Jan. 9 – DC Motors and Induction

Let's finish the question from yesterday.

Ex: Two parallel wires carry a current.

a) If the currents are in the same direction…

     wires attract!

b) If the currents are in opposite directions…

     wires repel!

Electric DC Motors

- consists of a loop of wire in a magnetic field.

- when current flows through the loop, a magnetic force rotates the loop.

- a brush connector allows current to travel in the same direction at all times and freely rotate.

How do you increase the force of rotation?

  - stronger current, I

  - longer length, L

  - stronger field, B

Easiest is to increase L by coiling.

Two-pole motor replaces the loop with a solenoid and uses opposing magnetic fields to generate more force.

  - this force is not constant.

Example of two pole motor.

To have more consistent force, add more poles.  ie. three-pole motor.

This is the small motor that is commonly used in toy cars.

Here's what the above motor looks like inside.  You can clearly see the three poles.

INDUCTION

Discovered by Michael Faraday.

Faraday’s Law of electromagnetic induction:

A changing magnetic field in the region of a closed-loop conductor will induce an electric current.

- When a wire moves through a magnetic field, a current can be induced.

- There is a potential difference across the wire.

- This potential difference is called an EMF (electromotive force).

- EMF is not a force!  It is a potential difference measured in Volts.

We can use this principle to generate electricity.

  - a motor turns electric energy into kinetic.

  - a generator turns kinetic energy into electric

Handout

• Induction Examples

Lenz’s Law

Gives the direction of an induced current.  

An induced current and emf are in a such a direction as to oppose the change producing them.

Here are some examples.  Try to figure out which way the current flows before scrolling down to see the answers.

Solutions: