FISIKA Kelas 12 - Gaya Magnetik | GIA Academy

Summary

The video introduces magnetic force, also called the Lorentz force, and explains how to determine its magnitude and direction in three situations: a current-carrying wire in a magnetic field, a moving electric charge in a magnetic field, and two parallel current-carrying wires. A loudspeaker is mentioned as an everyday application of magnetic force.

Direction of the Lorentz Force

The video presents two right-hand rules:

• Three-finger rule: Point your thumb in the direction of conventional current (I), your index finger in the direction of the magnetic field (B), and your middle finger in the direction of the force (F).
• Open-hand rule: Point your thumb along (I), your other fingers along (B), and use the direction indicated by your palm for (F).

For a moving charged particle, use the conventional-current direction. It is parallel to the particle’s velocity for a positive charge and opposite to its velocity for a negative charge.

Lorentz Force in Three Situations

1. Current-Carrying Wire in a Magnetic Field

[ F = BIL\sin\theta ]

Here, (B) is the magnetic-field strength, (I) is the current, (L) is the length of wire in the field, and (\theta) is the angle between the current and the field. If the wire and field are parallel, the force is zero.

2. Moving Electric Charge in a Magnetic Field

[ F = |q|vB\sin\theta ]

Here, (q) is the charge, (v) is its speed, and (\theta) is the angle between its velocity and the field. The force direction depends on the sign of the charge.

If the velocity is perpendicular to the field, the particle follows a circular path. The Lorentz force acts as the centripetal force, giving:

[ r = \frac{mv}{|q|B} ]

3. Two Parallel Current-Carrying Wires

[ F = \frac{\mu_0 I_1 I_2 L}{2\pi a} ]

Here, (a) is the separation between the wires and (\mu_0) is the permeability of free space. The wires attract when their currents flow in the same direction and repel when their currents flow in opposite directions.

Worked Examples Shown

• For current along (+z) and a magnetic field along (+y), the force is along (-x).
• A 1.5 m wire carrying 3 A perpendicular to a 0.2 T field experiences a force of 0.9 N, directed upward in the pictured setup.
• For a negatively charged particle moving right while the magnetic field points into the page, the force is downward.
• Using the stated electron values (v=4.5\ \text{m/s}), (q=1.6\times10^{-19}\ \text{C}), (\theta=30^\circ), and (F=5\ \text{N}), the video calculates (B=1.39\times10^{19}\ \text{T}).
• For a particle with (m=9\times10^{-28}\ \text{kg}), (q=3\times10^{-9}\ \text{C}), (B=2\ \text{T}), and (r=3\ \text{cm}), the video calculates a speed of (2\times10^{17}\ \text{m/s}).
• For parallel wires 0.5 m long, carrying 4 A and 5 A and separated by 0.1 m, the calculated force is (2\times10^{-5}\ \text{N}). The wires attract because their currents run in the same direction.
• For wires 0.2 m apart, with (I_1=6\ \text{A}) and a force per unit length of (4.8\times10^{-5}\ \text{N/m}), the video finds (I_2=8\ \text{A}). Since the wires repel, their currents are opposite; the pictured (I_2) points downward.

The unusually large results in the two particle examples follow from the numerical values stated in the subtitles.

Speakers and Sources

• Speaker: One presenter/narrator from GIA Academy; the subtitles do not give the presenter’s name.
• Source/channel: GIA Academy YouTube channel.
• Historical figure mentioned: Hendrik Antoon Lorentz, credited in the video with discovering the Lorentz force.