Physics Chapter 22

What are the 3 steps for reasoning through Lenz's law problems?

1. Determine whether the penetrating magnetic flux is increasing or decreasing 2. Find what the direction of the induced magnetic flied must be so that it can oppose the change in magnetic flux by adding or subtracting from the original field. 3.Now that you know the direction the induced field must be to oppose the flux, use RHR2 to determine the current's direction. Assign the polarity of the emf so that the conventional current is directed out of the positive terminal through the external circuit.

What do electric generators consist of in their simplest form?

A coil of N loops that rotates in a uniform magnetic field. This produces an emf.

What is a transformer?

A device that is used to increase or decrease an ac voltage

What kind of emf is this called? Why?

A motional emf because it originates from the motion of charges through a magnetic field.

What is mutual inductance?

A primary coil is connected to an ac current. The primary coil is an electromagnet, and creates a magnetic field in the surrounding region as it carries the current. When a secondary coil is close enough, a significant fraction of this magnetic field will penetrate the second coil, producing a magnetic flux. The flux is changing, and thus an emf is induced in the secondary coil. Put simply: the effect in which a changing current in one circuit induces an emf in another circiut

What is always needed to produce a current?

A source of emf

What is an induced current?

Any current that results from the induced emf

Why do we use Cos(phi) in the equation for magnetic flux? Recall: Mag. flux = BAcos(phi)

Because this is the component of the magnetic field that is perpendicular to the surface.

What's another way to induce an emf in a coil in a constant magnetic field?

Changing the area of the coil

How do generators produce electrical energy?

From mechanical work

If the rod stops moving, what happens to the emf?

It goes away.

Can this happen? Why or why not?

No because this would create energy, thus violating conservation of energy.

Is the induced field always opposed to the external field? Why or why not?

No, the magnetic field induced must oppose change in magnetic flux, not the external field itself

If a magnet is held still next to a still coil, will there be an emf induced in the coil?

No. Either the coil or magnet must be moving

How does it work?

The coil is rotated in the magnetic field by some mechanical means. A current arises because their is a magnetic force exerted on the charges in the wire due to the coil moving with a velocity in the field.

Why is there almost always a minus sign in the equation relating emf and magnetic flux? (may not be that important)

The direction of the current induce in the circuit is such that the magnetic force will oppose the motion, slowing the rod. The minus sign ensure that the polarity of the induced emf directs the induced current properly so it will create the magnetic force opposed to the motion

What does Lenz's law tell us?

The direction of the induced current is such that it opposed the change producing it by making a magnetic field that pushes the original change in flux.

What is self-inductance?

The effect in which a changing current in a circuit induces an emf in the same circuit

How does a current appear?

The induced emf causes an induced current to appear

How are the induced emf and the magnetic flux related?

The induced emf equals the time rate of change of the magnetic flux

When a magnet is pulled toward a coil, a current is induced. When the magnet is pulled away, what is the direction of the current produced?

The opposite

What is electromagnetic induction?

The phenomenon in which an emf is induced in a piece of wire or a coil of wire with the aid of a magnetic field.

How does it work?

The primary coil is connected to an ac generator, and receives current. The changing magnetic flux produced by the current in the primary coil induces and emf in the secondary coil.

What direction is the second force opposed to? What is the result?

The second force will oppose the velocity and will thus slow down the rod.

What would happen instead if the direction of the induced current were reversed?

The second magnetic force would be in the same direction as the velocity, and the rod would accelerate without needing an external force. This would generate a motional emf.

What is the unit of magnetic flux?

The weber (Wb)

What are the key parts of a transformer?

There is a prmary coil, a secondary coil, which are both wound to an iron core

How long do the + and - charges accumulate at both ends of the rod?

Until the attractive electric force that they exert on each other becomes equal in magnitude to the magnetic force. Equilibrium is reached

What are the units of an emf?

Volts

What has to happen to the magnetic field to induce an emf?

We must change the magnetic field to induce an emf

How do we avoid this?

We need a counterbalancing force that is equal to the second magnetic force and in the opposite direction or else the rod will slow and eventually stop.

How is an emf induced when a conducting rod moves through a constant magnetic field?

When a conducting rod is moved through a constant magnetic field, and the velocity of the rod is also constant and perpendicular to the uniform magnetic field, each charge within the rod will also move with the same velocity. From the equation F=qvB, we can see that each charge will experience a magnetic force. This force causes opposite charges to appear at the ends of the rod, giving rise to the induced emf. (The forces pushes electrons to one end, thus leaving positive charges at the other)

How does a second magnetic force come into the picture?

When the emf causes a current, the second force will arise because the current is perpendicular to the magnetic field

What did Faraday discover?

Whenever there is a change in flux through a loop of wire, an emf is induced in the loop. By change, we mean change as time passes.

What is the conclusion? (about direction of magnetic force from induced current)

\When a motional emf leads to an induced current, a magnetic force always appears that opposes the motion (velocity)

(can prove this with RHR)

draw picture

The magnetic flux is _____ to the number of filed lines that pass through a surface

proportional