-What is a 'circuit'?
- ) end. When you touch a wire onto both ends of the battery at the same time, you have created a circuit. What just happened? Current flowed from one end of the battery to the other through your wire. Therefore, our definition of circuit can simply be a never-ending looped pathway for electrons (the battery counts as a pathway!).
-Oh, so I get it Demosthenes, you could just put a wire onto one end of a battery, and the electrons would still bump each other?
No, you could not. As stated before, in our definition of the circuit, a continuous loop is required. But think about it scientifically: If you did attach the wire to only one end of the battery, where would the electrons go that got bumped to the opposite end of the wire? That is why there needs to be that continuous loop of wire: the electrons need somewhere to go.
Voltage = Current times Resistance
If you are having trouble, think back to the baseball example: you can have a high chance of winning (voltage) by either scoring a lot of runs (high current) or having good defense/pitching (resistance).
Have you ever pumped up a super----
(that last one is more advanced)
A little review, perhaps:
The three concepts of Voltage, resistance, and current are all interrelated through this basic formula:
V = I R
or Voltage = Current times Resistance
Okay, whatever Demosthenes, I don't really care, why do I have to learn this formula anyways? Like I care about this 'omg' guy or whoever he is.
You have to learn Ohm's law because it helps you to 'analyze' circuits. That means you can use this law to find voltage, resistance, or current, if you have two of the three. Let's look at how to apply this formula:
The application of this formula is pretty easy, once you get the hang of it. Basically, imagine a wire, a battery, and a resistor somewhere along the wire. If the battery has a voltage of 10 volts, and the resistor has a resistor of 30 ohms, you simply use Ohm's law to find the current:
V= IR .....write your equation, so you know what you're doing here...
10 volts = (I)(30 ohms) ...Set up the equation, plugging in the values....
(10 volts)/(30 ohms) = I ...Divide both sides by '30 ohms' so that you can isolate the variable, I, or the current....
10 volts/30 ohms = 1/3 amperes do out the math....fun!...
But wait Demosthenes, what if they ask for voltage or resistance?
Don't get scared, my young padawan. The equation can be set up so that no matter which two of the three variables you know, you can figure out the other one easily. Suppose there's a circuit with a 6 volt battery and 2 amps of current, how would you set that up? What's your answer? (You try it first, and see if it agrees with mine!!)
Alright, let's see how you did:
V = IR ....okay, first write out the equation so you know what you're doing
R = (V)/(I) .....Manipulate the equation so you have the two knowns on one side
R = 6 volts / 2 amps ....Plug in the values
R = 3 ohms ....solve by dividing 6 by 2.
Here are the 3 general forms of the law you'll need to know:
- V = IR
- R = V/I
- I = V/R
Whichever value you're searching for, simply make that the 'lone' variable, then plug in the values, and see what you get. Pretty simple.
There's some nice little quiz questions at the bottom of the page in the following link which you can test yourself further with...
Let's consider the 'series' and 'parallel' concepts that were mentioned, because they are vitally important.
We have gone over how to calculate Ohm's law in basic circuits, with one resistor, but suppose we have a circuit with multiple resistors. How would the calculation of resistance work?
This is a very important question - because no circuit you'll encounter is going to have just one component. Before we figure out how to calculate, however, we need to look into the concept of components in 'series' and components in 'parallel.'
I have no visual way of showing you how these circuits look on here, so I will refer you to a website for all of the diagrams.Series + Parallel Circuits
Okay, now, looking at this website, we quickly see two diagrams. The first is a series circuit - all of the components are connected in one line, one direct path, from one end of the battery to the other.
The second circuit is a parallel circuit - there are different ways for the current to go from one end of the battery to the other.
Understanding these two things is crucial - because we must recognize whether or not components are 'in series' or 'in parallel' with each other in order to make the correct mathematical calculations. I suggest to all reading this chapter - it is ON POINT with what you need to know for Circuit Lab.
I'll assume that the earlier concepts are understood, and move on to the mathematics part.
To calculate the total resistance of two resistors in series is quite easy - you simply add the resistance of all the resistors in series, and you get the total resistance.
For parallel circuits, however, you can find the inverse of the total resistance by adding the inverse of the resistors in parallel together. This is shown by this formula:
From this formula, we can solve for a direct formula for resistance in parallel.
It sounds confusing here, yes, but simply look through this chapter and it will become more clear to you.
Here are some questions for you, from me, and if you want, you can email me your answers and I will check them for you:
1. What is the total resistance of two 3 ohm resistors which are in parallel? What would the current be in this circuit if there is a 6 volt battery?
