Chapter 12: Magnetic Effects of Electric Current
[IMAGE PLACEHOLDER: Bar Magnet Field Lines & Right-Hand Thumb Rule]
1-Mark Questions & Solutions
Q1 | 1 Mark
A metal rod PQ is placed in a magnetic field. The ends of the rod are connected with a battery using wires. Where will the rod move?
(a) Into the field (b) Upward (c) Downwards (d) Out of the field
(a) Into the field (b) Upward (c) Downwards (d) Out of the field
Answer: (d) Out of the field (perpendicular to both magnetic field and current vectors according to Fleming's Left-Hand Rule).
Q2 | 1 Mark
A student places iron filings around a magnet. The iron filings arrange themselves in patterns labeled P, Q, R, S. Where will the magnetic field be strongest?
(a) S (b) R (c) Q (d) P
(a) S (b) R (c) Q (d) P
Answer: (b) R (Magnetic field strength $B$ is strongest at the poles where field lines are most dense).
Q3 | 1 Mark
An electron enters a magnetic field at right angles to it. The direction of force acting on the electron will be:
(a) to the right (b) to the left (c) out of the page (d) into the page
(a) to the right (b) to the left (c) out of the page (d) into the page
Answer: (d) into the page (Conventional current direction is opposite to electron movement; applying Fleming's Left-Hand Rule gives force directed into the page).
Q4 | 1 Mark
A current through a horizontal power line flows in East to West direction. The direction of magnetic field at a point directly below it is from:
(a) North to South (b) South to North (c) West to East (d) North to West
(a) North to South (b) South to North (c) West to East (d) North to West
Answer: (a) North to South (By applying the Right-Hand Thumb Rule).
Q5 | 1 Mark
According to international convention of colour coding in a wire:
(a) live is red, neutral is black and earth is green
(b) live is brown, neutral is blue and earth is green/yellow
(c) live is brown, neutral is green and earth is black
(d) live is red, neutral and earth is blue
(a) live is red, neutral is black and earth is green
(b) live is brown, neutral is blue and earth is green/yellow
(c) live is brown, neutral is green and earth is black
(d) live is red, neutral and earth is blue
Answer: (b) live is brown, neutral is blue and earth is green/yellow (New International Code; traditional Indian code is Live: Red, Neutral: Black, Earth: Green).
Q6 | 1 Mark
Appliances that have metallic bodies are connected to the earthing wire. What is the reason to earth these appliances?
(a) to prevent excess of current (b) to prevent leakage of current / electric shock (c) to provide extra current (d) to provide high resistance
(a) to prevent excess of current (b) to prevent leakage of current / electric shock (c) to provide extra current (d) to provide high resistance
Answer: (b) to prevent electric shock by providing a low resistance path to ground for leakage current.
Q7 | 1 Mark
Which diagram correctly shows the concentric magnetic field lines around a current-carrying circular loop?
Answer: (b) [Diagram showing concentric circular lines near the wire becoming straight and parallel at the center].
Q8 | 1 Mark
The direction of force acting on a current-carrying wire placed in a magnetic field depends on:
(a) direction of current only (b) direction of field only (c) direction of current as well as direction of field (d) neither
(a) direction of current only (b) direction of field only (c) direction of current as well as direction of field (d) neither
Answer: (c) direction of the current as well as the direction of the field ($\vec{F} = I(\vec{L} \times \vec{B})$).
Q9 | 1 Mark
What is the direction of magnetic field at a point A above a horizontal wire carrying current $I$ towards right?
(a) Out of the page (b) Into the page (c) Up the page (d) Down the page
(a) Out of the page (b) Into the page (c) Up the page (d) Down the page
Answer: (a) Out of the page (Applying Right-Hand Thumb Rule).
Q10 | 1 Mark
The magnetic field inside a long straight solenoid carrying current:
(a) is zero (b) decreases towards ends (c) increases towards ends (d) is the same at all points
(a) is zero (b) decreases towards ends (c) increases towards ends (d) is the same at all points
Answer: (d) is the same at all points (Uniform internal field $$B = \mu_0 n I$$).
