PS-6.6 Explain the relationships among voltage, resistance, and current in Ohm’s law.
A NOTE ON CURRENT FLOW
"...it has long been the convention to take the direction of electric current as if it were the positive charges which are moving. Some texts reverse this convention and take electric current direction as the direction the electrons move, an obviously more physically realistic direction..." Quote from Hyperphysics.com
Voltage is sometimes called electrical force. Just as an applied force can do work by moving objects through a distance, voltage can do electrical work by moving electrical charge along an electrical conductor. Most electrical conductors are metals. This is because metal atoms have a fairly loose hold on their outer electrons, making it fairly easy for them to move from atom to atom.
In diagram "A" below, two metal plates labeled "A" and "B" are separated by a space. Plate "A" has an abundance of electrons on it making it negatively charged. Plate "B" has a deficiency of electrons, making the atoms here positively charged. We know that opposite charges attract, so there must be a force of attraction, "F" between plates "A" and "B". This force of attraction is caused by the difference in charge between plates "A" and "B". The force created by this difference in charge is called voltage. Since there are no electrons actually moving between the plates, this force represents potential electrical energy, that is electrical energy waiting to move electrons.
In diagram "B" shown below, we see that the switch has now been closed, creating a conductor path between plates "A" and "B". The electrons, now free to move under the influence of the voltage (electrical force), move along the conductor from the negative plate towards the positive plate. These moving electrons form current electricity. As the negative electrons move from the negatively charged plate to the positive plate, the difference in charge decreases and so does the voltage. Eventually, the charge difference will be almost zero and the electrons will no longer move.
A voltage is created created by a chemical cell when it changes chemical potential energy to electrical energy. In chemical cells, a chemical reaction within the cell produces a difference in charge between the two terminals of the cell. The negative terminal has an abundance of electrons and the positive terminal has a deficiency of electrons. As illustrated in diagram "C" below, the electrons will move from the negative terminal to the positive terminal under the influence of the voltage. Again, as the electrons move from negative to positive, the difference in charge between the two terminals, and therefore the voltage, will decrease over time, and eventually the cell will be "dead", the electrons will no longer flow along the conductor.
A voltage can be created by an electrical generator when it changes mechanical energy to electric energy. It does this by passing a wire conductor through a magnetic field ( See diagram "D"). As the magnetic field moves across the conductor, the electrons in the wire conductor are pushed by the magnetic field. This process is called electromagnetic induction. Generators can produce both direct current (DC) or alternating current (AC). Electrical generators that produce alternating current are called alternators.
Diagram "E" shows how a steam turbine at an electrical power plant is connected to an electrical generator that produces electrical energy. The steam enters the turbine, spinning it at a high speed. The turbine spins the wire coils between the magnets of the generator. This produces the electric current through electromagnetic induction. The electrical current is then carried to the consumers (you) by transmission lines.
Click here to read about electrical generators and see an animation. Click here for more about electrical generation methods
Burning fossil fuels to produce steam is not the only way to turn a generator and produce electricity. Wind, water, Nuclear, and Geothermal energy can also be converted into electrical energy.
When a wire connects the terminals of a battery or generators then the voltage will push and pull electrons through a conductor. One terminal has extra electrons thus a negative charge. The other terminal has a deficit of electrons and thus a positive charge. Electrons in the wire are pushed by the negative terminal and pulled by the positive terminal through the wire.
Electric current refers to the flow of electrons or charge through a conductor. Since it is very difficult to visualize a single electron moving along a conductor, we will measure electrical charge in coulombs. One coulomb of electrical charge is equal to 6.25 x 1018 electrons. Electric current is a measure of the amount of electrical charge that flows through a conductor per second and is measured in amperes (A). One ampere is equal to one coulomb of charge per second.
All electrical circuits produce waste heat because of electrical resistance. Electrical resistance is a force, similar to friction, that opposes the flow of electrical charge. All substances, even metal conductors, have varying amounts of electrical resistance. When the electrons flowing through the wire continually run into atoms in the wire and bounce around, electrical resistance is produced, and the result is wasted heat energy. Electrical resistance is measured in Ohms. The symbol for the Ohm is the Greek letter Omega (Ω ).
Electrical resistance varies with material, wire diameter, and wire length. Since electrical charge moves on the surface of the wire conductor, wires with a larger diameter generally have less electrical resistance than wires with a smaller diameters because they have larger surface areas. Shorter wires also have less electrical resistance than longer wires. Diagrams "F" and "G" illustrate this. Click here and see what happens when resistance produces so much heat the wire actually glows.
Click here to see the effect of wire length on electrical resistance. In the video a length of wire is attached to a meter stick. The resistance is checked every decimeter. The resistance is just about 1Ω per centimeter.
While it is true that electrical wires provide much of the resistance in a circuit, the devices that the circuit operates also produce a substantial amount of electrical resistance. Light bulbs, toasters, motors, and other devices can produce substantial electrical resistance and this in turn heat.
Ohms law describes the relationship between voltage, current, and resistance. Ohm's Law states that the voltage is the product of the current and resistance as defined by the equation V = I R. In this equation "V" stands for voltage measured in volts, "I" stands for current, measured in amperes, and "R" stands for electrical resistance measured in Ohms. In electrical circuits, it takes one volt of electrical force to move one ampere of current through one ohm of resistance. Two very important factors illustrate this relationship between voltage, current, and electrical resistance, if the voltage ( electrical force ) increases the current will increase ( diagram "H") and if the voltage stays the same and the resistance increases then the current will decrease. In other words, current is proportional to voltage and inversely proportional to resistance.
To view a Powerpoint® presentation on electricity please click here.