# Flash Cards

Magnetism

## Difference between electric and magnetic forces

The comparison between electric and magnetic forces in physics for high school students is presented briefly.  Definition and formulas: Electric force is repulsion or attraction between two charged objects or particles, moving or at rest, and is calculated by Coulomb’s law with the following formula  $\vec{F}_E =k\,\frac{|q|\,|q^{'}|}{r^{2}}\,\hat{r}$ where $\hat{r}$ is the unit vector that indicates the direction of the electric force and $r$ is the distance between the two charges. According to Coulomb's law, this force is proportional to the product of the magnitudes of the charges that are $|q|$ and $|q'|$. But magnetic force is a repulsion

Magnetism

## Helical Path: Charges in Magnetic Field with Solved Example

Helical path is the path of the motion of a charged particle when enters at an angle of $\theta$ in a uniform magnetic field $B$. In this short tutorial, we explain the factors that cause this type of motion. On a moving charged particle in a uniform magnetic field, a magnetic force of magnitude $F_B=qvB\,\sin \theta$ is acted where $\theta$ is the angle of velocity vector $v$ with the magnetic field vector $B$. This is the main factor that creates a spiral or helical path.  A charged particle (say, electron) can enter a region filled with uniform $B$ either with right angle $\theta=90^\circ$ or at angle $\theta$. In the former case, its path results in a circul

Magnetism

## Direction of Magnetic Force on a Positive Charge: Right Hand Rule

The direction of the magnetic force on a moving positively charged particle or a wire carrying current $i$ in a uniform magnetic field is determined by the right-hand rules with different versions stated below.  Version 1 (right-hand rule): point the fingers of your right hand in the direction of $\vec v$ and curl them (through the smaller angle) toward $\vec B$. Your upright thumb shows the cross product $\vec v \times \vec B$ or the magnetic force $\vec F_B$. This force is perpendicular to the plane of $\vec v-\vec B$ Version 2 (right-hand rule): point your fingers in the direction of $\vec B$ so that the thumb points toward the velocity $\vec v$, your palm shows the direct

Magnetism

## Magnetic field Lines

The space around a magnet where the forces of attraction or repulsion on a magnetic object can be detected is called magnetic field.  Magnetic field around a magnet or other magnetic object is visualized by magnetic field lines. Magnetic field around a magnet or other magnetic object can be displayed by iron filling patterns.  Magnetic field around a magnet or other magnetic object can be detected by a little compass needle at that point.  Small compass needle is aligned parallel to the magnetic field, with the north pole of the compass shows the direction of the magnetic field at that point.  The direction of magnetic field is from the north pole to the sout

Magnetism

## Drawing the field lines

To draw the magnetic field lines around a magnet or other magnetic objects, one can use the alignment of compass needle near those. The direction of these lines, in a magnet outside it, is away from N pole toward the S pole. Using small iron filing one can display magnetic field patterns around magnetic objects. The compass needles using its alignment with the magnetic field find shows the direction of the magnetic fields.

Magnetism

## Charged particle in magnetic field

If a particle of charge q and velocity $\vec v$ enters a region of space occupied by magnetic field $\vec B$, which is establishes by some source, it experiences a magnetic force $\vec F_B$ given by  $\vec F_B=q\vec v \times \vec B$ Using definition of the cross product, we obtain its magnitude as $|\vec F_B|=|q|vB\, \sin \theta$ Where $\theta$ is the smaller angle between $\vec v$ and $\vec B$.

Magnetism

## Properties of magnetic force

(1) The magnitude of a magnetic force depends only on the magnitude of the charge i.e. $F\propto |q|$ (2)  Magnetic force is always perpendicular to the plane containing $\vec v$ and $\vec B$. (3)  A charge moving parallel $\theta=0$ to a magnetic field experiences zero magnetic force.  (4)  A charge moving perpendicular $\theta =90{}^\circ$ to a magnetic field experiences a maximum magnetic force $F_B=qvB$. (5)  The magnetic force on a positively charge particle is in opposite direction to that of a negatively charge particle i.e. $\vec F_{B(q)}=\vec F_{B(-q)}$