Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards


Magnetism

Electromagnetism: Moving Charges, Magnetic Forces, and Changing Magnetic Fields
Electricity and magnetism are two aspects of a single electromagnetic force. Moving electric charges produce magnetic forces, and moving magnets produce electric forces. These effects help students to understand electric motors and generators.


Further Description:

The relationship between electricity and magnetism is one of the most important in all of science. The facts that a changing magnetic field can produce an electric field and a changing electric field can produce a magnetic field lead to applications involving transformers, solenoids, motors, generators, and many other devices. These phenomena are also the basis of electromagnetic waves.


Concepts Needed:

Grade 9

Magnet, magnetic pole, electromagnet, electric circuit

Grade 10

Magnetic field, electric current

Grade 11

Voltage, resistance, potential difference, parallel and series electric circuits

Grade 12

Vector fields, electromagnetic force, induced EMF, back EMF, hysteresis


Empirical Laws or Observed Relationships:
:

A magnetic field is created around a moving charge. The magnitude of the field depends on the magnitude of the charge, the speed of motion of the charge (the first right hand rule), and the angle between the velocity of the charge and the radius vector from the point where the field is being observed to the charge. A changing magnetic field around a conductor produces an opposing electric field in the conductor (Lenz=s law, back EMF).

A force is created on a charge moving through an existing magnetic field. The magnitude of the electromagnetic force on a moving charge is proportional to the magnitude of the charge on the moving particle, the strength of the magnetic field through which it moves, the speed at which the charge is moving relative to the field, and the angle between the velocity and the field. The magnetic field around a wire carrying a current has a simple geometry, and the extension of this idea to coils of wires can be developed.

Current flow depends on voltage; flow also depends inversely on resistance. Flow divides at a junction depending inversely on the resistance along each route. Two routes (each with one unit of resistance) will be half as resistive (will allow twice the flow). Adding multiple routes (of equivalent resistance) will increase the flow proportionately.


Theories or Models:

Field theory of electricity and magnetism, electric circuits and their symbols


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Micro-Unit Description:

Magnetism
Students should observe that an unmarked bar magnet suspended at its midpoint revolves to align with an approximate north-south direction, and that this is the basis for calling a pole north (seeking). They should observe properties of permanent magnets they have poles, and like poles repel and unlike poles attract and with iron filings and small compasses observe that we can visualize something called magnetic lines of force.


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