10th Class Physics Notes 2026 — Chapter 17
Chapter 17: Electromagnetism / Magnetic Effects of Current
Class: 10th
Subject: Physics
Board: Punjab
Exercise Solutions
A. Multiple Choice Questions (MCQs)
17.1 Which statement is true about the magnetic poles?
(a) Unlike poles repel
(b) Like poles attract
(c) Magnetic poles do not affect each other
(d) A single magnetic pole does not exist ✓
17.2 A D.C. motor converts:
(a) Mechanical energy into electrical energy
(b) Mechanical energy into chemical energy
(c) Electrical energy into mechanical energy ✓
(d) Electrical energy into chemical energy
17.3 The part of a D.C. motor that reverses the direction of current through the coil every half-cycle is:
(a) The armature
(b) The commutator ✓
(c) The brushes
(d) The split rings
Note: The split rings form the commutator.
17.4 The two main layers of the Earth’s core that contribute to the generation of its magnetic field are:
(a) Crust and mantle
(b) Inner core and outer core ✓
(c) Lithosphere and asthenosphere
(d) Troposphere and stratosphere
17.5 What does a steady electric current in a conductor generate around it?
(a) Electric field
(b) Light
(c) Magnetic field ✓
(d) Sound waves
17.6 What happens to the magnetic field direction when the direction of the current is reversed?
(a) The magnetic field remains unchanged
(b) The magnetic field also reverses direction ✓
(c) The magnetic field becomes stronger
(d) The magnetic field disappears
B. Short Answer Questions
17.1 What is the direction of the magnetic field around a straight current-carrying wire?
Answer:
The magnetic field lines around a straight current-carrying wire are in the form of concentric circles centred on the wire. Their direction is determined by the right-hand grip rule. If you grip the wire with your right hand and point your thumb in the direction of conventional current, your curled fingers indicate the direction of the magnetic field lines.
17.2 What happens to the magnetic field if the magnitude of the current in a wire is increased?
Answer:
If the magnitude of the electric current in a wire increases, the strength of the magnetic field surrounding the wire also increases proportionally. The magnetic field becomes stronger.
17.3 How can you identify the north and south poles of an unmarked magnetized steel bar?
Answer:
There are two methods to identify the poles of a magnetized steel bar:
Suspension Method: Suspend the steel bar freely using a thin, non-magnetic thread. When it comes to rest, the end pointing towards the geographic North Pole is the north pole of the bar, while the other end is the south pole.
Bar Magnet Interaction: Bring a bar magnet with known poles close to the steel bar. Like poles repel, while unlike poles attract. This helps identify the poles of the bar.
17.4 What rule determines the direction of force on a current-carrying conductor placed in a magnetic field? Explain briefly.
Answer:
The direction of the force is determined by Fleming’s Left-Hand Rule.
Explanation: Hold the thumb, forefinger, and middle finger of your left hand mutually perpendicular to one another. If the forefinger points in the direction of the magnetic field (north to south) and the middle finger points in the direction of conventional current, the thumb indicates the direction of force or motion acting on the conductor.
17.5 What role does the commutator play in a D.C. motor?
Answer:
The commutator, consisting of split rings, reverses the direction of electric current flowing through the armature coil after every half-turn (\(180^\circ\)). This reversal ensures that the torque continues to act in the same rotational direction, allowing the motor to rotate continuously in one direction.
17.6 What is the source of the Earth’s magnetic field according to the theory?
Answer:
According to the dynamo theory, the Earth’s magnetic field is generated by convection currents of molten iron and nickel circulating in the Earth’s outer core. These liquid metals are good electrical conductors. Their movement, combined with the Earth’s rotation, produces electric currents that generate the Earth’s magnetic field.
C. Constructed Response Questions
17.1 Why does a magnetic field form around a conductor when current flows through it, but not when the current is OFF? What does this reveal about the nature of electricity and magnetism?
Answer:
Reason: A magnetic field is produced by moving electric charges. When current flows through a conductor, free electrons drift through it, producing a magnetic field. When the current is switched off, the net drift of electrons stops, and the magnetic field associated with the current disappears.
