Electric and Magnetic Fields
Students model how electric and magnetic fields exert forces and transfer energy without direct contact between objects.
Illustrations are auto-generated and may be placeholders. They can be refreshed to match the narration.
Fields and Forces
A field is a region in which an object can exert a force on another object without touching it. An electric charge creates an electric field, while a magnet or moving charge creates a magnetic field. Field strength describes how strongly the field can push or pull an appropriate object at a particular location. The field becomes weaker as distance from the source increases. For example, two positively charged balloons repel each other even though they do not touch. Each balloon produces an electric field that exerts a force on the charge of the other balloon. The forces form an interaction pair: they are equal in strength and opposite in direction. Field diagrams use arrows to model both the direction of a possible force and changes in field strength.
Electric Field Models
An electric field model shows the direction and relative strength of the force that a positive test charge would experience. Electric field lines point away from positive charges and toward negative charges. Lines never cross because the field at one location has only one direction. A greater density of lines represents a stronger field. Around a single point charge, the lines spread radially, so the field weakens with distance. Between two oppositely charged parallel plates, the central field is nearly uniform: the lines are straight, parallel, and evenly spaced. For example, a small positive test charge placed between the plates accelerates toward the negative plate. A negative charge would experience a force in the opposite direction because electric force depends on both the field direction and the sign of the charge.
Magnetic Poles and Fields
Every ordinary magnet has a north pole and a south pole. Like poles repel, while opposite poles attract. Magnetic field lines form closed loops: outside a magnet they point from the north pole to the south pole, and inside the magnet they return from south to north. Closely spaced lines represent a stronger magnetic field, especially near the poles. A compass needle aligns with the local magnetic field because its small north pole points in the field direction. For example, if the north pole of one bar magnet faces the south pole of another, the magnets accelerate toward each other. Each magnet experiences an equal-strength force in the opposite direction. The field model shows how this attraction occurs across the gap, without requiring the magnets to touch.
Moving Charges
Moving electric charges create magnetic fields. Around a straight wire carrying current, magnetic field lines form concentric circles centered on the wire. The right-hand rule gives their direction: point your right thumb in the direction of conventional current, and your curled fingers show the magnetic field direction. A charge moving through an external magnetic field can experience a magnetic force. This force is perpendicular to both the charge’s motion and the magnetic field, so it often bends the path rather than directly speeding up the charge. A stationary charge experiences no magnetic force. For example, a positive particle moving to the right through a magnetic field directed into the page feels an upward force and follows a curved path. A negative particle under the same conditions bends in the opposite direction.
Energy in Field Interactions
Electric and magnetic interactions can transfer energy between stored field energy and the kinetic energy of objects. For two like electric charges, pushing them closer together requires work because they repel. That work increases the electric potential energy of the system. If the charges are released, the field forces them apart, electric potential energy decreases, and kinetic energy increases. A similar change occurs when two like magnetic poles are pushed together and released. The objects speed up as they move apart, showing that stored energy has been converted into motion. For example, compressing two facing north poles with your hands transfers energy into the magnet system. After release, the magnets accelerate apart. Energy is conserved: ignoring friction, the decrease in potential energy equals the increase in total kinetic energy, even though the objects never touch each other.