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Test prep for ap courses
Which of the following can be explained on the basis of conservation of charge in a closed circuit consisting of a battery, resistor, and metal wires?
- The number of electrons leaving the battery will be equal to the number of electrons entering the battery.
- The number of electrons leaving the battery will be less than the number of electrons entering the battery.
- The number of protons leaving the battery will be equal to the number of protons entering the battery.
- The number of protons leaving the battery will be less than the number of protons entering the battery.
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When a battery is connected to a bulb, there is 2.5 A of current in the circuit. What amount of charge will flow though the circuit in a time of 0.5 s?
- 0.5 C
- 1 C
- 1.25 C
- 1.5 C
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Two students calculate the charge flowing through a circuit. The first student concludes that 300 C of charge flows in 1 minute. The second student concludes that 3.125 × 10
19 electrons flow per second. If the current measured in the circuit is 5 A, which of the two students (if any) have performed the calculations correctly?
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Section summary
- Electric current
is the rate at which charge flows, given by
where
is the amount of charge passing through an area in time
.
- The direction of conventional current is taken as the direction in which positive charge moves.
- The SI unit for current is the ampere (A), where
- Current is the flow of free charges, such as electrons and ions.
- Drift velocity
is the average speed at which these charges move.
- Current
is proportional to drift velocity
, as expressed in the relationship
. Here,
is the current through a wire of cross-sectional area
. The wire's material has a free-charge density
, and each carrier has charge
and a drift velocity
.
- Electrical signals travel at speeds about
times greater than the drift velocity of free electrons.
Conceptual questions
Car batteries are rated in ampere-hours (
). To what physical quantity do ampere-hours correspond (voltage, charge, . . .), and what relationship do ampere-hours have to energy content?
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If two different wires having identical cross-sectional areas carry the same current, will the drift velocity be higher or lower in the better conductor? Explain in terms of the equation
, by considering how the density of charge carriers
relates to whether or not a material is a good conductor.
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In cars, one battery terminal is connected to the metal body. How does this allow a single wire to supply current to electrical devices rather than two wires?
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Why isn't a bird sitting on a high-voltage power line electrocuted? Contrast this with the situation in which a large bird hits two wires simultaneously with its wings.
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Source:
OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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