Quick Review Flashcards - Click to flip and test your knowledge!
Question
What did Hans Oersted observe in 1820 regarding an electric current passed through a conducting wire?
Answer
He observed that a magnetic field is produced around the wire.
Question
How can the presence of a magnetic field at any point around a current-carrying wire be detected?
Answer
It can be detected with the help of a compass needle.
Question
In the absence of a magnetic field, in which direction does a compass needle typically rest on the surface of the earth?
Answer
It rests along the north-south direction due to the earth's magnetic field.
Question
What is the effect on the deflection of a compass needle if the current in a nearby wire is increased?
Answer
The deflection of the needle increases.
Question
What happens to the deflection of a compass needle if the direction of current in a wire is reversed?
Answer
The direction of deflection of the needle reverses.
Question
The strength of a magnetic field produced by a current depends on the _____ of the current.
Answer
magnitude
Question
What is the geometric shape of the magnetic field lines around a straight current-carrying wire?
Answer
The field lines form concentric circles around the wire.
Question
In what plane do the magnetic field lines of a straight wire lie relative to the wire itself?
Answer
They lie in a plane perpendicular to the straight wire.
Question
How is the direction of the magnetic field at any point on a field line determined?
Answer
It is along the tangent drawn on the magnetic field line at that point.
Question
Rule: Right hand thumb rule
Answer
If the thumb of the right hand points in the direction of current, the fingers encircle the wire in the direction of magnetic field lines.
Question
How does the density of magnetic field lines change as the current in a wire increases?
Answer
The magnetic field lines become denser.
Question
What is a 'neutral point' in the context of the magnetic fields of a wire and the earth?
Answer
A point where the magnetic fields of the wire and the earth are equal and opposite, resulting in a zero resultant field.
Question
What is the direction of magnetic field lines at the centre of a current-carrying loop?
Answer
The field lines are nearly parallel to each other and normal to the plane of the loop.
Question
Rule: Clock rule for an anticlockwise current
Answer
If the current around a face of a loop is in an anticlockwise direction, that face behaves as a north pole.
Question
Rule: Clock rule for a clockwise current
Answer
If the current around a face of a loop is in a clockwise direction, that face behaves as a south pole.
Question
Define: Solenoid
Answer
A conducting wire wound in the form of a cylindrical coil whose diameter is less than its length.
Question
The magnetic field lines inside a current-carrying solenoid are nearly straight and parallel to its axis, indicating the field is _____.
Answer
uniform
Question
How does the magnetic field of a solenoid change if a soft iron rod is placed along its axis?
Answer
The magnetic field strength increases significantly.
Question
Why is a current-carrying solenoid considered a temporary magnet?
Answer
It loses its magnetic behaviour as soon as the current is switched off.
Question
What are the two main ways to increase the magnetic field of an electromagnet?
Answer
By increasing the number of turns of the winding or by increasing the current through the solenoid.
Question
Concept: Permanent Magnet
Answer
A strong magnet made by passing current through a coil wound around a piece of steel, which retains magnetism for a long period.
Question
Why is steel used for permanent magnets instead of soft iron?
Answer
Steel has higher magnetic retentivity than soft iron.
Question
In an electric bell, what component strikes the gong to produce sound?
Answer
The hammer attached to the armature.
Question
Why does the hammer in an electric bell strike the gong repeatedly when the switch is held down?
Answer
Because the circuit is continuously made and broken as the armature moves to and fro.
Question
What is the Lorentz force?
Answer
The force experienced by a charge moving in a magnetic field in a direction other than the direction of the field.
Question
State the formula for the magnitude of force $F$ on a current-carrying wire in a magnetic field.
Answer
$F = KIBL$ (or $F = IBL$ in S.I. units).
Question
What is the S.I. unit of magnetic field $B$?
Answer
The tesla (symbol $T$).
Question
Rule: Fleming's left hand rule
Answer
Stretch the forefinger (field), central finger (current), and thumb (force) mutually perpendicular; the thumb indicates the direction of force.
Question
In a d.c. motor, what is the function of the split ring commutator?
Answer
It reverses the direction of current in the coil after every half rotation to keep the direction of rotation unchanged.
Question
What is the energy transformation that occurs in a d.c. motor?
Answer
Electrical energy is converted into mechanical energy.
