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NCERT Class 10 Science Magnetic Effects of Electric Current: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 10 Science book, chapter “Magnetic Effects of Electric Current”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   The magnetic field near a long straight wire is (a) straight lines perpendicular to it (b) straight lines parallel to it (c) radial lines from it (d) concentric circles around it.

   1. A current-carrying conductor produces a magnetic field around it.
   2. The direction of this field is determined by the right-hand thumb rule.
   3. If you hold the wire in your right hand with your thumb pointing in the direction of the current, your fingers curl around the wire.
   4. This curling motion indicates that the magnetic field lines form closed loops encircling the wire.
   5. Therefore, the field lines are concentric circles centered on the wire.

   Answer: (d) concentric circles around it

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2. Exercises Q2

   During a short circuit the current (a) falls a lot (b) does not change (c) rises sharply (d) varies continuously.

   1. A short circuit occurs when the live wire touches the neutral wire directly, bypassing the appliance.
   2. This creates a path with very low resistance between the power source and the return line.
   3. According to Ohm's law, current is inversely proportional to resistance for a constant voltage.
      why?

      Ohm's law: at a steady temperature the current through a conductor is proportional to the potential difference across it, so V = IR (R = V/I, I = V/R).

   4. Since the resistance in the circuit drops to nearly zero, the current flowing through the wires increases drastically.
   5. This sudden and large increase in current is what causes the heating and potential fire hazard in a short circuit.

   Answer: (c) rises sharply

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3. Exercises Q3

   True or false: (a) the field at the centre of a long circular coil carrying current is parallel straight lines (b) a wire with green insulation is usually the live wire.

   1. (a) The magnetic field lines inside a long circular coil are nearly parallel and straight, so the statement is true.
   2. (b) In standard wiring, the green insulated wire is the earth wire, not the live wire, so the statement is false.

   Answer: (a) True; (b) False

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4. Exercises Q4

   List two ways of producing magnetic fields.

   1. A magnetic field is produced around a current-carrying conductor.
   2. The strength of this field depends on the amount of current flowing through the wire.
   3. A magnetic field is also produced around a permanent magnet.
   4. The field lines emerge from the north pole and enter the south pole of the magnet.

   Answer: (i) By passing an electric current through a conductor; (ii) By using a permanent magnet.

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5. Exercises Q5

   When is the force on a current-carrying conductor in a magnetic field largest?

   1. The force on a current-carrying conductor in a magnetic field depends on the angle between the current direction and the magnetic field lines.
   2. The magnitude of this force is given by the formula F = BIL sin θ, where θ is the angle between the current and the magnetic field.
   3. The value of sin θ is maximum when θ is 90°.
   4. Therefore, the force is largest when the conductor is placed perpendicular to the magnetic field lines.

   Answer: The force is largest when the current-carrying conductor is placed perpendicular to the magnetic field.

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6. Exercises Q6

   You sit with your back to a wall; an electron beam moving horizontally from the back wall to the front wall is deflected to your right by a magnetic field. What is the field's direction?

   1. The electrons move from the back wall to the front wall, so the conventional current is in the opposite direction: from the front wall towards the back wall.
   2. The beam is pushed to your right, so the force on this current is towards your right.
   3. Fleming's left-hand rule: hold the forefinger (field), middle finger (current) and thumb (force) at right angles.
   4. Point the middle finger towards the back wall (current) and the thumb to the right (force): the forefinger then points vertically downwards.
   5. So the magnetic field is directed vertically downwards.

   Answer: The magnetic field is vertically downward.

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7. Exercises Q7

   State the rule for the direction of (i) the magnetic field around a straight current-carrying conductor (ii) the force on a current-carrying conductor in a perpendicular field (iii) the current induced in a coil rotating in a magnetic field.

   1. (i) The direction of the magnetic field around a straight current-carrying conductor is given by the Right-Hand Thumb Rule.
   2. (i) Hold the conductor in your right hand such that the thumb points in the direction of the current.
   3. (i) The curled fingers then show the direction of the magnetic field lines around the conductor.
   4. (ii) The direction of the force on a current-carrying conductor in a perpendicular magnetic field is given by Fleming's Left-Hand Rule.
   5. (ii) Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other.
   6. (ii) If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, the thumb points in the direction of the force.
   7. (iii) The direction of the induced current in a coil rotating in a magnetic field is given by Fleming's Right-Hand Rule.
   8. (iii) Stretch the thumb, forefinger, and middle finger of your right hand mutually perpendicular to each other.
   9. (iii) If the forefinger points in the direction of the magnetic field and the thumb in the direction of the motion of the conductor, the middle finger points in the direction of the induced current.

   Answer: (i) Right-Hand Thumb Rule; (ii) Fleming's Left-Hand Rule; (iii) Fleming's Right-Hand Rule

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8. Exercises Q8

   When does an electric short circuit occur?

   1. An electric short circuit occurs when the live wire and the neutral wire come into direct contact with each other.
   2. This usually happens when the insulating material covering the wires gets damaged or wears out over time.
   3. Because the resistance of the connecting wire is very low, a very large current flows through the circuit.
   4. This sudden surge in current can generate excessive heat, which may damage the appliances or cause a fire.

   Answer: A short circuit occurs when the live and neutral wires touch directly, causing a very large current to flow due to low resistance.

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9. Exercises Q9

   What does an earth wire do? Why should metallic appliances be earthed?

   1. The earth wire is a safety wire that connects the metallic body of an appliance to the ground.
   2. It provides a low-resistance path for current to flow directly into the earth.
   3. If the live wire touches the metallic body, the current flows through the earth wire instead of the person.
   4. This prevents the metallic body from becoming live and avoids electric shock.
   5. Therefore, metallic appliances must be earthed to ensure user safety.

   Answer: The earth wire provides a low-resistance path to ground for leakage current, preventing electric shock by keeping the metallic body at earth potential.

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