An electromagnetic wave is transporting energy in the negative $z$ direction At a certain point and certain time the direction of electric field of the wave is along positive $y$ direction What will be the direction of the magnetic field of the wave at that point and instant?
- Positive direction of $z$
- Negative direction of $y$
- Positive direction of $x$
- Negative direction of $x$
The Correct Option is C
Solution and Explanation
1. The energy transport in an electromagnetic wave is given by the Poynting vector: \[ \vec{S} = \vec{E} \times \vec{H}, \] where \(\vec{E}\) is the electric field vector and \(\vec{H}\) is the magnetic field vector.
2. Given: - Energy transport (\(\vec{S}\)) is in the negative \(z\)-direction: \(\vec{S} = -\hat{k}\). - Electric field (\(\vec{E}\)) is in the positive \(y\)-direction: \(\vec{E} = +\hat{j}\).
3. Substituting into the cross-product: \[ \vec{S} = \vec{E} \times \vec{H} \implies -\hat{k} = (+\hat{j}) \times \vec{H}. \]
4. Solving for \(\vec{H}\): \[ \vec{H} = -\hat{i}. \]
Thus, the magnetic field is in the positive direction of \(x\).
In an electromagnetic wave, the directions of the electric field, magnetic field, and energy transport (Poynting vector) are mutually perpendicular, forming a right-handed coordinate system.
Top Questions on Electromagnetic waves
- In an electromagnetic wave travelling in free space, the amplitude of magnetic field is \( 6.0 \times 10^{-4} \ \text{T} \). The amplitude of its electric field is:
- CBSE CLASS XII - 2025
- Physics
- Electromagnetic waves
- Briefly explain how and where the displacement current exists during the charging of a capacitor.
- CBSE CLASS XII - 2025
- Physics
- Electromagnetic waves
- State any three characteristics of electromagnetic waves.
- CBSE CLASS XII - 2025
- Physics
- Electromagnetic waves
- The amplitude of electric field in an electromagnetic wave in free space is \( 1000 \, \text{Vm}^{-1} \). The amplitude of the magnetic field in this electromagnetic wave is:
- CBSE CLASS XII - 2025
- Physics
- Electromagnetic waves
- Which of the following electromagnetic waves has photons of largest momentum?
- CBSE CLASS XII - 2025
- Physics
- Electromagnetic waves
Questions Asked in JEE Main exam
- A force of 49 N acts tangentially at the highest point of a sphere (solid of mass 20 kg) kept on a rough horizontal plane. If the sphere rolls without slipping, then the acceleration of the center of the sphere is:
- JEE Main - 2025
- Quantum Mechanics
Let $ P_n = \alpha^n + \beta^n $, $ n \in \mathbb{N} $. If $ P_{10} = 123,\ P_9 = 76,\ P_8 = 47 $ and $ P_1 = 1 $, then the quadratic equation having roots $ \alpha $ and $ \frac{1}{\beta} $ is:
- JEE Main - 2025
- Relations and functions
- If \( \alpha + i\beta \) and \( \gamma + i\delta \) are the roots of the equation \( x^2 - (3-2i)x - (2i-2) = 0 \), \( i = \sqrt{-1} \), then \( \alpha\gamma + \beta\delta \) is equal to:
- JEE Main - 2025
- Complex numbers
- The sum of all local minimum values of the function \( f(x) \) as defined below is:
\[ f(x) = \begin{cases} 1 - 2x & \text{if } x < -1, \\[10pt] \frac{1}{3}(7 + 2|x|) & \text{if } -1 \leq x \leq 2, \\[10pt] \frac{11}{18}(x-4)(x-5) & \text{if } x > 2. \end{cases} \]- JEE Main - 2025
- Functions
- Let \( \langle a_n \rangle \) be a sequence such that \( a_0 = 0 \), \( a_1 = \frac{1}{2} \), and \( 2a_{n+2} = 5a_{n+1} - 3a_n \).n= 0,1,2,3.... Then \( \sum_{k=1}^{100} a_k \) is equal to:
- JEE Main - 2025
- Sequences and Series
Concepts Used:
Electromagnetic waves
The waves that are produced when an electric field comes into contact with a magnetic field are known as Electromagnetic Waves or EM waves. The constitution of an oscillating magnetic field and electric fields gives rise to electromagnetic waves.
Types of Electromagnetic Waves:
Electromagnetic waves can be grouped according to the direction of disturbance in them and according to the range of their frequency. Recall that a wave transfers energy from one point to another point in space. That means there are two things going on: the disturbance that defines a wave, and the propagation of wave. In this context the waves are grouped into the following two categories:
- Longitudinal waves: A wave is called a longitudinal wave when the disturbances in the wave are parallel to the direction of propagation of the wave. For example, sound waves are longitudinal waves because the change of pressure occurs parallel to the direction of wave propagation.
- Transverse waves: A wave is called a transverse wave when the disturbances in the wave are perpendicular (at right angles) to the direction of propagation of the wave.