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Course: High school physics - NGSS > Unit 2

Lesson 5: Electric and magnetic fields
Science
High school physics - NGSS
Types of interactions
Electric and magnetic fields
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Electric and magnetic fields

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Review your understanding of electric and magnetic fields in this free article aligned to NGSS standards.

Key terms

TermMeaning
FieldA field models what an object would experience related to a force at a given point in space.
Magnetic FieldA magnetic field is a field explaining the magnetic influence on an object in space.
Electric FieldA electric field is a field defined by the magnitude of the electric force at any given point in space.
CurrentCurrent is the rate of charge moving past a region.

Forces at a distance and fields

Forces at a distance, such as gravitational, electric, and magnetic forces, can be represented using vector fields. These fields describe a relationship a given object might experience to the forces at any point in space.
Fields are often represented in two dimensions using field lines. The density of these field lines indicates the strength of the field at a particular point - the more dense the lines, the stronger the field. The conventions for how to show gravitational, electric, and magnetic field lines are all slightly different to model the unique aspects of each force. Some common models are shown below.
A diagram shows four types of fields. The first has two vertical bars, the left labeled with a positive sign and the right a negative sign. Several horizontal lines run between the bars and each has an arrow that points right. The second type of field includes a circle with a positive sign on the left and a circle with a negative sign on the right. One horizontal line runs between them, then curved lines come out from all around each circle and form concentric loops that connect the circles. There is one arrow along each line. All arrows point away from the positive circle and toward the negative circle. The third type of field also includes a positive and negative circle connected by lines in a similar pattern as the second type of field, but has over twice the number of lines. On the left side of the positive circle and right side of the negative circle, the lines curve away from the circles rather than connecting them together as they do in the second type. The fourth type of field includes two circles, both with a positive sign. Between the circles is a point labeled P. Lines come out of each circle, begin to curve toward the point labeled P, then bend away from it as they get farther away from the circles. Arrows along the lines point away from the circles.

Electric fields

Electric fields arise from electric charges and changing magnetic fields.
An electric charge, or a collection of charges, will have an associated electric field. Any charged object placed in this field will experience an electrostatic force as the field interacts with the charge of the object. Field lines represent the force a positively charged particle would experience if it were in the field at that point.
A changing magnetic field can also cause electric charges to move. This phenomenon is commonly used in electric generators to induce electric currents in wires. The induced current can be increased by causing larger changes in the magnetic field or by coiling the wire so that more wire is affected by the changing magnetic field.

Magnetic fields

Magnetic fields arise from permanent magnets and electric charges in motion.
Magnets can occur naturally (such as the Earth’s magnetic field), or they can be made by magnetizing ferromagnetic materials.
A diagram of Earth’s magnetic field includes curved lines coming from Earth. The overall pattern formed by these lines are concentric loops. The loops are small near the equator and larger as you go farther north or south. Arrows on the lines point away from Earth in the Southern Hemisphere and toward Earth in the Northern Hemisphere. A vertical line runs through the North and South Poles and is labeled axis. A circular arrow around the axis indicates that Earth is rotating counterclockwise from a perspective looking down on the North Pole.
Magnetic fields ultimately are the result of the motion of charges. A typical representation of this can be seen in the figure below. We can see the magnetic field surrounding the straight wire as current is moving through it. We use this phenomenon to run motors and even to store information in computers.
The magnetic field around a current-carrying wire can be increased by coiling the wire or increasing the current running through it. In a magnetic field, the field lines represent the force the north side of a magnet might experience if it were in the field at that point.
A narrow horizontal cylinder represents a straight wire and is labeled conductor. Concentric magnetic field lines circle around the wire along a vertical plane perpendicular to the wire. Arrows on the field lines indicate a clockwise direction if looking toward the field lines from the left end of the wire. An arrow above the wire points right and is labeled electric current.

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