Nature Revealed: This demonstrates that electricity and magnetism are closely connected. The phenomenon is known as electromagnetism. It shows that moving electric charges produce a magnetic field.
17.2 In a D.C. motor, why does the coil stop turning after \(90^\circ\) if the direction of the current is not reversed? What mechanical feature solves this issue?
Answer:
Why it stops: As the coil rotates, the turning effect of the magnetic forces changes. Without reversing the current, the torque may oppose the rotation after the coil passes its effective turning position, causing it to slow down, return, or oscillate instead of rotating continuously.
Solution: The commutator (split-ring) reverses the current direction after every half-turn. This reverses the magnetic forces on the sides of the coil so that the torque continues to drive the coil in the same rotational direction.
17.3 How does placing a wire at different angles to a magnetic field affect the magnetic force it experiences, and why is this important in designing motors?
Answer:
Effect of Angle: The magnetic force on a current-carrying conductor depends on the sine of the angle \(\theta\) between the wire and the magnetic field.
Where:
\(F\) = magnetic force
\(B\) = magnetic field strength
\(I\) = electric current
\(L\) = length of the conductor in the magnetic field
\(\theta\) = angle between the conductor and the magnetic field
The force varies as follows:
Perpendicular position (\(\theta=90^\circ\)): The force is maximum.
Parallel position (\(\theta=0^\circ\) or \(180^\circ\)): The force is zero.
Importance in Motors: Motor armatures are designed so that their current-carrying sides experience suitable magnetic forces to produce maximum rotational torque and improve efficiency.
D. Comprehensive Questions
17.1 How is a magnetic field created around a current-carrying wire, and what factors affect its strength?
Answer:
Creation of the Magnetic Field: When electric charges flow through a wire, they produce circular magnetic field lines around the conductor.
Factors Affecting Magnetic Field Strength:
1. Magnitude of Current (\(I\)): Increasing the current increases the magnetic field strength.
2. Distance from the Wire (\(r\)): The magnetic field is stronger near the wire and becomes weaker as the distance increases.
3. Core Material: Placing a ferromagnetic material, such as iron, inside a coil greatly increases the magnetic field strength.
17.2 Explain the concept of the right-hand grip rule and how it helps determine the direction of the magnetic field around a current-carrying conductor.
Answer:
Concept: The right-hand grip rule is used to determine the direction of magnetic field lines in relation to the direction of current flow.
Applications:
For a Straight Wire: Imagine gripping the wire with your right hand so that your thumb points in the direction of conventional current. Your curled fingers indicate the direction of the circular magnetic field lines.
For a Solenoid: Curl the fingers of your right hand in the direction of conventional current through the coil. Your thumb points towards the north pole of the electromagnet.
17.3 Describe an experiment that demonstrates the magnetic field produced by a current-carrying conductor using a compass or iron filings.
Answer:
Apparatus:
A thick copper wire, a horizontal cardboard sheet, a battery, a switch, and iron filings or a small plotting compass.
Procedure:
Pass the copper wire vertically through a hole in the horizontal cardboard sheet.
Connect the wire to a battery and a switch.
Sprinkle iron filings evenly on the cardboard around the wire.
Close the switch to allow current to flow through the wire.
Gently tap the cardboard.
Observation:
The iron filings arrange themselves in concentric circles around the wire, demonstrating that a magnetic field exists around a current-carrying conductor. A compass placed at different points around the wire can be used to determine the direction of the magnetic field lines.
17.4 Discuss the theory behind the Earth’s magnetic field and the role of the Earth’s molten core in generating it.
Answer:
The Dynamo Theory: The dynamo theory explains that the Earth’s magnetic field is generated within its outer core and is maintained by the movement of electrically conducting molten material.
Role of the Molten Core:
The Earth’s outer core consists primarily of molten iron and nickel, which are good electrical conductors.
Heat from the inner core drives convection currents in the liquid metal.
These circulating currents, combined with the Earth’s rotation and the Coriolis effect, produce organized electric currents.
The electric currents generate the Earth’s magnetic field, which helps protect the planet from harmful charged particles in the solar wind.
Notes by Sir Dilawar
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