Lesson 01: Electricity

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What are Electronics?

Electronics are things that people have made to control the flow of electricity. Using the flow of electricity, our electronic devices do things for us. Our telephones and televisions and automobiles and computers all contain electronics that we human beings have designed to perform specific tasks. We use electronics to communicate, to store information, to allow us to perceive things beyond direct human sensation, and to control household and manufacturing systems.

If you want to make something using electronics, it helps to understand some of how electronics work. Let’s begin by looking at the nature of electricity itself, so that we understand the thing we want to control.

What is Electricity?

The word electricity comes from the Greek word for amber, elektron. Amber is fossilized tree sap, which is comparable to modern hard plastic in terms of strength and the ease with which it can be cut. If you rub a piece of plastic on some cat fur, you can get a pretty dramatic spark of electricity, so I like to think of the Greeks using amber combs on cats, and seeing sparks. I have no evidence for that happening, but I think the image is helpful for remembering that we humans named the sparky zappy useful electricity stuff after some fossilized tree sap. That understanding takes the mystery out of the word. Electricity isn’t magic, it’s just a thing we use, that you can learn how to use.

We humans found there was a property of matter that sometimes made it attract other pieces of matter and sometimes made it repel other pieces of matter, and we had to name it something, so we chose the word charge, meaning that the matter carries with it some kind of load, a property distinct from weight.

All atoms of ordinary matter are made of things we call protons, neutrons, and electrons.

Electric charge is a property of electrons and protons. Neutrons do not carry a charge. Electricity is the movement of electric charge.

On May 1 in 1911, Earnest Rutherford published a paper on the structure of the atom. For only a little over 100 years, we humans have known how atoms are organized. There is a very tiny volume in the very middle of an atom where we find the protons and neutrons. We call it the nucleus of the atom. There is a larger volume around the nucleus in which we find the electrons. That’s really all there is to an atom. The electrons are not orbiting the nucleus like the Moon orbits the Earth. The behavior of electrons and protons and neutrons is so strange that it launched two new branches of science called Quantum Physics and Physical Chemistry. For our purposes, it’s enough to know that the electrons are on the outside.

This drawing is not to scale at all. Atoms are extremely tiny. The nucleus of an atom is extremely tiny compared to the overall size of the atom, which makes it extremely extremely tiny.

This drawing is not to scale at all. Atoms are extremely tiny. The nucleus of an atom is extremely tiny compared to the overall size of the atom, which makes it extremely extremely tiny.

Conductors and Insulators

In a solid material, the atoms are stuck in place. Though they do wiggle, they don’t slide past each other like they do in a liquid, or fly away from each other like they do in a gas.

Atoms in a solid are stuck in place. They can wiggle, but they cannot change their location.

Atoms in a solid are stuck in place. They can wiggle, but they cannot change their location.

In some solid materials, some of the electrons, usually only one or two per atom, are free to move around inside the whole piece of material. When an electron moves from one place to another, it takes its negative charge with it, so there is a movement of charge. That movement of charge is electricity, and we can quantify it as electric current. Materials in which electric current can flow we call conductors. Most metals are conductors.

The drawing is not to scale. The free electrons are very much smaller than the nucleus.

If the electrons are firmly stuck to their atoms, they are not free to move, so there is no movement of charge, so there is no electrical activity. Solids that do not conduct an electric current we call insulators. Most plastics, rubber, and glass are insulators.

There are no free electrons in an insulator.

There are no free electrons in an insulator.

Electronic components are made of metal parts, which conduct electricity, plastic and rubber and glass parts that insulate, preventing the flow of electricity, and some special materials called semiconductors. Under certain conditions, semiconductors act like conductors and allow the flow of electrons from one place to another in the material. Under other conditions, semiconductors act like insulators, locking the electrons to the atoms so that they cannot move, so no electricity can flow.

Electric Current

When electrons move from one place to another, they take their electric charge with them, and that flow of charge is what we call electric current. If more electrons go by in a given amount of time, there is more charge going by in that time, so there is more electric current flowing.

We count the charge where crosses an imaginary plane, so there can be no confusion about where we are measuring the current. current is charge across a plane

Each electron carries the same amount of charge as every other electron in the universe. That amount of charge is very small, so, in practice, we use a much larger unit of charge which we call a Coulomb, which is equal to the charge on more than a billion billion electrons. (A Coulomb is a little more than 6 billion billion electrons, which, in scientific notation, is 6 x 109 x109 = 6 x 1018 electrons.)

We measure current in Coulombs per second, which we call Amperes, and abbreviate “A”. One thousandth of an Ampere is a milliampere, which we abbreviate as “mA”.current equation

Resistance

If we want to control the flow of electrical current, we use a resistor. A resistor is a carefully designed conductor that gives electrons a particular amount of trouble when they move through it. You can think of it like a long hallway that turns left and right and right and left, making a zig-zag path. It’d be hard to run fast down that hallway. You would hit the walls and that would slow you down.

