Only very special substances, called superconductors (they're actually special substances placed in special situations, namely extreme cold) offer no resistance to the flow of electric current. All other substances resist the flow current to some degree.
Because of the way that electrons are arranged around their nuclei, metals tend to be excellent conductors. Some, like copper (Cu), silver (Ag) and gold (Au) are better than others, like lead (Pb), but all resist the flow of electric charge (current) to some degree.
Electrical resistance of a material is a property of that material, and its size and temperature. It is a result, like most of physics and all of chemistry, of the way electrons behave when bound to atoms.
If you know a bit about chemistry, the conducting properties of metals arises from how their electrons in d-orbitals arrange and behave in bulk materials.
The unit we use to measure and compare electrical resistance in materials and in electrical and electronic circuits is the Ohm, defined in the box and referred to by the Greek letter omega.
Resistance can range from a few pico-Ohms (1 pΩ = 1 × 10-12Ω) in good conductors to many Giga-Ohms (1 GΩ = 1 × 109 Ω) in insulators.
The base units of Ohms (1 Ω = 1 Kg·m2s-3A2) seem a bit complicated, but you generally won't need them except in special situations. For now, just know the rough range of resistance for conductors and insulators. We'll use Ohms a lot to do circuit calculations.
By the way, Ohms is capitalized because it's a person's name: Georg Simon Ohm.
The resistance of materials depends on their size (more about that below), so when we compare resistance to the flow of electricity, we need to put everything on the same scale – in other words, we need to make size not matter. The unit used in the table on the left is Ohms per meter (Ω/m) and it's called resistivity.
You can see that the things you would expect to conduct electricity have low resistivity, and the things you hope don't, insulators like glass and rubber, have very high resistivities.
Notice that the difference in resistivity between a good conductor and a good insulator can be many orders of magnitude of resistivity.
Also notice that many materials commonly held to be good conductors, like water, are actually not so good.
Silicon (and other semi-metallic elements around it in the periodic table) can either conduct or resist current in different circumstances, and they are called semiconductors – and there's a whole industry built around them!
Conductors have low resistance to the flow of electric current, while insulators have high resistance. Some materials in the middle, like silicon and germanium can act, in the right circumstances, as either a conductor or an insulator. These are called semiconductors.
The resistance of a material is a property of the identity (atomic & molecular structure) of the material and its size and shape. You can think of resistance to the flow of electric current much like resistance to the flow of water in a pipe:
If the pipe is narrow, the amount of water you will be able to push through will be limited.
If the pipe is long, the small amount of attraction of the water for the walls will add up and slow it down.
In a wire like the one shown below, the resistance is proportional to the length, and inversely proportional to the cross-sectional area.
The longer the wire the greater the resistance, and thicker wires have less resistance. Mathematically, the resistance proportionality statement is:
The constant of proportionality is ρ (the Greek letter "rho"), the resistivity of the material. Resistivity is an intensive property, and length and area are extensive.
Let's say you have a long and very conductive wire through which you're running an electric current. The wire is a good conductor so the resistance is low, and we would say that the resistance of the whole system (we'll call it a circuit later) is low.
Now interrupt that wire with a small element that has a high resistance – it could just be a thin piece of wire like the fine wires in an incandescent light bulb – and the resistance of the whole system will just be the resistance of that least conductive piece. It's a choke point for the flow of current.
You can think of electricity like water flowing through pipe. You can get a lot of water to flow through a 10 cm diameter pipe, but if there's just one small section that has a diameter of 1 cm, then that section will determine the overall flow. It's the rate-limiting or current-limiting element of the system.
Later we'll use this idea to create different "local" currents around the same loop of current in circuits.
We can make a handy analogy between the flow of electric current and the flow of water in a closed circuit of pipes.
The figure on the left shows a closed pipe system which includes a water pump (pumping counterclockwise), a valve for shutting the whole thing down and a constriction in the pipe. If we set the pump at a certain level (volume of water pumped per unit time), then the flow of water through the system is determined by the constriction. The water in the pipe can flow no faster than it does through the narrow part.
