The Motor Principle and the DC Motor

Question 1

In 1821, physicist Michael Faraday was intrigued by
____________ discovery that an electric current in a wire could cause a compass needle to move.

Answer 1

Oersted’s

Question 2

Since a current carrying wire has a magnetic
field around it, an _______ magnetic field
around the wire can cause the _____ to move.

Answer 2

external; wire

Question 3

This is because of the _______ or _______
of the two magnetic fields (the one around the
wire and the external one).

this property is called the _____________

Answer 3

attraction; repulsion; motor principle

Question 4

motor principle

Answer 4

When a current-carrying conductor (wire) is in
a magnetic field, and the field is perpendicular to the wire, the wire feels a force that is perpendicular to both itself and the magnetic field.

Question 5

How __ the electromagnetic force is
depends on the amount of _______ in
the wire, and the __________ of the
external magnetic field (called B from
now on).

Answer 5

big; current; magnitude

Question 6

It can also depend on the angle between the conductor and the magnetic field if they are at an angle that is not __˚.

Answer 6

90

Question 7

The force is caused because the magnetic field lines on the magnet, and on the wire, are going in the _____ direction at the _____ and opposite
directions at the top.

Therefore there is a _______ at the bottom and an ________ at the top. This pushes the wire up.

Answer 7

same; bottom; repulsion; attraction

Question 8

The _________ of the force on the
conductor depends on the direction of
the __________ current and the direction of
the _________ field.

Answer 8

direction; electric; magnetic

Question 9

The direction of the force can be determined using what is called the …

Answer 9

right-hand rule for the motor principle

Question 10

Right-hand Rule for the Motor Principal:

Answer 10

If the fingers of your open right hand point in the direction of the external magnetic field, and your thumb points in the direction of the conventional
current, then your palm faces in the direction of the force on the conductor.

Question 11

A student sets up a successful demonstration of
the motor principle, but notices that the force on
the conductor is very weak. What two changes
could the student make to increase the force?

Answer 11

Increasing the strength of the magnet or the
current in the wire would increase the size of the
force.

Question 12

The size of the force that acts on the
wire depends on:

Answer 12

The magnitude/strength of the external
magnetic field

The magnitude of the current

Question 13

A DC Motor uses an ______ _______ in
a conductor which generates a magnetic
field that interacts with an external
magnetic field to cause _______.

Answer 13

electric current; rotation

Question 14

DC motors and other electrical motors
are used all around us in many ________ ________.

Answer 14

electric devices

Question 15

If the wire forms a loop, then it will move in _______ directions on one side of the loop
than on the other side. This sets up
a ________.

Answer 15

different; rotation

Question 16

Once the loop has rotated half way, the ____ ____ _________ causes the circuit to be open. No
charges flow. But the ring will still continue to rotate due to ______.

Answer 16

split ring commutator; inertia

Question 17

Once the loop has spun around enough, the
commutator will allow electricity to
flow again. This will cause the ______ _________ to
continue again.

Answer 17

same rotation

Question 18

What two things can be done to increase the force on a current-carrying conductor according to the motor principle?

Answer 18

To increase the force on a current-carrying
conductor, increase the magnitude of the external
magnetic field or increase the magnitude of the
electric current through the conductor.

Question 19

What is the purpose of the split ring commutator?

Answer 19

The purpose of the split ring commutator is to
interrupt the circuit when the loop is perpendicular
to the magnetic field of the external magnets.

This allows the current to flow in the opposite direction in the loop once the split ring comes in contact with the brushes again.

This, in turn, changes the direction of the magnetic field and keeps the motor spinning continuously

Question 20

What effect would each of the following changes have on a DC motor? Consider each of the changes separately.
(a) increasing the number of loops in the coil

Answer 20

Increasing the number of loops in the coil
would increase the strength of the motor, since the
magnetic field of the coil would be stronger. This
causes a greater force on the armature

Question 21

What effect would each of the following changes have on a DC motor? Consider each of the changes separately.

(b) using a plastic core instead of a soft-iron core

Answer 21

Using a plastic core instead of a soft-iron core
would decrease the strength of the motor, since the magnetic field of the coil would be weaker. This
causes a lesser force on the armature.

Question 22

What effect would each of the following changes have on a DC motor? Consider each of the changes separately.

(c) decreasing the current

Answer 22

Decreasing the current would decrease the
strength of the motor, since the magnetic field of
the coil would be weaker. This causes a lesser
force on the armature

Question 23

What effect would each of the following changes have on a DC motor? Consider each of the changes separately.

(d) reversing the polarity of the external magnets

Answer 23

Reversing the polarity of the external magnets
would cause the armature to rotate in the opposite
direction.

If the external magnetic pole nearest to
the north pole of the coil were attracting the north
pole of the coil, it would now be repelling it.

If it were repelling the north pole of the coil, it would now be attracting it.

This is similar to what would happen for the external magnetic pole nearest to the south pole of the coil, so the armature would begin to rotate in the opposite direction

Question 24

What effect would each of the following changes have on a DC motor? Consider each of the changes separately.

(e) reversing the polarity of the external magnets and reversing the direction of the current

Answer 24

Reversing the polarity of the external magnets
and reversing the direction of the current would
have no effect on a DC motor.

The magnetic fields from the external magnet and the loop would remain the same strength but change to the opposite direction.

These changes in magnetic field direction mean that if magnetic fields are aligned, they stay aligned, and if they are in opposite directions, they stay in opposite directions.

So the DC motor would not be affected.