Circuit Lab
Circuit Lab | ||||
Physics & Lab Event | ||||
Forum Threads | 2019 | |||
2014 | 2013 | |||
2011 | 2010 | 2009 | ||
Tests | 2019 | |||
There are no images available for this event | ||||
Question Marathons | 2019 | |||
2014 | ||||
Division B Champion | Springhouse Middle School | |||
Division C Champion | Thomas Jefferson High School for Science and Technology |
Circuit Lab is a Division C and Division B event that is expected to rotate in for the 2019 season. It was previously an event in 2013 and 2014, when it was called Shock Value in Division B. Circuit Lab is a laboratory event which deals with the various components and properties of direct current (DC) circuits. Historically, the fields which have been tested in this event are DC circuit concepts and DC circuit analysis (both theory and practice).
Contents
What is a Circuit?
Let's take an example of a battery, for now. The battery has a positive (+) end, and a minus ( - ) end. When you touch a wire onto both ends of the battery at the same time, you have created a circuit. (It is generally ill advised to attempt this experiment. Not only will there be nothing to see, but short-circuiting a battery is potentially dangerous). What just happened? Current flowed from one end of the battery to the other through the wire. Therefore, the definition of a circuit can simply be a never-ending looped pathway for electrons (the battery counts as a pathway!).
The Requirement of a Closed Conducting Path
There are two requirements which must be met to establish an electric circuit. The first is clearly demonstrated by the above activity. There must be a closed conducting path which extends from the positive terminal to the negative terminal. It is not enough that there is a closed connecting loop; the loop itself must extend from the positive terminal to the negative terminal of the electrochemical cell. An electric circuit is like a water circuit at a water park. The flow of charge through the wires is similar to the flow of water through the pipes and along the slides of the water park. If a pipe gets plugged or broken such that water cannot make a complete path through the circuit, then the flow of water will soon cease. In an electric circuit, all connections must be made and made by conducting materials capable of carrying a charge. Metallic materials are conductors and can be inserted into the circuit to successfully light the bulb. There must be a closed conducting loop from the positive to the negative terminal in order to establish a circuit and to have a current.
Basic Electrical DC Circuit Theory
Current Flow and Direction
"Conventional Current Flow" vs. "Electron Flow" - This has to do with how circuit diagrams are interpreted. The question here deals with: Do electrons 'flow' from the positive end of the battery, or the negative end of the battery?
Just as where in mathematics subtracting a negative is equivalent to adding a positive, so also a flow of positive charges in one direction is the exact same current as a flow of negative charges in the opposite direction. As such in most applications, the choice of current direction is an arbitrary convention.
Conventional current flow, devised by Benjamin Franklin, views the current as a "flow" of positive charges. Therefore, this concept holds that current "flows" out of the positive end of the battery. Electron flow, on the other hand, deals with the ACTUAL route of the electrons (the primary carrier of electric charge in most circuits). Being negatively charged particles, electron currents moves out of the negative end of the battery.
Current
What is an "electron?" To put it simply, an electron is an atomic particle which carries a negative charge. These electrons spin around the nucleus of an atom, which has a positive charge, and is located in the very center of the atom. The concept of "electricity" has to do with these electrons and with their "electron flow." In the example of the battery, the battery takes these negatively charged electrons from a chemical reaction inside the battery, pushes them out of the negative end of the battery, and into the wire. These electrons then bump electrons in the atoms of the wire over and over until finally, electrons arrive back at the positive end of the battery. Elements which allow this process of "bumping" those electrons on over determine how conductive the element is. So, when there's a current, it's just electrons bumping each other from atom to atom and flowing on. The individual electrons generally move very slowly, but the electric current moves at the speed of light.
A circuit requires a loop for the electrons to travel on (think of "circle"). This means you can not simply attach a wire to one end of a battery and expect electrons to flow through it. As stated before, in our definition of the circuit, a continuous loop is required. But think about it scientifically: If a wire was attached 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, Resistance, and Amperes
For more in-depth information, see Circuit Lab (Episodes), written by a user for the old wiki (pre-2009 season)
Amperes
To understand Amps, the coach of a baseball team can be used as an analogy. A coach wants to make his/her team the best that it can be. There are two ways they can do this: making the team score as much as possible and making the opposing team score as little as possible. Focusing on both would be impossible so naturally, the coach is going to have to choose one area to focus on. Say the coach wants to score more runs; this circumstance can be related to the concept of "amperes." The number of runs the team makes is the score - the more they get, the better their chance of winning. Similarly, amperes measure the amount of current flowing per second through an area.
