This MCQ module is based on: Force on a Current-Carrying Conductor and the Electric Motor
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Force on a Current-Carrying Conductor and the Electric Motor
🎓 Class 10
Science
CBSE
Theory
Ch 12 — Magnetic Effects of Electric Current
⏱ ~20 min
🌐 Language: [gtranslate]
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12.6 Force on a Current-Carrying Conductor in a Magnetic Field
Part 1 showed how a current creates a magnetic field. The converse is equally beautiful — a magnet exerts a force on a current. This is the idea behind every electric motor in your home.
In 1820, Andre-Marie Ampere reasoned as follows: a current-carrying conductor itself produces a magnetic field, so if it is placed in another magnetic field it should experience a force, just as any other magnet would. Experiment confirmed his hunch.
Experiment shows that the force \(F\) on the conductor is:
- largest when the current direction is perpendicular to the field;
- zero when the current is parallel to the field;
- proportional to the current \(I\), the field strength \(B\), and the length \(L\) of the conductor in the field: \(F = BIL\sin\theta\).
12.6.1 Fleming's Left-Hand Rule (for motors)
The direction of the force on the conductor can be found from a simple rule put forth by John Fleming.
Fleming's Left-Hand Rule (FBI rule)
Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the:
Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the:
- Forefinger points along the magnetic Field (B),
- Middle finger points along the direction of the Current (I),
Activity 12.2 — The Jumping Aluminium RodL3 Apply
Materials: a strong horseshoe magnet, a light aluminium rod (AB) suspended by two flexible wires, a 6 V battery, a plug-key, a rheostat.
- Suspend rod AB horizontally between the poles of the horseshoe magnet so that a horizontal magnetic field passes through it.
- Connect A and B to the battery through the key. Close the key — the rod jerks in one direction (say, away from you).
- Reverse the battery connections and close the key again. Observe the direction.
- Keep connections the same but flip the magnet so that the field direction reverses. Observe again.
Predict: In which direction will the rod move if you reverse both the current and the field together?
When current flows, the rod experiences a force perpendicular to both I and B — Fleming's left-hand rule gives the direction. Reversing I or B alone reverses the force, so the rod jumps the opposite way. Reversing both together brings the rod back to its original direction of motion. This confirms that force direction depends on both I and B.
12.7 Electric Motor — Turning Electricity into Rotation
An electric motor uses the force on a current-carrying coil placed in a magnetic field to produce continuous rotation. The key trick is to reverse the direction of current in the coil every half rotation, so that the coil always feels a torque in the same rotational sense.
12.7.1 Construction of a DC Motor
Main parts
- Armature coil (ABCD): a rectangular coil of insulated copper wire wound around a soft iron core, free to rotate about an axle.
- Field magnets: a strong permanent magnet whose poles provide a steady magnetic field in which the coil turns.
- Split-ring commutator (P, Q): two half-cylinders of a metal ring separated by a thin gap; rigidly attached to the coil so they turn with it.
- Brushes (X, Y): two graphite strips pressed against the split-rings. They supply current from the external battery to the rotating coil.
- Battery and key: external DC source.
12.7.2 Working — The Role of the Commutator
Suppose current enters the coil from X, flows through arm AB (left to right). By Fleming's left-hand rule:
- In AB, current flows one way in field B → force \(F_1\) pushes AB upward.
- In CD, current flows the opposite way in the same field → force \(F_2\) pushes CD downward.
The two equal, opposite, parallel forces form a couple that rotates the coil.
After half a rotation, AB is now where CD was, and CD is where AB was. At this exact moment the split-rings also swap the brushes that touch them, so the current direction in the coil reverses. Now AB (at the new bottom) is pushed down and CD (at the new top) is pushed up — the couple continues to act in the same rotational sense, and the coil keeps spinning.
Split-ring commutator = rotating reverser. Without it, after half a turn the coil would feel a torque in the opposite sense and would simply oscillate back and forth. The commutator makes sure the current in the coil reverses at the right moment, producing steady one-way rotation.
How to make the motor spin faster
- Increase the current I through the coil.
- Increase the number of turns N of the coil.
- Use a stronger magnetic field B (bigger magnets).
- Use a soft iron core in the armature — it greatly increases the flux through the coil.
Commercial motors
In real commercial motors (the ones in your ceiling fan or water pump) the armature coil is replaced by several coils wound on a soft iron core, and the permanent magnets are often replaced by electromagnets. This greatly increases the torque and power output.
12.7.3 Applications of Electric Motors
Everyday uses of motors — electric fans, washing machines, mixers and grinders, water pumps, refrigerators, computers (disc drives), electric cars, electric trains, drones, toys, industrial robots, power tools (drills, saws), hair dryers, hospital equipment (ventilators, pumps), CNC machines, cranes and elevators.
Competency-Based Questions
Scenario: Neha assembles a small DC motor for her school project. It has a rectangular coil of 50 turns placed between the poles of a horseshoe magnet. She connects the split-ring commutator to a 6 V battery through two carbon brushes, and the motor spins slowly. She wants to make it spin faster and smoother.
Q1. (MCQ) The main purpose of the split-ring commutator in Neha's motor is to L2 Understand
(b) — After every half turn, the split rings swap brushes, which reverses the current direction in the coil. This keeps the torque acting in the same rotational sense, so rotation is continuous.
Q2. (MCQ) Which change will not increase the speed of Neha's motor? L3 Apply
(d) — Reversing the battery merely reverses the direction of rotation; the speed does not increase.
