Explain electromagnetic induction (a changing magnetic field produces a current in a conductor) and how a generator converts kinetic energy into electrical energy (MA STE Introductory Physics, Motion and Forces, Energy, HS-PS2-5, HS-PS3-5).

What this topic is asking

This topic closes the module and the subject with the reverse of the electromagnet idea, and it spans two reporting categories. You must explain electromagnetic induction, that a changing magnetic field produces a current in a conductor (HS-PS2-5), and how a generator converts kinetic energy into electrical energy (HS-PS3-5). The crosscutting ideas are cause and effect (a changing field causes a current) and energy and matter (rotation becomes electricity). Induction is how almost all of the world's electricity is generated.

Electromagnetic induction

This is the partner idea to "a current produces a magnetic field" from the magnetism topic: there, a current makes a field; here, a changing field makes a current. The MCAS tests the condition carefully. Push a bar magnet into a coil connected to a meter and the meter twitches, a current is induced. Pull it out and the current reverses. But hold the magnet still inside the coil and the current stops, because the field through the coil is no longer changing. The word "changing" is the whole point: motion or change is required.

What makes the induced current larger

These factors are a frequent recall item, and they all come down to how quickly and strongly the field through the coil changes:

• A stronger magnet gives a bigger field change for the same motion.
• Faster motion changes the field more quickly.
• More turns means each change in the field is felt by more loops, adding up to a larger induced current.

Reversing the direction of the motion reverses the direction of the induced current, which is why a magnet rocked back and forth in a coil produces a current that keeps changing direction (alternating current).

Generators convert kinetic energy to electrical energy

This is the HS-PS3-5 energy side and the payoff of the whole module. Whatever turns the coil, falling water, steam from burning fuel or a nuclear reactor, or wind, supplies the kinetic energy, and induction converts it into electrical energy delivered to the grid. Because the coil turns continuously, the field through it rises and falls over and over, inducing a current that typically alternates in direction. The generator and the motor are mirror images, and together they show energy flowing both ways between motion and electricity, always conserved, with some lost as heat as covered in energy conversion devices.

Worked example

Reference-sheet note

The reference sheet does not print anything for induction; like the rest of magnetism it is tested qualitatively and through diagrams. What you recall is that a changing magnetic field induces a current (a steady field does not), the factors that increase the induced current (stronger magnet, faster motion, more turns), and that a generator converts kinetic energy into electrical energy, the reverse of a motor.

Try this

Q1. State the condition needed to induce a current in a coil with a magnet. [2]

• Cue. The magnetic field through the coil must be changing, for example by moving the magnet in or out; a stationary magnet in a steady field induces no current.

Q2. State the energy transformation a generator performs and one way to increase the current it produces. [2]

• Cue. It converts kinetic energy into electrical energy; turning the coil faster (or using a stronger magnet, or more turns) increases the current.