Skip to main content
Ch 27: Current and Resistance
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 27, Problem 65b

You've decided to protect your house by placing a 5.0-m-tall iron lightning rod next to the house. The top is sharpened to a point and the bottom is in good contact with the ground. From your research, you've learned that lightning bolts can carry up to 50 kA of current and last up to 50 μs. You don't want the potential difference between the top and bottom of the lightning rod to exceed 100 V. What minimum diameter must the rod have?

Verified step by step guidance
1
Determine the resistance of the lightning rod using Ohm's Law. The potential difference (V) is related to the current (I) and resistance (R) by the formula: V=IR. Rearrange to find resistance: R=V/I. Substitute the given values: V=100 volts and I=50kA (convert to amperes: 50×10^3 A).
Relate the resistance of the rod to its material properties and geometry. The resistance of a cylindrical conductor is given by the formula: R=ρL/A, where ρ is the resistivity of the material, L is the length of the rod, and A is the cross-sectional area. Rearrange to solve for the cross-sectional area: A=ρL/R.
Substitute the known values into the formula for the cross-sectional area. The resistivity of iron is approximately 1.0×10^-7 Ωm, the length of the rod is 5.0 m, and the resistance is calculated from Step 1. Substitute these values into the formula for A.
Relate the cross-sectional area to the diameter of the rod. The cross-sectional area of a cylindrical rod is given by the formula: A=π×d^2/4, where d is the diameter. Rearrange to solve for the diameter: d=A×4/π. Substitute the value of A from Step 3.
Simplify the expression to find the minimum diameter of the rod. Ensure that all units are consistent and the result is expressed in meters. This will give the minimum diameter required to ensure the potential difference does not exceed 100 V.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electric Field and Potential Difference

The electric field is a region around charged particles where other charges experience a force. The potential difference, or voltage, between two points in an electric field is the work done to move a unit charge from one point to another. In the context of a lightning rod, the potential difference must be controlled to prevent excessive voltage that could lead to a dangerous discharge.
Recommended video:
Guided course
07:33
Electric Potential

Current and Resistance

Current is the flow of electric charge, measured in amperes (A), while resistance is the opposition to this flow, measured in ohms (Ω). According to Ohm's Law, the relationship between voltage (V), current (I), and resistance (R) is given by V = I × R. Understanding this relationship is crucial for determining the necessary diameter of the lightning rod to ensure that the resistance is low enough to keep the potential difference below the desired threshold.
Recommended video:
08:17
Microscopic View of Current

Lightning Rod Design and Function

A lightning rod is designed to protect structures from lightning strikes by providing a low-resistance path for the electrical discharge to follow. The rod's height, shape, and material influence its effectiveness. A sharpened point enhances the electric field strength at the tip, which helps to attract lightning, while the diameter must be sufficient to handle the high current without exceeding the safe voltage limit.
Recommended video:
Guided course
08:30
Intro to Wave Functions
Related Practice
Textbook Question

A 300 μF capacitor is charged to 9.0 V, then connected in parallel with a 5000 Ω resistor. The capacitor will discharge because the resistor provides a conducting pathway between the capacitor plates, but much more slowly than if the plates were connected by a wire. Let t=0 s be the instant the fully charged capacitor is first connected to the resistor. At what time has the capacitor voltage decreased by half, to 4.5 V?

1
views
Textbook Question

A 2.0-mm-diameter wire formed from a composite material has a resistivity that decreases with distance along the wire as ρ=ρ₀e−αx, where ρ₀=4.0×10−5 Ω m, x (in m) is measured from one end of the wire, and the constant α=4.0 m−1. What is the resistance of a 50-cm-long length of this wire?

2
views
Textbook Question

You've decided to protect your house by placing a 5.0-m-tall iron lightning rod next to the house. The top is sharpened to a point and the bottom is in good contact with the ground. From your research, you've learned that lightning bolts can carry up to 50 kA of current and last up to 50 μs. How much charge is delivered by a lightning bolt with these parameters?

1
views
Textbook Question

An aluminum wire consists of the three segments shown in FIGURE P27.64. The current in the top segment is 10 A. For each of these three segments, find the current density J. Place your results in a table for easy viewing.

2
views
Textbook Question

Two 10-cm-diameter metal plates 1.0 cm apart are charged to ±12.5 nC. They are suddenly connected together by a 0.224-mm-diameter copper wire stretched taut from the center of one plate to the center of the other. Does the current increase with time, decrease with time, or remain steady? Explain.

2
views
Textbook Question

An aluminum wire consists of the three segments shown in FIGURE P27.64. The current in the top segment is 10 A. For each of these three segments, find the drift velocity vd. Place your results in a table for easy viewing.

1
views