NCERT Class 10 Science Solutions: Magnetic Effects of Electric Current
Question:
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
Concept in a Minute:
The magnetic field produced by a current-carrying wire is described by the right-hand rule. For a long straight wire, the magnetic field lines form concentric circles around the wire, with the direction of the magnetic field given by the direction a thumb points if it’s aligned with the current.
Explanation:
When an electric current flows through a long straight wire, it generates a magnetic field around it. Experiments and theoretical calculations (using Ampere’s Law) show that this magnetic field forms closed loops. Specifically, for a long straight wire, the magnetic field lines are concentric circles, with the wire passing through the center of these circles. The plane of these circles is perpendicular to the wire. The direction of these circular magnetic field lines is determined by the right-hand rule: if you point your thumb in the direction of the current, your fingers curl in the direction of the magnetic field lines. Options A, B, and C are incorrect because they describe different geometries that are not characteristic of the magnetic field around a long straight current-carrying wire.
The final answer is $\boxed{D}$.
Question:
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
Concept in a Minute:
The magnetic field produced by a long straight solenoid is uniform inside it. This is because a solenoid can be approximated as a stack of current loops, and the magnetic fields from these loops add up to create a uniform field in the central region. The field lines inside are parallel and equally spaced.
Explanation:
A long straight solenoid is designed such that the magnetic field inside it is very nearly uniform. This uniformity is a consequence of the geometry of the solenoid and the current distribution. For an infinitely long solenoid, the magnetic field inside is constant and parallel to the axis. For a practically long solenoid (where its length is much greater than its diameter), the field inside is approximately uniform and constant across its cross-section, especially away from the ends. As we approach the ends of the solenoid, the magnetic field lines begin to spread out, and the field strength starts to decrease. Therefore, the magnetic field is the same at all points inside the solenoid, except very close to the ends.
The final answer is $\boxed{D}$.
Question:
State whether the following statement is true or false.
A wire with a green insulation is usually the live wire of an electric supply.
A. True
B. False
Concept in a Minute:
The color coding of electrical wires is a safety standard used to identify different types of wires in an electrical system. The live wire, neutral wire, and earth wire are distinguished by specific colors to prevent accidents.
Explanation:
In most electrical wiring systems, particularly in residential and industrial settings, there are standard color codes for wires to ensure safety and proper identification. The live wire, which carries the electrical current from the power source, is typically insulated with a brown or red color. The neutral wire, which completes the circuit and returns the current, is usually insulated with black or blue. The earth wire, also known as the ground wire, is used for safety and is almost universally insulated with green or green and yellow striped insulation. Therefore, a wire with green insulation is generally the earth wire, not the live wire.
The statement “A wire with a green insulation is usually the live wire of an electric supply” is false.
Final Answer: B
Question:
State whether the following statement is true or false
The field at the centre of a long circular coil carrying current will be parallel straight lines.
A. True
B. False
Concept in a Minute:
Magnetic field inside a long solenoid (or a long circular coil) is uniform and parallel to the axis of the solenoid. This is a consequence of Ampere’s Law.
Explanation:
A long circular coil carrying current behaves like a solenoid. For a long solenoid, the magnetic field lines inside are very nearly uniform and parallel to the axis of the solenoid, especially in the central region. At the center of the coil, these field lines are essentially straight and parallel to each other, pointing along the axis of the coil. Therefore, the statement is true.
The final answer is $\boxed{A}$.
Question:
An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Concept in a Minute:
The core concepts involved are:
1. Electric Power (P), Voltage (V), and Current (I) relationship: P = V * I
2. Understanding the rating of domestic circuits and appliances.
3. Identifying potential issues when an appliance draws more current than the circuit can safely handle.
Explanation:
The electric oven has a power rating of 2 kW (kilowatts), which means it consumes 2000 Watts of power. The domestic electric circuit operates at a voltage (V) of 220 Volts and has a current rating of 5 Amperes (A).
First, we need to calculate the current drawn by the electric oven. We can use the formula P = V * I.
Rearranging the formula to find the current (I): I = P / V.
Given:
Power (P) = 2 kW = 2 * 1000 Watts = 2000 Watts
Voltage (V) = 220 Volts
Calculating the current drawn by the oven:
I = 2000 W / 220 V
I ≈ 9.09 A
Now, we compare the current drawn by the oven with the current rating of the domestic circuit.
Current drawn by the oven ≈ 9.09 A
Current rating of the circuit = 5 A
Since the current drawn by the oven (approximately 9.09 A) is significantly greater than the maximum current the domestic circuit can safely handle (5 A), we expect the circuit to overload. This overload will likely cause the fuse to blow or the circuit breaker to trip, interrupting the power supply to prevent damage and potential fire hazards. The oven will not be able to operate at its full capacity or at all.