[IMAGE PLACEHOLDER: Magnetic Field Pattern of a Solenoid]
Q11 - Q15 | Additional 1-Mark MCQs
• Q11: Most suitable core material for electromagnet $\rightarrow$ (c) Soft iron (high magnetic permeability and low retentivity).
• Q12: Straight conductor carrying current $\rightarrow$ (a) Circular magnetic field lines around it.
• Q13: Moving magnet continuously in and out of a coil $\rightarrow$ (b) Current will change its direction continuously (Alternating induced current).
• Q14: Magnetic field near long straight wire $\rightarrow$ (d) Concentric circles centered on the wire.
• Q15: Correct magnetic field lines around bar magnet $\rightarrow$ Emerging from North pole and entering South pole externally.
• Q11: Most suitable core material for electromagnet $\rightarrow$ (c) Soft iron (high magnetic permeability and low retentivity).
• Q12: Straight conductor carrying current $\rightarrow$ (a) Circular magnetic field lines around it.
• Q13: Moving magnet continuously in and out of a coil $\rightarrow$ (b) Current will change its direction continuously (Alternating induced current).
• Q14: Magnetic field near long straight wire $\rightarrow$ (d) Concentric circles centered on the wire.
• Q15: Correct magnetic field lines around bar magnet $\rightarrow$ Emerging from North pole and entering South pole externally.
Q16 - Q27 | Conceptual MCQs & Assertions
• Q21: Magnetic field lines inside current-carrying solenoid are (b) along the axis and parallel to each other.
• Q22: Assertion: Strength of electromagnet depends on current magnitude. Reason: Used for lifting heavy loads. $\rightarrow$ Answer: (b) Both A and R are true, but R is not the correct explanation of A.
• Q23: Assertion: Metallic appliances have 3-pin connections. Reason: 3-pin connections reduce heating. $\rightarrow$ Answer: (c) A is true, but R is false (3rd pin is for safety earthing).
• Q24: Magnet NS moved away from coil $\rightarrow$ Induced current flows (b) Clockwise (Lenz's Law).
• Q25: When a bar magnet is broken into two pieces $\rightarrow$ (c) Each piece will have two unlike poles (N and S).
• Q26: Incorrect statement $\rightarrow$ (d) Outside magnet, field lines go S to N (Outside: N to S; Inside: S to N).
• Q21: Magnetic field lines inside current-carrying solenoid are (b) along the axis and parallel to each other.
• Q22: Assertion: Strength of electromagnet depends on current magnitude. Reason: Used for lifting heavy loads. $\rightarrow$ Answer: (b) Both A and R are true, but R is not the correct explanation of A.
• Q23: Assertion: Metallic appliances have 3-pin connections. Reason: 3-pin connections reduce heating. $\rightarrow$ Answer: (c) A is true, but R is false (3rd pin is for safety earthing).
• Q24: Magnet NS moved away from coil $\rightarrow$ Induced current flows (b) Clockwise (Lenz's Law).
• Q25: When a bar magnet is broken into two pieces $\rightarrow$ (c) Each piece will have two unlike poles (N and S).
• Q26: Incorrect statement $\rightarrow$ (d) Outside magnet, field lines go S to N (Outside: N to S; Inside: S to N).
2-Mark Questions & Solutions
Q28 | 2 Marks
(a) How is the direction of magnetic field at a point determined? (b) Direction at center of current-carrying circular loop?
Answer:
(a) By placing a small compass needle at that point; the direction in which the North pole of the compass points gives the magnetic field direction.
(b) Perpendicular to the plane of the circular loop along its axis.
(a) By placing a small compass needle at that point; the direction in which the North pole of the compass points gives the magnetic field direction.
(b) Perpendicular to the plane of the circular loop along its axis.
Q29 | 2 Marks
An electron and a proton move parallel to each other and enter a uniform magnetic field with same velocity along the field direction. How will their paths be affected?
Answer: Their paths will remain **unaffected** (deflection force $F = qvB \sin \theta = 0$ when moving parallel, $\theta = 0^\circ$). They will continue in straight lines.
