Motors and Generators

Physics 12 Adv

Comprehension Check

1. A coil that consists of 500 turns has an edge 15cm long that carries a current of 1.0A to the right. If the edge of the coil is inside a magnetic field of 0.20T pointing out of the page , what is the force the field exerts on the coil?

2. The magnetic field of the Earth is 0.030mT and is approximately horizontal in the room (pointing south). A 1.0m horizontal wire experiences a .15mN force toward the floor. What is the current in the wire? (Bonus – what is the orientation of the wire and which way is the current flowing)

Comprehension Check

1. 15N2. 5.0A, wire runs east-west and current

is flowing to the west

DC Electric Motor If a current carrying coil is placed in a

magnetic field, it will experience torque

This will cause it to rotate so that it no longer experiences a force that causes it to rotate

At this point, a split ring commutator will reverse the current direction so that the coil will continue to rotate

AC Electric Motor

The design of the AC motor is similar to the DC motor except the split-ring commutator is replaced by two slip-ring commutators

As AC current is alternating, the voltage source will reverse the current in the loop so the split-ring commutator is no longer required

DC Motor – Step by Step

Motor Questions

1. How could you increase or decrease the frequency of rotation of a motor?

2. How could you increase the rate of energy output for a given motor (assuming no changes in the actual construction)?

Lenz’s Law

When a conductor interacts with a magnetic field, there must be an induced current that opposes the interaction, because of the law of conservation of energy

Eddy Currents

These are an application of Lenz’s law and can be used in balances in order decrease the time it takes to come to rest

Generator Effect

With a motor, a moving charge in a magnetic field results in a force on the wire

In a similar manner, moving a magnet past a conducting wire will result in a force on the charge carriers (i.e. a current)

Right Hand Rule for Generator Effect

The fingers point in the direction of the magnetic field

The thumb points in the direction of motion of the wire

The palm shows the direction of the emf (electromotive force)

DC Generator A DC generator is

essentially a DC motor that is designed to convert motion to electricity instead of vice versa

The voltage will alternate between a max and zero

AC Generator The difference with

an AC generator is that in does not require a split-ring commutator but instead uses two slip-ring commuators

This is because the direction of current in an AC device alternates (positive/negative)

Electromotive Force

The emf is a function of: Magnetic Field (B) Length of wire (L) Speed of wire (v)

LvBemf

Section Review

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