The water analogyA common analogy for electrical circuits is a closed hydraulic circuit. There are simple hydraulic devices corresponding to the three common passive electronic components: resistors, capacitors, and inductors. With effort you can imagine correspondences for more devices, but if you can do that, you can probably already understand the electronic devices directly.
Potential difference / emf / voltageThe analogy for potential difference is so direct that, to a physicist at least, you could use the same word whether you're talking about electrical or hydraulic circuits. Water at the top of a dam has a lot of (gravitational) potential energy compared to the water flowing in the river below. At the bottom of the dam wall this potential energy manifests as a high water pressure, which a turbine can convert into mechanical work, as in a hydroelectric power station.
Just as you can feel water pressure when you try to block a hosepipe with a finger, you can also sometimes feel electric potential. On a dry day, after you take off a polyester jersey, you can feel the potential difference between your body and the garment: the static charge on the jersey attracts the fine hairs on your forearms, which bend slightly and give you that "electric" sensation.
We measure water pressure in pascal (Pa); typically in kilopascal (kPa), bar (multiples of atmospheric pressure), or pounds per square inch (psi) in some old-fashioned circles. Electrical "pressure", which we typically call "voltage", is in volts (V). In modern hobby electronics, you're likely also to work in millivolts (mV) or even microvolts (µV), and only rarely kilovolts (kV).
ChargeCharge is the word we use for a quantity of electric charge. It tells you how much of the electric "stuff" you have: how many charge carriers (usually electrons, but also "holes" or even ions). Where you might measure a quantity of fluid in liters (ℓ), gallons, cubic meters (m³), electric charge is in units of the coulomb (C), which is a very large unit if you were to have that amount of charge isolated from the rest of the world. For example, the gate charge required to fully turn on a big power MOSFET might be several hundred nanocoulomb (nC).
Where charges are not isolated, however, it's easy to find loads of coulombs flowing around. One ampere of current (see below) is once coulomb passing a point in a circuit per second: If your electric power is at 230V, your kettle might experience 1500 coulomb passing through its heating element when you make tea for four! Moving electric stuff around is clearly easier than isolating it.
CurrentElectrical current is the rate at which charge carriers (electrons, holes, or even ions, in an electrochemical cell) flow past a particular point in a circuit. The hydraulic analogy is also just a flow rate - of fluid particles (atoms or molecules, ultimately) instead of charge carriers.
You might see a hydraulic flow rate expressed as liters per second ℓ/s or m³/s; electric current is in ampere (A) (usually only in high-power circuits), milliampere (mA), microampere (µA) or even smaller units. For example, the 1N4148 diode has a reverse leakage current measured in nanoampere (nA).
|An 8.2Ω power resistor|
When a resistor resists the current flowing through it, the electrical energy has to go somewhere. Resistors simply dissipate this energy as heat; sometimes a little too much:
|A 22Ω power resistor|
|A 560Ω resistor|
|Air air-core solenoid|
|Almost all of the magnetic field is inside the toroidal ferrite core|
|The ferrite core in this solenoid increases its inductance|
|6800000pF at 250V - 5 for 5 zeroes after 68|
|Turn the screw to adjust the amount of air in the bladder|
Just as a hydraulic accumulator stores energy in the compressed air inside the rubber bladder, a capacitor stores energy in the electric pressure - the potential difference - between its plates of conductive material. I've never seen the "capacitance" of a hydraulic accumulator stated, but it's obvious that there are big ones and small ones. If you insisted, you could state it in liters or cubic meters per bar: the more water you need to pump into the accumulator to produce a given pressure change, the higher its capacitance. To achieve a large hydraulic capacitance, you would need a physically large unit - you need space to put all the stored water.
|An electrolytic capacitor that stores a lot of energy: 6.6 joule|
|Old-fashioned markings: colour bands|