30.1
誘導
棒磁石をコイルに押し込んだり押し出したりすることによってコイル内の磁場が変化すると、起電力が誘導されます。 反対方向の運動によって反対符号の起電力が生成され、極を反転すると起電力の方向も逆転します。 磁石ではなくコイルを動かしても同じ結果が得られます。重要なのは相対的な動きです。 動きが速くなるほど…
Electromagnetic induction is produced when an electrically conducting material is exposed to a time-varying magnetic field.
For example, consider a bar magnet placed close to a coil. The galvanometer connected to this coil shows zero current passing through it.
If the bar magnet moves toward the coil, a current flows through the coil. Furthermore, if the bar magnet is moved away from the coil, current flows in the opposite direction.
Similarly, if the bar magnet is kept fixed and the coil is moved in either direction, then current flows through the coil.
Suppose a bar magnet is replaced with a second coil connected to a battery. If one of the coils is moved, a current flows through the other coil.
If a switch controls the current passing through the one coil, then an instantaneous current pulse is induced in the other coil while opening or closing the switch.
Here the current flowing through the other coil is called an induced current, and the corresponding emf is called an induced emf.
View the full transcript and gain access to JoVE Core videos
Q1: What happens when a bar magnet moves toward or away from a coil?
When a bar magnet moves toward a coil, current flows through it. If the magnet moves away, current flows in the opposite direction. This occurs because the changing magnetic field induces an emf in the coil. The direction of the induced current depends on the direction of motion, and faster motion produces greater emf.
Q2: Why does relative motion between a magnet and coil produce an induced current?
Induced current flows due to changing magnetic flux through the coil. Whether the magnet or coil moves, it is the relative motion that matters. When the magnet is stationary relative to the coil, no emf is induced. The faster the relative motion, the greater the induced emf and resulting current.
Q3: How can two coils induce current in each other without direct contact?
When a current-carrying coil moves toward or away from a stationary coil, emf is induced in the second coil. Similarly, opening or closing a switch in the first coil induces an instantaneous current pulse in the second coil. This mutual induction occurs because the changing magnetic field from one circuit affects the other nearby circuit.
Q4: What is the difference between induced emf and induced current?
Induced emf is the electromotive force generated when magnetic flux changes through a coil. Induced current is the actual flow of charge resulting from that induced emf. The induced emf drives the induced current through the circuit. Both occur whenever there is relative motion between a magnetic field and a conducting coil.
Q5: What is the common factor in all electromagnetic induction scenarios?
The common factor in all induction scenarios is changing magnetic flux. Whether produced by a time-dependent magnetic field or by motion changing the flux through a circuit, induction occurs because of the non-static nature of the forces involved. This distinguishes induction from static electric fields produced by stationary charge distributions.
Q6: How does reversing a magnet's poles affect the induced emf?
Reversing the poles of a bar magnet reverses the direction of the induced emf in the coil. The magnitude of the induced emf depends on the rate of change of magnetic flux, while the polarity depends on which pole approaches the coil. This directional relationship is fundamental to understanding electromagnetic induction behavior.
Q7: What role does a switch play in inducing current between two coils?
When a switch controls current in one coil, opening or closing it induces an instantaneous current pulse in a nearby second coil. The switch creates a time-varying magnetic field by changing the current flow, which alters the magnetic flux through the second coil and triggers induction. This demonstrates that changing magnetic flux, not steady fields, produces induced currents.