6. Electromagnetism Flashcards
How does current affect the magnetic field produced around it?
When a current flows through a conductor, a magnetic field is produced around the conductor.
- When magnitude of the current is increased, a stronger magnetic field is formed.
- When the direction of the current is reversed, the direction of the magnetic field is also reversed.
How to increase the magnetic field strength of a solenoid
The strength of the magnetic field in a solenoid can be incraesed by:
- Increasing the magnitude of current flowing through the solenoid
- Increasing the number of turns per unit length of the solenoid
- Placing a soft iron core within the solenoid
B = µ0µeff × N/L × I
How do circuit breakers work?
If the current becomes too high, the solenoid becomes an electromagnet that is strong enough to attract the magnetic material and separate the contacts and break the circuit.
After the fault has been rectified, the contacts are pushed back and the circuit breaker is switched on again by depressing the reset button.
What causes the motor effect/catapult effect acting on a straight conductor from A to B?
- When a current flows through the conductor, a magnetic field is produced around the conductor.
- The magnetic field around the current-carrying conductor interacts with the magnetic field of the permanent magnet.
- Magnetic fields acting in the same direction interact to give a stronger field at A.
- Magnetic fields acting in the opposite direction interact to give a weaker field at B.
- The difference between the magnetic field strengths at A and at B produces a magnetic force on the conductor, acting from the stronger magnetic field at A towards the weaker magnetic field at B.
How to increase the motor effect / catapult effect?
- Increase the current through the straight conductor
- Use a stronger permanent magnet with a stronger magnetic field
How to reverse the direction of the motor effect?
- Reverse the direction of current through the straight conductor
- Reverse the direction of the external magnetic field
How to predict the direction of force on a current-carrying conductor?
- Using Fleming’s left hand rule,
- The index finger points in the direction of the magnetic field from North to South.
- The middle finger points in the direction of conventional current from (A) to (B).
- The thumb will point in the direction of the resultant force acting on the current-carrying conductor.
- The direction of the force, magnetic field or current are perpendicular to one another.
Function of Split-Ring Commutator
The split-ring commutator reverses the direction of current in the coil every half a revolution, every time the coil passes the vertical.
Function of Carbon Brushes
Carbon brushes maintain electrical contact between the coil and exteral circut.
Why does the coil rotate in a d.c. motor?
Why the coil rotates:
- Using Fleming’s left-hand rule, a downward force acts on AB and an upward force acts on CD.
- This pair of equal and opposite forces on the sides of the coil next to the magnets produce a clockwise/anticlockwise moment about the axis of the coil.
Why does the coil rotates past the veritcal in a d.c. motor?
Why the coil rotates past the vertical:
- When the coil is at the vertical position, the current is cut off from the coil because the split-ring commutator is not in contact with the carbon brushes.
- The inertia of the coil, however, carries it past the vertical position.
Why does the coil continue to rotate after passing the veritcal in a d.c. motor?
Why the coil continues to rotate past the vertical:
- Once the coil passes the vertical, the split-ring commutator comes into contact with the carbon brushes again.
- The presence of the split-ring commutator reverses the direction of the current in the coil every time the coil passes the vertical.
- This ensures that the force on the side of the coil next to the North pole continues to be in the same direction.
- Coil continues to rate clockwise/anticlockwise about the axis of the coil.
How to increase the turning effect of a d.c. motor?
- Inreasing the number of turns in the coil
- Increasing the magnitude of the current passing through the coil
- Inserting a soft iron core within the coil to concentrate the magnetic field lines
- Using a stronger magnet to increase the strength of the magnetic field