2. What is the total resistance of three 2 ohm resistors which are in series? What would the voltage of the power source be if the current is 3 amps?
3. What is the total resistance of two 1 ohm resistors in series, and two 2 ohm resistors in parallel? What would the current flow be if there was a 12 volt battery powering the circuit?
Okay, we've been talking a lot about resistors - but we don't even know what one looks like yet. The Mighty Resistor
There you have it - pretty simple eh? It's just a piece of metal, and the piece in the center there is what provides the resistance.
But if you think about resistance - remember we said it was the force against the flow of the electrons - you must realize an important concept.
Resistors release heat....Don't worry, I'll explain.
Imagine our electrons - merrily flowing along the wire, pushing new electrons to flow on, and so on. This wire is not very hard to flow in - it's made of a material that's very conductive. But what would happen if we placed something in the middle of the wire that was harder for the electrons to flow through? They're going to be bumping into all the atoms in the material, which will cause the atoms to vibrate. This, in turn, will cause nearby air molecules to take some energy.
That energy is in the form of heat. Where did it come from again? From the electrons bumping into atoms inside the resistor.
But, if you think about this even further, wires are matter too - they have atoms. So you can't say that these are perfect conductors either - because they aren't.
Have you ever wondered why the US government wouldn't just put a bunch of solar panels out in New Mexico, Nevada, and Arizona, then ship the electricity everywhere and make it cheaper for us all? The reason is that electrons can't flow in wires for a really long distance without losing a lot of energy. There's just too many atoms along the way; the amount of energy lost is going to be huge.
That's something very important to realize - and the rules sheet also tells us to pay attention to that.
Despite this, during our calculations with Ohm's Law, we usually disregard wire resistance, because in small circuits the amount of heat energy lost is negligible. However, it's still important to consider.
And what about batteries, the sources of the power which are required for our lovely event?batteries! *cue angel choir music*
Believe it or not - these aren't perfectly efficient either (what kind of world is this?)! The resistance in batteries is also a topic to understand for Circuit Lab. First, let's examine what a battery actually is.
A battery works by producing an excess of electrons through a kind of chemical reaction known as a 'redox' reaction. Basically, without jargon - you have some chemicals inside the battery, they react together, and their reaction creates electrons (we're not concerned with chemistry here, but circuitry). The resistance in a battery comes through the chemicals' ability to react smoothly, and the 'electrode's' ability to get the electrons out smoothly.
In a newer battery, this is not a problem. It is just with older batteries that we find serious internal resistance problems.
These explanations were not meant to have you become the master of science - I just wanted to touch upon some things, because the event rules do. These are some of the more advanced things in the rules - and I'm not looking to explain that in this guide. For further explanations, see the following websites:
Voltage Drops and POWER!
Okay guys, we should all have a firm grasp on our three basic concepts - V, I, and R. Now let's talk about some things that happen in a circuit with these numbers.
Take a look at this circuit - the voltage of the battery is 9 volts, the resistor has a resistance of 100 ohms, so by Ohm's Law, the people who made the picture know that...
9 volts / 100 ohms = A
A = .09 amps
The thing to understand about current - it is ALWAYS the same everywhere you go in a circuit. Before the resistor -- .09 amps. After the resistor -- .09 amps. In Canada -- .09 amps. Always the same!
However, voltage at different points in a circuit is NOT the same. Remember that we said voltage is a measure of 'potential energy,' think of it as the amount of push is behind the electrons to push them forward. So there's 9 volts of push before the resistor - the battery is giving those electrons a real shove.
But, now you have a resistor in the way - it's like trying to bike up a hill. It was easy at first, you were giving the same amount of push to go at, say 10 mph, but to continue to go that speed (think of it as amps), you need to increase the effort. You're going to be tired coming off the hill.
Now, relate that back to voltage drops with resistors - as the current goes past a resistor, it has LESS potential to push it along, because some was lost in going through the resistor.
Just the exact amount of voltage lost can be calculated using Ohm's Law: just take the current at the resistor, and the resistance of each resistor, multiply them, and you have the voltage drop.
Take this example and I will help you through it.
There are three resistors, each with different resistance values. If you remember our rule about adding series resistors, you can just add all three values together to the get the resistance over the whole circuit. Now, you've added these values, and you know the voltage for the whole circuit, so using Ohm's Law, find the current by dividing voltage by resistance.
Next, use the rule as I stated above to find the voltage drop for each resistor (if you did it right, it should add up to 45 volts).
Now what about POWER (I capitalized it just because it looks cooler that way)?
Well, power's a pretty easy concept : to calculate the power of a circuit, just multiply the voltage and the current.
P = IV