Voltage (V) - amount of available energy per coulomb of charge. The unit is volt (also V).
Current (I) - how quickly charge travels (or charge per time, q/t). The unit (a coulomb per second) is called the ampere (or amp, A).
Resistance (R) - a way of expressing how much charge is resisted through a device. It is expressed as a ratio of applied voltage to the resulting current (V/I). The unit (a volt per amp) is called an ohm (represented as the Greek symbol omega).
Often, the relationship between V, I and R is expressed as Ohm's Law:
V = I R
Batteries and other sources (such as wall sockets) "provide" voltage, which is really a difference between TWO points (marked + and - on a battery). A wall outlet is a bit more complex - there are 2 prongs, but often also a third prong (the "ground", for safety purposes).
Some folks like analogies. Consider the water analogy discussed in class. Voltage is like a tank of water (how much water). Resistance is provided by a drain or faucet. The rate at which water comes out is the current. It's only an analogy, but it gets the gist of circuit terminology ok.
What exactly *IS* a circuit?
An electrical circuit can be thought of as a complete "loop" through which charge can travel. Therefore, it actually has to be physically complete - there can be no openings. That is, the current actually has to have a full path to take.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
OK, so about regular circuits:
The images represent SERIES CIRCUITS and PARALLEL CIRCUITS.
In a series circuit, the current is constant and is set by the total resistance of the circuit (the sum of the resistors). If you remove one resistor (or light bulb, as in the first image), the current stops. If the resistors were identical bulbs, having more bulbs would result in dimmer bulbs, since the battery voltage is distributed among them. Note that the sum of the voltages "over" the bulbs is equal to the total voltage provided by the battery (give or take some minor losses). Identical bulbs (or resistors) have identical voltages "over" them - 3 identical bulbs connected to a 9-V battery would have roughly 3-V each over them.
In parallel circuits, current has multiple paths to take, so the total resistance of the circuit is actually LESS than if the resistors were alone or in series with other resistors. Since the bulbs are connected equally to the battery, they experience the same as the battery voltage - they are, therefore, of equal brightness (and the same brightness they would have if there were only ONE bulb connected). Of course, bulbs in parallel draw more current and thus cause a battery to die sooner. You could have 10 bulbs or resistors connected in parallel to a battery - each will be as bright as if only 1 were connected to the battery (same voltage over each), though 10 bulbs will kill the battery 10 times faster.
Does this have anything to do with holiday lights?
What I've written above is primarily geared toward identical bulbs. In series, add up the resistances to get the total resistance. In parallel, it is more complicated. There is a formula one can use (1/Rp = 1/R1 + 1/R2 + ...), but we will only concern ourselves with the case of identical resistors in parallel. In that case, divide the value of the resistor by the number of resistors to get the total effective resistance. For example, two identical 50-ohm resistors in parallel is the same as one 25-ohm resistor. This seems strange, but it's a little like toll booths - when one toll booth is open, it can get crowded (the current is small). With multiple toll booths open, the resistance is effectively less, so the current can be greater.
In parallel circuits, current has multiple paths to take, so the total resistance of the circuit is actually LESS than if the resistors were alone or in series with other resistors. Since the bulbs are connected equally to the battery, they experience the same as the battery voltage - they are, therefore, of equal brightness (and the same brightness they would have if there were only ONE bulb connected). Of course, bulbs in parallel draw more current and thus cause a battery to die sooner. You could have 10 bulbs or resistors connected in parallel to a battery - each will be as bright as if only 1 were connected to the battery (same voltage over each), though 10 bulbs will kill the battery 10 times faster.
Does this have anything to do with holiday lights?
What I've written above is primarily geared toward identical bulbs. In series, add up the resistances to get the total resistance. In parallel, it is more complicated. There is a formula one can use (1/Rp = 1/R1 + 1/R2 + ...), but we will only concern ourselves with the case of identical resistors in parallel. In that case, divide the value of the resistor by the number of resistors to get the total effective resistance. For example, two identical 50-ohm resistors in parallel is the same as one 25-ohm resistor. This seems strange, but it's a little like toll booths - when one toll booth is open, it can get crowded (the current is small). With multiple toll booths open, the resistance is effectively less, so the current can be greater.
In the images below, the first graphic represents the schematic view of a parallel circuits, with 2 resistors. Note that 2 possible paths are available for current to take - current runs through EACH path, though there will be more current where there is less resistance. The total current from the battery is equal to the sum of the currents through the 2 resistors. It follows V = I R, though the V over each R is the same. The I through each will therefore be V/R.
The second image illustrates the series circuit concept: identical resistors in series will effectively give MORE resistance (the sum of the resistances, actually) to the battery, so the current will be LESS (and exactly the same in each resistor or bulb). It also easily follows V = I R, with more R yielding less I (when V is constant). Think of V = I R this way: I = V/R. More R, less I.
The second image illustrates the series circuit concept: identical resistors in series will effectively give MORE resistance (the sum of the resistances, actually) to the battery, so the current will be LESS (and exactly the same in each resistor or bulb). It also easily follows V = I R, with more R yielding less I (when V is constant). Think of V = I R this way: I = V/R. More R, less I.
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