Do magnetic forces obey Newton's third law? Verify for two current elements $\overrightarrow{dl_1} = dl(\hat{i})$ located at the origin and $\overrightarrow{dl_2} = dl(\hat{j})$ located at $(0, R, 0)$. Both carry current $I$.

Vedclass pdf generator app on play store
Vedclass iOS app on app store
(N/A) According to the Biot-Savart law,the magnetic field $d\vec{B}$ produced by a current element $I\overrightarrow{dl}$ at a position vector $\vec{r}$ is given by $d\vec{B} = \frac{\mu_0}{4\pi} \frac{I\overrightarrow{dl} \times \vec{r}}{r^3}$.
$1$. Magnetic field at the position of element $1$ due to element $2$:
Element $2$ is at $(0, R, 0)$ with $\overrightarrow{dl_2} = dl\hat{j}$. The position vector of element $1$ (at origin) relative to element $2$ is $\vec{r}_{12} = -R\hat{j}$.
Since $\overrightarrow{dl_2} \times \vec{r}_{12} = (dl\hat{j}) \times (-R\hat{j}) = 0$,the magnetic field $\vec{B}_2$ at the origin is $0$. Thus,the force $\vec{F}_{12} = I\overrightarrow{dl_1} \times \vec{B}_2 = 0$.
$2$. Magnetic field at the position of element $2$ due to element $1$:
Element $1$ is at $(0, 0, 0)$ with $\overrightarrow{dl_1} = dl\hat{i}$. The position vector of element $2$ relative to element $1$ is $\vec{r}_{21} = R\hat{j}$.
The magnetic field $\vec{B}_1$ at $(0, R, 0)$ is $\frac{\mu_0}{4\pi} \frac{I(dl\hat{i}) \times (R\hat{j})}{R^3} = \frac{\mu_0 I dl}{4\pi R^2} \hat{k}$.
The force on element $2$ is $\vec{F}_{21} = I\overrightarrow{dl_2} \times \vec{B}_1 = I(dl\hat{j}) \times (B_1\hat{k}) = I dl B_1 \hat{i}$.
Since $\vec{F}_{12} = 0$ but $\vec{F}_{21} \neq 0$,magnetic forces between current elements do not obey Newton's third law in the strong sense (action-reaction pairs are not equal and opposite).

Explore More

Similar Questions

Two circular coils $P$ and $Q$ are made from two identical wires of the same length. The number of turns in coils $P$ and $Q$ are $4$ and $2$,respectively. The magnetic inductions at the centers of $P$ and $Q$ are $B_P$ and $B_Q$,respectively. The ratio $\frac{B_P}{B_Q}$ is

An element $\overrightarrow{\Delta \ell} = \Delta x \hat{i}$ is placed at the origin and carries a current of $10 \ A$. Find the magnitude of the magnetic field on the $Y$-axis at a distance of $0.5 \ m$,given $\Delta x = 1 \ cm$. (Use $\frac{\mu_0}{4 \pi} = 10^{-7} \ T \cdot m/A$)

What should be the current $i$ (in $A$) in a circular coil of radius $5\,cm$ to annul the horizontal component of Earth's magnetic field ${B_H} = 5 \times {10^{ - 5}}\,T$?

Two long thin,parallel conductors carrying equal currents $I$ in the same direction are fixed parallel to the $x$-axis,one passing through $y = a$ and the other through $y = -a$. The resultant magnetic field due to the two conductors at any point is $B$. Which of the following are correct?

Difficult
View Solution

Two long parallel wires carrying currents $8\,A$ and $15\,A$ in opposite directions are placed at a distance of $7\,cm$ from each other. $A$ point $P$ is equidistant from both the wires such that the lines joining the point $P$ to the wires are perpendicular to each other. The magnitude of magnetic field at $P$ is $............\times 10^{-6}\,T$. (Given : $\sqrt{2}=1.4$)

Vedclass Products

For Students

Vedclass Test Series

Mock tests in real JEE/NEET style with performance analysis. 5-day free trial.

Start Free Trial
For Teachers

Exam Paper Generator

Generate Set A/B/C/D exam papers from 7.5L+ questions in 2 minutes. 3 chapters free.

Try Free
For Institutes

Online Exam Module

Live online exams with unlimited students, 360° analytics & white-label branding.

See Demo