Electromagnetism Flashcards
How does a magnetic field produced by a current
Magnetic field is produced by a current flowing through conductors. When magnitude of current increases, stronger magnetic field formed. When direction of current is reversed, direction of magnetic field is also reversed
How to increase magnetic field in solenoid
Increase magnitude of current flowing through solenoid
Increase number of turns per unit length of solenoid and place a soft iron core within the solenoid
What is the motor effect
Current-carrying conductor experiences a force if magnetic field applied is perpendicular to direction of flow of current. When magnetic field of current-carrying conductor interacts with magnetic field, magnetic attraction or repulsion can take place. If forces of attraction or repulsion are large enough, they will cause a motion known as the motor or catapult effect.
What causes the motor effect
When current-carrying conductor is placed inside uniform magnetic field, magnetic field due to current in wire interacts with magnetic field of magnet. Magnetic fields acting in same direction give stronger field and magnetic fields acting in opposite directions give weaker field. Difference between magnetic field strength produces a force on conductor, acting from stronger field to the weaker field
How to describe movement of charged particle in uniform magnetic field
Charged particle is deflected upwards/downwards in a circular path when magnetic field is applied perpendicularly to beam
How does current-carrying coil in magnetic field experience a turning effect
Current-carrying coil experiences equal force on parts AB and CD of wire. Force is created by motor effect on each side of coil. Using Fleming’s left-hand rule, downward force acts on side AB and upward force acts on side CD. This force produces turning effect that is equal in magnitude on each wire. Causes coil to turn in anticlockwise direction about the axis PQ
What is the function of a split-ring commutator
Reverse the direction of current in the coil every half a revolution
What is the function of carbon brushes
Maintain electrical contact between coil and external circuit
How does d.c. motor work when coil is horizontal
Using Fleming’s left-hand rule, downward force acts on wire section AB and upward force acts on wire section CD
Anticlockwise moment about axis causes coil to rotate.
How does d.c. motor work when coil is vertical
When coil is at vertical position, the current is cut off from coil because split-ring commutator is not in contact with carbon brushes. Inertia of coil carries it past vertical position
How does d.c. motor work after coil passes the vertical
Direction of current flowing through each side of the coil reverses. This ensures the force on the side of coil next to north pole continues to be in same direction. Coil continues to rotate anticlockwise about the axis.
How to increase the turning effect of a d.c. motor
Increase the number of turns in the coil, increase the magnitude of current passing through the coil, inserting a soft iron core within coil to concentrate magnetic field lines
How does soft iron core increase turning effect of motor
A soft iron core concentrates the magnetic field lines and increases strength of magnetic field. Stronger magnetic field gives rise to stronger force.
Why can’t the carbon brushes be in contact with both halves of the split-ring at the same time
There will be a short circuit and no current will flow through coil so external circuit will overheat
Why does the motor not start when switched on even if there is no friction
The carbon brushes are positioned next to the gaps in the split-ring, resulting in no current passing through coil