Q3. (Short answer) In the coil, arm AB carries current from A to B. The field is from N (left) to S (right). State the direction of the force on AB. L3 Apply
Apply Fleming's left-hand rule. Forefinger along field (left to right), middle finger along current (A → B, i.e. into/out-of the page depending on orientation). The thumb then gives the direction of the force. In the standard diagram orientation of Fig 12.9, the force on AB points upward.
Q4. (Short answer) Why are the brushes of a motor usually made of carbon (graphite)? L2 Understand
Graphite is a good conductor of electricity, it is soft enough not to wear out the metal rings, and it has self-lubricating properties that reduce friction between the brushes and the rotating commutator.
Q5. (HOT) A student replaces the split-ring commutator by a pair of continuous slip rings. What will the coil do now, and why? L4 Analyse
Slip rings do not reverse the current in the coil. After half a rotation, the direction of the torque on the coil reverses, and the coil simply oscillates back and forth about its rest position (or, with DC supply, settles with its plane perpendicular to the field). It cannot rotate continuously. Slip rings are instead used in AC generators, where the supply itself changes direction each half cycle.
Assertion–Reason Questions
Options: (A) Both A & R true, R correctly explains A. (B) Both A & R true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Assertion (A): A current-carrying conductor parallel to a uniform magnetic field experiences no force.
Reason (R): The magnetic force on a current-carrying wire is proportional to \(\sin\theta\), where \(\theta\) is the angle between I and B.
(A) — Both true; R correctly explains A (\(\sin 0^\circ = 0\)).
Assertion (A): Fleming's left-hand rule is used to find the direction of current in a motor coil.
Reason (R): In Fleming's left-hand rule, the thumb represents the direction of the force on the conductor.
(D) — A is false; R is true. The left-hand rule gives the direction of force when current and field are known — not the direction of current.
Assertion (A): An electric motor converts electrical energy into mechanical energy.
Reason (R): A current-carrying coil placed in a magnetic field experiences a torque.
(A) — Both true; R correctly explains A. The torque rotates the coil, which drives the shaft — mechanical energy is produced.
Frequently Asked Questions — Force on Conductor & Electric Motor
What is force on conductor & electric motor in Class 10 Science (CBSE board)?
Force on Conductor & Electric Motor is a key topic in NCERT Class 10 Science Chapter 12 — Magnetic Effects of Electric Current. It explains force experienced by a current-carrying conductor in a magnetic field, fleming's left-hand rule and the working of an electric motor. Core ideas covered include force on conductor, Fleming's left-hand rule, electric motor, armature. Mastering this subtopic is essential for scoring well in the CBSE Class 10 Science board exam because board papers repeatedly test these concepts through MCQs, short answers and long-answer questions. This part gives a complete, exam-ready explanation with activities, diagrams and competency-based practice aligned to NCERT.
Why is force on conductor important in NCERT Class 10 Science?
Force on conductor is important in NCERT Class 10 Science because it forms the foundation for understanding force on conductor & electric motor in Chapter 12 — Magnetic Effects of Electric Current. Without a clear idea of force on conductor, students cannot answer higher-order CBSE board questions involving Fleming's left-hand rule, electric motor, armature. Board papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link force on conductor to real-life situations. Building clarity here pays off directly in board marks.
How is force on conductor & electric motor tested in the Class 10 Science CBSE board exam?
The CBSE Class 10 Science board exam tests force on conductor & electric motor through a mix of 1-mark MCQs, 2-mark short answers, 3-mark explanations with examples, 5-mark descriptive questions (often with diagrams or balanced equations) and 4-mark competency-based questions. Expect direct questions on force on conductor, Fleming's left-hand rule, electric motor and application-based questions drawn from NCERT activities. Students who follow NCERT thoroughly and practice this chapter's questions consistently score in the 90%+ range.
What are the key terms to remember for force on conductor & electric motor in Class 10 Science?
The key terms to remember for force on conductor & electric motor in NCERT Class 10 Science Chapter 12 are: force on conductor, Fleming's left-hand rule, electric motor, armature, commutator, brushes. Each of these concepts carries exam weightage and regularly appears in the CBSE board paper. Write clear one-line definitions of every term in your revision notes and revisit them before the exam. Linking these terms visually through a flowchart or concept map makes recall easier during the Class 10 Science board exam.
Is Force on Conductor & Electric Motor included in the Class 10 Science syllabus for 2025–26 CBSE board exam?
Yes, Force on Conductor & Electric Motor is a part of the NCERT Class 10 Science syllabus (2025–26) prescribed by CBSE. It falls under Chapter 12 — Magnetic Effects of Electric Current — and is examined in the annual board paper. The current syllabus retains the full treatment of force on conductor, Fleming's left-hand rule, electric motor as per the NCERT textbook. Because CBSE bases every board question on NCERT, studying this part thoroughly ensures complete syllabus coverage and guarantees marks from this chapter.
How should I prepare force on conductor & electric motor for the CBSE Class 10 Science board exam?
Prepare force on conductor & electric motor for the CBSE Class 10 Science board exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of force on conductor, Fleming's left-hand rule, electric motor. Second, solve every in-text question and end-of-chapter exercise — CBSE questions often come directly from NCERT. Third, practice competency-based and assertion-reason questions to sharpen reasoning. Write answers in the exam-style format (point-wise with diagrams) and time yourself. This method delivers confidence and full marks in the board exam.
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