Physics 112 Homework 5(Ch18 & Ch19)

Ch18

  1. An ac voltage, whose peak value is 180 V, is across a resistor. What are the rms and peak currents in the resistor?

Solution

Find the peak current from Ohm’s law, and then find the rms current from the relationship between peak and rms values.

  1. The peak value of an alternating current in a 1500-W device is 5.4 A. What is the rms voltage across it?

Solution

  1. A 0.65-mm-diameter copper wire carries a tiny current of What is the electron drift speed in the wire?

Solution

We follow exactly the derivation in Example 18-14, which results in an expression for the drift velocity.

Ch19

  1. Calculate the terminal voltage for a battery with an internal resistance of and an emf of 8.50 V when the battery is connected in series with (a) an resistor, and (b) an resistor.

Solution

The current in the circuit is .

(a), or

(b)

  1. What is the internal resistance of a 12.0-V car battery whose terminal voltage drops to 8.4 V when the starter draws 75 A? What is the resistance of the starter?

Solution:Use equations forterminal voltage:

  1. Calculate the current in the circuit of thefigure below and show that the sum of all the voltage changes around the circuit is zero.

Physics 112 Homework 5(Ch18 & Ch19)

Solution:All of the resistors are in series, so the equivalent resistance is just the sum of the resistors. Use Ohm’s law then to find the current, and show all voltage changes starting at the negative pole of the battery and going counterclockwise.

Physics 112 Homework 5(Ch18 & Ch19)

  1. What is the potential difference between points a and d in the figure below (the same circuit as Fig. 19–13, Example 19–8 in the textbook), and (b) what is the terminal voltage of each battery?

Physics 112 Homework 5(Ch18 & Ch19)

Solution

From Example 19-8, we have .

If another significant figure had been kept, the values would be .

We use those results.

Physics 112 Homework 5(Ch18 & Ch19)

(a)To find the potential difference between points a and d, start at point a and add each individual potential difference until reaching point d. The simplest way to do this is along the top branch.

Slight differences will be obtained in the final answer depending on the branch used, due to rounding. For example, using the bottom branch, we get the following.

(b) For the 80-V battery, the terminal voltage is the potential difference from point g to point e. For the 45-V battery, the terminal voltage is the potential difference from point d to point b.

  1. The RC circuit has and The capacitor is at voltage at when the switch is closed. How long does it take the capacitor to discharge to 1.0% of its initial voltage?

Solution

The voltage of the discharging capacitor is given by . The capacitor voltage is to be .

  1. An ammeter whose internal resistance is reads 5.25 mA when connected in a circuit containing a battery and two resistors in series whose values are and What is the actual current when the ammeter is absent?

Solution

The total resistance with the ammeter present is . The voltage supplied by the battery is found from Ohm’s law to be . When the ammeter is removed, we assume that the battery voltage does not change. The equivalent resistance changes to , and the new current is again found from Ohm’s law.