Vardaan Watermark

VARDAAN LEARNING INSTITUTE

Level 2 Answer Key: Magnetic Effects

Class: 10 Science Topic: Logic, Numericals & Reasoning Max. Questions: 50
Section A: Direction & Logic (1-15)
  1. Upwards. (Using Fleming's Left Hand Rule: Motion is West, Force is North -> Field is Upwards). Note: Current direction is same as positive charge motion (West).
  2. Towards South. (Electron moves West to East -> Current is East to West. Field is Down. Left Hand Rule: Forefinger Down, Middle Finger West -> Thumb points South).
  3. (a) Into the page (Downwards). (b) Out of the page (Upwards). (Using Right Hand Thumb Rule: Current East to West -> Fingers curl into page at North side).
  4. Downwards. (Electron from Back to Front -> Current Front to Back. Force is Right. Left Hand Rule: Middle Finger Front to Back, Thumb Right -> Forefinger points Down).
  5. Towards South. (Proton East -> Current East. Field Up. Left Hand Rule: Middle Finger East, Forefinger Up -> Thumb South).
  6. Right-Hand Thumb Rule.
  7. Towards North. (Current East to West. Below wire, curl fingers -> Points North).
  8. South Polarity. (Clock Face Rule: Clockwise Matches South pole field entry).
  9. Into the page. (Electron Lower to Upper -> Current Upper to Lower. Field Left to Right. Left Hand Rule: Middle Down, Forefinger Right -> Thumb into page).
  10. No. Because neutron is electrically neutral (q=0), so Magnetic Force F = qvB is zero.
  11. Attract. (Currents in same direction attract).
  12. East. (Wait. Current Up. Magnetic field lines are anti-clockwise. At East of wire, tangent points North. So North pole points North).
  13. The direction of magnetic field lines is reversed, and the polarity of the faces is reversed (North becomes South and vice versa).
  14. No. Monopoles do not exist. Magnetic poles always exist in pairs (North and South). Breaking a magnet creates two smaller magnets.
  15. The magnetic field lines would reverse their direction periodically with the frequency of the AC source.
Section B: Circuit Analysis & Diagrams (16-30)
  1. [Diagram: Mains (L, N, E) -> Fuse/Meter -> Distribution Board -> Switches -> Appliances in Parallel. Earth connected to socket top pin]
  2. Fuse is placed in the Live wire, before the electricity meter or at the beginning of the distribution board.
  3. Live, Neutral, Earth. The top pin is Earth. It is thicker and longer so that it connects first (providing safety) and cannot be inserted into the wrong holes.
  4. Bring a bar magnet's North pole near one face of the coil. If it attracts, that face is South. If repels, it is North. Or suspend it freely; it aligns N-S.
  5. [Diagram: Solenoid field lines parallel inside, curving outside N to S. Resembles Bar Magnet]
  6. If switch is in neutral, the appliance remains connected to Live wire even when switch is off. Touching it inside causes shock. Hazard: Shock risk even when switched off.
  7. Parallel ensures voltage (220V) for all. If connected in series, voltage divides, and if one breaks (fuses), the circuit opens and all stop working.
  8. [Diagram: Rectangular coil between magnet poles, split rings, brushes, battery]
  9. Brushes pass current from static battery to rotating split rings. Without them, there is no electrical connection to the rotating coil, so it won't work.
  10. [Diagram: Stronger, denser field lines through the core]
    The core increases magnetic permeability, making field lines much denser (stronger field).
  11. Short circuit is when L and N touch directly (no resistance path). Current becomes $I = V/R \approx V/0 \rightarrow \infty$ (Very high).
  12. [Diagram: Circular loop. At center, lines go Into page (Cross) for Clockwise current]
  13. North Pole (Anti-clockwise = North).
  14. [Diagram: Magnet moving into/out of Coil connected to Galvanometer]
  15. The wire should be placed parallel to the needle and very close to it (over or under).
Section C: Numericals & Safety (31-40)
  1. $P = 2000 W, V = 220 V$. Current $I = P/V = 2000/220 \approx 9.09 A$. Since rating is 5A, the fuse will blow or the circuit will trip. It cannot be operated.
  2. $I = P/V = 1000/220 \approx 4.54 A$. Ideal fuse rating should be slightly higher, i.e., 5 A.
  3. In parallel, Voltage is same. $P = VI \Rightarrow I = P/V$. Current is directly proportional to Power. So 100W bulb draws more current. Ratio = 100:60 = 5:3.
  4. Total Power = $10 \times 40 = 400 W = 0.4 kW$. Time = 5 h. Energy = $P \times t = 0.4 \times 5 = 2 kWh$ (or 2 Units).
  5. $I = 1500/220 \approx 6.8 A$. The current (6.8A) > Fuse Rating (5A). The fuse will melt immediately. It will NOT work.
  6. $R = V^2/P = (220 \times 220)/100 = 48400/100 = 484 \Omega$.
  7. Heat $H = V^2/R \times t$. If V becomes half (110 vs 220), Heat becomes $(1/2)^2 = 1/4$ times. Heat produced reduces to one-fourth.
  8. For single turn radius $R$, $B_1 \propto I/R$.
    For double loop (2 turns), length $2\pi r \times 2 = 2\pi R \Rightarrow r = R/2$.
    $B_2 = N \times (\text{Field for } r) \propto 2 \times I/(R/2)$ $= 4 I/R$.
    So field becomes 4 times.
  9. $P = V \times I = 220 \times 4.5 = 990 W$.
  10. Total Power $P_{total} = 1000 + 500 = 1500 W$. $I = 1500/220 \approx 6.81 A$.
Section D: Scientific Reasoning (41-50)
  1. To offer minimum resistance. In case of a fault, the huge leakage current should flow through the earth wire to ground instantly to blow the fuse/MCB, rather than through the user or appliance. Thicker wire = Less Resistance.
  2. Because the magnetic field lines inside are parallel and equidistant, indicating that the magnetic field strength is the same at all points inside the solenoid.
  3. (1) Voltage division: Appliances won't get rated 220V. (2) Reliability: If one fails, circuit breaks and all fail.
  4. Physics of Protection: Metallic body is connected to Earth. If live wire touches the body, potential of body becomes zero (Earth potential) and current flows to Earth (low resistance path). This prevents shock to the user touching the metal.
  5. The tangent to a magnetic field line gives the direction of the field. If lines intersect, there would be two tangents at one point, implying two directions of the field at the same point, which is physically impossible.
  6. Soft iron has low retentivity (loses magnetism easily), making it suitable for temporary magnets. Steel has high retentivity (retains magnetism), making it a permanent magnet (cannot be switched off).
  7. A current carrying wire produces a magnetic field around it. This magnetic field exerts a magnetic force/torque on the compass needle (which is a small magnet), causing it to deflect.
  8. The force $F = BIl \sin(\theta)$. When conductor is perpendicular, $\theta = 90^\circ$, $\sin(90) = 1$ (Maximum). When parallel, $\sin(0) = 0$ (Zero force).
  9. Because they are good conductors of electricity (low resistivity), minimizing energy loss as heat ($I^2R$) during transmission.
  10. AC can be stepped up (voltage increased, current decreased) using transformers. Transmission at high voltage/low current reduces $I^2R$ heating loss significantly. DC cannot be stepped up/down easily by transformers.