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CALC An airplane propeller of total length \(L\) rotates around its center with angular speed \(\omega\) in a magnetic field that is perpendicular to the plane of rotation. Modeling the propeller as a thin, uniform bar, find the potential difference between (a) the center and either end of the propeller and (b) the two ends. (c) If the field is the earth's field of 0.50 \(\mathrm{G}\) and the propeller turns at 220 \(\mathrm{rpm}\) and is 2.0 \(\mathrm{m}\) long, what is the potential difference between the middle and either end? It this large enough to be concerned about?

Short Answer

Expert verified
The potential difference between the center and either end is 0.288 mV, which is too small to be concerning.

Step by step solution

01

Understanding the Setup

The problem involves a rotating airplane propeller modeled as a thin, uniform bar in a magnetic field that is perpendicular to the plane of rotation. It requires using the principles of electromagnetic induction to find potential differences.
02

Determine the Potential Difference Between Center and End

The potential difference between the center of the propeller and its endpoint is given by the integral of the electromotive force (emf): \[ V = \int_0^{L/2} B \omega x \, dx = \frac{B \omega L^2}{8} \]where \( B \) is the magnetic field strength, \( \omega \) is the angular velocity, \( x \) is the distance to the center, and \( L \) is the length of the propeller.
03

Calculate Potential Difference Between Two Ends

Using the properties of emf in a rotating wire, the potential difference between the two ends is:\[ V = \int_{-L/2}^{L/2} B \omega x \, dx = 0 \]This is because the section from the midpoint to one end has an equal and opposite contribution from the midpoint to the other end, canceling each other out.
04

Substituting Given Values into Formula

Convert the magnetic field from gauss to tesla: \( 0.50 \, \text{G} = 0.50 \times 10^{-4} \, \text{T} \). Convert angular speed from rpm to rad/s: \[ 220 \, \mathrm{rpm} = \frac{220 \times 2 \pi}{60} \, \mathrm{rad/s} \approx 23.04 \, \mathrm{rad/s} \]Substitute into the formula for potential difference between the center and the end:\[ V = \frac{0.50 \times 10^{-4} \times 23.04 \times (2.0)^2}{8} = 2.88 \times 10^{-4} \, \text{V} \]
05

Interpreting Results

The calculated potential difference between the midpoint and either end is \( 2.88 \times 10^{-4} \, \text{V} \) (or 0.288 mV). This potential difference is very small and is not large enough to be a concern in practical applications.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Rotating Systems
In the context of electromagnetic induction, rotating systems can be fascinating to explore. Rotating systems, like our propeller, help us understand how movement in a magnetic field can produce electromotive force (emf). As the propeller spins, every part of it moves through the magnetic field at a different speed depending on its distance from the center of rotation. This creates potential differences, which are fascinating for both learning and practical applications.
When analyzing rotating systems, one must consider both the angular velocity and the geometry of the object. These factors determine how the speed varies along the length, which affects the potential difference induced along the propeller. In problems like these, it's key to model the rotating body accurately, recognizing that each segment of the system may contribute differently to the overall electromagnetic effects.
Magnetic Fields
Magnetic fields play a crucial role in electromagnetic induction. In our exercise, we deal with a magnetic field perpendicular to the rotation plane of the propeller. The magnetic field is usually characterized by its strength, given in units like tesla or gauss.
The interaction between the moving parts of the system and the magnetic field is what generates the potential difference. It's important to understand that the magnetic field's strength determines how much force it can exert on the electrons in the metal, inducing an electromotive force (emf) as they move across the field lines.
In practice, magnetic fields interact with moving conductors to produce electric currents, which is the essence of how generators work. Recognizing the effects of the Earth's magnetic field, as in the problem, highlights the omnipresence of these fields and their subtle influence on mechanical systems.
Potential Difference
Potential difference is a key concept in circuits and electromagnetism. In this problem, we explore the potential difference generated across different parts of a rotating system within a magnetic field. Potential difference, measured in volts, is the work done per charge unit to move charges from one point to another along a circuit's path.
For the rotating propeller, the potential difference between different parts results from the combination of the propeller's rotation speed, its length, and the magnetic field strength. By integrating the emf along specific parts of the propeller, we can calculate these potential differences.
It's fascinating how even such a simple system can demonstrate potential differences and reflect the larger principles that guide currents and voltages in all electrical circuits. Exploring these differences helps deepen our understanding of how electricity and magnetism intertwine.
Electromotive Force (emf)
Electromotive force (emf) is the term for the voltage created by any energy source in a circuit. Unlike static voltage, emf is induced by changing magnetic fields as conductors move through them. In our rotating propeller example, the emf depends on the magnetic field, the speed of rotation, and the propeller’s length.
The principle of emf in rotating systems can be expressed by Faraday's law of induction, which states that the induced emf in any closed circuit is equal to the rate of change of the magnetic flux through the circuit. In simpler terms, as parts of the propeller keep moving through the magnetic field, they "cut" through magnetic lines, inducing an emf.
Understanding emf requires recognizing the dynamic interactions between movement, geometry, and magnetic fields, helping us grasp how mechanical motion can be converted into electrical energy. This concept underpins the operation of most mechanical generators.

