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Level 1 Answer Key: Magnetic Effects

Class: 10 Science Topic: Concepts & Rules Max. Questions: 60
Section A: Magnetic Field & Field Lines
  1. Magnetic field is the region around a magnet where its force can be detected. It is a vector quantity (has both magnitude and direction).
    [Diagram: Magnetic field lines around a bar magnet showing the region of influence]
  2. (i) They emerge from North Pole and merge at South Pole (outside). (ii) They form continuous closed curves. (iii) They never intersect.
  3. If two field lines were to intersect, there would be two tangents at the point of intersection, indicating two directions of magnetic field at the same point, which is impossible.
  4. [Diagram: Bar Magnet with field lines emerging N to S outside and S to N inside]
  5. Inside: South to North. Outside: North to South.
  6. At the poles, where the field lines are crowded.
  7. To find the direction of magnetic field and to detect magnetic fields.
  8. Oersted observed that a compass needle placed near a current-carrying wire experienced deflection, proving that electric current produces a magnetic field.
  9. [Diagram: Concentric circles around a straight wire]
  10. Right-Hand Thumb Rule.
  11. (i) Directly proportional to current. (ii) Inversely proportional to distance.
  12. [Diagram: Curved lines around circular loop, straight in the center]
  13. If current is Clockwise: South Pole. If current is Anti-clockwise: North Pole.
  14. Parallel and equidistant straight lines.
  15. It gets deflected due to the magnetic force exerted by the field of the wire.
Section B: Electromagnets & Solenoids
  1. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
  2. [Diagram: Solenoid field lines similar to bar magnet, parallel inside]
  3. (i) Field lines pattern is similar. (ii) Has North and South poles at ends.
  4. (i) Increase magnitude of current. (ii) Increase number of turns. (iii) Use a soft iron core.
  5. A temporary magnet formed by placing a soft iron core inside a current carrying solenoid.
  6. Permanent Magnet: Retains magnetism for long, fixed strength. Electromagnet: Temporary, variable strength (depends on current).
  7. Soft iron has high magnetic permeability and low retentivity, meaning it gains magnetism quickly and loses it instantly when current is stopped.
  8. Steel has high retentivity; it becomes a permanent magnet and does not lose magnetism when current is switched off.
  9. South Pole.
  10. North Pole.
  11. Electric Bells, Cranes for lifting heavy iron loads, Speakers.
  12. The magnetic field disappears instantly (if core is soft iron).
  13. No, Copper is non-magnetic. It will not enhance the magnetic field. Soft iron is used as it is ferromagnetic.
  14. [Diagram: Solenoid coil wrapped around a nail, connected to battery/switch]
  15. The field inside a long solenoid is uniform (parallel lines), similar to the concept of a uniform field, whereas a bar magnet field lines are curved except at the very axis. Externally they appear similar.
Section C: Force on Conductor & EMI
  1. When the conductor carries current and is placed in a magnetic field (provided current is not parallel to the field).
  2. Stretch thumb, forefinger, and middle finger of left hand mutually perpendicular. Forefinger = Field, Middle Finger = Current, then Thumb = Force (Motion).
  3. (i) Maximum when current is perpendicular to magnetic field. (ii) Minimum (zero) when parallel.
  4. Electric Motor (or Loudspeaker).
  5. An electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force.
  6. To reverse the direction of current in the coil every half rotation to ensure continuous unidirectional rotation.
  7. To make sliding contact with the rotating commutator and supply current to the coil.
  8. The phenomenon of inducing an electric current in a coil by changing the magnetic field linked with it.
  9. Stretch thumb, forefinger, central finger of right hand perpendicular. Forefinger = Field, Thumb = Motion of conductor, Middle Finger = Induced Current.
  10. Electric Generator.
  11. Left Hand Rule: Used to find direction of Force (Motor). Right Hand Rule: Used to find direction of Induced Current (Generator).
  12. (i) Moving a magnet inside a coil. (ii) Changing current in a nearby coil.
  13. AC changes direction periodically. DC flows in one direction only.
  14. [Diagram: DC is straight line, AC is sine wave]
  15. Galvanometer detects presence and direction of small currents. Ammeter measures magnitude of current.
Section D: Domestic Electric Circuits
  1. Live (Red/Brown), Neutral (Black/Blue), Earth (Green/Yellow).
  2. It provides a low resistance path for leakage current to ground. It protects the user from electric shock if insulation fails in metallic appliances.
  3. 220 Volts.
  4. When Live and Neutral wires come in direct contact (due to damaged insulation). Resistance becomes zero, current becomes huge.
  5. When current drawn exceeds the rating. Causes: (i) Connecting too many appliances to one socket, (ii) Accidental voltage hike.
  6. Safety device that breaks circuit if current exceeds limit. Works on Joule's Heating Effect ($H=I^2Rt$).
  7. If current exceeds the rating, the heat generated melts the fuse wire, breaking the circuit and stopping current flow.
  8. So that it melts and breaks quickly when temperature rises due to excess current.
  9. It implies the maximum current that can flow through it safely is 5 Amperes.
  10. (i) Each appliance gets full voltage (220V). (ii) Switching off one does not affect others.
  11. $P = 1500W, V = 220V. I = P/V = 1500/220 \approx 6.8 A$. So, a fuse rating of 7A or 10A should be used.
  12. 50 Hz. DC has 0 Hz (constant direction).
  13. Because our body conducts electricity and potential difference between live wire and earth causes severe shock.
  14. Don't connect too many high-power appliances to a single socket. Use fuse/MCB of proper rating.
  15. So that when switch is off, the live connection is broken and the appliance is at zero potential (safe to touch).