Q30 | 2 Marks
How does magnetic field strength at the center of a circular coil depend on: (a) radius of coil $r$, (b) number of turns $N$?
Answer:
(a) Inversely proportional to radius ($B \propto \frac{1}{r}$).
(b) Directly proportional to number of turns ($B \propto N$). $$\text{Combined relation: } B = \frac{\mu_0 N I}{2r}$$
(a) Inversely proportional to radius ($B \propto \frac{1}{r}$).
(b) Directly proportional to number of turns ($B \propto N$). $$\text{Combined relation: } B = \frac{\mu_0 N I}{2r}$$
Q33 | 2 Marks
Explain two different ways to induce current in a stationary or moving coil.
Answer:
1) Relative Motion: Moving a bar magnet rapidly towards or away from a stationary coil.
2) Changing Current: Changing current in a neighboring primary coil (Electromagnetic Induction).
1) Relative Motion: Moving a bar magnet rapidly towards or away from a stationary coil.
2) Changing Current: Changing current in a neighboring primary coil (Electromagnetic Induction).
[IMAGE PLACEHOLDER: Fleming's Left-Hand & Right-Hand Rules]
3-Mark Questions & Solutions
Q37 | 3 Marks
What precautions should be taken to avoid overloading of domestic circuits?
Answer:
1) Do not connect too many high-power appliances (heaters, ACs, irons) to a single socket.
2) Always use appropriate fuses or Miniature Circuit Breakers (MCBs) rated correctly for the circuit.
3) Regularly inspect wiring for damaged insulation to prevent direct live-neutral contact (short circuiting).
1) Do not connect too many high-power appliances (heaters, ACs, irons) to a single socket.
2) Always use appropriate fuses or Miniature Circuit Breakers (MCBs) rated correctly for the circuit.
3) Regularly inspect wiring for damaged insulation to prevent direct live-neutral contact (short circuiting).
Q38 | 3 Marks
(a) Purpose of soft iron core in electromagnets? (b) How is it different from a permanent magnet?
Answer:
(a) Soft iron increases magnetic field strength significantly and instantly loses its magnetism when current is switched off.
(b)
(a) Soft iron increases magnetic field strength significantly and instantly loses its magnetism when current is switched off.
(b)
- Electromagnet: Temporary magnet, variable strength, reversible polarities.
- Permanent Magnet: Permanent magnetism, fixed strength, fixed N-S polarities.
Q39 | 3 Marks
What happens to force on a current-carrying conductor in a magnetic field when:
(a) Field direction is reversed? (b) Current direction is reversed? (c) Both directions are reversed?
(a) Field direction is reversed? (b) Current direction is reversed? (c) Both directions are reversed?
Answer:
(a) Direction of force is **reversed**.
(b) Direction of force is **reversed**.
(c) Direction of force **remains unchanged** ($\vec{F} \propto (-\vec{I}) \times (-\vec{B}) = \vec{I} \times \vec{B}$).
(a) Direction of force is **reversed**.
(b) Direction of force is **reversed**.
(c) Direction of force **remains unchanged** ($\vec{F} \propto (-\vec{I}) \times (-\vec{B}) = \vec{I} \times \vec{B}$).
Q44 | 3 Marks
A coil is connected to a galvanometer. What happens when a bar magnet is: (i) pushed into coil, (ii) withdrawn from coil, (iii) held stationary inside?
Answer:
(i) Galvanometer deflects momentarily in one direction (e.g., right).
(ii) Galvanometer deflects momentarily in the opposite direction (e.g., left).
(iii) Galvanometer deflection drops to zero (no change in magnetic flux linkage).
(i) Galvanometer deflects momentarily in one direction (e.g., right).
(ii) Galvanometer deflects momentarily in the opposite direction (e.g., left).
(iii) Galvanometer deflection drops to zero (no change in magnetic flux linkage).
[IMAGE PLACEHOLDER: Domestic Electric Wiring Circuit Diagram]
4-Mark & 5-Mark Questions & Solutions
Q48 | 4 Marks
(a) State Right-Hand Thumb Rule. (b) How is magnetic field affected on: (i) increasing current, (ii) reversing current direction?
