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| | Glossaryampere (amp) - the basic unit of current. It is abbreviated A. atom - the basic building block of all materials. An atom is made of a small, compact, central nucleus surrounded by electrons. Inside the nucleus are positively charged protons and uncharged neutrons.* Negatively charged electrons orbit the nucleus. Neutral atoms have equal numbers of protons and neutrons and have no overall charge. *All atoms have protons and neutrons except for hydrogen in its most common form. It has a proton but no neutron.
B capacitor - a device that stores a charge temporarily, then releases the charge when in a closed circuit. Thus, in a circuit without a power source, a charged capacitor acts as a temporary source. Capacitors are also called condensers. circuit - a closed path through which current flows. Parts of a circuit include a power source, conducting wire, various devices that have loads (resistance to current flow), and a switch or switches. Usually insulating materials (around the wire) and circuit protection devices (resistors and fuses or circuit breakers) are also found in circuits.
current - the rate of flow of charge through a circuit. Current is abbreviated I. The unit of current is the ampere, or amp, abbreviated A. An amp is one coulomb/second (C/s), which is equal to the movement of 6.24 x 1018 electrons/second. The definition of current arose from a guess made about the direction of movement of positive charge. It turned out that what actually happens is the movement of negative charge (electrons) in the opposite direction. Either way, the result is the same - electricity!
current in series circuits - all parts of a series circuit have the same current. Multiple devices connected in series share the current, thus each gets less current than if it were the only device in the circuit (each device in the circuit provides resistance to the flow of charge).
current in parallel circuits - the total current equals the sum of the individual branch currents.
Also, the sum of the currents entering a circuit junction (point where circuits intersect) equals the sum of the currents leaving the intersecting wires. This is known as Kirchhoff's current law. diode - a device that allows electricity to flow in only one direction. Diodes prevent the flow of current when hooked up to a circuit in the wrong direction. dry cell - a portable power source similar to a battery,
but with a paste rather than a liquid between the two metal plates of the
dry cell. The D, C, AA, and AAA "batteries" with which we are
familiar are actually combinations of 1.5 V dry cells. Voltages greater
than 1.5 V are gained by wiring the cells in series. Wiring dry cells in
parallel does not change the voltage, but it can result in a greater number
of electrons available to flow and longer battery life.
electricity - the flow of electrons through a circuit. Electricity is the same as the flow of current or the flow of charge. Each electron (represented by e-) has a charge of -1. valence electrons - the electrons in an atom's outermost electron orbits (referred to as the valence shell). They can gain energy and escape to be free electrons, or they can join other atoms to complete their valence number of electrons (the most stable number of electrons in an orbit). Negatively charged electrons are attracted to positive charges. The closer to a nucleus, the stronger the attraction. When electrons are freed, an atom becomes a positively charged ion. If an atom accepts one or more extra electrons, it becomes a negatively charged ion. Positively charged ions attract negatively charged electrons. Negatively charged ions repel negatively charged electrons. To learn more, see the background section on atoms and electricity.
electron flow - is from an area of lower voltage to an area of higher voltage. The difference in electron potentials between the two materials connected to the battery terminals inside the battery is the voltage of the battery. It is this difference between the metals that allows electricity (the flow of electrons) to happen. The greater the potential difference, the greater the voltage. You might think of voltage as a force that pushes the electrons through a circuit.
energy - the ability to do work, or the amount of work done. It is abbreviated E. Energy = Power x Time. Units of energy include joules (J), watt·seconds (Ws), kilowatt·hours (kWh), calories, and British termal units (BTU). Kilowatt·hours are the units used to measure energy consumption in homes. Watt·seconds (Ws), which equal joules (J), can be converted to kilowatt·hours (kWh) as follows:
Thus, dividing watt·seconds or joules by 3,600,000 will give the kWh unit used by power companies to charge us for energy usage. That's right, power companies charge us for energy, not power! fuse - a device used to limit the amount of current flowing through a circuit. Fuses consist of conducting materials that heat and break at defined current values. Fuses are used to protect devices wired into circuits from receiving too much electricity (such as might happen during a power surge or a short circuit). When the fuse breaks, the circuit opens, and all devices wired to the circuit turn off. joule - the System Internacional, or SI, unit of energy. The joule is abbreviated J. A joule is equal to one watt·second (1 J = 1 Ws). kilo - 1,000 of whatever comes after it. It is abbreviated k. For example, 57 kilovolts (57 kV) is 57,000 volts. (In other words, put a comma and three zeros at the end of the number to convert from kilovolts to volts.) light emitting diode - a lamp that gives off light of a particular wavelength when current runs through it, thus converting electrical energy to light energy. It is abbreviated LED. Each wavelength produces a characteristic color. Red LEDs are commonly used in electric devices to show that the device is turned on. load - the amount by which a device resists the flow of electric current. Lightbulbs, motors, fans, buzzers, resistors, etc., all have loads. Wires and batteries are not considered to have loads (although they have very small resistances). measuring
"relative" voltage, current, and resistance without using a meter.
measuring
voltage, current, and resistance using a meter. The diagrams below show correct positions for connecting meters to measure voltage, current, and resistance.
