Magnetic Effects of Electric Current-Exercise
Physics - Exercise
Q1. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire. (b) The field consists of straight lines parallel to the wire. (c) The field consists of radial lines originating from the wire. (d) The field consists of concentric circles centred on the wire.
Answer
Answer: (d) The field consists of concentric circles centred on the wire.
Explanation
When an electric current flows through a long straight wire, it generates a magnetic field around the wire due
to
the motion of charges. This field forms closed loops that appear as concentric circles in planes
perpendicular
to the wire, with the wire acting as the centre. The direction of these field lines follows the right-hand thumb rule: point the thumb along the current direction, and the curled fingers indicate the circular path of the
field.
Q2.At the time of a short circuit, the current in the circuit (a) reduces substantially. (b) does not change. (c) increases heavily. (d) vary continuously.
Answer
Answer: (c) increases heavily.
Explanation
A short circuit happens when the live wire touches the neutral wire directly, bypassing the normal
resistance of
appliances in the circuit. This creates a path of almost zero resistance, allowing current to surge
dramatically
as per Ohm's law (I = V/R), where voltage remains constant but resistance drops sharply.
Q3. State whether the following statements are true or false. (a) The field at the centre of a long circular coil carrying current will be parallel straight lines. (b) A wire with green insulation is usually the live wire of an electric supply.
Answer
(a) True. The field at the centre of a long circular coil carrying current consists of nearly parallel, straight magnetic field lines along the coil's axis.
(b) False. A wire with green insulation is not usually the live wire; it is typically the earth or grounding wire in electrical wiring. The live wire often has red or brown insulation, while the neutral wire is blue or black and green is reserved for grounding to ensure safety by providing a path for fault current
Q4. List two methods of producing magnetic fields.
Answer
Two methods of producing magnetic fields are:- Using a permanent magnet: A permanent magnet naturally produces a magnetic field around itself. This magnetic field can be observed by sprinkling iron filings around the magnet, which align along the magnetic field lines.
- Using a current-carrying conductor: When electric current flows through a conductor such as a straight wire, circular coils, or a solenoid, it produces a magnetic field around the conductor. The shape and strength of the magnetic field depend on the shape of the conductor and the amount of current flowing through it.
Q5. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Answer
The force experienced by a current-carrying conductor placed in a magnetic field is largest when the direction of the current is perpendicular to the direction of the magnetic field. Explanation The magnitude of the magnetic force on the conductor follows the relation \[F=BILsin\theta\]where,
\(B\) is the magnetic field strength,
\(I\) is the current,
\(L\) is the length of the conductor in the field, and
\(\theta\) is the angle between the current direction and the magnetic field. This force reaches its maximum
value when
\(\sin\theta=1\), which occurs at \(\theta=90^\circ\)
At this perpendicular orientation, every moving charge in the conductor experiences the full Lorentz force without any component cancellation, resulting in the strongest deflection observable in experiments like those with a straight wire between magnet poles. When \(\theta=0^\circ\) or \(\theta =180^\circ\) \(\sin \theta =0\), so the force drops to zero.
This principle underpins devices such as electric motors, where perpendicular placement maximizes torque from the force.
Q6. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer
When an electron beam travels horizontally from the back wall toward the front wall, and it is deflected to your right by a strong magnetic field, the direction of the magnetic field must be vertically downward.
Explanation:
The force on a moving charged particle in a magnetic field is given by the Lorentz force, which is
perpendicular
to both the velocity of the particle and the magnetic field direction. For an electron (negative charge),
the
force direction is opposite to that predicted by the right-hand thumb rule. Here, with the electron moving
horizontally forward (from back to front), and deflected to your right, the magnetic field must be directed
vertically downward (toward the floor) to produce such a force.
If the current direction were considered instead (opposite to electron motion), the magnetic field direction can be verified using Fleming’s left-hand rule, confirming the magnetic field is vertically downward (toward the floor) for the observed deflection to the right.
Thus, the magnetic field direction is downward, perpendicular to the electron’s velocity and the deflection direction.
Q7. State the rule to determine the direction of a
(i) magnetic field produced around a
straight conductor-carrying current,
(ii) force experienced by a current-carrying
straight conductor placed in a magnetic field which is perpendicular to it, and
(iii) current induced in a coil due to its rotation in a magnetic field.
Answer
- Right-hand thumb rule: Hold the straight current-carrying conductor in the right hand with the thumb pointing in the direction of the current; the direction of the curled fingers gives the direction of the magnetic field around the conductor.
- Fleming's left-hand rule: Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to each other such that the forefinger points in the direction of the magnetic field, the middle finger points in the direction of the current, and then the thumb points in the direction of the force experienced by the conductor.
- Fleming's right-hand rule: Stretch the thumb, forefinger, and middle finger of the right hand mutually perpendicular to each other such that the thumb points in the direction of the force (motion), the forefinger points in the direction of the magnetic field, and then the middle finger points in the direction of the induced current.
Q8. When does an electric short circuit occur?
Answer
An electric short circuit occurs when the live wire comes into direct contact with the neutral wire or the earth wire in an electric circuit, creating a path of very low resistance.
Explanation
This direct connection bypasses the appliances and their normal resistance, causing the current to increase
heavily since current \(I=V/R\) rises sharply as resistance \(R\) drops near zero, while voltage \(V\) stays
constant. The excessive current produces intense heat, sparks, or potential fire hazards until a fuse or
circuit breaker interrupts the flow.
Common causes include damaged insulation on wires allowing unintended contact, loose connections, or faulty appliances exposing live parts. In household wiring, this fault demands immediate protection mechanisms like MCBs to prevent damage.
Q9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer
The function of an earth wire is to provide a low resistance path for the leakage of electric current from the metallic body of an appliance to the ground. This prevents the electric shock to anyone who touches the appliance if a fault occurs, such as when the live wire touches the metal casing.
It is necessary to earth metallic appliances because the metallic body may become live due to insulation failure or damage, and without earthing, touching the appliance can cause a dangerous electric shock. By connecting the metallic parts to the earth, any fault current safely flows into the ground, causing the fuse to blow or the circuit breaker to trip, thus protecting users from electric shock and preventing possible fire hazards.
