Direct Current and Transient Analysis 209
The following relations may be of help:
1 Btu = 1055 J = 778.2 ft · lbf
1 J = 0.737 ft · lbf
1 N = 0.225 lbf
1 in. = 2.54 cm
1 Hp = 746 W
1 ft = 0.3048 m
1 Btu = 252 Cal
1 Hp = 550 ft ∙ lbf/s = 33,000 ft ∙ lbf/m i n = 0.07067 Btu/s
1 kW/h = 3.6(106) J = 3.414 Btu/h
1 Cal = 3.088 ft ∙ lbf
1 C = 6.24 * 1018 electrons
P.2.2 Two electric charges given by Q 1 = + 500 μC and Q 2 = − 600 μC experience a force of
1500 N. Determine the distance separating the charges and the direction of the
force (attraction or repulsion).
P.2.3 An electric current I consists of 25 * 1016 electrons/s. Express I in amperes (A), mil-
liampere (mA), and microampere (μA).
P.2.4 An energy E of 650 J is required to move 12 C from point A to point B. Determine
the potential difference between the points (A and B).
P.2.5 Obtain tables and plots of the force of attraction versus the separation (force versus
distance) of two charges Q 1 = +1.500 μC and Q 2 = − 600 μC, where the separation
distance r is over the range 0.5 m ≤ r ≤ 5 m, in intervals of 0.25 m, where the vari-
ables are evaluated using the following units:
a. Newtons versus meters
b. Pound force versus inches
c. Newtons versus foot
d. Pound force versus centimeter
P.2.6 The current through a 2.7 kΩ resistor R is 3 mA. Determine the current direction
and the polarity of the voltage VR across the resistor.
P.2.7 A light bulb is rated 100 W at 120 V. Find the current through the bulb and calculate
its resistance.
P.2.8 If the cost of electrical energy is $0.09 kW/h, what would be the cost of using a
100 W bulb during the following time periods?
a. One day
b. One week
c. One month
d. One year
P.2.9 Convert the MATLAB programs of Examples 2.2 and 2.3 into script fi les.
P.2.10 Determine the equivalent resistances of each of the networks shown in Figure 2.109
by hand and by using MATLAB.