To learn more about meters, try the hands-on activities, Using Meters, Advanced Meter Measurements, and Using CBLs and Probes to Measure Voltage and Current. milli - 1/1000 of whatever comes after it. It is abbreviated m. For example, 480 milliamps (480 mA) is 0.480 amps. (In other words, move the decimal three places to the left to convert from milliamps to amps.) multimeter - a meter that measures current, voltage, and resistance.
ohm - a unit of resistance. The ohm is abbreviated Ω, the capitalized Greek letter omega. Ohm's law - the law that relates voltage, current, and resistance in conductors. Ohm's law is expressed as : Voltage = Current x Resistance, or Resistance = Voltage / Current, or Current = Voltage / Resistance
I is used for current because current was originally referred to as electrical intensity. periodic table of elements - arrangement of elements into a table organized by atomic number and also by columns or families by numbers of valence electrons. To learn more, see the background section, Atoms and Electricity.
photovoltaic cell - a solar cell. It converts light energy directly into electric energy (current). power - the rate at which work is done or energy is transformed. Power is abbreviated P. One unit of power is the watt (abbreviated W). Another unit of power is the joule/second (abbreviated J/s). The initial unit for power was the horsepower, abbreviated HP. 1 horsepower = 746 watts.
resistance - the opposition to the flow of electrons, with the conversion of some electrical energy into heat energy (heat). The unit of resistance is the ohm, abbreviated Ω, which is the capitalized Greek letter, omega (the last letter of the Greek alphabet). Resistance in objects like motors is called "impedance" because the device impedes (interferes with) the flow of electric current. Whether you call it resistance or impedance, the result is the same! Resistance of conductors is affected by four factors in particular: cross -sectional area of wire, length of wire, temperature, and resistivity (specific resistance) of the material. Resistance of a material is directly proportional to its length (as length increases, resistance increases). Resistance is inversely proportional to cross-sectional area (as the thickness of a wire increases, the resistance decreases). Thus, short and fat wires have less resistance than long and thin wires. A practical application for this is to use thick wires to move large currents. Thus power plants are more efficient if kept close to their users.
total resistance (RT) in parallel circuits - calculated by using the reciprocal formula. resistor - a device that protects an electrical device by controlling the amount of current reaching the device. Resistors decrease how much current passes through the circuit so that the electricity won't damage electronic devices. In the process some of the electrical energy is converted to heat energy. The position and color of the lines on the resistor represent the value of resistance in ohms (Ω).
schematic
diagram - a sketch using various symbols and lines to represent different
devices in circuits.
semiconductor - a device or material that allows the flow of current but offers a higher resistance than a conductor. solar array - a collection of solar panels. The eight solar arrays on the International Space Station consist of more than 250,000 solar cells. solar cell - a silicon surface that converts light energy directly into electricity. Solar cells are also known as photovoltaic (photo + voltage) cells. When exposed to light, solar cells are effective power sources. They do not provide power when in the dark. However, while in the light, power can be saved by wiring solar cells to rechargeable batteries. Solar cells provide only direct current (dc) voltage. That means the current always flows in only one direction (as opposed to ac, or alternating current, which continuously changes direction—that is how electricity is delivered to most homes). solar panel - a collection of solar cells. These panels come in various sizes and are used on rooftops and even cartops to convert light energy into electricity. static electricity - the existence of charges that don't flow characteristically. It is high voltage electricity. Almost everyone has experienced this type of electricity. When the weather is dry and your hairs fly apart from each other, when you touch a door handle and you get a shock, or when lightning strikes, it's because of static electricity. Static electricity tends to be uncontrollable. It is the result of charges building up so high at two locations that the large potential difference results in a jump of charge from one object to another. switch - a wired device that will either open (turn OFF) or close (turn ON) a circuit, depending on the switch position.
time - a necessary variable to measure for many scientific studies. We can't see time, but we can measure it as it passes. Time is abbreviated t. The base unit for time is the second (abbreviated s), but minutes (min), hours (h), and days (d) are commonly used units. unit - the word following a number that lets us know what the number represents. For example, in the expression 5 feet, the value is 5 and the unit is feet. In the expression 10 volts, the value is 10 and the unit is volts. valence electrons - the electrons in an atom's outermost electron orbits (referred to as the valence shell). They can gain energy and escape to be free electrons, or they can join other atoms to complete their valence number of electrons (the most stable number of electrons in an orbit). Negatively charged electrons are attracted to positive charges. The closer to a nucleus, the stronger the attraction. When electrons are freed, an atom becomes a positively charged ion. If an atom accepts one or more extra electrons, it becomes a negatively charged ion. Positively charged ions attract negatively charged electrons. Negatively charged ions repel negatively charged electrons. To learn more, see the background section on atoms and electricity.
voltage - an electrical potential difference between the two metals in a dry cell or battery power source. This potential difference forces electrons to flow. Voltage is abbreviated V. Voltage can be described as the force that pushes electrons. The size of the power source determines how many electrons (and thus how much current) is available to flow. The unit of voltage is the volt. voltages in series circuits - In a series circuit multiple devices share the total voltage from the power source (such as a battery), thus each gets less voltage than if it were the only device in the circuit. Total voltage (VT) in a series circuit is calculated as follows:
This is known as Kirchhoff's voltage law. It is named after the German physicist, Gustav R. Kirchhoff. voltages in parallel circuits - In a parallel circuit each circuit branch is independent of the other. Thus, each branch receives the total voltage of the source. Total voltage (VT) in a parallel circuit equals the voltage of each circuit branch:
watt - the unit of power. It is abbreviated W.
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