NCERT Solutions For Class 10 Physics: Magnetic Effects of Electric Current

Intext Questions (Page 196)

Question 1: Why does a compass needle get deflected when brought near a bar magnet?

Answer-
A compass needle is itself a small bar magnet. The region surrounding a bar magnet has a magnetic field. When the compass needle is brought near this field, the magnetic field of the bar magnet exerts a force on the poles of the compass needle. Since like poles repel and unlike poles attract each other, this force causes the compass needle to deflect.

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Intext Questions (Page 201)

Question 1: Draw magnetic field lines around a bar magnet.

Answer-
(The answer describes the pattern shown in Figure 12.4 in the source material).
The magnetic field lines around a bar magnet are represented by closed curves. By convention, the field lines emerge from the north pole and merge at the south pole outside the magnet. Inside the magnet, the direction of the field lines is from the south pole to the north pole. The lines are crowded near the poles where the magnetic field is stronger.

Question 2: List the properties of magnetic field lines.

Answer-
The properties of magnetic field lines are:

1. They are closed curves.
2. By convention, they emerge from the north pole and merge at the south pole outside the magnet.
3. Inside the magnet, the direction of the field lines is from the south pole to its north pole.
4. The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger where the lines are crowded.
5. No two field-lines are found to cross each other.

Question 3: Why don’t two magnetic field lines intersect each other?

Answer-
If two magnetic field lines were to cross each other (intersect), it would mean that at the point of intersection, the compass needle would point towards two directions simultaneously. Since a compass needle can only point in a single direction at any given point, pointing in two directions is not possible.

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Intext Questions (Page 202)

Question 1: Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

Answer-
Applying the right-hand thumb rule to a circular loop carrying a clockwise current:

1. Outside the loop: The magnetic field lines will be directed coming out of the plane of the table (or loop).
2. Inside the loop: The magnetic field lines will be directed going into the plane of the table (or loop).
   (By applying the rule to successive sections of the loop, it is easy to check that every section of the wire contributes to the magnetic field lines in the same direction within the loop).

Question 2: The magnetic field in a given region is uniform. Draw a diagram to represent it.

Answer-
(The answer describes the representation of a uniform field, similar to that found inside a solenoid).
A uniform magnetic field is represented by parallel straight lines. This diagram would show a series of straight lines that are equally spaced and parallel to one another.

Question 3: Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current
(a) is zero.
(b) decreases as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.

Answer-
The magnetic field lines inside a long straight solenoid-carrying current are in the form of parallel straight lines. This pattern indicates that the magnetic field is uniform, meaning it is the same at all points inside the solenoid.
The correct option is (d) is the same at all points.

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Intext Questions (Page 203/204)

Question 1: Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) mass (b) speed (c) velocity (d) momentum

Answer-
When a charged particle (like a proton) moves in a magnetic field, the force acting on it is perpendicular to both its direction of motion and the magnetic field. This force changes the direction of motion, but not the speed.

1. The mass of the proton remains constant.
2. The speed remains constant.
3. The velocity (which has both magnitude/speed and direction) will change because the direction of motion changes.
4. The momentum ($\text{p} = \text{mv}$, where $\text{v}$ is velocity) will change because the velocity changes.

The correct options are (c) velocity and (d) momentum.

Question 2: In Activity 12.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?

Answer-
The force (and thus the displacement) exerted on a current-carrying conductor in a magnetic field is largest when the current, conductor length, and magnetic field strength are maximized.
(i) If the current in rod AB is increased, the displacement of the rod will increase (the force magnitude increases).
(ii) If a stronger horse-shoe magnet is used, the displacement of the rod will increase (the magnetic field strength increases).
(iii) If the length of the rod AB is increased, the displacement of the rod will increase (the force magnitude increases).

Question 3: A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) towards south (b) towards east (c) downward (d) upward

Answer-
This requires the application of Fleming’s left-hand rule.

1. Middle Finger (Current, $\text{I}$): The particle is positively charged and moving West, so the direction of conventional current is West.
2. Thumb (Motion/Force, $\text{F}$): The particle is deflected (Force) North.
3. Forefinger (Magnetic Field, $\text{B}$): Orienting the thumb (Force: North) and the middle finger (Current: West) of the left hand shows that the forefinger (Magnetic Field) points upward.
   The direction of the magnetic field is (d) upward.

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Intext Questions (Page 205)

Question 1: Name two safety measures commonly used in electric circuits and appliances.

Answer-
Two safety measures commonly used in electric circuits and appliances are:

1. Electric Fuse: A fuse is an important safety component that prevents damage to appliances and the circuit due to overloading or short-circuiting. The fuse melts and breaks the circuit if the current becomes unduly high.
2. Earth Wire: This wire, typically with green insulation, is connected to the metallic body of certain appliances (like an electric press or refrigerator). It provides a low-resistance conducting path for the current, ensuring that any leakage current keeps the metallic body's potential at that of the earth, preventing the user from getting a severe electric shock.

