Magnetism Flashcards
What is the law of attraction/repulsion for magnets?
Like poles repel; unlike poles attract.
What are the basic properties of a magnet?
Magnets have a north and a south pole (there are no monopole magnets), are made of iron or steel, and do not affect non-magnetic materials.
What are permanent (hard) magnets and what are they made of?
Permanent magnets are made of steel and keep their magnetism for a long time.
What are temporary (soft) magnets and what are they made of?
Temporary magnets are made of iron and lose their magnetism after a short time.
How can steel be magnetized by stroking?
By moving a magnet on a steel bar in one direction many times.
How can steel be magnetized using DC current?
By moving a steel bar inside a coil that is carrying a DC current.
What are the methods to demagnetize a magnet?
- By hammering
- By heating B
- By moving a magnet inside a coil carrying AC.
What is a magnetic field?
It is the region surrounding a magnet where magnetic force acts.
How do magnetic field lines run around a magnet?
They come out of the north pole in all directions and go into the south pole from all directions.
How does the spacing of magnetic field lines relate to the field’s strength?
As the spacing between lines increases, the magnetic field strength decreases.
Where is the magnetic field strongest and weakest?
It is strongest near the poles and weakest near the center.
How can you find the strength of a magnetic field using iron filings?
Place the magnet on paper sprinkled with iron filings; the pattern shows the field strength.
How can you determine the direction of a magnetic field using needle compasses?
Place needle compasses at different positions around the magnet to observe the orientation of the needles.
How can the magnetic field produced by a current-carrying wire be increased?
- Increase the current/voltage
- Increase the length of the wire
- Move closer to the wire
How can the magnetic field from a coil be increased?
- Increase the current/voltage
- Increase the number of turns in the coil
- Use an iron core
Inside the coil the field lines are straight and parallel.
What are electromagnets and how do they operate?
- Electromagnets are magnets that can be switched on or off. When switch is closed, current passes through the coil and magnetizes an iron core (producing a strong field)
- When open, no current flows and no field occurs.
How can you increase the magnetic field of an electromagnet?
- Increase the current/voltage
- Increase the number of turns in the coil
- Use an iron core
What is a relay and how does it use magnetic fields?
- A relay uses a small current circuit to operate a circuit that uses a large current
- Closing the switch sends current through a coil, creating a magnetic field that attracts a free contact to close the circuit and power a motor.
What is the function of a circuit breaker in a magnetic field?
It uses the magnetic effect to reduce the current to its normal value when excess current flows through a wire in a magnetic field.
How can the magnetic force acting on a moving wire be increased?
- By increasing current/voltage
- By increasing the number of turns of the coil
- By using a stronger magnet
What causes a coil to rotate in a magnetic field?
Two equal and opposite forces (one acting downward on AB and one upward on CD) create a moment that rotates the coil.
How can you increase the speed of rotation of a coil?
- By increasing current/voltage
- By increasing the number of turns of the coil
- By using a stronger magnet
How can the direction of rotation of a coil be reversed?
- Reverse the current
- Reverse the poles of the magnet
What is the function of a DC motor?
A DC motor converts electric energy into kinetic energy.
What is the role of split ring commutators in a DC motor?
They reverse the direction of current every half cycle, ensuring that the coil rotates continuously in one direction.
What is the purpose of carbon brushes in a motor?
Carbon brushes connect the rotating coil with the battery.
How is an induced current generated?
When a wire moves inside a magnetic field, the rate at which magnetic lines are cut changes, inducing a current.
How can you increase the induced current (emf) in a wire?
- By increasing the length of the wire
- By increasing the strength of the magnets
- By moving the wire faster
What does Faraday’s law of induction state?
The induced emf is directly proportional to the rate at which magnetic field lines are cut by the wire.
What does Lenz’s rule state about the direction of induced current?
The induced current is opposite to the normal current.
Is the induced current produced by a moving wire AC or DC?
It is alternating current (AC).
How can the induced current (emf) in a coil be increased?
- By increasing the number of turns in the coil
- By increasing the strength of the magnets
- By rotating the coil faster
What is an AC generator and how does it operate?
- An AC generator converts kinetic energy into electrical energy
- The current is zero when the coil is vertical and maximum when the coil is horizontal
What is mutual induction in electromagnetic induction?
It is the process by which a changing magnetic field in one circuit induces a current in a nearby circuit.
How does the movement of a magnet relative to a stationary coil affect induced current?
- When the magnet moves toward the coil, current is induced
- When it moves away, induced current occurs in the opposite direction
- If both move together at the same speed in same direction, no current is induced.
How does induced current occur when the coil moves relative to a stationary magnet?
- The coil moving toward the magnet induces a current, and moving away induces current in the opposite direction
- If both remain at rest, no induced current occurs.
How do transformers work?
Transformers use mutual induction with AC. An alternating current in the primary coil creates an alternating magnetic field (via an iron core), which induces a current in the secondary coil.
What is the relationship between primary and secondary coils in a transformer?
- Power is constant: 𝐼(𝑃) x 𝑉(𝑃) = 𝐼(𝑆) x 𝑉(𝑆)
- In step-up transformers: 𝑁(𝑃) < 𝑁(𝑆) so 𝑉(𝑃) < 𝑉(𝑆) and 𝐼(𝑃) > 𝐼(𝑆)
- In step-down transformers: 𝑁(𝑃) > 𝑁(𝑆) so 𝑉(𝑃) > 𝑉(𝑆) and 𝐼(𝑃) < 𝐼(𝑆)
Why is high alternating voltage used in the national grid instead of high alternating current?
- To reduce current
- To reduce heating effect
- To reduce energy lost
- Use thinner wires
- Cheaper
- Less metal used
- Less insulating material used
What are the key methods to increase magnetic effects in various devices?
- Increase current/voltage
- Increase the number of turns in coils
- Use stronger or iron cores
- Adjust proximity or movement speed
- When needed, reverse current or magnetic poles
What does the Right‑Hand Rule for Magnetic Fields around a straight wire state, and how do you use it?
Extend your right thumb in the direction of the conventional current (from positive to negative). Your curled fingers then show the circular magnetic field lines around the wire. Use this rule to determine the field direction around any straight current‑carrying conductor.
How is the Right‑Hand Rule for Magnetic Fields in Coils (Solenoids) applied, and what does it reveal?
Curl the fingers of your right hand in the direction of the conventional current flowing through the coil loops. Your thumb will point in the direction of the magnetic field inside the coil, indicating the north pole of the electromagnet. Use this rule to determine the polarity and field direction inside coils or solenoids.
What is Fleming’s Left‑Hand Rule (Motor Rule), and when should you apply it?
Hold your left hand so that the thumb, forefinger, and middle finger are perpendicular. The index finger shows the direction of the magnetic field, the middle finger the direction of the current, and the thumb the direction of the force (motion) on the conductor. Use this rule to predict the force direction on a current‑carrying conductor in electric motors.
Describe Fleming’s Right‑Hand Rule (Generator Rule) and its use in determining induced current direction.
Extend your right hand so that your thumb points in the direction of the conductor’s motion relative to the magnetic field, your index finger points in the direction of the magnetic field, and your middle finger then indicates the direction of the induced current. Use this rule when a conductor moves within a magnetic field, as in generators, to find the direction of the induced current.
Describe the magnetic field inside a solenoid.
- The magnetic field inside a solenoid is strong, uniform, and consists of straight, parallel lines.
- It resembles the field of a bar magnet, with distinct north and south poles at the ends.
