[gn_pullquote align="left"]Much of the technical work of physical computing is about figuring out what form energy a person is putting out, and what kind of transducer you can buy or build to read that energy.[/gn_pullquote] Electricity refers to the presence of electric current, which is a flow of electric charge through a conductive medium. Electronics refers to the generation, distribution, switching, storage and conversion of electrical energy to and from other energy forms. Electronic sensors convert some form of energy such as light, heat, and sound pressure into electrical energy so that we can interpret what’s going on electronically. The process of converting this energy is called transduction, and transducers are the devices that perform this conversion. In order to understand this conversion, it is important to understand some things about electricity, the components frequently used in electronics, and the relationships between them.
Current is a flow of electric charge through a conductive medium, and in electric circuits this charge is often carried by moving electrons in a wire. It is measured in Amperes, or Amps. Many people explain electrical flow by using water flow as an analogy. Following that analogy, current would be how much water (or electricity) is flowing past a certain point. The higher the amperage, the more water (or electricity) is flowing.
Voltage is a measure of the electrical energy of a circuit. Specifically, it is the electric potential difference between two points. Voltage is equal to the work which would have to be done to move the charge between two points. It is measured in Volts. In the water analogy, voltage would be the water pressure. Think of a geyser as high voltage, and the the trickle from a partially clogged faucet as low voltage. This is why a voltage that is too high can damage a circuit.
Resistance is a measure of a material’s ability to oppose the flow of electricity (the inverse quantity is electrical conductance, the ease at which an electric current passes). Electrical resistance shares some conceptual parallels with the mechanical notion of friction. It is measured in Ohms. A valve that would narrow the opening of a pipe would act as a resistor, limiting the amount of water flowing through.
A circuit is a type of network that has a closed loop giving a return path for the current. At the very least, it contains a voltage source, and a load. In closed circuits, all of the electrical energy has to get used by the load. The load will convert the electrical energy to some other form of energy (a light bulb will convert the energy into light and heat). A circuit with no load is called a short circuit. In a short circuit, the power source feeds all of its power through the wires and back to itself, and this can cause damage to your circuit.
Direct Current (DC) is the unidirectional flow of electric charge. Batteries are a common source of direct current. In a DC circuit, current always flows one direction
Alternating Current (AC) refers to the the movement of electric charge which periodically reverses direction. AC is the form in which electric power is delivered to businesses and residences. In an AC circuit, poles of the circuit are reversed in a regular repeating cycle. In one part of the cycle, one pole is at a higher potential (positive) and the other is at a lower (negative). In the next part of the cycle, the second pole is more positive, and the first pole is more negative.
Schematic diagrams are diagrams of circuits, with symbols representing the components in the circuit. Many of the typical symbols are shown below.
Resistors resist the flow of electricity. They are used to control the flow of current. Current can move either way through a resistor, so it doesn’t matter which way they’re connected in a circuit. They are symbolized like this:
Capacitors store up electricity while current is flowing into them, then release the energy when the incoming current is removed. Sometimes they are polarized, meaning current can only flow through them in a specific direction, and sometimes they are not. If a capacitor is polarized, it will be marked as such on the diagram. Capacitors are symbolized like this:
Diodes permit the flow of electricity in one direction, and block it in the other direction. Because of this, they can only be placed in a circuit in one direction. In the diagram, the direction of the arrow mimics the direction of current, from high potential (voltage) to low potential (ground). They are symbolized like this:
Light-Emitting Diodes (LEDs) are special types of diodes which emit light when current flows through them. They are symbolized like this:Switches control the flow of current through a junction in a circuit. They can be represented in a few ways. Here is a Normally Open switch, which is the kind you’d find on a pushbutton:
Transistors and relays are switching devices:
Thermistors change resistance in reaction to varying temperature:
Flex sensors change resistance in reaction to being bent or flexed;
Piezoelectric devices create a varying voltage in reaction to slight changes in pressure.
For a more detailed look at electronic components, view this reference.
One of the most important equations in the understanding of electricity is Ohm’s Law, in which Voltage (V), Current (I), and Resistance are related (R) are all related by the following formula:
Volts = Amps x Ohms
V = I x R
Current (I), voltage (V), and resistance (R) are also related to electrical power (P) (measured in watts), as follows:
Watts = Volts x Amps
W = V x A
Remembering that current flows from point of high potential (positive) to points of low potential (negative), ground is the place in a circuit with where the potential energy of the electrons is zero. Sometimes this point is connected to the actual ground, either through a grounded electrical circuit, water pipe, or some other method.
Important Rules of Thumb
Current follows the path of least resistance to the ground. So if it has a choice of two paths in a circuit, and one has less resistance, that’s the path it’ll take.
In any given circuit, the total voltage around the path of the circuit is zero. Each component that offers a resistance lowers the voltage, and by the time we reach the end of the circuit loop, there will be no voltage left.
The amount of current going into any point in a circuit is the same as the amount coming out of that point.
Series and Parallel
When components are in line with one another, they are in series. When they are side by side, they are parallel. Depending on how they are arranged, current and voltage across the components will vary.
When resistors are in series, the voltage drops across each resistor, and the total resistance is equal to the sum of all the resistors. We know in the above circuit, the current anywhere is constant. We know the voltage drops across each resistor, and we know that the total of all the voltage drops equals the voltage across the battery. So Vin = V1 + V2. If we know the values of the resistors, we can use the formula V= I x R to calculate the exact voltages at each point.
For resistors in parallel, the voltage across them is equal, but the current is divided between them. The total current is constant, however, so we know that the divided current across the parallel resistors is equal to the total current. So I1 + I2 = Itotal.
Though it’s sometimes useful to think about the mathematical relationships of parallel and series circuits, it’s often more useful to think about them in terms of practical effects. Again, think of the water metaphor. For the series example, if one resistor lowers the voltage (water pressure), only a smaller voltage (trickle of water) gets through to the next. For the parallel example, the amount of water from the main stream (total current) gets divided into two streams, but the total amount of water flowing through those two streams is equal to the original amount of water.