Question 2: An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

Answer-
Given: Oven Power $P = 2 \text{ kW} = 2000 \text{ W}$; Supply Voltage $V = 220 \text{ V}$; Circuit Current Rating $I_{max} = 5 \text{ A}$.
First, calculate the current drawn by the oven:
$I_{oven} = \frac{P}{V} = \frac{2000 \text{ W}}{220 \text{ V}} \approx \underline{9.09 \text{ A}}$
Result Expected: Since the oven draws approximately $\underline{9.09 \text{ A}}$ of current, which is much higher than the circuit’s maximum current rating of 5 A, the circuit will experience overloading.
Explanation: The electric fuse in the circuit will melt due to the Joule heating caused by the unduly high current, thereby breaking the circuit. This prevents damage to the circuit and the appliance.

Question 3: What precaution should be taken to avoid the overloading of domestic electric circuits?

Answer-
Overloading occurs when the current in the circuit exceeds the rated maximum limit. Precautions to avoid overloading include:

1. Avoid Connecting Too Many Appliances: Do not connect too many appliances to a single socket.
2. Avoid Short-Circuiting: Ensure that the insulation of wires is not damaged and that the live wire and the neutral wire do not come into direct contact.
3. Use Appropriately Rated Circuits: Ensure that appliances with higher power ratings (like geysers) are connected only to circuits designed for them (e.g., the 15 A circuit).

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Exercise Questions (Page 207)

Question 1: Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.

Answer-
The magnetic field lines around a straight conducting wire carrying an electric current form a pattern of concentric circles centred on the wire.
The correct option is (d) The field consists of concentric circles centred on the wire.

Question 2: At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.

Answer-
Short-circuiting occurs when the live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit abruptly increases (or increases heavily).
The correct option is (c) increases heavily.

Question 3: State whether the following statements are true or false.
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.

Answer-
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines. True. At the centre of the loop, the arcs of the magnetic field circles appear as straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply. False. The live wire is usually covered with red insulation. The earth wire has green insulation.

Question 4: List two methods of producing magnetic fields.

Answer-
Two methods of producing magnetic fields are:

1. Using a permanent magnet (like a bar magnet), which naturally exerts its influence and creates a magnetic field in the surrounding region.
2. By passing an electric current through a metallic conductor, which produces a magnetic field around it.

Question 5: When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

Answer-
Experiments show that the magnitude of the force acting on the current-carrying conductor is the highest (largest) when the direction of current is at right angles (mutually perpendicular) to the direction of the magnetic field.

Question 6: 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?

Answer-
We use Fleming’s left-hand rule:

1. Middle Finger (Current, $\text{I}$): The electron beam moves from back to front. Recall that the direction of conventional current is opposite to the direction of motion of electrons. Thus, the current ($\text{I}$) is flowing from front wall to back wall.
2. Thumb (Motion/Force, $\text{F}$): The beam is deflected to the right side (Force is Right).
3. Forefinger (Magnetic Field, $\text{B}$): Orienting the thumb (Right) and the middle finger (Back wall) of the left hand shows that the forefinger (Magnetic Field) points vertically downward.
   The direction of the magnetic field is downward.

Question 7: State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

Answer-
(i) Magnetic field produced around a straight conductor: The direction is determined by the Right-Hand Thumb Rule (or Maxwell’s corkscrew rule). If you hold the conductor in your right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the field lines.

(ii) Force experienced by a current-carrying straight conductor: The direction is determined by Fleming’s Left-Hand Rule. Stretch the thumb, forefinger, and middle finger of the left hand so they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, the thumb will point in the direction of the force (motion).

(iii) Current induced in a coil due to its rotation in a magnetic field: (This is governed by Fleming’s Right-Hand Rule, which is introduced later in the full textbook chapter, but since the source only provided content up to Fleming's Left-Hand Rule, we rely on the rule mentioned in the prompt). The rule used here would be Fleming’s Right-Hand Rule. (Note: The specific statement of Fleming's Right-Hand Rule is not detailed in the provided source material, only the Left-Hand Rule).

Question 8: When does an electric short circuit occur?

Answer-
An electric short circuit occurs when the live wire and the neutral wire come into direct contact. This usually happens when the insulation of wires is damaged or when there is a fault in an appliance. In this situation, the current in the circuit abruptly increases heavily.

Question 9: What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Answer-
Function of an Earth Wire:
The earth wire, typically covered with green insulation, is connected to a metal plate placed deep in the earth. Its function is to provide a low-resistance conducting path for the current.

Necessity of Earthing Metallic Appliances:
It is necessary to earth metallic appliances (like an electric press or refrigerator) as a safety measure. If there is any leakage of current to the metallic body of the appliance, the earth wire ensures that the potential of the metallic body remains at that of the earth. This prevents the user from receiving a severe electric shock.