Back To CourseCLEP Natural Sciences: Study Guide & Test Prep
25 chapters | 277 lessons
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Jim has taught undergraduate engineering courses and has a master's degree in mechanical engineering.
A new Mega-Mart just opened up in my neighborhood last month. Being the curious type, I decided to go check it out on opening day. Something I found unique about the store was the way that they had the checkout registers set up. There were multiple registers, but instead of forming a line at each one, everyone waited in a single line and then went to the next available register one at a time. While I was waiting to make my purchases, I started thinking about how the checkout lines would be a great analogy for the different configurations of electric circuits. Let's talk about my day at the store as we learn about series and parallel electric circuits.
When I visited the store on opening day, they still hadn't hired enough cashiers, so only one register was open. Everyone waiting in line had to go through this one register if they wanted to get checked out. Being conscientious of the fact that there was a long line, each customer rushed as fast as they could to get their merchandise unloaded, paid for, and bagged. This left them quite exhausted and without any energy by the time they were on their way out to the parking lot. To pass the time while I waited, I kept track of how quickly people were checking out and counted two customers per minute. This meant that every minute, two people were leaving and the line was getting shorter by two people.
If we were to build an electric circuit that represented the checkout line, it would look like a series circuit because it provides only one path for the electrons to flow through the resistance. Electrons flow through the circuit because they are trying to get from the negative end of the battery to the positive end. This is similar to the customers trying to get from the shopping area and out to the parking lot by passing through the checkout. Therefore, the bulb is like the checkout register because they both act as a resistance that impedes flow. Like the customers at the register, the electrons go through the resistance of the bulb as fast as they can, and as a result, they lose nearly all of their energy. Voltage is basically a measurement of how much energy an electron has, so when the energy drops, so does the voltage. In this case, the electrons start out with as much voltage as the battery and lose nearly all of it as they pass through the bulb.
Back at the store, my luck took a turn for the worse. As if only having one register open wasn't bad enough, the store manager decided to set up a security checkpoint at the door. Each customer had to unload their bags and let the security guard check off each item on their receipt before they could leave. Needless to say, this slowed things down considerably. A funny thing happened as a result of this added resistance. Since the checkpoint was backing things up, the customers at the register were no longer rushing because they knew they would end up waiting at the checkpoint if they went too fast. As a result of slowing down, they weren't expending all of their energy at the register and were left with just enough energy to get through the checkpoint. However, by the time they did get through the checkpoint and out to the parking lot, they were again completely exhausted. Facing an even longer wait, I again counted how quickly people were moving through and found that the added resistance of the checkpoint had slowed things down to only one customer per minute.
To represent the security checkpoint in our electric circuit, we could do so by adding a second light bulb, which adds another resistance. The total resistance of the circuit now becomes the sum of the resistances of the two bulbs, which determines how quickly the electrons can flow through the circuit. Every time a resistance is added to a series circuit, the total resistance goes up, which means the current will go down. As a result of the reduced current, the electrons passing through the first bulb don't lose as much voltage as before. This means they still have some voltage left when they get to the second bulb. However, passing through the bulb uses up the remaining voltage, and they're back to zero by the time they return to the battery. The amount of voltage lost in each bulb will depend on the resistances, but one thing is always certain: The sum of the voltages lost in each resistance will always equal the voltage of the battery. This holds true no matter how many bulbs, or resistances, are added to the circuit.
An important thing to note is that even though the voltage lost in each bulb can be different, the current flowing through them is exactly the same. In fact, the current is the same in all parts of a series circuit. Just like the next customer couldn't go to the register until the previous one was done, electrons can't flow into a bulb unless other electrons flow out. It is this sequential movement of electrons that gives a series circuit its name.
Undaunted by my first experience at the Mega-Mart, I returned the next week to find there were now two registers open and there was no more security checkpoint. As one might expect, the new cashier was not as fast as the other one. While the more experienced cashier could ring up two customers every minute, the new cashier could only ring up one customer in a minute. This meant that every minute, three customers left for the parking lot while the waiting line got shorter by three people. So, even though the new cashier was slower, there were still more customers checking out than if she hadn't been there. Another thing I noticed was that since the security checkpoint had been removed, the customers were back to rushing through the registers as fast as they could, not wanting to hold everyone up. In turn, they were using up all of their energy and going to the parking lot completely exhausted.
If we modified our electric circuit to represent the two registers being open, it would become a parallel circuit, which provides multiple paths for the electrons to pass through the resistances. Since there is only one resistance in each path, the electrons will go through the bulb as quickly as they can and lose all of their voltage. This means that the voltage across each resistance will always be equal to the voltage of the battery.
The current flowing through each bulb will depend on that bulb's resistance. As we saw at the cash registers, more customers made it through the register with the cashier that offered the least resistance. In our electric circuit, even though the current will be higher through the bulb with the lower resistance, current will still flow through both bulbs. If we add up these two currents, the sum will equal the amount of current leaving and returning to the battery. In other words, the current coming from the negative end of the battery simply splits into the different paths based on how much resistance each path offers. These individual currents then rejoin on the other side and return to the battery. This is similar to the way the customers all came from the same line, split up to the different registers, and then rejoined on the other side to go out to the parking lot.
The total resistance of a parallel circuit is a little tricky to calculate, but the important thing to understand is that the total resistance of a parallel circuit will always go down as more resistors are added. It may seem counterintuitive that adding a resistance to a circuit can actually reduce the total resistance, but as we saw at the store, even adding a slow cashier increased the number of customers checking out each minute. Adding a resistor to a parallel circuit, no matter how high the resistance, will always increase the total current. The fact that some electrons can flow through one path, while other electrons simultaneously flow through another path, is where the parallel circuit gets its name.
We just covered a whole bunch of new ideas, but we can sum them up in terms of what happens to the voltage, current, and resistance in each type of circuit. A series circuit provides only one path for the electrons to get through the resistive part of the circuit. The total resistance of a series circuit is equal to the sum of all the individual resistances, and adding a resistance will always cause the total resistance to increase. The current through each resistance, and through every part of the circuit for that matter, is the same. The voltage lost in each resistance can be different, but the sum of the voltages will always equal the voltage of the battery.
A parallel circuit provides multiple paths for the electrons to get through the resistive part of the circuit. Each time a new path is added to a parallel circuit, the total resistance will decrease no matter how high the resistance is of the new path. If the total resistance decreases, then the total current leaving and returning to the battery will increase. The current through each path can be different, but the sum of all the currents will always equal the total current. Finally, the voltage across each resistance will always be equal to the battery voltage.
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Back To CourseCLEP Natural Sciences: Study Guide & Test Prep
25 chapters | 277 lessons