On the other hand, if the coach wants to win the game, he/she doesn't necessarily have to have the team score a whole lot of runs, the team just needs to score more than the opponent. So, maybe the coach's resistance to their scoring of runs will be high, which means that the number of runs needed to achieve the same goal is less.Resistance to current flowing is also one of the important terms.
Now, how do these concepts of amperes and resistance relate? If resistance is multiplied by the amperes, the product would be the voltage of a circuit. This relationship was discovered by Georg Simon Ohm, and it says, simply, that:
[math]V = I \times R[/math]
Or
Voltage = Current times Resistance
- Sometimes E is used in place of V, for electromotive force (EMF)
Voltage
Imagine a battery as a super-soaker, and the water that comes out of it as voltage. The harder someone pumps that super-soaker, the harder the stream is going to be when it comes out of the gun. Voltage is the potential for that water exit the gun quickly: the more the gun is pumped, more "voltage" is added in, the faster that water will go.
But sometimes, there will be a "multi-functioning" nozzle which even allows for adjustments of the water speed even further. For a "wider" and "bigger" stream of water, the nozzle may have to be changed to one with a bigger opening. A nozzle with a bigger opening increases the amount of space that the water is allowed to go through. The water still has a high "voltage", or potential for speed and force but the overall pressure of the water is decreased due to the stream's increased spread. The bigger the nozzle gets (think of it like the resistance), the smaller the hitting power (current, which is a speed in electricity too) is.
Voltage is technically electrical potential. While in many cases it is treated as an absolute, it is important to remember that in circuits mostly the difference in voltage is discussed, a potential difference, and that things like Ohm's laws only apply to potential differences, not just electrical potential. However, in the context of circuits, Voltage is often used in reference to potential difference.
Resistance (Ω)
A resistor is just a piece of metal, and the piece in the center is what provides the resistance. A resistor limits the flow of electrical current.
And as for what resistance is itself - it is the force against the flow of the electrons. They transform the electrical energy they absorb into heat energy.
Imagine electrons - 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 something was placed 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. Thus, heat would be created from the electrons bumping into atoms inside the resistor.
Other Analogy
The other way that these three are explained is using water as an example. Imagine the basic components of a circuit, a battery, wire, and a resistor. In the water analogy, this translates to a pump (because the battery pushes electrons around the circuit), some large pipe (wire), and a section of much smaller pipe (resistor). In the water analogy, the flow rate of the pump is the same as the voltage of the battery, and the pressure in the tubing if the same as the current in the circuit. This is a pretty simple way to explain voltage, current, and resistance. If the voltage is increased, but the resistance (pipe size) remains the same, it logically takes more pressure. However, if the flow rate the remains same and put in large pipes, it takes a lot less pressure to so the same job. Conversely, if the pressure is dropped, but the pipe size remains the same, the flow rate goes down, and if a constant pressure is maintained, but the pipe size increases, the flow rate goes up. And that's all there is to it. Thus one can easily comprehend the relationships in Ohm's law.
Application of Ohm's Law
This section doesn't teach any theory behind Ohm's law, but this is one of the easiest ways to apply the law (or the power law, P=IV, or any similar law). Basically, take a circle and divide into half, then divide one of the halves in half again, so there is half a circle at the top, and two quarters at the bottom.
Then an equation would be added (any equation in the form a=bc). In the case of the power law, P would go into the half circle, and I and V would go into quarter circles. Now a certain value can be covered up to determine how to solve for the covered value. For example, for finding I using P and V, P and V are uncovered since P is on top of V, it can be determined that [math]I=P/V[/math]. If the letters are next to each other (i.e. finding P from I and V) then simply multiply. Sure, the math behind it is very simple, but in a competition, this method goes a lot quicker than rearranging equations.
Base and Derived Units
SI base units are the base quantities that are independent. There is a total of seven units, but the ones important to this event are meters (m, length), kilograms (kg, mass), amperes (A, electric current), and seconds (s, time). Derived units are units that come from a combination of the base units. The ones important to this event are newtons, joules, watts, coulombs, volts, farads, siemens, and ohms. The table below shows how each of the units is related.
