 # Magnetism

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More Lessons for High School Physics

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

• magnetic fields
• magnetic domains
• Lorentz force
• right hand palm rule and right hand curl rule
• electric currents and magnetic fields

Magnetic Fields
Magnetic fields are vector fields associated with magnetic forces. Magnetic fields are generated by Hard Ferromagnets. Unlike electric fields, magnetic fields always make closed loops and are never created nor destroyed. Though magnetic field lines go out of north poles and end at south poles, within the magnet they form a closed loop. The closer together the field lines are, the stronger the field. When opposite ends of two magnets interact, their field lines connect the north pole of one with the south pole of another, and the two attract. When the same ends of two magnets interact, they repel because the lines cannot cross and are compressed by each other.

Understanding magnetic fields
How magnetism is caused by moving electric charges.
Describe magnetic poles and the interaction between magnets.
Draw magnetic field lines.

Magnetic Domains
Magnetic domains are collections of magnetic fields in the same direction. They are often found in ferromagnetic materials because their atoms align with magnetic fields in a process called a ferromagnetic phase transition. At high temperatures the atoms in ferromagnetic materials have random orientations but at low temperatures their orientations naturally naturally. Hard ferromagnetic materials' orientations stay after the removal of a magnetic field. Soft ferromagnetic materials' orientations are naturally random and are affected by magnetic fields.

Understanding ferromagnetic substances and magnetic domains.
Lorentz Force
Lorentz force is the force on charge in electromagnetic field. Lorentz force is determined by the formula F = qv x B, in which q is the charge, v is the velocity, and B is the magnetic field density. Lorentz force is perpendicular to both velocity and magnetic field. The right hand rule is applied when determining Lorentz force.

Understanding the Lorentz force.
A wire carrying current in a magnetic field experiences a force - the Lorentz force.

Right Hand Rule
The right hand rule is a way to predict the direction of a force in a magnetic field. To predict the behavior of positive charges, use your right hand. To predict the behavior of negative charges, use your left hand. If your thumb points in the direction of the velocity and your fingers point in the direction of the magnetic field, your palm points in the direction of the force.

How to use the right hand rule to predict forces in magnetic fields.
Right hand palm rule
The right hand palm rule shows the direction of force on a positive moving charge in a magnetic field.
Right hand curl rule
An explanation of the magnetic field around a current-carrying wire using the right hand curl rule.

Electric Currents - Magnetic Fields
The interactions between electric currents and magnetic fields create forces. The magnetic force on a current in a magnetic field = current × displacement across the magnetic field. This force can be predicted using the right hand rule. When two currents are directed through magnetic fields in the same direction, they attract. When they are directed in the opposite directions, they will repel.

How to predict the behavior of electric currents in magnetic fields.
Moving charges (i.e. electrical current) create magnetic fields. This video demonstrates this using a battery, a coil of wire, and a compass.

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