Chapter 12 · Magnetic Effects of Electric Current

NCERT Class 10 Science Chapter 12 Exercises

All 9 questions of NCERT Class 10 Science Chapter 12 Exercises (Magnetic Effects of Electric Current), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Which of the ​following ​correctly ​describes the magnetic field ​near ​a long straight ​wire?

   1. (a)The field consists of straight lines perpendicular to the wire.
   2. (b)The field consists of straight lines parallel to the wire.
   3. (c)The field consists of radial lines originating from the wire.
   4. (d)The field consists of concentric circles centred on the wire.
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   (d) The field consists of concentric circles centred on the wire.

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2. Q2
   At the ​time of ​short circuit, the ​current ​in ​the circuit

   1. (a)reduces substantially.
   2. (b)does not change.
   3. (c)increases heavily.
   4. (d)vary continuously.
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   (c) increases heavily.

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3. Q3
   State whether the following statements are true or false.

   1. (a)The field at the ​centre ​of a long circular coil carrying current ​will be parallel straight ​lines. ​
   2. (b)A wire with a green insulation is usually the live wire of ​an electric ​supply.
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   (a) True: the field inside a long coil is uniform. (b) False: green insulation marks the earth wire, not the live wire.

   (a) True. A long circular coil carrying a current is a solenoid, and inside a long solenoid the magnetic field lines are straight, parallel and equally spaced — which means the field there has the same magnitude and direction at every point, that is, it is uniform. (b) False. Green, or green with a yellow stripe, is the colour of the **earth** wire, which runs from the metal body of an appliance to a plate buried in the ground and carries no current in normal working. The live wire is red, or brown in newer wiring.

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4. Q4
   List ​two methods of ​producing magnetic ​fields.
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   A permanent magnet and an electric current in a conductor each produce a magnetic field.

   Two methods are: (i) By using a permanent magnet. A bar magnet or a horseshoe magnet sets up a magnetic field in the space around it; outside the magnet the field lines run from its north pole to its south pole, and the field cannot be switched off. (ii) By passing an electric current through a conductor. A current-carrying straight wire, a circular loop or a solenoid each produces a magnetic field around it — this is Oersted’s magnetic effect of a current. A solenoid wound on a soft-iron core makes an electromagnet, whose field is strong and can be switched on and off with the current.

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5. Q5
   When ​is the force ​experienced by ​a ​current–carrying ​conductor placed in ​a magnetic ​field largest?
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   The force is largest when the current is at right angles to the field ($\theta =90^{\circ }$), where $F=BIL$.

   The force is largest when the current-carrying conductor is placed at right angles to the magnetic field — that is, when the direction of the current is perpendicular ($90^{\circ }$) to the direction of the field. The magnitude of the force is $F=BIL\sin \theta$, where $\theta$ is the angle between the current and the field, so $F$ takes its greatest value $F=BIL$ when $\sin \theta =1$, at $\theta =90^{\circ }$. When the conductor lies along the field, $\theta =0^{\circ }$ and the force is zero.

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6. Q6
   Imagine that you are sitting in ​a chamber with your back to one wall. An electron beam, moving ​horizontally ​from back wall towards the front wall, is ​deflected by a ​strong magnetic field to your ​right ​side. What is the direction of magnetic field?
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   The field is vertically downwards, found by Fleming's left-hand rule with the current taken opposite to the electron beam.

   The magnetic field is directed vertically downwards. The beam is made of electrons, which are negative, so the conventional current runs opposite to the beam — from the front wall back towards you. The force on the beam is to your right and is at right angles to that current, and the field must be at right angles to both, so the field can only be vertical. Applying Fleming’s left-hand rule with the middle finger pointing towards you (the current) and the thumb pointing to your right (the force), the first finger — the field — points straight down.

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7. Q7
   State ​the rule to determine the direction of a

   1. (i)magnetic ​field produced around ​a straight conductor-carrying current,
   2. (ii)force experienced by a current-carrying straight conductor placed in a magnetic field which ​is perpendicular to it, and ​
   3. (iii)current ​induced in ​a coil due to its ​rotation in a magnetic field.
   Show answer

   The right-hand thumb rule gives the field, Fleming’s left-hand rule the force, and Fleming’s right-hand rule the induced current.

   (i) Right-hand thumb rule (also called Maxwell’s corkscrew rule). Hold the straight current-carrying conductor in your right hand so that the thumb points along the direction of the conventional current; the fingers wrapped around the conductor then give the direction of the magnetic field lines. (ii) Fleming’s left-hand rule. Stretch the thumb, the forefinger and the middle finger of the left hand so that they are mutually perpendicular; if the forefinger points along the magnetic field and the middle finger along the current, the thumb points in the direction of the force on the conductor. (iii) Fleming’s right-hand rule. Stretch the thumb, the forefinger and the middle finger of the right hand so that they are mutually perpendicular; if the forefinger points along the magnetic field and the thumb along the direction of motion of the conductor, the middle finger points in the direction of the induced current.

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8. Q8
   When does ​an electric ​short ​circuit occur?
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   A short circuit occurs when the live wire and the neutral wire come into direct contact.

   An electric short circuit occurs when the live wire and the neutral wire come into direct contact with each other. This happens when the insulation of the wires is damaged or worn through, when there is a fault inside an appliance, or at a loose connection. The current then bypasses the appliance and returns to the supply through a path whose resistance is only a fraction of an ohm. Since the supply voltage stays at about $220$ V, by $I=V/R$ the current rises heavily — from a few amperes to hundreds of amperes — and the heat produced, $H=I^{2}Rt$, can melt the wires and start a fire. The fuse in the live wire melts first and breaks the circuit.

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9. Q9
   What is ​the function of an ​earth ​wire? ​Why is it ​necessary to ​earth ​metallic appliances?
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   The earth wire gives a low-resistance path to the ground, keeping the metal body at earth potential and carrying away fault current.

   Function of the earth wire: The earth wire is the green (or green-and-yellow) wire of a domestic supply. It is connected at one end to the metallic body of the appliance and at the other to a metal plate buried deep in the ground near the house, and it provides a low-resistance conducting path from the body to the earth. In normal working it carries no current; its job is to hold the metal body at the potential of the earth, which is taken as $0$ V, and to carry away any current that reaches the body during a fault. Why metallic appliances must be earthed: If the insulation inside a metal-bodied appliance fails and the live wire touches the casing, the casing is raised to about $220$ V. Anyone who then touches it completes a path to the ground through their own body and receives a severe, possibly fatal, electric shock. With the body earthed, the fault current instead flows through the earth wire, whose resistance is a fraction of an ohm against the thousands of ohms of a human body, so almost none of it passes through the person and the casing never rises above the earth’s potential. The large fault current also melts the fuse in the live wire and disconnects the supply. This is why metallic appliances such as an electric iron, a toaster or a refrigerator must always be earthed.

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