Quantity measured | Unit name | Unit symbol | Expression in other SI Units | Base SI Units |
---|---|---|---|---|
Electrostatic Force | Newton | N | - | kg*m*s^{-2} |
Energy, work | Joule | J | N*m | m^{2}*kg*s^{-2} |
Power | Watt | W | J/s | m^{2}*kg*s^{-3} |
Electric Charge | Coulomb | C | - | s*A |
Electric Potential Difference | Volt | V | W/A | m^{2}*kg*s^{-3}*A^{-1} |
Capacitance | Farad | F | C/V | s^{4}*A^{2}*m^{-2}*kg^{-1} |
Electric Resistance | Ohm | Ω | V/A | m^{2}*kg*s^{-3}*A^{-2} |
Electric Conductance | Sieman | S | A/V | s^{3}*A^{2}*m^{-2}*kg^{-1} |
Another important derived quantity that does not have a special unit name is the electric field strength, measured in V/m.
One coulomb is also equal to the charge of 6.24 x 10^{18} electrons.
Circuit Elements
Conductors and Insulators
A pre-requisite to understanding what conductors and insulators are is understanding the phrase "electron conductivity." This phrase is essentially the measure of a substance to transport electrons. Conductors have a high electron conductivity while insulators have a low electron conductivity. Conductors are substances which transmit electrons (electricity) relatively easily with little resistance. On the other hand, insulators do not transmit electrons easily and if they do, they have a lot of resistance. Bear in mind, not all conductors have the same electron conductivity and not all insulators have the same electron conductivity. Some common examples of conductors and insulators are below.
Sources
- Voltage Source: a theoretical component which outputs a precise, constant voltage regardless of current. Their primary usage is in modeling real components. For example, a battery can be modeled as a voltage source in series with a resistor equal to its internal resistance.
- Current Source: a theoretical component which outputs a precise, constant current, regardless of the voltage.
Resistors
The color bands around the resistor signify what the resistance is, and what the tolerance is (how accurate it is). The color codes are:
Color | Value |
---|---|
Black | 0 |
Brown | 1 |
Red | 2 |
Orange | 3 |
Yellow | 4 |
Green | 5 |
Blue | 6 |
Purple (Indigo) | 7 |
Gray | 8 |
White | 9 |
Gold | .1 |
Silver | .01 |
Color | Percent |
---|---|
Silver | 10% |
Gold | 5% |
Red | 2% |
Brown | 1% |
The most common tolerance is Gold, followed by Brown, but this doesn't rule out the other possibilities. To convert the color codes into resistance values on a resistor with 3 bands and a tolerance band, read the first two bands off in order, and then multiply that by 10^(color of third band). In the picture it would be green, then blue, thus 56, and then multiply it by 10^0 which is 56 x 1, or 56 ohms. If the resistor has 4 or more bands, read the first however many necessary, usually 3, until you only have one color (not tolerance) left, and multiply by 10^(color of last band).
Resistor Networks
Networks of resistors between two points can be simplified into an equivalent single resistor, for which the resistance can be calculated according to the configuration and values of the resistors within the network.
Series Resistance
The resistance of a resistor is directly proportional to the length of the resistive material. As such, because placing resistors in series effectively adds the lengths, resistances add in series. Therefore, for a chain of resistors, the equivalent resistance is equal to the sum of individual resistors.
Parallel Resistance
In parallel, it is not the resistances that add, but the conductances. An analogy for this is to imagine a crowd of people trying to get through a door. A single door will allow so many people per minute, but if a second, adjacent, identical door is opened, the same number of people per minute will simultaneously move through that door. Therefore, twice the number of people will move through the doors per minute. Similarly, two identical resistors in parallel will conduct twice the current as a single one. Therefore the total conductance is equal to the sum of individual conductances in parallel. As conductance is the reciprocal of resistance, the usual formula is that [math]\frac{1}{R_t}=\frac{1}{R_1}+\frac{1}{R_2}+...+\frac{1}{R_n}[/math] for n resistors in parallel.
Networks Containing Both Series and Parallel
Many real circuits will contain a combination of both series and parallel components. To simplify these networks, one must find parts of the networks that are purely one or the other and simplify them according to the formulas above. One can repeat this process until the network is simplified into a single equivalent resistor.
