Current is the rate flow of electric charges. The unit of electric charge is the coulomb (pronounced ku’-lom). The unit for current is the ampere. We use the term amps for short. An amp is the amount of electrical current that exists when a number of electrons having one coulomb of charge move past a given point in one second. One coulomb equals 6,240,000,000,000,000,000 electrons. That’s a lot of electrons moving past a given point in a second!
In physics we describe the flow of current conventionally. The flow of positive hole charges are the result of the flow of electrons. Although it is electrons which are the mobile charge carriers which are responsible for electric current in conductors such as wires, in physics it has been the convention to take the direction of electric current as if it were the positive charges which are moving. As electrons move through the wires a positive hole charge moves in the opposite direction. This positive hole charge is created when the electron moves away from the atom, making the atom momentarily positive.
I is the current, Q is the quantity of charge, and t is the time. 1 ampere = 6.24 x 1018 elementary chages per second
Example 1: How much current is flowing if 8 coulombs of charge flow past a point in 6 seconds?
Example 2: If the fuse can withstand 10 Amps and the object it is connected to can provide a continuous 20 coulombs, how long should it take to flow through the fuse if you want the maximum amount of current to pass through the fuse and you do not want the fuse to blow?
I = 10 Amps
Q = 20 coulombs
t = ?
I = Q/t
t = Q/I
I = 20/10
I = 2 seconds
therefore 20 coulombs will flow through the fuse every 2 seconds.
Types of Current:
Two types of current exist. Direct current (DC) involves the continuous flow of electrons in the same direction. Batteries produce direct current. Alternating current (AC) is the second type of current which involves a periodic reversal in the direction of electron flow. The outlets in your house provide alternating current.
Direct current is constant, non-changing.
Alternating current is measured at the maximum (peek) to the minimum of the cycle. Another measurement of alternating current is frequency. Ex: 60 Hz. This means that the waveform oscillates through 60 cycles per second.
Measuring current may be accomplished by using a meter called an Ammeter. An ammeter may be analogue (needle) or digital (numbers). This meter is placed in the path of the electron flow, and reads the flow in amperes. Click on the toggle switch to turn the circuit on. The direction of the current (hole charges) moves from the positive side of the battery through the ammeter, the light, the toggle switch, and through the battery. The letter "I" is the symbol/variable for current.
The flow of electrons move from the negative terminal on the battery through the toggle switch, the light, ammeter to the positive terminal. The flow of electrons are caused by the completion of the circuit providing a path for the electrons to flow. The Law of Electric Charge states the like charges repel and opposite charges attract. The excess of negative charges (electrons) on the negative terminal of the battery provide the 'push' force on the electrons. At the same time the electrons feel a 'pull' towards the positive terminal of the battery (negative charge deficit). This push - pull effect moves the electrons through the path (conductors) of the circuit. Consequently the hole charges move in the opposite direction to the flow of electrons. This hole charge flow is indicated by the moving "I" in the circuit. This is the conventional flow of current.
The circuit above can be represented by a drawing called a schematic. A schematic replaces the pictures of the items such as the battery, switch, light, ammeter, and wires. Symbols are used to represent the various objects of electrical circuits.
The symbol of the ammeter is .
The symbol of the battery is .
The symbol of the light is .
The symbol of the switch is .
Schematic of the above circuit.
Symbols are universal, like chemistry nomenclature (the periodic table), electronic symbols are used world wide to communicate the various components and circuit paths. Schematic diagrams are often included in electronic device user manuals for future trouble shooting if the device encounters problems. Electrical technicians, engineers, and various other people use schematic diagrams in the first phases of planning a circuit and performing calculations. The schematic diagram is then used in the manufacturing process. Just as an architect designs house plans and the building contractor builds the house, the engineer designs the electrical schematic diagram, and the manufacturer builds the electronic device.
A brief explanation of atomic structure and electrical conduction
Electrical conduction is caused by electrons breaking free of their atoms and moving around.
Atoms of some elements let go of their outer electrons pretty easily, which makes
these elements good conductors.
In other elements, the atoms hold on to their electrons, so these elements don't conduct
electricity as well.
Copper and silicon are used here as examples. The same general ideas apply to other elements.
In these sketches, positive charge is shown in red, negative in black, and neutral
The atomic number of copper is 29, which means it has 29 protons in the middle and 29
electrons moving around the outside. (The 29 negative charges of the electrons and
the 29 positive charges of the protons balance out, so the atom is neutral when all
of its electrons are in place.)
Copper has two electrons in the innermost shell,
eight in the next shell, eighteen in the third shell, and one in the fourth shell.
This means that the first three shells each have as many electrons as they can hold,
and the fourth shell has
one lonely electron. (The fourth shell can hold up to 32 electrons.)
Because this one lonely electron is all by itself in the outer shell,
it can easily separate from the rest of the
atom and go roaming around, which makes copper a very good conductor.
The atomic number of silicon is 14, which means it has 14 protons in the middle and 14
electrons moving around the outside. Silicon has two electrons in the innermost shell,
eight in the next shell, and four in the third shell.
This means that the first two shells are completely full, and the third shell has
four electrons, out of the 18 that can fit in the third shell of an atom.
Something about having four electrons in that outer shell makes the shell
more stable than copper's outer shell with its one lonely electron, so the electrons
in the silicon atom don't wander off as easily. Since the silicon atom
has a fairly firm grip on its electrons, silicon is not as good a conductor as copper is.
Most house wiring is made of copper, because copper
is a very good conductor of electricity and is not as expensive as other good
conductors, such as gold and platinum.
Silicon can conduct electricity, but not nearly
as well as copper does. The conductivity of silicon depends a great deal on what
is mixed with it, because this affects how tightly it holds on to its outer electrons.
Silicon is the main ingredient in glass,
which does not conduct electricity. Silicon is also used to make semi-conductors,
which do conduct electricity, but still not as well as copper does. (That's why they're
called semi-conductors instead of conductors.)