7.2 The motor effect Flashcards
Electromagnets
An electromagnet is a solenoid with an iron core
The magnetic field produced by the electromagnet can be switched on and off
When the current is flowing there will be a magnetic field produced around the electromagnet.
When the current is switched off there will be no magnetic field produced around the electromagnet.
The strength of the electromagnet can be changed by:
Increasing the current will increase the magnetic field produced around the electromagnet.
Decreasing the current will decrease the magnetic field produced around the electromagnet.
Motor effect
The motor effect occurs when:
A wire with current flowing through it is placed in a magnetic field and experiences a force.
This effect is a result of two interacting magnetic fields.
One is produced around the wire due to the current flowing through it.
The second is the magnetic field into which the wire is placed, for example, between two magnets.
As a result of the interactions of the two magnetic fields, the wire will experience a force.
Factors affecting force
The size of the force exerted by the magnetic fields can be increased by:
Increasing the amount of current flowing through the wire.
This will increase the magnetic field around the wire.
Using stronger magnets.
This will increase the magnetic field between the poles of the magnet.
Placing the wire at 90o to the direction of the magnetic field lines between the poles of the magnet.
This will result in the maximum interaction between the two magnetic fields.
Note: If the two magnetic fields are parallel there will be no interaction between the two magnetic fields and therefore no force produced.
Calculating Magnetic Force on a Current-Carrying Conductor
F = BIL
F = force acting on current-carrying wire in Newtons (N)
B = magnetic flux density (which is the strength of the magnetic field) in Tesla (T)
I = current flowing through the conductor in Amps (A)
L = length of the conductor that is in the magnetic field in metres (m)
Fleming’s left hand rule
The direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field.
All three will be perpendicular to each other.
This means that sometimes the force could be into and out of the page (in 3D).
The direction of the force (or thrust) can be worked out by using Fleming’s left-hand rule.
Electric motors
The motor effect can be used to create a simple d.c. electric motor
The force on a current-carrying coil is used to make it rotate in a single direction
The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field
The coil of wire, when horizontal, forms a complete circuit with a cell
The coil is attached to a split ring (a circular tube of metal split in two)
This split ring is connected in a circuit with the cell via contact with conducting carbon brushes.
Factors affecting D.C motor
The speed at which the coil rotates can be increased by:
Increasing the current.
Use a stronger magnet.
The direction of rotation of coil in the d.c. motor can be changed by:
Reversing the direction of the current supply.
Reversing the direction of the magnetic field by reversing the poles of the magnet.
The force supplied by the motor can be increased by:
Increasing the current in the coil.
Increasing the strength of the magnetic field.
Adding more turns to the coil.
Loudspeakers
Loudspeakers and headphones convert electrical signals into sound.
They work due to the motor effect.
A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet.
An alternating current passes through the coil of the loudspeaker
This creates a changing magnetic field around the coil.
As the current is constantly changing direction, the direction of the magnetic field will be constantly changing.
The magnetic field produced around the coil interacts with the field from the permanent magnet.
The interacting magnetic fields will exert a force on the coil.
As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction.
This makes the coil oscillate.
The oscillating coil causes the speaker cone to oscillate.
This makes the air oscillate, creating sound waves.