Wheatstone Bridge
A wheatstone bridge is used to measure an unknown resistance value to a high degree of accuracy. It uses 4 resistors set up in a diamond fashion (shown below) and a voltmeter. In the schematic below, Rx is the unknown resistance, R1 and R3 are fixed resistance values (generally the same, but they don't have to be the same, also generally >1% tolerance, but again, not always) and R2 is a variable resistor (potentiometer, this is not always the case, see below). By adjusting R2 until the voltmeter reads 0 volts, you know that the ratio between the R1/R2 and R3/Rx is equal.
To understand this, think of a circuit with two resistors of equal value in series, connected to a +5v source, because the resistances are equal, the voltage drop is equal, this kind of circuit is called a voltage divider, because the voltage in between the two resistors is 1/2 the input voltage. Again, imagine a circuit with 2 resistors in series connected to a +5v source, however this time, the resistors are 50 ohms and 25 ohms, because the total resistance is 75 ohms, at 5v, we can calculate the current, and from there calculate the voltage drop from each resistor, you should have gotten 3.33 volts across the first, and 1.66 for the second one; the voltage happens to be in the same ratio as the resistance values; now that we've proved that, we can apply it to the wheatstone bridge.
With that in mind, we now know that the ratio of the resistors is what controls the voltage at the midpoint, so if two sets of resistors have the same ratio, then they would have the same voltage. When the voltage across the bridge is 0, the sets of resistors (R1/R2 and R3/Rx) have the same voltage, and thus the same ratio of resistance values! Since we know the ratio of the first leg (R1/R2, remember we set R2 to a known value to balance the bridge) and we know R3, it's fairly simple to solve for Rx.
Meters
Teams may be asked to measure certain values in a circuit, using a Voltmeter, Ammeter, Ohmmeter or Multimeter. There are three things that teams could be asked to measure: voltage, current, or resistance.
The Voltmeter measures the voltage between two points in a circuit. To use a Voltmeter, place the meter 'parallel' to the circuit element. The Voltmeter has very high resistance, around [math]10\text{M}\Omega[/math], so it has minimal impact to the circuit when placed in parallel.
The Ammeter measures the current in a circuit. To use an Ammeter, place the meter in 'series' with the circuit. The Ammeter has very low resistance, so it has minimal impact to the circuit when placed in series.
The Ohmmeter measures the resistance of a circuit element. It can be placed either in series or parallel to the element. If an Ohmmeter is not provided, one can also place a Voltmeter parallel to the element and an Ammeter in series with the element and apply Ohm's Law to calculate the resistance.
The Multimeter can measure voltage, current, and resistance. Set the Multimeter to the mode corresponding to the unit of measurement. Then, place the meter in series if measuring current, or in parallel if measuring voltage.
When using the above meters, it is important to keep track of units. Misjudging the units being measured can cause calculations to be off by several orders of magnitude. Also, if using a meter that has different precision levels, make sure to use the level of precision that provides the most unique digits; a reading of 1 V is less helpful than a reading of 1123 mV. Finally, as with all pieces of equipment, typical safety procedures and measures should be taken when using meters. Generally, most multimeters are easy and safe to use, but common sense still applies.
Capacitors
Capacitors are, in DC at least, a device that stores a charge. When capacitors are in a circuit, they are said to resist change in voltage (i.e. if the voltage in a circuit goes up, the capacitor charges, taking away the excess voltage. If the voltage drops, the capacitor discharges, adding back to the circuit to make up the difference. There are many types of capacitors (Mylar, polystyrene, electrolytic, etc), but they all do the same basic job. At the most basic level, a capacitor is comprised of two plates separated by a dielectric (insulating material) that stores a charge, there are a few basic concepts it may be helpful to know. First off, look at the charging circuit below, the capacitor is uncharged in the beginning, but when the switch is closed, it begins to charge, as it starts to charge, the resistance across it is small (thus a current flows through the circuit, charging the capacitor), however as the voltage of the capacitor reaches [math]V_0[/math], the current decays exponentially, because the voltage is smaller, less current flows (remember?). This can also be shown by trying to measure the resistance of a capacitor (see below, because the meter puts out a small current, that charges the capacitor). It's useful to in some cases calculate the voltage for a capacitor as it is charging or discharging, for which 2 formulas are incredibly helpful. For a charging capacitor in an RC circuit:
For a discharging one:
Other Analogy
In the water analogy, a capacitor is simulated as a piece of rubber blocking the pipe. Using this example, we can see that a DC current would flow for a time, but when the rubber reached its elastic limit, it would stop, the same as the capacitor charge curve discussed above. However, an AC current (imagine the water moving back and forth very fast) would simply move the rubber back and forth, never stopping the rubber on the other side from flowing (this is true for electricity, but in an effort to not drone on too long, it's not in there).