Question
When is the deflecting couple on the coil of a d.c. motor at its maximum?
Answer
When the plane of the coil is parallel to the direction of the magnetic field.
Question
Define: Electromagnetic Induction
Answer
The phenomenon in which an e.m.f. is induced in a coil due to a change in the magnetic flux linked with it.
Question
What is 'magnetic flux'?
Answer
The total number of magnetic field lines linked with a conductor.
Question
State Faraday's second law of electromagnetic induction regarding the magnitude of induced e.m.f.
Answer
The magnitude of induced e.m.f. is directly proportional to the rate of change of magnetic flux linked with the coil.
Question
What two factors affect the magnitude of induced e.m.f. in a coil?
Answer
The rate of change of magnetic flux and the number of turns in the coil.
Question
Rule: Lenz's law
Answer
The direction of induced e.m.f. is such that it opposes the cause (change in magnetic flux) which produces it.
Question
What physical law is Lenz's law based upon?
Answer
The law of conservation of energy.
Question
Rule: Fleming's right hand rule
Answer
Stretch the thumb (motion), forefinger (field), and central finger (induced current) mutually perpendicular to determine the direction of induced current.
Question
What is the function of an a.c. generator?
Answer
It converts mechanical energy into electrical energy using the principle of electromagnetic induction.
Question
In an a.c. generator, what components are used to connect the rotating coil to the external circuit?
Answer
Slip rings and stationary carbon brushes.
Question
What is the standard frequency of a.c. household supplies in India?
Answer
$50 \text{ Hz}$.
Question
How does alternating current (a.c.) differ from direct current (d.c.) regarding its direction?
Answer
D.c. flows in one direction, while a.c. reverses its direction periodically.
Question
Give one advantage of a.c. over d.c. regarding long-distance transmission.
Answer
A.c. can be transmitted over long distances without much loss of energy in the line wires.
Question
Define: Transformer
Answer
A device by which the magnitude of an alternating e.m.f. can be increased or decreased.
Question
On what principle does a transformer work?
Answer
The principle of electromagnetic induction (mutual induction).
Question
Why can a transformer not be used with a direct current (d.c.) source?
Answer
Because d.c. produces a constant magnetic field, so there is no change in magnetic flux to induce e.m.f. in the secondary coil.
Question
Formula: Transformer turns ratio ($n$)
Answer
$n = \frac{N_s}{N_p} = \frac{E_s}{E_p}$ where $s$ is secondary and $p$ is primary.
Question
What defines a 'step-up' transformer in terms of its turns ratio?
Answer
The number of turns in the secondary coil is greater than the number of turns in the primary coil ($N_s > N_p$).
Question
In a step-down transformer, how does the output voltage ($E_s$) compare to the input voltage ($E_p$)?
Answer
The output voltage is lower than the input voltage ($E_s < E_p$).
Question
Why is the core of a transformer laminated?
Answer
To prevent the loss of energy due to induced eddy currents in the core.
Question
What material is typically used for a transformer core to minimise hysteresis loss?
Answer
Soft iron.
Question
In an ideal transformer with no energy loss, how does the output power compare to the input power?
Answer
The output power is equal to the input power ($E_s I_s = E_p I_p$).
Question
Where is a step-up transformer commonly used in the electrical grid?
Answer
At power generating stations to step up the voltage for transmission.
Question
Where are step-down transformers commonly used by consumers?
Answer
At power sub-stations or in devices like electric bells and mobile chargers.
Question
How does the thickness of wire in the secondary coil of a step-up transformer compare to the primary coil?
Answer
The secondary wire is thinner because it carries less current ($I_s < I_p$).
Question
What is 'copper loss' in a transformer?
Answer
The energy loss as heat in the copper coils due to their resistance.
Question
Under what condition is the force on a current-carrying conductor in a magnetic field zero?
Answer
When the conductor is placed parallel to the direction of the magnetic field.
Question
In an electric bell, what happens to the electromagnet when the armature touches the adjusting screw?
Answer
The circuit is completed, and the electromagnet becomes magnetised.
Question
What is the purpose of the rheostat in the experiments described?
Answer
To vary the magnitude of the current flowing through the circuit.
Question
How many times does the polarity of a $50 \text{ Hz}$ a.c. supply change per second?
Answer
$100$ times a second.