Answer:
(a) Right-Hand Thumb Rule: Imagine holding a straight current-carrying conductor in your right hand such that the thumb points in the direction of electric current. Then, the fingers curled around the conductor point in the direction of magnetic field lines.
(b) (i) Magnetic field strength $B$ **increases** ($B \propto I$).
(ii) Direction of magnetic field lines **reverses**.
(a) Right-Hand Thumb Rule: Imagine holding a straight current-carrying conductor in your right hand such that the thumb points in the direction of electric current. Then, the fingers curled around the conductor point in the direction of magnetic field lines.
(b) (i) Magnetic field strength $B$ **increases** ($B \propto I$).
(ii) Direction of magnetic field lines **reverses**.
Q51 | 4 Marks
Domestic electric circuits and safety measures:
1) Colors/signs of Live and Neutral wires? 2) Potential difference in India? 3) What is short circuiting? 4) Main purpose of fuse?
1) Colors/signs of Live and Neutral wires? 2) Potential difference in India? 3) What is short circuiting? 4) Main purpose of fuse?
Answer:
1) Live wire: Red (Positive), Neutral wire: Black (Negative).
2) Potential difference $= 220\text{ V}$ at frequency $50\text{ Hz}$.
3) **Short Circuiting:** Sudden drastic increase in current when live wire comes into direct physical contact with neutral wire due to insulation failure.
4) **Electric Fuse:** Safety device that melts due to Joule heating ($H = I^2 R t$) to break the circuit and prevent damage to appliances during overloading or short circuits.
1) Live wire: Red (Positive), Neutral wire: Black (Negative).
2) Potential difference $= 220\text{ V}$ at frequency $50\text{ Hz}$.
3) **Short Circuiting:** Sudden drastic increase in current when live wire comes into direct physical contact with neutral wire due to insulation failure.
4) **Electric Fuse:** Safety device that melts due to Joule heating ($H = I^2 R t$) to break the circuit and prevent damage to appliances during overloading or short circuits.
Q53 | 5 Marks
What are magnetic field lines? How is field direction determined? Mention three important properties of magnetic field lines.
Answer:
• Magnetic Field Lines: Continuous imaginary curves drawn in a magnetic field such that the tangent at any point gives the direction of magnetic field at that point.
• Properties:
1) Continuous closed curves (Outside: North to South; Inside: South to North).
2) Two field lines **never intersect** each other because at the point of intersection, compass needle would point in two different directions simultaneously, which is impossible.
3) Relative closeness of field lines indicates field strength (dense near poles $\rightarrow$ stronger field).
• Magnetic Field Lines: Continuous imaginary curves drawn in a magnetic field such that the tangent at any point gives the direction of magnetic field at that point.
• Properties:
1) Continuous closed curves (Outside: North to South; Inside: South to North).
2) Two field lines **never intersect** each other because at the point of intersection, compass needle would point in two different directions simultaneously, which is impossible.
3) Relative closeness of field lines indicates field strength (dense near poles $\rightarrow$ stronger field).
Q54 | 5 Marks
Coil AB of many circular turns of insulated copper wire wrapped over soft iron core:
(a) Special name given to coil AB? (b) Nature of field inside AB? (c) Factors affecting strength? (d) Effect of soft iron core?
(a) Special name given to coil AB? (b) Nature of field inside AB? (c) Factors affecting strength? (d) Effect of soft iron core?
Answer:
(a) **Solenoid**.
(b) **Uniform** magnetic field (represented by parallel, straight, equally-spaced field lines).
(c) Factors: Magnitude of current $I$, number of turns per unit length $n$. Formula: $$B = \mu n I$$
(d) Placing a soft iron core inside transforms it into a powerful **electromagnet** by increasing total magnetic flux density.
(a) **Solenoid**.
(b) **Uniform** magnetic field (represented by parallel, straight, equally-spaced field lines).
(c) Factors: Magnitude of current $I$, number of turns per unit length $n$. Formula: $$B = \mu n I$$
(d) Placing a soft iron core inside transforms it into a powerful **electromagnet** by increasing total magnetic flux density.
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