Kirchhoff's Laws
Kirchhoff has two well-known laws of circuits: Kirchhoff's Current Law (KCL) and Kirchhoff's Voltage Law (KVL). They are simplifications of Maxwell's Laws of Electromagnetism that are valid for most practical circuits.
Kirchhoff's Current Law
A node is a junction in a circuit where two or more electrical components meet. Kirchhoff's Current Law states that sum of currents entering a node is equal to the sum of currents leaving the node. This is based on the assumption that charges cannot accumulate in a node of the circuit. Equivalently, if one designates the direction of the currents with a sign (eg all currents leaving the node are negative), the sum of currents at each node equals zero.
Node Method
The Node Method is a powerful tool of circuit analysis that is based upon Kirchhoff's Current Law. Basically, one writes the equation for every node in the circuit based upon unknown variables. Then from the resultant system of equations, one can calculate all the unknown variables and solve the circuit. It is often unnecessary for simple circuits but becomes quite convenient for large circuits.
Detailed method:
1. Select a node to be your ground and assign it a voltage of zero. (N.B. The term "ground" in the context of circuit analysis does not necessarily mean that it is connected to the ground. Instead, it is a node designated at zero electrical potential from which all other voltages are measured.)
2. Assign every other node in the circuit a variable voltage. You may in certain cases be able to calculate a voltage for a few nodes (e.g. if the negative terminal of the battery is connected to the ground, the node connected to the positive terminal will have a known, positive voltage).
3. Write the KCL equation for every node in the equation. [Example soon].
4. Solve the resulting system of equations for all the unknown variables.
At this point, you know the voltage for every node in the circuit and should be able to easily calculate anything else.
Kirchhoff's Voltage Law
Kirchhoff's Voltage Law (KVL) states that for any closed loop in a circuit, the sum of voltages will be zero. One must be very careful with sign convention for this to work. For example, in a simple series circuit of resistors and voltage sources, one must choose either the voltage sources or the resistors to have negative voltages. This is based on the assumption that there is no changing magnetic field.
Mesh Method
The Mesh Method is another technique of circuit analysis based upon Kirchhoff's Voltage Law. Essentially, one designates a variable for a current circulating through every loop in the circuit, and then writes an equation for each of these in terms of KVL.
Equivalent circuits
Just as networks of individual resistors can be simplified into a single equivalent resistor, so also can more complicated networks. Any network containing resistors, current sources, and voltage sources can be transformed into a Thevenin or Norton equivalent. This is especially useful when analyzing circuits containing other components, as the entire rest of the circuit around the component may be a network which has an equivalent.
Thevenin equivalent
The Thevenin equivalent circuit between two points consists of a voltage source in series with a resistor. In order to find the Thevenin voltage, you must find the open-circuit voltage across the two points (ie when it is broken open). The resistance is found by removing all the power sources (replacing current sources with shorts and voltage sources with breaks) and finding the equivalent resistance of the resultant resistor network.
Norton equivalent
The network can also be represented by a Norton equivalent. It consists of a resistor in parallel with a current source. The Norton resistance is equal to the Thevenin resistance. The Norton equivalent current is equal to the current that passes between the two points if you short circuit them.
Digital Logic
In digital logic in circuits, a current corresponds to a "true" or "1", and no or very little current corresponds to a "false" or "0". A logic gate will take 1 or more of these signals, perform a logical operation on it, and then either send a true or a false on its way.
A simple example of a logic gate is a transistor. If it receives very little current (false/0), then it does not allow current to pass through it (false/0). If it receives a current (true/1), then it allows current to pass through it (true/1).
Here is a list of logic gates:
- AND AND must have two trues in order for current to pass through it.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A AND B |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
Boolean Algebra: [math]A \cdot B[/math] or [math]A[/math] & [math]B[/math].
Distinctive Shape:
Rectangular Shape:
- OR OR will allow current to pass through it if any true is given to it.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A OR B |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
Boolean Algebra: [math]A+B[/math].
