Introductory Physics Volume Two

74 Circuits 3.10
Because the R 3 = 4Ω resistor in connected in series with the parallel
combination of R 1 and R 2 , the current through it is I 123 :
−→ I 3 = 1.2A
§ 3.10 Homework
⊲ Problem 3.8
In the Bohr model of the hydrogen atom, an electron in the lowest
energy state follows a circular path, 5.29 × 10 −11 m from the proton.
Show that the speed of the electron is 2.19×10 6 m s
. What is the effective
current associated with this orbiting electron?
⊲ Problem 3.9
In a particular cathode ray tube, the measured beam current is 20µA.
How many electrons strike the tube screen every 1 30 seconds?
⊲ Problem 3.10
The quantity of charge q (in coulombs) passing through a surface of
area 2.0cm 2 varies with time as q = 4t 3 + 5t + 6 where t is in seconds.
What is the instantaneous current through the surface at t = 1.0s ?
What is the value of the current density?
⊲ Problem 3.11
An electric current is given by I(t) = 100.0 sin(120πt), where I is in
amperes and t is in seconds. What is the total charge carried by the
current from t = 0 to t = 1/240s ?
⊲ Problem 3.12
Calculate the average drift speed of electrons traveling through a copper
wire with a cross-sectional area of 1.00mm 2 when carrying a current
of 1.00A. It is known that about one electron per atom of copper
contributes to the current. The atomic weight of copper is 63.54 and
its density is 8.92g/cm 3 .
⊲ Problem 3.13
Eighteen gauge wire has a diameter of 1.024mm. Calculate the resistance
of 15.0m of 18-gauge copper wire at 20.0 ◦ C.
⊲ Problem 3.14
Suppose that a voltage surge produces 140V for a moment. By what
percentatge will the ouput of a 120V, 100W lightbulb increase, assuming
its resistance does not change?
⊲ Problem 3.15
Two cylindrical copper wires have the same mass. Wire A is twice as
long as wire B. What is the ratio of their resistances?