Question:
List two methods of producing magnetic fields.
Concept in a Minute:
A magnetic field is a region around a magnetic material or a moving electric charge within which the force of magnetism acts. Magnetic fields can be produced by electric currents and by magnetic materials.
Explanation:
There are two primary methods of producing magnetic fields:
1. By Electric Currents: Moving electric charges create magnetic fields. This is the fundamental principle behind electromagnetism. Any time an electric current flows through a conductor, it generates a magnetic field around that conductor. The strength and direction of this magnetic field depend on the magnitude and direction of the current. Examples include:
* A straight wire carrying current.
* A coil of wire (solenoid) carrying current, which produces a stronger and more uniform magnetic field.
* An electromagnet, which is essentially a coil of wire wrapped around a ferromagnetic core.
2. By Permanent Magnets: Certain materials, called ferromagnetic materials (like iron, nickel, and cobalt), can be permanently magnetized. In these materials, the intrinsic magnetic moments of electrons align in a particular direction, creating a net magnetic field that persists even without an external electric current. These materials have their own inherent magnetic poles (North and South).
Question:
When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Concept in a Minute:
The force experienced by a current-carrying conductor in a magnetic field is described by the Lorentz force law. This force is perpendicular to both the direction of the current and the direction of the magnetic field. The magnitude of this force depends on the strength of the magnetic field, the current flowing through the conductor, the length of the conductor within the field, and the angle between the direction of the current and the magnetic field. The formula for the force is F = BILsinθ, where B is the magnetic field strength, I is the current, L is the length of the conductor, and θ is the angle between the current and the magnetic field.
Explanation:
The force experienced by a current-carrying conductor placed in a magnetic field is largest when the conductor is perpendicular to the magnetic field. This is because the sine of the angle between the current and the magnetic field is at its maximum value of 1 when the angle is 90 degrees (perpendicular). According to the formula F = BILsinθ, if sinθ is maximum, then the force F will also be maximum, assuming B, I, and L remain constant. Therefore, the force is largest when the current-carrying conductor is at a 90-degree angle to the magnetic field lines.
Question:
When does an electric short circuit occur?
Concept in a Minute:
An electric short circuit occurs when there is an unintended, low-resistance path that allows electric current to bypass the normal circuit. This bypass typically happens when the insulation of electrical wires degrades or is damaged, causing direct contact between conductors that should be separated. The low resistance leads to a sudden and drastic increase in current flow.
Explanation:
An electric short circuit occurs when there is a direct connection between two points in an electric circuit that are supposed to be at different electrical potentials, and this connection has a very low electrical resistance. Normally, electricity flows through a designated path with a specific resistance. However, if the insulating material around wires gets damaged or worn out, the bare conductors can touch each other. This creates a low-resistance pathway that allows a much larger amount of current to flow than the circuit is designed to handle. This excessive current can generate a lot of heat, leading to overheating, melting of wires, sparks, fire, and damage to electrical appliances and the power supply. This phenomenon is called a short circuit.
Question:
State the rule to determine the direction of a force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
Concept in a Minute:
The question asks for the rule to determine the direction of force on a current-carrying conductor in a magnetic field. This is directly related to the Fleming’s Left-Hand Rule, which establishes the relationship between the direction of magnetic field, the direction of current, and the direction of the force.
Explanation:
To determine the direction of the force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, we use Fleming’s Left-Hand Rule. This rule states that if you stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other, then:
1. The forefinger points in the direction of the magnetic field (B).
2. The middle finger points in the direction of the current (I).
3. The thumb then points in the direction of the force (F) acting on the conductor.
This rule is applicable when the magnetic field and the current are perpendicular to each other, as stated in the question. By orienting your left hand according to the given directions of the magnetic field and the current, you can accurately predict the direction of the force.
Question:
State the rule to determine the direction of a magnetic field produced around a straight conductor-carrying current.
Concept in a Minute:
The question asks about the rule used to find the direction of the magnetic field around a current-carrying straight conductor. This relates to the fundamental principle of electromagnetism that a moving charge (which is electric current) produces a magnetic field. The direction of this magnetic field is determined by a specific rule.
Explanation:
The rule used to determine the direction of the magnetic field produced around a straight conductor carrying current is the Right-Hand Rule.
Here’s how to apply it:
1. Imagine you are holding the straight conductor in your right hand.
2. Point your thumb in the direction of the electric current flowing through the conductor.
3. Your fingers will then curl around the conductor. The direction in which your fingers curl represents the direction of the magnetic field lines.