A “low resistance” hallway, easy to run through.

A “low resistance” hallway, easy to run through.

A “high resistance” hallway, difficult to run through.

A “high resistance” hallway, difficult to run through.

The symbol for a resistor is below. In Europe, the symbol is just a rectangle, which is easier to draw, but less evocative.

Resistor Symbols for Electrical Schematic Diagrams

Resistor Symbols for Electrical Schematic Diagrams

 The resistance of a resistor is a measure of how hard it is to get electrons through it. A very low resistance means that it is easy to move electrons through. Most wires have very low resistances. A very high resistance means that it is difficult to get electrons through. If you can’t get any electrons through at all, the resistance is infinitely high.

Measuring Resistance

To measure the resistance of a resistor (or a wire, or your skin, which has high-but-not-infinite resistance), you need a measuring device, and a unit of measurement. The unit of measurement is the Ohm, named for Georg Ohm, who worked out the basic mathematical relationship for electricity. That relationship is called Ohm’s Law, and we will get to it soon.

The symbol for Ohm is Ω, so a resistance of 5 Ohms is written 5Ω. Because resistance varies from 0 to infinity, engineers and scientists use k as an abbreviation for 1000 (kilo-), and M as an abbreviation for 1000000 (one million) (Mega-). We write 11000 Ohms as 11kΩ, and say “eleven kilo-ohms” or “eleven k”. A typical resistance for dry skin on a human being is a couple of Mega-ohms, 2 to 10 MΩ. If your skin is wet, or you are wearing hand lotion, it will probably be a bit lower.

The measuring device we will use is called an Ohmmeter. Since measuring resistance, current, and voltage are all things people want to do when they work with electricity, people make meters that measure all three, and call them Multimeters.

If you want to measure your skin resistance, so long as your meter is not connected to anything else, it is safe to use a multimeter on your skin. Pinch the probes in your fingers, or lay them flat on your skin. Don’t poke them into your skin. Leave all subcutaneous electrical measurements to medical professionals, who will make them with special equipment that is safe for such use. 

Two different brands of multimeters, showing their “this resistance is too high for me to measure” messages on their displays.

Two different brands of multimeters, showing their
“this resistance is too high for me to measure” messages on their displays.

Prepare to Measure Resistance:

  1. Plug the black probe into the COM hole on the meter. That hole is used by the meter for all measurements, so it is the common port, abbreviated “com”.

  2. Plug the red probe into the port with the Ω symbol. It will usually also have the V symbol.

  3. Turn the meter on, either by pushing the power button or by turning the dial, depending on the kind of meter you have.

  4. Find the Ω section on the dial of your meter.

  5. Set the dial to the lowest value in the Ω section, usually 200. Numbers with no letter are the lowest settings, numbers with a “k” are the middle settings, and numbers with “M” are the largest settings.

  6. Check the measurement for an OPEN CIRCUIT: The display will show a message to indicate that the resistance is too high for it to measure. That message is different for each kind of meter, and may be something confusing like a 1 on the far left of the screen, with no other digits showing.

  7. Check the measurement for a CLOSED CIRCUIT: Touch the two probe tips together. The reading should show between 0.1 and 0.3Ω. You may need to press the metal parts together quite firmly, or saw them against each other to get a stable, low reading.

Open CircuitClosed Circuit Reading should be higher than meter can measure. Reading should be less than 0.5Ω

Open Circuit                                                                                        Closed Circuit
Reading should be higher than meter can measure.        Reading should be less than 0.5Ω

Measure Resistance:

  1. Connect the probe tips to either side of the resistor. You can use alligator test leads, or wrap the resistor’s leads around the probes, or pinch them with your fingers, or press the probes onto the resistor’s leads against an insulating surface.
  2. If the “too high to measure” symbol is showing, turn up the range setting one step, wait a couple of seconds, to see if you get a reading.
  3. If the display still shows “too high to measure”, turn the range setting up another notch, and wait a couple of seconds again, to see if you get a reading.
  4. If you get all the way to the highest setting and it still reads “too high to measure”, you may have a problem with your connections, or a bad resistor. Try another resistor, preferably one with a value that you know, or another meter, to see if it is the meter or the resistor that is faulty.
  5. Write down the reading, including the “k” or “M” symbol from the dial.
0.198kΩ = 198Ω ≈ 200Ω200.6Ω ≈ 200Ω Both meters are reading the same 200Ω resistor.

0.198kΩ = 198Ω ≈ 200Ω                                                 200.6Ω ≈ 200Ω
Both meters are reading the same 200Ω resistor.