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Most popular questions from this chapter

As a new electrical engineer for the local power company, you are assigned the project of designing a generator of sinusoidal ac voltage with a maximum voltage of 120 \(\mathrm{V}\) . Besides plenty of wire, you have two strong magnets that can produce a constant uniform magnetic field of 1.5 T over a square area of 10.0 \(\mathrm{cm}\) on a side when they are 12.0 \(\mathrm{cm}\) apart. The basic design should consist of a square coil turning in the uniform magnetic field. To have an acceptable coil resistance, the coil can have at most 400 loops. What is the minimum rotation rate (in \(\mathrm{rpm} )\) of the coil so it will produce the required voltage?

CALC In Fig. 29.22 the capacitor plates have area 5.00 \(\mathrm{cm}^{2}\) and separation 2.00 \(\mathrm{mm}\) . The plates are in vacuum. The charging current \(i_{\mathrm{C}}\) has a constant value of 1.80 \(\mathrm{mA} .\) At \(t=0\) the charge on the plates is zero. (a) Calculate the charge on the plates, the electric field between the plates, and the potential difference between the plates when \(t=0.500 \mu \mathrm{s}\) (b) Calculate \(d E / d t\) , the time rate of change of the electric field between the plates. Does \(d E / d t\) vary in time? (c) Calculate the displacement current density \(j_{\mathrm{D}}\) between the plates, and from this the total displacement current \(i_{\mathrm{D}} .\) How do \(i_{\mathrm{C}}\) and \(i_{\mathrm{D}}\) compare?

A long, thin solenoid has 900 turns per meter and radius 2.50 \(\mathrm{cm} .\) The current in the solenoid is increasing at a uniform rate of 60.0 \(\mathrm{A} / \mathrm{s}\) . What is the magnitude of the induced electric field at a point near the center of the solenoid and (a) 0.500 \(\mathrm{cm}\) from the axis of the solenoid; (b) 1.00 \(\mathrm{cm}\) from the axis of the solenoid?

CALC A coil 4.00 \(\mathrm{cm}\) in radius, containing 500 turns, is placed in a uniform magnetic field that varies with time according to \(B=(0.0120 \mathrm{T} / \mathrm{s}) t+\left(3.00 \times 10^{-5} \mathrm{T} / \mathrm{s}^{4}\right) t^{4} .\) The coil is connected to a \(600-\Omega\) resistor, and its plane is perpendicular to the magnetic field. You can ignore the resistance of the coil. (a) Find the magnitude of the induced emf in the coil as a function of time. (b) What is the current in the resistor at time \(t=5.00 \mathrm{s?}\)

emf in a Bullet. At the equator, the earth's magnetic field is approximately horizontal, is directed toward the north, and has a value of \(8 \times 10^{-5} \mathrm{T}\) . (a) Estimate the emf induced between the top and bottom of a bullet shot horizontally at a target on the equator if the bullet is shot toward the east. Assume the bullet has a length of 1 \(\mathrm{cm}\) and a diameter of 0.4 \(\mathrm{cm}\) and is traveling at 300 \(\mathrm{m} / \mathrm{s} .\) Which is at higher potential: the top or bottom of the bullet? (b) What is the emf if the bullet travels south? (c) What is the emf induced between the front and back of the bullet for any horizontal velocity?

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