Class 10 Science · Chapter 12 NotesMagnetic Effects of Electric Current
Comprehensive NCERT notes for Class 10 Science Chapter 12. Learn about magnetic fields, solenoids, Fleming's Left-Hand Rule, and domestic electric circuits.
In this chapter, we explore the fascinating link between electricity and magnetism, a discovery first made by Hans Christian Oersted in 1820. You will learn how an electric current flowing through a conductor generates a magnetic field and how this field's pattern changes based on the shape of the conductor, such as a straight wire, a circular loop, or a solenoid. The chapter also explains the physical force exerted on a current-carrying conductor when placed within a magnetic field, guided by Fleming's Left-Hand Rule. Beyond theoretical concepts, we examine practical applications including electromagnets and the essential safety components of domestic electric circuits. Understanding these principles is fundamental to grasping how modern electrical appliances and safety systems, like fuses and earthing, function in our daily lives to prevent hazards like short-circuiting and overloading.
What you'll learn
1Describe the properties and behavior of magnetic field lines around a bar magnet
2Apply the Right-Hand Thumb Rule to determine the direction of magnetic fields
3Explain the magnetic field patterns produced by straight conductors, circular loops, and solenoids
4Define the concept of an electromagnet and its construction
5Predict the direction of force on a conductor using Fleming's Left-Hand Rule
6Identify the components and safety measures of domestic electric circuits
Chapter at a glance
01Chapter Overview
02Magnetic Field Due to Current
03Force on Current-Carrying Conductor in Magnetic Field
04Domestic Electric Circuits and Safety Measures
05Magnetic Field and Field Lines
06Magnetic Field due to a Current through a Straight Conductor
07Right-Hand Thumb Rule
08Magnetic Field due to a Current through a Circular Loop
09Magnetic Field due to a Current in a Solenoid
Detailed chapter notes
01
Magnetic Field and Field Lines
A magnetic field is the region surrounding a magnet where its force can be detected. We represent this field using imaginary lines called field lines. By convention, these lines emerge from the north pole and merge at the south pole outside the magnet, while inside, they move from south to north, forming closed curves. The density of these lines indicates the strength of the field; the closer the lines, the stronger the magnetic force. A key property is that no two field lines ever intersect, as this would imply a compass needle pointing in two directions at once.
North PoleThe north-seeking end of a magnet
South PoleThe south-seeking end of a magnet
Field StrengthProportional to the degree of closeness of field lines
IntersectionField lines never cross each other
02
Magnetic Field due to Current-Carrying Conductors
Every electric current produces a magnetic field. For a straight wire, the field lines form concentric circles around the conductor. The direction of these circles is determined by the Right-Hand Thumb Rule. If the wire is bent into a circular loop, the field lines at the center appear as straight lines because the contributions from every part of the loop add up. In a solenoid—a long coil of many circular turns—the internal magnetic field is uniform and represented by parallel straight lines, behaving much like a bar magnet.
Right-Hand Thumb RuleThumb points to current, wrapped fingers show field direction
SolenoidA cylinder-shaped coil of insulated copper wire
Uniform FieldThe magnetic field is the same at all points inside a solenoid
ElectromagnetA soft iron core magnetized by being placed inside a current-carrying solenoid
03
Force on a Conductor in a Magnetic Field
When a current-carrying conductor is placed in an external magnetic field, it experiences a mechanical force. This force is a result of the interaction between the magnetic field of the current and the external field. The magnitude of this force is highest when the current direction is perpendicular to the magnetic field. This principle is the foundation for various electrical devices like motors and loudspeakers. The direction of this force can be predicted using Fleming's Left-Hand Rule.
Maximum ForceOccurs when current and field are at 90 degrees to each other
Force ReversalChanging the direction of either current or field reverses the force direction
04
Domestic Electric Circuits
Household power is supplied via three main wires: the live wire (red insulation), the neutral wire (black insulation), and the earth wire (green insulation). In India, the potential difference between the live and neutral wires is 220 V. Appliances are connected in parallel so that each receives the same voltage and can be operated independently. Safety is maintained through earthing, which protects against shocks from metallic bodies, and fuses, which prevent damage from excessive current flow.
Live WireCarries 220 V potential (Positive)
Neutral WireCompletes the circuit (Negative)
Earth WireSafety wire connected to a metal plate deep in the ground
Potential Difference220 V in domestic circuits
05
Safety Measures: Hazards and Fuses
Electrical hazards primarily include short-circuiting and overloading. Short-circuiting occurs when the live and neutral wires come into direct contact due to damaged insulation. Overloading happens when too many high-power appliances are connected to a single socket or when there is a sudden hike in supply voltage. The electric fuse is the most critical safety device; it contains a wire that melts due to Joule heating when current exceeds a safe limit, breaking the circuit and protecting appliances.
Short-circuitAbrupt increase in current due to direct live-neutral contact
OverloadingExcessive current draw beyond the circuit's capacity
Electric FuseSafety device that breaks the circuit during high current flow
Joule HeatingThe principle used by fuses to melt and stop current
Want the complete chapter resources?Topic notes, quizzes and flashcards for Magnetic Effects of Electric Current.
