26 - Cabrillo College

Chapter 26: Current & Resistance
Example Questions & Problems
i   J  dA
J  (ne)v d
V  iR
P  iV  i2R 
E  J
Questions 26
 Why do electric power companies reduce voltage during times of heavy demand?
What is being saved? Explain.
 The textbook states that good thermal conductors are also good electrical
conductors. If so, why don’t the cords used to connect toasters, irons, and similar
heat-producing appliances get hot by conduction of heat from the heating element?
 Is there an electric field in the space around an ordinary flashlight battery if it is not
connected to anything? Explain.
Example 26.1
An 18-gauge copper wire (the size usually used by lamp cords) has a diameter of 1.02
mm. This wire carries a constant current of 1.67 A to a 200-watt lamp. The density of
free electrons is 8.5 × 1028 electrons/m3. Find magnitudes of the (a) current density and
(b) drift velocity.
Example 26.2
A wire 4.00 m long and 6.00 mm in diameter has a resistance of 15.0 m. A potential
difference of 23.0 V is applied between the ends. (a) What is the current in the wire? (b)
What is the magnitude of the current density? (c) Calculate the resistivity of the wire
material. (d) Using Table 26-1, identify the material.
Example 26.3
The power rating of a light bulb is the power it dissipates when connected across a 120V potential difference. What is the resistance of (a) a 100-W and (b) 60-W bulb? (c) How
much current does each bulb draw in normal use? (d) Car headlights are either set at a
low or high beam; is there more or less resistance in the high-beam filament?
Example 26.4
a. Find the rates of (i) energy conversion (chemical to electrical) and (ii) energy
dissipation in the battery, (iii) the rate of energy dissipation in the 4Ω resistor, and (iv)
the battery’s net power output.
b. For the short-circuit situation (set R = 0), find the rates of energy conversion and
energy dissipation in the battery and the net power output of the battery.