Magnetic Effects of Electric Current-QnA

Q2: List the properties of magnetic field lines.

Magnetic field lines are a convenient way to represent the direction and relative strength of a magnetic field. The important properties you should write in an exam are:

1. Direction: Field lines emerge from the north pole of a magnet and enter the south pole outside the magnet.
2. Inside the magnet: Within the magnet, field lines run from the south pole back to the north pole, forming closed continuous loops.
3. Never intersect: Magnetic field lines never cross each other; at any point the field has a single definite direction.
4. Density indicates strength: The closeness (density) of field lines shows field strength — closer lines mean a stronger magnetic field.
5. Tangent gives direction: A tangent drawn to a field line at any point indicates the direction a north pole would point at that point.
6. Continuous loops: Field lines are continuous closed curves; they do not start or end in space.
7. Representation tool: Field lines are imaginary lines used for visualization and do not represent physical wires or forces themselves.
8. Effect of external fields: The pattern of field lines near a magnet can change when other magnets or current-carrying conductors are nearby (resultant field is the vector sum).

Q3: Why don’t two magnetic field lines intersect each other?

Two magnetic field lines do not intersect because at any point in space the magnetic field has one unique direction. If two field lines were to cross at a point, that point would have two different directions for the magnetic field simultaneously, which is impossible. Field lines are drawn so that a tangent to a line at any point shows the direction of the magnetic field there; a single point cannot have two different tangents representing two different directions.

Clear points to write for full marks

• Magnetic field at a point is represented by a single vector (one direction and magnitude).
• A tangent to a field line at any point indicates the direction of the field at that point.
• If lines intersected, a compass placed at the intersection would have to point in two directions at once, which cannot happen.
• Therefore field lines are drawn as continuous non-crossing curves to reflect the uniqueness of the field direction at every point.

Q4: Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

To determine the direction of the magnetic field produced by a circular current loop, we use the Right-Hand Thumb Rule. According to this rule, if you curl the fingers of your right hand in the direction of the current in the loop, your outstretched thumb gives the direction of the magnetic field along the axis of the loop.

Applying the Rule to the Given Situation

• The current in the loop is flowing clockwise when viewed from above.
• Curl the fingers of your right hand in the clockwise direction (as if tracing the current through the loop).
• Your thumb now points downward, i.e., into the table.

Conclusion (Magnetic Field Direction)

• Inside the loop (center region): The magnetic field is directed into the table.
• Outside the loop: The magnetic field lines spread outward and return as closed loops, so the field is directed out of the table at many points outside the loop.

Q5: The magnetic field in a given region is uniform. Draw a diagram to represent it.

A uniform magnetic field has the same magnitude and direction at every point in the region. In diagrams it is represented by a set of straight, parallel, and equally spaced arrows all pointing in the same direction.

Q6: Choose the correct option. The magnetic field inside a long straight solenoid-carrying current (a) is zero. (b) decreases as we move towards its end. (c) increases as we move towards its end. (d) is the same at all points

Correct option: (d) is the same at all points.

Exam-style Explanation (concise)

For a long solenoid (length much greater than its diameter) the magnetic field inside is nearly uniform and directed along the axis. In the idealised case, the contributions from many closely spaced turns add up so the field at interior points has essentially the same magnitude and direction. Mathematically the ideal interior field is given by B = µ₀ n I (where n is turns per unit length and I the current), which does not depend on the position inside the solenoid.

Brief note on edge effects (for full-credit answers)

Near the ends of a real finite solenoid the field lines begin to bulge and the field strength falls slightly — these are called edge effects. For a long solenoid such variations are negligible in the central region, so option (d) remains the correct idealised choice expected in NCERT-level questions.

One-line conclusion: Inside a long solenoid the magnetic field is effectively uniform and the same at all interior points (option d).

Q7: Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum

Correct answer

(c) velocity and (d) momentum

Exam-style explanation (concise)

A magnetic field exerts a force on a moving charged particle given by F = q(v × B). This force is always perpendicular to the instantaneous velocity v of the proton.

• The mass of the proton is an intrinsic property and does not change. (So (a) is incorrect.)
• The magnetic force is perpendicular to velocity and therefore does no work on the proton; it cannot change the kinetic energy or the speed. (So (b) is incorrect.)
• Because the force is perpendicular to velocity, it continuously changes the direction of the velocity vector — hence the velocity (a vector) changes. (So (c) is correct.)
• Momentum p = m v is a vector proportional to velocity; since the direction of v changes, the proton’s momentum also changes (its magnitude stays same, direction changes). (So (d) is correct.)

Q8: A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (c) downward (b) towards east (d) upward

Answer

Correct option: (d) upward.

Exam-style explanation (concise)

The magnetic force on a moving positive charge is given by F = q (v × B). Here the particle's velocity v is towards west and the observed deflection (force) F is towards north.

Using the right-hand rule for v × B: point the index finger of your right hand along the velocity (west), and rotate your middle finger so that the thumb points in the direction of the force (north). You will find the middle finger (the direction of B) must point upward. Thus the magnetic field is upward.

Alternate vector check (brief)

Treat west as -x and north as +y. We need (-x) × B = +y. This is satisfied when B has a +z (upward) component. Hence upward is correct.

Q9: Name two safety measures commonly used in electric circuits and appliances.

Two commonly used safety measures are:

• 1. Fuse (or Miniature Circuit Breaker — MCB): A fuse contains a thin wire that melts when current exceeds a safe limit, breaking the circuit and preventing overheating or fire. An MCB performs the same protective function but can be reset after tripping.
• 2. Earthing (Grounding): Earthing provides a low-resistance path for fault current to flow to the ground. This prevents metal parts of appliances from remaining at dangerous potentials and reduces the risk of electric shock.

Q10: An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

Step 1 — Calculate the current drawn by the oven:

Electric power P = 2 kW = 2000 W. Supply voltage V = 220 V. Required current I = P / V = 2000 / 220 ˜ 9.09 A.

Step 2 — Compare with the circuit rating:

The circuit is rated for 5 A, but the oven needs about 9.09 A. This is well above the safe current rating of the wiring/circuit protection.

Step 3 — Expected result:

• Because the oven draws significantly more current than the circuit rating, the overcurrent protection (fuse or MCB) on that circuit will trip or blow to interrupt the supply. This prevents sustained overheating.
• If the protective device were absent or faulty, the excessive current could cause the wiring to overheat, risking damage to insulation, fire, or permanent harm to the circuit.
• Therefore the practical outcome is either the circuit protection operates (power cut) or, if protection fails, dangerous overheating occurs.

Q11: What precaution should be taken to avoid the overloading of domestic electric circuits?

Overloading of a domestic electric circuit occurs when too many electrical appliances draw current from the same circuit at the same time, more than its safe current-carrying capacity. This can cause excessive heating of wires, damage to appliances, and even electric fires.

Precautions to Prevent Overloading

• Do not connect too many high-power appliances (like heaters, irons, geysers, ovens) to a single socket or to the same circuit at the same time.
• Use only proper-rated fuses or MCBs (Miniature Circuit Breakers) in each circuit so that the supply is automatically cut off if the current exceeds the safe limit.
• Avoid using multiple plug adapters or extension boards from one socket to run many appliances together.
• Ensure that the wiring in the house is of correct thickness (proper wire gauge) and is checked regularly by a qualified electrician for any damage or loose connections.
• Switch off and unplug appliances when they are not in use, especially high-power devices.