Introduction: Voltage, Current, Resistance, and Ohm's Law
Covered in this Tutorial
How electrical charge relates to voltage, current, and resistance.
What voltage, current, and resistance are.
What Ohm's Law is and how to use it to understand electricity.
A simple experiment to demonstrate these concepts.
Step 1: Electric Charge
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges: positive and negative (commonly carried by protons and electrons respectively). Like charges repel and unlike attract. An absence of net charge is referred to as neutral. An object is negatively charged if it has an excess of electrons, and is otherwise positively charged or uncharged. The SI derived unit of electric charge is the coulomb (C). In electrical engineering, it is also common to use the ampere-hour (Ah); while in chemistry, it is common to use the elementary charge (e) as a unit. The symbol Q often denotes charge. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that don't require consideration of quantum effects.
The electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by or produces electromagnetic fields. The interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces (See also: magnetic field).
Twentieth-century experiments demonstrated that electric charge is quantized; that is, it comes in integer multiples of individual small units called the elementary charge, e, approximately equal to 1.602×10−19 coulombs (except for particles called quarks, which have charges that are integer multiples of 1/3e). The proton has a charge of +e, and the electron has a charge of −e. The study of charged particles, and how their interactions are mediated by photons, is called quantum electrodynamics.
Step 2: Voltage：
Voltage, electric potential difference, electric pressure or electric tension (formally denoted ∆V or ∆U, but more often simplified as V or U, for instance in the context of Ohm's or Kirchhoff's circuit laws) is the difference in electric potential energy between two points per unit electric charge. The voltage between two points is equal to the work done per unit of charge against a static electric field to move the test charge between two points. This is measured in units of volts (a joule per coulomb).
Voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three. A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; often a common reference potential such as the ground of the system is used as one of the points. A voltage may represent either a source of energy (electromotive force) or lost, used, or stored energy (potential drop)
When describing voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the water amount, voltage is represented by the water pressure, and current is represented by the water flow. So for this analogy, remember:
Water = Charge
Pressure = Voltage
Flow = Current
Consider a water tank at a certain height above the ground. At the bottom of this tank, there is a hose.
So, the current is lower in the tank with higher resistance.
Step 3: Electricity：
Electricity is the presence and flow of electric charge. Its best-known form is the flow of electrons through conductors such as copper wires.
Electricity is a form of energy that comes in positive and negative forms, which occurs naturally (as in lightning), or is produced (as in generator). It is a form of energy which we use to power machines and electrical devices. When the charges are not moving, electricity is called static electricity. When the charges are moving they are an electric current, sometimes called 'dynamic electricity'. Lightning is the most well-known and dangerous kind of electricity in nature, but sometimes static electricity causes things to stick together.
Electricity can be dangerous, especially around water because water is a form of conductor. Since the nineteenth century, electricity has been used in every part of our lives. Until then, it was just a curiosity seen in a thunderstorm.
Electricity can be created if a magnet passes close to a metal wire. This is the method used by a generator. The biggest generators are in power stations. Electricity can also be generated by combining chemicals in a jar with two different kinds of metal rods. This is the method used in a battery. Static electricity is created through the friction between two materials. For instance, a wool cap and a plastic ruler. Rub them together may make a spark. Electricity can also be created using energy from the sun as in photovoltaic cells.
Electricity arrives to homes through wires from the place where it is generated. It is used by electric lamps, electric heaters, etc.. Many home appliances such as washing machines and electric cookers use electricity. In factories, there are electricity powers machines. People who deal with electricity and electrical devices in our homes and factories are called "electricians".
Let's say now that we have two tanks, each tank with a hose coming from the bottom. Each tank has the exact same amount of water, but the hose on one tank is narrower than the hose on the other.
We measure the same amount of pressure at the end of either hose, but when the water begins to flow, the flow rate of the water in the tank with the narrower hose will be less than the flow rate of the water in the tank with the wider hose. In electrical terms, the current through the narrower hose is less than the current through the wider hose. If we want the flow to be the same through both hoses, we have to increase the amount of water (charge) in the tank with the narrower hose.
Step 4: Electrical Resistance and Conductance
In the hydraulic analogy, current flowing through a wire (or resistor) is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop that pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. (Conductance and resistance are reciprocals.)
The voltage drop (i.e., difference between voltages on one side of the resistor and the other), not the voltage itself, provides the driving force pushing current through a resistor. In hydraulics, it is similar: The pressure difference between two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe. If these pressures are equal, no water flows. (In the image at right, the water pressure below the pipe is zero.)
The resistance and conductance of a wire, resistor, or other element is mostly determined by two properties:
- geometry (shape), and
Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In the same way, a long, thin copper wire has higher resistance (lower conductance) than a short, thick copper wire.
Materials are important as well. A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. Similarly, electrons can flow freely and easily through a copper wire, but cannot flow as easily through a steel wire of the same shape and size, and they essentially cannot flow at all through an insulator like rubber, regardless of its shape. The difference between copper, steel, and rubber is related to their microscopic structure and electron configuration, and is quantified by a property called resistivity.
In addition to geometry and material, there are various other factors that influence resistance and conductance.
It stands to reason that we can't fit as much volume through a narrow pipe than a wider one at the same pressure. This is resistance. The narrow pipe "resists" the flow of water through it even though the water is at the same pressure as the tank with the wider pipe.
In electrical terms, this is represented by two circuits with equal voltages and different resistances. The circuit with the higher resistance will allow less charge to flow, meaning the circuit with higher resistance has less current flowing through it.
Step 5: Ohm's Law：
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:
where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. Ohm explained his experimental results by a slightly more complex equation than the modern form above (see History).
In physics, the term Ohm's law is also used to refer to various generalizations of the law originally formulated by Ohm.