Which of the following correctly describes the magnetic field near a long straight current-carrying wire?
AThe field consists of straight lines perpendicular to the wire.
BThe field consists of straight lines parallel to the wire.
CThe field consists of radial lines originating from the wire.
DThe field consists of concentric circles centred on the wire.
Show answer
Answer: (D) The field consists of concentric circles centred on the wire.
The iron filings around a straight current-carrying wire align in concentric circles, showing that the magnetic field lines are concentric circles centred on the wire.
Question 02
A long straight current-carrying conductor is placed vertically. The magnetic field lines produced around it are:
Astraight lines parallel to the conductor
Bconcentric circles centred on the conductor
Cradial lines emerging from the conductor
Dstraight lines perpendicular to the conductor
Show answer
Answer: (B) concentric circles centred on the conductor
The iron filings around a straight current-carrying wire align in concentric circles, which represent the magnetic field lines.
Question 03
A current-carrying conductor is placed in a magnetic field. The force acting on it is largest when the direction of current is:
Aparallel to the magnetic field
Bat 45 degrees to the magnetic field
Cat right angles to the magnetic field
Dopposite to the magnetic field
Show answer
Answer: (C) at right angles to the magnetic field
According to the NCERT text, the displacement of the rod is largest when the direction of current is at right angles to the direction of the magnetic field.
Question 04
In an electric motor, which of the following components is responsible for reversing the direction of current in the coil?
AArmature
BSplit ring commutator
CPermanent magnet
DBrush
Show answer
Answer: (B) Split ring commutator
The split ring commutator reverses the direction of current every half rotation, ensuring the coil continues to rotate in the same direction.
Question 05
What is the standard voltage of domestic electric supply in India?
A100 V
B220 V
C440 V
D110 V
Show answer
Answer: (B) 220 V
The standard domestic electric supply voltage in India is 220 V AC (alternating current).
Ready for more practice?Unlock the full quiz for this chapter.
Q1. State the right-hand thumb rule and use it to describe the magnetic field lines produced around a long straight current-carrying conductor.
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Model answer
Right-hand thumb rule: Imagine holding a current-carrying straight conductor in your right hand such that the thumb points in the direction of current. Then the fingers wrap around the conductor in the direction of the magnetic field lines. For a long straight conductor, the magnetic field lines are concentric circles centred on the wire, lying in a plane perpendicular to the wire. The direction of the field is given by the rule, and the field strength decreases as we move away from the wire.
Sample question3 marks
Q2. Describe the pattern of magnetic field lines produced around a long straight current-carrying conductor. State how the direction of the field is determined.
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Model answer
The magnetic field lines around a long straight current-carrying conductor are concentric circles centred on the wire. The direction of the field is given by the right-hand thumb rule: if the conductor is held in the right hand with the thumb pointing in the direction of current, the fingers wrap around the conductor in the direction of the field lines. The field is stronger near the wire and becomes weaker as the distance from the wire increases.
Sample question3 marks
Q3. State Fleming's left-hand rule and explain how it helps determine the direction of force on a current-carrying conductor placed in a magnetic field.
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Model answer
Fleming's left-hand rule states that if we stretch the thumb, forefinger and middle finger of our left hand mutually perpendicular, with the forefinger pointing in the direction of the magnetic field and the middle finger in the direction of current, then the thumb points in the direction of the force (motion) acting on the conductor. This rule is used when the current and magnetic field are perpendicular to each other.
Sample question3 marks
Q4. State the principle of an electric motor and explain the function of the split ring commutator in it.
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Model answer
An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force. The split ring commutator reverses the direction of current in the coil after every half rotation, so the coil continues to rotate in the same direction.
Sample question3 marks
Q5. Name the three wires in a domestic electric circuit and state the colour of insulation of each. What is the potential difference between the live and neutral wires in our country?
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Model answer
The three wires are the live wire (red insulation), the neutral wire (black insulation) and the earth wire (green insulation). In our country the potential difference between the live and neutral wires is 220 V.
Want more questions with answers?Get the full practice set for this chapter.
Why don't two magnetic field lines ever intersect each other?
If two magnetic field lines intersected, it would mean that at the point of intersection, a compass needle would point in two different directions simultaneously. Since this is physically impossible, field lines can never cross.
What is the difference between a short circuit and overloading?
A short circuit occurs when the live and neutral wires touch directly, causing current to spike instantly. Overloading occurs when the total current drawn by appliances exceeds the circuit's capacity, often by connecting too many devices to one socket.
How does an earth wire protect us from electric shocks?
The earth wire connects the metallic body of an appliance to the ground. If there is a current leakage to the metal body, the earth wire provides a low-resistance path for the current to flow into the earth, keeping the potential of the body at zero and preventing a shock to the user.
What are the properties of magnetic field lines around a bar magnet?
Field lines emerge from the north pole and enter the south pole outside the magnet. They are closed curves, they are crowded where the field is strong, and they never intersect each other.
What is a solenoid and how does it behave when current passes through it?
A solenoid is a coil of many circular turns of insulated copper wire wrapped in a cylinder shape. When current passes through it, it produces a magnetic field similar to a bar magnet, with a uniform field inside represented by parallel straight lines.