46.5kΩ ≈ 47kΩ = 47000Ω46.8kΩ ≈ 47kΩ = 47000Ω Both meters are reading the same 47000Ω resistor.

46.5kΩ ≈ 47kΩ = 47000Ω                                                  46.8kΩ ≈ 47kΩ = 47000Ω
Both meters are reading the same 47000Ω resistor.

If you go through the Prepare to Measure Resistance routine every time you use a meter, you will catch any problems with the meter before you make a measurement. If you have not checked the meter before making a measurement, and if there are problems with the meter, such as a low battery or damaged wire, you may get an incorrect measurement, which can lead to much time wasted trying to figure out a problem with your circuit that doesn’t exist.

Contact Resistance

You may have noticed that the resistance value read by the meter is sometimes inconsistent. If you press harder with the probes, you may get a lower and more stable reading.

In order for electricity to flow from the meter so it can measure the resistance of the resistor, you need to make good contact between the two metal parts (the tip of the probe and the lead wire of the resistor) so that the electrons can move easily from one part to the other.

If there is an insulator in the way, the electrons can’t flow, or can’t flow easily, and the resistance reading will be too high to read, or just higher than the design resistance of the resistor. If you can crush through the insulator, you get better electrical contact. The added resistance of the unintended insulation is known as contact resistance. Contact resistance can be caused by dirt, oil, or by the metal itself forming an insulating layer in air.

Almost all metals react with oxygen in the air to form a very thin layer of glass on their exposed surfaces. Glass doesn’t conduct electricity because the electrons in the glass are not free to move around. The glass layer is called an oxide, because it is made of metal mixed with oxygen. Iron oxide is rust. If you want to make good electrical contact to a piece of iron, you must clean off the rust. Tarnish is also a layer of oxide, usually on silver or copper. Copper forms copper oxides, aluminum forms aluminum oxides, and so on. Gold is one of the few metals that doesn’t form an oxide layer in air, which is why people use it in electronics. They use it to minimize contact resistance, so that connections are more consistently reliable.

When you are making electrical connections, you must apply enough force at the point of contact to break through the oxide glass layer to make good electrical contact. You can push or rub the contacts back and forth to make good contact.

Light force does not break oxide layers, makes poor electrical contact. Greater force breaks through oxide layers to make good electrical contact.

Light force does not break oxide layers,                                       Greater force breaks through oxide         makes poor electrical contact.                                                      layers to make good electrical contact.

When a piece of equipment isn’t working, if you give it a smart bang on the table, you may break through the contact resistance, which may get it to work again. You can get a little more use out of a battery by cleaning the ends on a piece of cloth. Natural fibers like cotton are more effective for this kind of cleaning than synthetic ones. You break up the oxide layer, and the electrons can then flow more easily.

Voltage

The flow of electrons from one side of the battery to the other is a current. The battery adds energy to the current as the electrons go through it, energy which we can use somewhere else in the circuit. The energy is stored in the crowding of electrons close to each other on the negative terminal. Electrons repel each other, so crowding them together is like compressing a spring. You can store energy in the compression, and use it later.

The amount of energy that a battery adds to the current depends on the amount of charge that flows through the battery and the capability of the battery, it’s voltage.

voltage equations 

1 Volt is equal to one Joule per Coulomb, which means we get one Joule of energy for every Coulomb of charge that flows through a 1 Volt battery. For a 1.5V battery, we get 1.5 Joules per Coulomb.

Since current is in Amperes, which is Coulombs per second, if we let 1 Ampere of current flow for 1 second, we get 1 Coulomb of charge flow. So if we let 1A flow through a 1.5V battery for 1 second, we get 1.5 Joules of energy out of it. 

A Joule is a unit of energy. There are a little more than 4 joules in a chemist’s calorie, and a thousand chemist’s calories in a food calorie. So each food calorie ~ 4000 joules.

This is what batteries are built for, to provide energy to electrical systems.

Batteries

A battery is a chemical electron-pusher. It is a pile of three pieces: a piece of some kind of metal, some active chemicals, and a piece of a different kind of  metal. We show this arrangement in the symbol, where the two lines represent the two different metals, and the gap represents the chemical layer:

Battery Symbol

Battery Symbol

You can make your own battery with a copper penny, a nickel or quarter or some other silver tone metal, and a piece of paper towel thoroughly damp (but not dripping) with spit or salt water or vinegar.

A battery made of a copper penny (negative plate), a piece of folded toilet paper wet with salt water (active chemistry), and the silver plating on a nickel (positive plate).

A battery made of a copper penny (negative plate),
a piece of folded toilet paper wet with salt water (active chemistry),
and the silver plating on a nickel (positive plate).

The chemicals corrode the positive side of the battery, and deposit electrons on the negative side of the battery. The positive side of the battery symbol has a longer line than the negative side, because that plate will be consumed by the chemistry, so we build batteries with extra material on that side.

