When two ends of an electric conductor are at different electric potentials, a charge will flow from the high end of the conductor to the low end. This current will flow as long as there is a potential difference. Electrical flow is measured in amperes. One ampere is equal to one coulomb of charge passing a fixed point in one second. To sustain a current in a conductor, it is necessary to have a pumping device to maintain a difference in electric potential-a voltage. Consider two metal spheres. If one is charged positively and the other negatively, a large potential difference can be made between them. However, they will not make a good electric pump, because if a conductor is placed between them, a current will flow between them only for the brief moment necessary to equalize the potential difference between the spheres. The spheres have no way to maintain their potential difference. A battery does. A chemical battery dissolves zinc in acid and converts energy in chemical bonds to electrical potential energy. A battery can maintain a voltage (difference in electric potential), and thus can maintain a current. Note that it is not voltage that flows through a conductor, but charge caused by voltage.

The amount of current that flows through a conductor depends not only on the voltage, but also on the electrical resistance of the conductor. The electrical resistance of a conductor is how much the conductor resists the flow of a charge. Electrical resistance is measured in units called ohms, after George Ohm. George Ohm discovered this relationship between current, voltage, and resistance called ohm's law: Current in a circuit is proportional to the voltage across the circuit and inversely proportional to the resistance in the circuit. Current=voltage/resistance. So one ohm is equal to one volt of voltage/one ampere of current. Note: The units of volts are used for two different things. It is used for the amount of electrical potential energy, and it is also used for voltage. Remember that voltage is the difference in electric potential, not electrical potential energy.

Electrical shock is caused by an electric current passing through the human body. The human body has a resistance of normally about 100,000 ohms. Moisture on the human body drastically reduces electrical resistance of the human body, sometimes to less than 100 ohms. A voltage that might not cause a harmful current in a dry person could provide a fatal current to a moist human. Remember, for a current to pass through something there must be a difference in electric potential on one end to the other. Birds can stand on electrical wires because both of their feet are on the same wire. So there is no electric potential difference between the two feet, so no current will flow. IF the bird had one foot on one wire and the other on a different wire, current would flow and the bird would be electrocuted.

Electric current can flow either as direct current(dc) or alternating current(ac). In direct current, charge flows only in one direction a battery will produce a direct current, because charge will always flow from the negative end to the positive end. In an alternating current, charge moves in different directions. Electrons move back and forth about fixed positions. Both alternating current and direct current have the same purpose: to transfer energy from one place to another. Alternating current can be changed to direct current by using a diode. A diode is a device that acts as a one way valve for electrons. An alternating current passing through a diode comes out as a direct current.

Flip a light switch, and the light turns on immediately. In fact, the electric signal travels through the circuit at the speed of light. However, it is not the electrons in the circuit that move at the speed of light. It is the electric field that moves at the speed of light. When two terminals of a battery are connected by a conductor, the electric field lines produced are "directed" by the conductor form one terminal to the other. If the current is dc, electrons in the conductor are accelerated by the electric field in one direction. These electrons almost immediately bump into other molecules, lowering their speed. So the electrons are actually moving very slowly. Electric power is not transmitted by electrons bumping into each other. How is it produced? Remember, electric field lines extend indefinitely. If our batter y is attached to a light, the electric field will accelerate the electrons already in the light filament! This makes the light filament heat up and shine. This makes sense. Consider an ac current. The electrons in an ac current are moving back and forth across fixed positions. They are not moving in a certain direction at all. In an ac current, no net migration of the electrons takes place. Instead, the electrons are vibrated by an electric field.

A charge moving in a circuit that is not composed of a superconductor expends some energy. The rate at which electrical energy is converted into another form (heat energy, light energy, mechanical energy) is called electric power. Electric power is equal to the product of current and voltage, and is measured in Watts. Power=current times voltage.

Any path along which electrons can flow is called a circuit. For a continuous flow of electrons, the circuit must have no gaps. Most circuits used by us contain more than one device that gathers electricity from the circuit. These devices can be connected to the circuit in series or in parallel. In a series circuit, electric current has only one pathway through the circuit, so the current passing through the different devices is the same. This means that if one electrical device "burns out" and breaks the circuit, all the other devices will cease to work. It also means that each device only receives a fraction of the total voltage, so if another device is added to the circuit, all the original devices will receive less voltage. In a parallel circuit, each electrical device is attached to the same two points. This means that the electrical current received by each device is the same, so adding more devices will not take energy away from the others. It also means that if one device burns out, the other devices will remain unaffected. A danger of parallel circuits is overloading. If too many devices are connected to the same circuit, the circuit may overheat and cause a fire. This can be prevented by the use of safety fuses. Safety fuses are designed to melt when the current passing through them reaches a certain point when the fuse melts, the circuit is broken and no electricity can pass through it, so no fire can occur.