Questions and Answers
Who was the first person to broadcast a message across the English Channel?
Guglielmo Marconi
In what year did Maxwell propose the electromagnetic theory of light?
1873
What is the relationship between electric and magnetic fields in a propagating electromagnetic wave?
The effect associated with charge movement can travel faster than the speed of light.
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The strength of a Hertzian dipole is known as the dipole moment p and is given by p = ______
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What is the speed of light denoted as in the context of electromagnetic waves?
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What does the Poynting vector represent?
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In terms of a Hertzian electric dipole, how do the charges behave in each cycle?
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What is the general equation for electromagnetic waves given in the lecture?
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Study Notes
Historical Background
- James Clerk Maxwell proposed the electromagnetic theory of light in 1873, revolutionizing physics.
- Guglielmo Marconi broadcasted the first message across the English Channel in 1899, pioneering long-distance radio communication.
- Heinrich Hertz demonstrated short-range radio links in 1887, laying the groundwork for later advancements in radio technology.
- Marconi's innovations earned him the Nobel Prize in Physics in 1909.
Electromagnetic Waves
- Electromagnetic waves are solutions to Maxwell’s equations, with electric (E) and magnetic (B) fields that vary in both space and time.
- In free space, E and B fields are perpendicular to each other and the direction of wave propagation.
- The wave equation is represented as kz = ωt + nπ (where n = ±0,1,2,3…).
Creation of Electromagnetic Waves
- Movement of electric charge creates an electric field that cannot change instantaneously; changes propagate at the speed of light (c = 3 x 10^8 m/s).
- Oscillating charge movements generate oscillating electromagnetic waves, while a single sharp movement results in a pulse.
Electric Dipoles
- A Hertzian electric dipole is created by alternating positive and negative charges in an electrical circuit, such as two charged spheres connected to an AC generator.
- The maximum charge on a sphere (q) and the distance between spheres (L) give rise to a dipole moment (p), defined as p = qL.
- Dipole antennas radiate electromagnetic radiation perpendicular to current flow.
Poynting Vector and Radiated Power
- The Poynting Vector (S) describes the power density of electromagnetic waves and is calculated as S = E x H, with units of Watts/m².
- Key equations for radiated electric and magnetic fields of a short dipole antenna include:
- Eθ (radiation) = (sin θ d²p) / (4πε₀rc²)
- Bφ (radiation) = (μ₀ sin θ d²p) / (4πrc²)
- These equations illustrate how the electric field and magnetic field strength relate to the distance and dipole moment.
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Description
This quiz covers Lecture 10 of the Antennae section in Electromagnetism (Engr. 227). It discusses key historical developments, including Maxwell's electromagnetic theory and Marconi's landmark broadcasts. Test your understanding of these foundational concepts in electromagnetism.
