<< Chapter < Page Chapter >> Page >

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?

  1. The number of electrons leaving the battery will be equal to the number of electrons entering the battery.
  2. The number of electrons leaving the battery will be less than the number of electrons entering the battery.
  3. The number of protons leaving the battery will be equal to the number of protons entering the battery.
  4. The number of protons leaving the battery will be less than the number of protons entering the battery.

(a)

Got questions? Get instant answers now!

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?

  1. 0.5 C
  2. 1 C
  3. 1.25 C
  4. 1.5 C
Got questions? Get instant answers now!

If 0.625 × 10 20 electrons flow through a circuit each second, what is the current in the circuit?

10 A

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!

Section summary

  • Electric current I size 12{I } {} is the rate at which charge flows, given by
    I = Δ Q Δ t ,
    where Δ Q is the amount of charge passing through an area in time Δ t .
  • 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 1 A = 1 C/s. size 12{1" A "=" 1 C/s."} {}
  • Current is the flow of free charges, such as electrons and ions.
  • Drift velocity v d size 12{v rSub { size 8{d} } } {} is the average speed at which these charges move.
  • Current I size 12{I } {} is proportional to drift velocity v d size 12{v rSub { size 8{d} } } {} , as expressed in the relationship I = nqAv d size 12{I = ital "nqAv" rSub { size 8{d} } } {} . Here, I size 12{I } {} is the current through a wire of cross-sectional area A size 12{A} {} . The wire's material has a free-charge density n size 12{n} {} , and each carrier has charge q size 12{q} {} and a drift velocity v d size 12{v rSub { size 8{d} } } {} .
  • Electrical signals travel at speeds about 10 12 size 12{"10" rSup { size 8{"12"} } } {} times greater than the drift velocity of free electrons.

Conceptual questions

Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.

Got questions? Get instant answers now!

Car batteries are rated in ampere-hours ( A h size 12{A cdot h} {} ). To what physical quantity do ampere-hours correspond (voltage, charge, . . .), and what relationship do ampere-hours have to energy content?

Got questions? Get instant answers now!

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 v d = I nqA size 12{v rSub { size 8{d} } = { {I} over { ital "nqA"} } } {} , by considering how the density of charge carriers n size 12{n} {} relates to whether or not a material is a good conductor.

Got questions? Get instant answers now!

Why are two conducting paths from a voltage source to an electrical device needed to operate the device?

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!
Practice Key Terms 3

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics for ap® courses' conversation and receive update notifications?

Ask