第11次课科技英语课件

Passage 11 Comprehensive praቤተ መጻሕፍቲ ባይዱtice
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If one starts getting technical about the meaning of electric current, one could argue that electric current is not something that flows past a point on a circuit, but rather something that exists to a given measure at a point on a circuit. TRUE - Technically speaking, current does not flow or move. Current is a quantity which is measured at a location and is based upon the rate at which charge flows. Teachers and textbooks often get sloppy with their language and say that the current flows. In reality, it is the charge that does the moving; the current is simply measured at a location to reveal the rate at which charge flows past that location.
Passage 11 Comprehensive practice
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The resistance of an electric circuit is a measure of the overall amount of hindrance to the flow of charge through the circuit. A large resistance value indicates that the charge is encountering a relatively large amount of difficulty in moving through the circuit. The unit of resistance is the ampere.
Passage 11 Comprehensive practice
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Suppose a miniature light bulb is connected to a battery in a circuit. A light bulb with a greater resistance will have a greater current. Wider conducting wires are capable of carrying larger currents.
Passage 11 Comprehensive practice
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The electric current in a circuit will increase as the electric potential impressed across a circuit is increased. The electric current in a circuit will triple in value as the electric potential impressed across a circuit is increased by a factor of three. TRUE - A higher electric potential impressed across the two terminals of a battery will cause the rate of charge flow to increase. This is consistent with the V = I•R equation. TRUE - Consistent with V = I•R, one could say that the current is directly proportional to the voltage. A tripling of the voltage (while R is held constant) would cause a tripling of the current.
Passage 11 Comprehensive practice
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Electric current is defined as the number of Coulombs of charge which move past a point on a circuit. Electric current is equal to the number of Coulombs of charge which move past a point on a circuit per unit of time.
Passage 11 Comprehensive practice
TRUE - The resistance of a resistor is the V/I ratio at that location. FALSE - The resistance of a resistor is the V/I ratio at that location. TRUE - The resistance of a wire is mathematically related to the resistively of the material, the length of the wire and the cross-sectional area of the wire. Longer wires have a greater resistance.
FALSE - A light bulb with a greater resistance will offer more hindrance to the flow of charge through it and as such lead to less current. Since I = V / R, the quantities current and resistance are inversely proportional. TRUE - A wider wire has a greater cross-sectional area and thus offers less resistance. As such, wider wires allow a greater current or flow rate.
Passage 11 Comprehensive practice
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A 5-ohm resistor would allow a current of 0.5 Amperes when 2.5 Volts is impressed across it. A 10-ohm resistor would allow a current of 2 Amperes when 5 Volts is impressed across it. The resistance of a conducting wire will increase as the length of the wire is increased.
TRUE - Remember this as the definition of resistance. TRUE - Resistance refers to the amount of hindrance (resistance) to the flow of charge. The more hindrance which charge encounters, the more resistance. FALSE - The unit of resistance is the ohm. Ampere or Amps is the unit of current.
Passage 11 Comprehensive practice
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Electric current provides a measure of how fast charge moves between two points on a circuit. The electric current diminishes in value as charge progresses to locations further and further from the + terminal of the battery. Charge is marching around a circuit at the same rate everywhere within the circuit. The number of Coulombs which exit the battery at the + terminal in a second is equal to the number of Coulombs which enter the battery at the - terminal in a second. Charge is not created; charge is not destroyed; and charge does not accumulate at a location in a circuit. Charge simply marches together around the loop of a circuit.
Passage 11 Comprehensive practice
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1 Which of the following statements are true about electric current? Electric current is measured in units of Amperes. Electric current and drift speed are synonymous terms. No! Current refers to how many Coulombs of charge pass a cross-sectional area in a wire in a second of time. Drift speed refers to the distance that a mobile charge carrier travels in a unit of time. One quantity - current has to do with how many? and the other quantity - drift speed - has to do with how far?
Current is a rate quantity much like velocity or power is a rate quantity. There is a per time basis to it. Current is defined as the Coulombs of charge which pass a point on a circuit per second of time.