In the electric circuit (right), the pump is replaced by a battery, a source of electric potential. It provides the force that pushes and pulls electrons around the circuit, which consists of wire. The valve in the water circuit is replaced by a switch in the electric circuit.
The constriction is replaced in the circuit by the squiggly line, the common symbol for a resistor. A resistor is a section of wire that resists the smooth flow of electrons. Electric current can flow no faster through a circuit than it can through the resistor.
Resistors in electric circuits usually look something like those pictured here. They consist of two lead wires connected to one or more of a variety of materials that don't conduct electric current as well as the wire.
Resistors are constructed to have a fixed resistance. The ones shown have a color code that you can look up. This makes it easy to read the resistance levels when looking at a circuit board.
Below is the common resistor color code table. It lists the color codes for numbers, the multiplier and the tolerance or precision of the resistance.
Two examples of banded resistors are shown below the table. Some resistors have five colored bands and others four. In the five-band case, the first three numbers give the resistance, the fourth band gives the multiplier (e.g. × 10 KΩ) and the fifth the tolerance in plus or minus some percentage of the resistance value.
The second example shows a four-band code. the difference is that only the first two bands are numbers. Everything else is the same.
Not all resistors use the color code; those will simply have a resitance written on them. And there are many kinds of specialty resistors, some with adjustable resistances called potentiometers.
One further important designation is the power capacity of the resistor. Circuits operating at high voltages with high currents can burn up low-power resistors, so one must be careful in choosing the right kind of resistors for the task at hand.
Here are two examples of how to read the resistor code on 5-band and 4-band resistors.
Incandescent light bulbs operate using the choke-point principle above.
Current is directed through a very fine wire (the horizontal section in the center of the bulb on the right) that is resistive because of the choice of material, and because it is very thin.
Because of the high resistance and high current, electrons moving in the wire experience a "friction" that heats the wire. In fact, it's heated so hot that it emits visible light.
You may have seen that metal objects heated in a fire glow red. It's the same principle here except that the wire gets much hotter. The hotter the wire, the more light is emitted in the visible and ultraviolet range.
Source: Wikipedia Commons
Very often, and to varying degrees, the electrical resistance of a material can depend on the temperature of the material.
When a material, such as a wire, is relatively cold, its atoms move more slowly and don't oscillate as far from their equilibrium positions in the metal. The path of a charged particle, like an electron, through the wire might look like this:
But if the substance is hotter, its atoms will oscillate more rapidly, and will travel farther from their equilibrium positions, and thus the probability of a collision with the traveling charge will be greater:
The path of such a charge will be more convoluted, so it will take longer, on average, to travel the same distance in cold material
This charge dependence of resistance can be important in many kinds of electric circuits, so it's worth keeping in mind.
In most applications of electricity, what we're really interested in is regulating the flow of electric current to do useful things. We'd like to make the flow of current predictable and to be able to manipulate it.
It turns out the the flow of current through a material is directly proportional to the potential, and it's inversely proportional to the resistance of the material to the flow of current. That's known as Ohm's law, and it's written like this:
Ohm's law is one of the most important relationships in all of the field of electricity and magnetism.
If we want more current, we can either
Later you will see electric circuits in which the potential, current and resistance are fixed, and those in which each can depend upon other things, such as frequency of switching current on and off. All of this leads to all of the wonderful electronic devices to which we've become so accustomed.
Ohm's law is usually written on one line as a product, V = IR.
Ohm's law: Potential is current multiplied by resistance.
V = IR
We'll put the resistivities on the first column and calculate the resistance of each wire in the second. We'll need to convert 2.5 mm2 to square meters:
Now the second column of our spreadsheet will be the resistance,
Here are the results. You can look up resistivities of substances in a number of places. They are also temperature-dependent.
The resistance is then:
When writing resistances, it's customary to keep the numerical part at three significant digits or less, so we express this resistance in kilo-ohms, 40 KΩ
Small resistances like this are common for motor-driven tools like a drill.
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