In a visual representation, if the current is flowing upwards, the magnetic field lines will be in a counter-clockwise direction when viewed from above. If the current is flowing downwards, the magnetic field lines will be in a clockwise direction.
Question:
The magnetic field in a given region is uniform. Draw a diagram to represent it.
Concept in a Minute:
Magnetic field lines represent the direction and strength of a magnetic field. Uniform magnetic field means the magnetic field has the same magnitude and direction at all points in the region.
Explanation:
A uniform magnetic field can be represented by straight, equally spaced, parallel lines. These lines indicate that the magnetic field strength is the same everywhere in the region and the direction of the field is also the same at every point.
Diagram:
Imagine drawing several straight lines. All these lines should be parallel to each other and should have the same distance between them. These parallel lines represent the uniform magnetic field. You can draw them pointing in any consistent direction (e.g., all pointing upwards, all pointing to the right, or all pointing in some diagonal direction).
For example, to represent a uniform magnetic field directed from left to right, you would draw several straight, equally spaced lines all parallel to each other and oriented horizontally from left to right.
Question:
Write any three properties of magnetic lines of force.
Concept in a Minute:
Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field. They are a visual tool to understand how magnetic forces act in space.
Explanation:
Here are three properties of magnetic lines of force:
1. Direction: Magnetic field lines are directed from the North pole to the South pole outside a magnet. Inside the magnet, they are directed from the South pole to the North pole, forming closed loops.
2. No Crossing: Magnetic field lines never intersect each other. If they were to intersect, it would mean that at the point of intersection, the magnetic field has two different directions simultaneously, which is impossible for a magnetic field at a single point.
3. Density indicates Strength: The relative density of magnetic field lines indicates the strength of the magnetic field. Where the field lines are crowded together, the magnetic field is strong, and where they are spread apart, the magnetic field is weak.
Question:
Why don’t two magnetic lines of force intersect each other?
Concept in a Minute:
Magnetic Field Lines: These are imaginary lines that represent the direction and strength of a magnetic field. They are used to visualize the magnetic field around a magnet or current-carrying conductor.
Direction of Magnetic Field: At any point in a magnetic field, the magnetic field line points in the direction of the magnetic field at that point.
Uniqueness of Direction: A magnetic field at a specific point has a unique direction.
Explanation:
Two magnetic field lines cannot intersect each other because if they did, it would imply that the magnetic field has two different directions at the same point of intersection. This is physically impossible. At any given point in space, the magnetic field can only have one definite direction. Therefore, magnetic field lines, which represent this direction, must never cross or intersect.
Question:
What is the function of an earth wire?
Concept in a Minute:
Electrical safety, current flow, resistance, conductivity, earthing.
Explanation:
The earth wire is a safety feature in electrical appliances. Its main function is to provide a low-resistance path for electrical current to flow to the ground in case of a fault. If the live wire inside an appliance accidentally touches the metal casing, the casing becomes live and could give a dangerous electric shock to anyone who touches it. However, if the appliance is earthed, the current will flow through the earth wire to the ground instead of through the person. This significantly reduces the risk of electric shock. The earth wire is usually connected to the metal casing of the appliance and then to an earth pin on the plug, which is connected to the earth terminal in the electrical socket.
Question:
Why does a compass needle get deflected when brought near a bar magnet?
Concept in a Minute:
A compass needle is a small magnet itself. Magnets exert forces on other magnets. Specifically, opposite poles attract and like poles repel.
Explanation:
A compass needle is a small magnet, which has its own north and south poles. When a bar magnet is brought near the compass, its magnetic field interacts with the magnetic field of the compass needle. The poles of the bar magnet (north and south) exert magnetic forces on the poles of the compass needle. These forces cause the compass needle to align itself with the magnetic field lines of the bar magnet, resulting in a deflection. For example, if the north pole of the bar magnet is brought near the compass, it will attract the south pole of the compass needle and repel its north pole, causing the needle to turn.
Question:
State the rule to determine the direction of a current induced in a coil due to its rotation in a magnetic field.
Concept in a Minute:
Electromagnetic Induction, Faraday’s Law of Induction, Fleming’s Right-Hand Rule
Explanation:
The rule used to determine the direction of a current induced in a coil due to its rotation in a magnetic field is Fleming’s Right-Hand Rule.
To apply Fleming’s Right-Hand Rule:
1. Stretch out your thumb, forefinger, and middle finger of your right hand mutually perpendicular to each other.
2. Point your forefinger in the direction of the magnetic field.
3. Point your thumb in the direction of the motion of the conductor (or the side of the coil).
4. Your middle finger will then point in the direction of the induced current.
Next Chapter: Metals and Non-metals
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