Physics · Class 12 Science
Ch 8Electromagnetic Waves — Class 12 Physics, concept-first.
Every chapter so far in this Semester -- electrostatics, current electricity, the magnetic effects of current, and electromagnetic induction -- has treated electric and magnetic fields as, in a sense, separate phenomena that happen to influence one another: a current produces a magnetic field (Oersted), a changing magn…
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Displacement Current
Ampere's circuital law, in its original form, links the magnetic field around a closed loop to the conduction current (moving charges) threading that loop:
Most relevant Q&A
- What is displacement current? Why did Maxwell find it necessary to introduce this concept into Ampère's circuital law?Free
- What do you mean by displacement current? Write down its expression. (1+1) **OR** Write the names of electromagnetic radiation, which are us…Preview
- What do you mean by displacement current? Write down its mathematical expression. (1+1)Preview
In previous exams
How often this chapter’s concepts have been examined — real appearance data, never estimated.
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
Every chapter so far in this Semester -- electrostatics, current electricity, the magnetic effects of current, and electromagnetic induction -- has treated electric and magnetic fields as, in a sense,…
Displacement Current
The trouble with the old Ampère's law. Ampère's circuital law, as met in the chapter on Moving Charges and Magnetism, states that the line integral of the magnetic field around any closed loop equals…
Sources of Electromagnetic Waves
A stationary charge is not enough. A charge sitting perfectly still produces only a static electric field around it, unchanging in time -- no magnetic field, and certainly no wave.
Transverse Nature of Electromagnetic Waves
Working out the geometry from Maxwell's equations. Carrying Maxwell's equations through for a simple plane electromagnetic wave travelling in vacuum (a calculation WBCHSE does not require in algebraic…
Speed of Electromagnetic Waves
The result Maxwell obtained. Solving Maxwell's equations for a plane electromagnetic wave travelling through vacuum (again, WBCHSE requires only the RESULT of this calculation, not its derivation) giv…
Energy Carried by Electromagnetic Waves
Both fields carry energy. An electromagnetic wave is not merely a geometric pattern of oscillating field DIRECTIONS -- the electric field and the magnetic field that make up the wave each store real e…
Electromagnetic Spectrum
One family, one wavelength axis. Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays are, physically, exactly the same kind of disturbance discusse…
Summary
Displacement current is Maxwell's addition to Ampère's circuital law, needed because a changing electric flux (e.g.
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
+−Show 13 questionsHide questions13 questions
- Q1Mention two characteristics of electromagnetic waves.Preview
- Q2The speed of electromagnetic waves in vacuum is (a) √(ε0μ0) (b) 1/√(ε0μ0) (c) ε0μ0 (d) 1/(ε0μ0)Preview
- Q3Name any two electromagnetic waves. State any one similarity and one dissimilarity between them.Preview
- Q4How are electromagnetic waves produced? How are the magnitudes of the electric and magnetic fields related to velocity of the electromagneti…Preview
- Q5The electromagnetic radiation which can be produced by colliding fast moving electron on metal target, is (a) X-rays (b) γ-rays (c) infrared…Preview
- Q6The highest wavelength among the following electromagnetic waves is (a) Infrared ray (b) γ-ray (c) X-ray (d) UV-rayPreview
- Q7What do you mean by displacement current? Write down its expression. (1+1) **OR** Write the names of electromagnetic radiation, which are us…Preview
- Q8The energy of an electromagnetic wave is distributed along its electric and magnetic fields in the ratio (a) 1 : 1 (b) C² : 1 (c) √C : 1 (d)…Preview
- Q9How are infrared waves produced? Why are these waves referred to as heat waves? [1+1] **OR** A plane electromagnetic wave travels in vacuum…Preview
- Q10What do you mean by displacement current? Write down its mathematical expression. (1+1)Preview
- Q11The velocity of electromagnetic radiation in a medium of permittivity ε₀/2 and permeability 2μ₀ is given by (a) 2√(ε₀/μ₀) (b) 2√(μ₀ε₀) (c) 1…Preview
- Q12Match Column-I with Column-II and select the correct option : Column-I (Name of Electromagnetic wave): (i) Infrared waves; (ii) Microwaves;…Preview
- Q13The electromagnetic wave travels in free space along X-direction. At a particular point in space and time, B is given by 2·4 × 10⁻⁸ k̂ (in T…Preview
More questions
18 Q+−Show 6 questionsHide questions6 questions
- Example 1What is displacement current? Why did Maxwell find it necessary to introduce this concept into Ampère's circuital law?Free
- Example 2Using the example of a parallel-plate capacitor being charged, explain qualitatively how a magnetic field can exist at a point between the p…Free
- Example 3State, in qualitative terms, how Maxwell's equations predict the existence of electromagnetic waves that can travel through empty space with…Preview
- Example 4Explain why an electromagnetic wave is called a transverse wave. Describe the relative orientation of the electric field vector, the magneti…Preview
- Example 5Show that the quantity $1/\sqrt{\mu_0\epsilon_0}$ has the dimensions of speed, and state what this quantity represents physically for an ele…Preview
- Example 6State qualitatively what carries the energy in an electromagnetic wave as it travels through space, and name the physical quantity that desc…Preview
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- Q7Write the modified (Ampère-Maxwell) form of Ampère's circuital law, including the displacement current term, and explain in words the physic…Free
- Q8An accelerating (oscillating) electric charge is said to be the source of an electromagnetic wave. Explain, in your own words, how an oscill…Free
- Q9In a plane electromagnetic wave travelling along the $x$-axis, the electric field oscillates along the $y$-axis. State along which axis the…Free
- Q10Explain why an electromagnetic wave is able to travel through empty vacuum, unlike a mechanical wave such as sound, which needs a material m…Preview
- Q11Prepare a table listing, in order of increasing frequency, the seven major regions of the electromagnetic spectrum, together with one typica…Preview
- Q12Gamma rays carry enormously more energy per photon than radio waves, yet radio waves, not gamma rays, are used for long-distance broadcastin…Preview
- Q13Explain, with a reason, why the ionosphere reflects medium-wave (AM) radio signals back towards the Earth's surface, allowing them to be rec…Preview
- Q14X-rays and gamma rays both lie at the extreme high-frequency end of the electromagnetic spectrum but are produced in very different ways. Br…Preview
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- Q15Taking $\mu_0=4\pi\times10^{-7}\ \text{T m A}^{-1}$ and $\epsilon_0=8.85\times10^{-12}\ \text{C}^2\text{N}^{-1}\text{m}^{-2}$, calculate the…Free
- Q16A medium-wave (AM) radio transmitter broadcasts at a wavelength of $300\ \text{m}$. Calculate the frequency of this radio wave, taking the s…Free
- Q17An X-ray tube produces radiation of frequency $3\times10^{18}\ \text{Hz}$. Calculate the wavelength of this radiation in nanometres, and sta…Preview
- Q18A household microwave oven generates radiation of frequency $2.45\ \text{GHz}$. Calculate the wavelength of this radiation, and state, with…Preview