data
Suppose that 2 identical photoelectric devices are connected to the same electrical circuit, and the devices are connected in parallel with each other. Compared to the voltage generated by a single device, the total voltage supplied by the parallel devices will be?

2. Relevant equations

Errr, not entirely sure....

3. The attempt at a solution

This is an MCAT practice test question, and boy am I stumped. The answer is supposed to be "the same," but I don't know why. Are photoelectric devices similar to capacitors, or resistors, or are they a completely other creature? I figured that the voltage should have been half. The explanation behind the solution was given as:

Identical voltage sources connected in parallel produce the same output voltage as a single source, whereas if they were connected in series, the source voltages would have been added.

I am utterly confused. MCAT's are thursday morning, and I am hopeful to not get any questions like that one. I've never even heard of photoelectric devices, but I apparently should have! Any help, as always, is greatly appreciated.

2. Jun 30, 2009

cepheid

Staff Emeritus
Hi catie1981,

One example of a photoelectric device is a solar cell (like the kind that are found in solar panels). However, the specific nature of the device is not important here. All you need to know is that it is somewhat like a battery. As the solution states, it's a source of voltage. Its job is to create a voltage (potential difference) across its two terminals. The only difference is that a battery uses chemical energy to create this potential difference, whereas a solar cell (also known as a photovoltaic cell) uses light energy to do so. In other words, when this device is illuminated, a voltage appears across its terminals, and therefore this device can act like a source.

So, this question is essentially asking, "what happens when you connect two voltage sources in parallel, assuming that they both have the same voltage?"

Does that help?

3. Jun 30, 2009

catie1981

How incredibly stupid will I sound if I say, "no, I still don't get it?" Why do the outputs equal each other if connected in parallel, but are not equal when connected in series? And what happens if the sources are NOT the same voltage? Argh.

4. Jun 30, 2009

cepheid

Staff Emeritus
Don't feel too bad. There are no stupid questions after all, and the point of this is to learn.

If two batteries are connected in parallel, then this means that the + terminals of the batteries are connected to each other, and the - terminals of the batteries are connected to each other. Do you agree?

If two terminals are electrically connected, then they have to be at the same potential. Do you agree? This means that both + terminals are at the same potential (let's call it + 5 V), and both minus terminals are at the same potential (let's call it 0 V). Because they are in parallel with their corresponding terminals connected, the voltage measured across each of the batteries is no different from the voltage measured across both of the batteries (draw a picture of two batteries in parallel...they share a common + terminal and a common - terminal, so the voltage across one battery is same as the voltage across the other).

What if the two batteries aren't at the same voltage to start with? At first it seems like we have a contradiction -- they are in parallel, sharing a common + and a common - terminal, so how could the voltages across them be different? The contradiction is resolved as follows: when we first connect the two batteries in parallel, the two + terminals are NOT at the same potential, meaning that a current will flow between them. The battery with the higher voltage will supply current to the battery with the lower voltage in an effort to "charge it up" until the two batteries are at the same voltage.

Now generalize this by replacing the word "battery" with "voltage source" in everything above.

EDIT: Just as a side note: connecting batteries of unequal voltage in parallel could potentially be problematic, because this "equalization" process I talked about might happen very fast and result in a very large current that would cause the batteries to heat up quite a bit (esp if the batteries are not the chargeable kind). Of course, this little factoid is not relevant to your problem, I just thought I'd throw it in there, because it aids in your understanding of what happens when you try to do something contradictory in real life. ;)

Last edited: Jun 30, 2009
5. Jun 30, 2009

catie1981

Wow. That is why you are the master, and I am but a lowly grasshopper. Thanks. I get it now. Sad addition to my story- my husband knew what was going on, and he didn't even take Physics in college :( If it is ANY consolation, biological sciences do not elude me the way physics do. I will be a good doctor, so long as no one comes to my office with a physics problem. ;)

6. Jun 30, 2009

cepheid

Staff Emeritus
Hi catie1981,

Talking to some of my med student friends, I gather that although physics is not specifically relevant to medicine, deductive reasoning and analytical problem-solving skills are. One of my friends even went so far as to say that he liked medicine because it offered the best of both worlds. On the one hand, you need to have wealth of knowledge and factual info at your disposal (like biology), but on the other hand, you need to be able to reason your way to a logical conclusion when confronted with a few basic pieces of data and using a few key principles (like physics or engineering).

Of course, technology is also becoming increasingly relevant in modern medicine. ;)

So take heart that even though you may not grasp the specific physics concepts immediately, exercising your brain in this way is helping you to become a better doctor.

Finally, best of luck on the MCAT! I'm sure you'll do fine. Hey, from a practical, test-taking point of view, if you can rock the bio section, then you don't need to get perfect on the physics part. All you have to do is keep a cool head and do the best you can.