Distinctive Shape:
Rectangular Shape:
- NOT NOT turns trues into falses and falses into trues. The NOT gate is commonly called an inverter.
Truth Table:
INPUT | OUTPUT |
---|---|
A | NOT A |
0 | 1 |
1 | 0 |
Boolean Algebra: [math]\overline A[/math] or ~[math]A[/math].
Distinctive Shape:
Rectangular Shape:
- NAND NAND blocks current only when two trues are given to it.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A NAND B |
0 | 0 | 1 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
Boolean Algebra: [math]\overline {A \cdot B}[/math] or [math]A | B[/math].
Distinctive Shape:
Rectangular Shape:
- NOR NOR only allows current to pass through it when it is given two falses.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A NOR B |
0 | 0 | 1 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 0 |
Boolean Algebra: [math]\overline {A + B}[/math] or [math]A - B[/math].
Distinctive Shape:
Rectangular Shape:
- XOR XOR only allows current to pass through it when the two signals it is sent are not the same.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A XOR B |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
Boolean Algebra: [math]A \oplus B[/math].
Distinctive Shape:
Rectangular Shape:
- XNOR XNOR only allows current to pass through it when the two signals it is sent are the same.
Truth Table:
INPUT | OUTPUT | |
---|---|---|
A | B | A XNOR B |
0 | 0 | 1 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
Boolean Algebra: [math]\overline {A \oplus B}[/math] or [math]A \odot B[/math].
Distinctive Shape:
Rectangular Shape:
Alternating Current (AC)
As of the 2013 season, Alternating Current is a disallowed topic under the rules. However, as of the 2018 season, Alternating Current is a allowed topic under the rules to a certain extent.""
Up until now, the entire discussion (minus a mention when talking about capacitors) has dealt with DC, or direct current. In a DC circuit, current flows from the positive to the negative terminals of a battery or other source, that it (electrons flow the opposite, see above). However, the power in your house is AC, not DC. AC, or alternating current, is much more complicated. Basically, the direction of the current flow change, if voltage vs time is plotted, instead of a line (for DC), a sine wave is graphed. This yields many advantages, namely, the use of transformers for voltage step-up/step-down.
Single Phase
Single phase AC (only one wire that's 'hot' or has voltage in it) is most of what's in your home. It has a hot and neutral line. The hotline varies between the minimum and maximum voltage, and neutral stays around 0v. The switch direction does not matter much for simple devices like light bulbs, but more complicated devices generally convert AC into DC before use through a rectifier (usually a diode bridge). As a side note, an inverter is used to convert DC into AC. The benefit of this is that AC enables the use of transformers to easily step-up and step-down the voltage.
Transformers
The diagram of a transformer is as shown below:
On the left hand side of the diagram is the primary circuit, where the the initial AC voltage source provides power to the circuit. The primary coil (on the left hand side of the transformer) and secondary coil (on the right hand side of the transformer) are wrapped around the iron core. These coils are both considered to be electromagnetic inductors. The changing magnetic field in primary coil then induces a voltage in the secondary coil according to the formula
where [math]V_p[/math] and [math]V_s[/math] are the voltages of the primary and secondary coils respectively, and [math]N_p[/math] and [math]N_s[/math] are the number of turns on the primary and secondary coils respectively. Thus, it can be seen that the voltage of the each coil is directly proportional to the number of turns in that coil.
It isn't possible to build a DC transformer because the magnetic field of the primary coil would be constant. Remember that, according to Faraday's law, constant magnetic fields can't induce the voltage needed to create a current in the secondary coil, only changing magnetic fields can.
Phase Shift
Imagine a sine wave. Now, put another sine wave on top of it, only flipped. These waves are said to be 180 degrees out of phase - either wave could be moved 180 degrees along the x-axis and the waves would line up. This is phase shift.
Power Factor
In a normal AC waveform graph, there is both a voltage vs. time waveform and a current vs. time one. The two are normally in sync and always have positive power (signs switch as the zero-crossing, so the product is always positive). This occurs in a purely resistive load like a perfect lightbulb, and when the two are in sync, the power factor is always 1, a perfect power factor. However, this never happens. On the other hand, when the voltage and current are 90 degrees out of phase, the power transfer is half negative and half positive, so there is no net power transfer. This occurs in a purely inductive or capacitive load (i.e. a motor), and the power factor is 0. This also never happens. In the real world, every wire has resistance, inductance, and capacitance (they are very negligible, small wire is in the 1-10 milliohms/ft), which causes the power factor to never be perfect.
