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circuit diagramELECTRICITY: ELECTRICAL CIRCUIT: simplified animation of the movement of electrons (the opposite of conventional current) in a simple electric circuit consisting of a battery, a switch and a light bulb.

ELECTRIC CURRENT is the flow of charge. Conventionally this is the flow of positive charge. However, in a simple circuit such as that illustrated the current in the wire is composed of electrons that flow from the negative pole of the battery (the cathode at the bottom of the battery) and return to the positive pole (the anode at the top of the battery, marked by a +). The diagram at the left shows the same circuit as the one in the animation but in a standardised diagrammatic form (i.e. it's a circuit diagram). The two parallel lines of different lengths marked battery actually represent a single cell. Single cells supply 1.5 volts and are what we generally mean when we say a battery (e.g. "I must change the batteries in my torch"). Technically, a battery is a series of single cells joined together to provide a greater voltage in, for example, a car battery. The "battery" in the animation is a single cell.

The flow of electrons is opposite to the direction of the conventional current. The battery provides the electromotive force (or e.m.f.) that "pushes" the electrons through the wires of the circuit. Electromotive force is measured in volts. In some ways it is similar to the potential energy stored in an object at the top of a hill. The object might roll down the hill and lose its potential energy and, in an analogous way, the electrons flow down the voltage drop (or potential difference) as they traverse the circuit.

NOTES: as the switch is turned on (about a second into the animation, the circuit is completed (closed) and current starts to flow. This makes the bulb light up. In this animation, it is the flow of electrons that is illustrated. Since this animation is designed to help you visualise a real process I thought it better to visualise the actual direction of flow of the charges. These electrons belong to the conduction band in the metal. The conduction band is an energy level in which the outermost electrons around the metal atoms are not fixed to a specific atom but are free to move around the metal. When the circuit is closed, by throwing the switch, the battery forces those electrons to flow around the wire, thereby creating the current.

SI UNITS used in electricity:
  • VOLTS (V): unit of potential difference, emf, or voltage
  • OHM (Ω): unit of resistance
  • AMPS (AMPERES) (A): unit of current
  • COULOMBS (C): unit of charge (= the charge moved when one amp of current runs for one second).
  • WATTS (W): unit of power (power energy per unit time). In electrical circuits, one watt is produced when a current of one amp flows down a potential difference of one volt.
  • JOULE (J): unit of energy.
these units are all related to each other by simple rules. For example, if the potential difference is 1 volt and the resistance is 1 ohm then the current that will flow will be 1 amp. One watt is one joule per second. One amp is one coulomb per second. All things being equal, if you double the voltage the current will double. If you halve the voltage the current will halve. If you double the resistance the current will halve. If you halve the resistance the current will double.


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