Batteries use the chemical energy of corrosion to push electrons from the positive side to the negative side, so the negative side winds up with more electrons, and the positive side ends up with a deficit of electrons. Notice that no electrons are created or destroyed, they are only moved from one side to the other. The difference in the population density of electrons will remain so long as the chemistry has energy to keep it that way. Once the store chemical energy is exhausted, the battery no longer pushes electrons, so we call it a “dead” battery.

The Voltage of a battery is a measure of how much more crowded the electrons are at the negative side, compared to the positive side.

Measure the voltage of a battery:

  1. Plug the black probe into the COM hole on the meter. That hole is used by the meter for all measurements, so it is the common port, abbreviated “com”.
  2. Plug the red probe into the port with the V symbol. It will usually also have the Ω symbol.
  3. Turn the meter on, either by pushing the power button or by turning the dial, depending on  the kind of meter you have.
  4. Find the VDC  section on the dial of your meter. It will have a label that looks like this: VDC symbol
  5. Set your meter to a number just a bit larger than the voltage you expect to measure.
  6. Connect your probes to either side of the battery. You will need to apply some pressure to make good contact. 

It is good practice to measure your battery voltage (or the voltage of the power supply, if you are using one instead of batteries) before you connect it to your circuit to avoid damage to your circuit from too-high voltages, and to avoid confusing circuit behavior caused by too-low voltages.

Here are some typical batteries, and the voltages you can expect if you connect your meter across them in VDC symbolmode. 

Four different kinds of batteries, each showing the expected voltage in Volts (V)

Four different kinds of batteries, each showing the expected voltage in Volts (V)

Measure the voltage across the battery or power supply you are planning to use. It will likely be different than the voltage listed on the package, which we call the nominal voltage. The voltage from a battery is dependent on the amount of energy stored in the battery chemistry, so when the chemistry is fresh, the voltage is a bit higher, and as we release energy from the battery by using it to power our circuits, the voltage will decrease in proportion to the energy released. 

Some circuits are particularly sensitive to the voltage, and will do some odd things if the supply voltage is just a little bit low. If your circuit is behaving badly, measure the supply voltage, and change the batteries if the voltage is lower than you expect it to be.

If the supply voltage is higher than the maximum rating for a particular component, that component may be damaged or destroyed by the heat created by the current flow.

Combining Batteries in Series

If you want a voltage higher than what each of your batteries produces, you can put one after the other, in the same orientation, to add their voltages.

The first battery in this case pushes electrons from left to right, crowding them to make a 1.5V difference from the positive terminal to the negative terminal.

A single battery

A single battery

We add another battery to the right of the first one. It receives electrons from the negative terminal of the first, and pushes them to the right, crowding them to another 1.5V. The voltage across each battery remain the same. The voltage from the far left terminal to the right terminal of the second battery is 1.5V + 1.5V = 3.0V.

We add a third battery in the same way, and we get a total of 4.5V from one side of the series to the other.

Three batteries in series

Three batteries in series

We tend to design packaging that is not directly in a line, because that shape is awkward. So long as the batteries are electrically in series, their physical orientation does not matter.

Here are three batteries in series in a battery holder. The negative end goes towards the spring. It’s the same circuit as the three in  series above.

Three batteries in series in a battery holder.

Three batteries in series in a battery holder.

For convenience, we use this symbol showing three batteries in one package.

4.5V battery symbol

4.5V battery symbol

We are going to be working with an Arduino, which is a little computer that was designed to be easy to use. It operates at around 5V, so we are going to use this package of batteries that makes 4.5V, which is a close as we can reasonably come to 5V with ordinary batteries in series.

Ohm’s Law

Now that you know about Current, Resistance, and Voltage, we can address the relationship between the three quantities.

Voltage is what causes current to flow. Apply more voltage, and you will get more current.

Resistance is what controls the amount of current that flows. Increase the resistance, and you will get less current.

ohms law

If a component follows Ohm’s law, then we call it an Ohmic resistor. In components that don’t follow Ohm’s law, the current flow is not predicted by the applied voltage divided by the resistance. Ohm’s law is not universal, like Newton’s Law of Gravity, it doesn’t apply all the time to everything.

Regular materials, like metals, and carbon, follow Ohm’s law. Resistors are built specifically to follow Ohm’s law, unless you put more current through them than they were designed to handle.

Diodes, including LEDs, do not follow Ohm’s law. Most electronic components, unless they are resistors, are designed and built to do something other than follow Ohm’s law. That’s what makes them useful.

Forward to Lesson 2: Basic Circuits

©Paul Mirel 2015: For educational purposes only, you may freely reproduce this information, provided you cite this page as the source. Commercial uses are prohibited.