Although there are many units that go along with AC, the most important are: watts, which are real power, the work that can be done through a motor; volt-amps (VA) are apparent power, the power that wires and cables must handle; volt-amps-reactive (VAR) are the power the wires must carry, but cannot do real work. These three main units are related in the power triangle shown to the right. Most power companies do not want reactive power, charging more for businesses with low power factor loads (bigger wires are needed), and inductors or capacitors can be placed to correct for the bad power factor. However, there are some uses for reactive power. For example, in drilling rigs, when a rig holds the top drive, most rigs using AC motors don't actually use any power. They only use enough to overcome the resistive losses in the wire/control system (VFD) and motor, approximately 50A at 60V, or 30KVA. This is a tiny amount of power to hold a block weighing a little over a million pounds. This occurs because about 1200A (720KVA) flows between the motor and the VFD, but the motor is almost a purely inductive load, so no real power is used.
Polyphase
Although a complex-sounding word, polyphase has a simple meaning. Whereas a single phase system has one wire with changing voltage, a polyphase (many-phase) system has multiple wires carrying current at a time, shifted (time delayed) by a certain amount. The most common is 3-phase power, which is used in many factories and industrial places, anywhere where large motors are involved. In this system, there are 3 wires carrying power, each of them shifted 120 degrees from the other. In this case, there is always a voltage between the phases (due to the shift) and a neutral is not required. The benefit to this is that the power through the system is constant, instead of varying like single phase (add up the magnitude of the 3 waves at any point and it will always be the same number). Polyphase is also useful in motors because the motors become self-starting; in an AC motor, the magnetic field must come from two points, and there must be a phase shift between the two so that the field 'rotates' around the stator (motor shaft). In a single phase system, this isn't possible, so most motors need to be moving to start (so the stator is already moving through the field), in three phase, the phase shift is already present, so the motor can start itself, it can also apply full torque without any speed, because the field can 'hold' the stator in place. This is useful in cranes and oil derricks, removing the need for a mechanical break. Systems that use more than 3 phases exist, but almost solely for motor applications where higher speeds are required.
Other Topics
Inductors
An inductor is essentially an electromagnet that exhibits special characteristics in a circuit. It can be described as the opposite of a capacitor, but this is slightly misleading. An inductor can store a charge in a magnetic field (capacitors store it in an electric field) and can maintain a constant current in a circuit (capacitors maintain a constant voltage). An inductor easily conducts DC (capacitors easily conduct AC), but it AC is put through an inductor, the magnetic field grows and collapses with the rise and fall of the current, which tends to opposes the flow of AC through an inductor.
In the water analogy from above, an inductor is a waterwheel, with a constant flow of water through it. The waterwheel spins normally. However, with an alternating flow, the water wheel continuously tries to turn back and forth, limiting the flow of water.
Diodes
Although teams only need to know that diodes conduct only in one way, it is helpful to know more about them, though they are not covered deeply by the rules. A diode is a semiconductor made of a junction of P-type and N-type silicon that only conducts in one direction. There are many different types of diodes (Schottky, Zener, light-emitting) that vary mainly in their forward voltage drop (amount of voltage lost while conducting) and avalanche voltage point (where diodes conduct in reverse). For example, Schottky diodes are used in power supplies to combine two supplies of the same voltage because of the low forward voltage drop. Zener diodes are used to detect when a voltage is above a certain point because of their low avalanche voltage. Light-emitting diodes are used for light. The arrow on a diode's symbol points to the direction of the positive (conventional) current through the diode.
In the water analogy, the diode is simply a check-valve (one-way valve).
Circuit Analysis with Ideal Diodes
Ideal diodes can generally be approximated by either a short or a break in the circuit. To analyze a circuit with an ideal diode, and educated guess must be made on whether or not the diode conducts. Once a solution is found for this circuit, one much check if it makes sense. If the diode is assumed to conduct, one must make sure it is conducting current in the correct direction. If the diode is assumed not to conduct, one must check to make sure the voltage across the non-conducting diode is such that it would not conduct. If this is not the case, the assumptions must be switched and the circuit recalculated.
Resources
Circuit Lab (Episodes) - written by a user for the old wiki (pre-2009 season)
The rules for a trial event form on Circuit Lab can be found here