This MCQ module is based on: Electric Charge, Current and Potential Difference
TOPIC 40 OF 50
Electric Charge, Current and Potential Difference
🎓 Class 10
Science
CBSE
Theory
Ch 11 — Electricity
⏱ ~20 min
🌐 Language: [gtranslate]
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Introduction — The Flow That Powers Modern Life
From the LED bulb lighting your room to the smartphone in your pocket, every electrical device relies on one invisible phenomenon — the controlled flow of electric charge. Chapter 11 explores this flow and the physical laws that govern it. In Part 1 we meet three essential quantities: electric charge, electric current, and potential difference.
11.1 Electric Charge
Matter is built from atoms that carry two kinds of charge: positive (protons in the nucleus) and negative (electrons in shells). When an object gains extra electrons it becomes negatively charged; when it loses electrons it becomes positively charged. The SI unit of charge is the coulomb (C), named after the French physicist Charles-Augustin de Coulomb.
Quantisation of charge: Any charge Q on a body is always an integral multiple of the elementary charge e:
\[ Q = n\,e, \qquad e = 1.6 \times 10^{-19}\text{ C} \]
Here n is a whole number (1, 2, 3, …). One coulomb is the charge carried by approximately \(6.25 \times 10^{18}\) electrons.
11.2 Electric Current
When charges move through a conductor in a specific direction, we say an electric current flows. The current tells us how much charge passes through a cross-section of the conductor in one second.
Definition — Electric Current (I): The rate of flow of electric charge through a given cross-section of a conductor.
\[ I = \dfrac{Q}{t} \]
where Q is charge in coulombs and t is time in seconds.
SI unit: ampere (A). \(1\text{ A} = 1\text{ C/s}\).
A current of 1 ampere means 1 coulomb of charge crosses a section every second.
SI unit: ampere (A). \(1\text{ A} = 1\text{ C/s}\).
A current of 1 ampere means 1 coulomb of charge crosses a section every second.
11.2.1 Direction of Current — Conventional vs Electron Flow
Inside a metal wire, the actual charge carriers are electrons, which are negative. They drift from the negative terminal of the cell to the positive terminal. But long before electrons were discovered, scientists assumed current flowed from + to −. This historical convention stuck.
Conventional current is taken to flow from the positive terminal through the external circuit to the negative terminal of the cell. Electrons actually move in the opposite direction. In problems and circuit diagrams, we always mark the conventional direction.
11.2.2 Measuring Current — The Ammeter
An ammeter measures the current flowing through a circuit. It is always connected in series with the component whose current we want to read, because the same current must pass through both. An ideal ammeter has zero resistance so it does not reduce the current it is measuring.
Worked Example 1 — Current from Charge and Time
Q. A current of 0.5 A flows through a conductor. Find the charge that crosses any section of the wire in 4 minutes.
Solution. Given \(I = 0.5\) A, \(t = 4 \times 60 = 240\) s.
From \(I = Q/t\), we get \(Q = I \times t = 0.5 \times 240 = 120\) C.
Charge = 120 coulombs.
Worked Example 2 — Number of Electrons
Q. How many electrons constitute a charge of 1 C? If a current of 1 A flows for 1 second, how many electrons pass a section of the wire?
Solution. Charge on one electron \(e = 1.6 \times 10^{-19}\) C. Using \(Q = n\,e\):
\[ n = \dfrac{Q}{e} = \dfrac{1}{1.6 \times 10^{-19}} \approx 6.25 \times 10^{18} \text{ electrons}. \]
For \(I = 1\) A over \(t = 1\) s, \(Q = 1\) C, so the same \(6.25 \times 10^{18}\) electrons cross the section.
Worked Example 3 — Time for a Given Charge
Q. How long will it take for 480 C of charge to flow through a wire carrying a steady current of 4 A?
Solution. \(t = Q/I = 480 / 4 = 120\) s = 2 minutes.
11.3 Electric Potential and Potential Difference
A cell maintains a difference of potential between its two terminals. This difference is what pushes charges around the circuit — just as a height difference pushes water down a slope.
Electric potential difference between two points of a current-carrying conductor is defined as the work done to move a unit positive charge from one point to the other.
\[ V = \dfrac{W}{Q} \]
SI unit: volt (V). 1 volt = 1 joule/coulomb. A potential difference of 1 volt means 1 joule of work is needed to carry 1 coulomb of charge between the two points.
Potential difference is measured with a voltmeter, which is always connected in parallel across the component whose voltage is being measured. An ideal voltmeter has infinite resistance so that it draws negligible current.
11.4 Circuit Components and Symbols
A circuit diagram is a schematic drawing that uses standard symbols instead of real pictures. Every student of electricity must recognise the following symbols at a glance.
| Component | Symbol meaning | Role |
|---|---|---|
| Cell | long line (+), short thick line (−) | source of potential difference |
| Battery | two or more cells in series | higher EMF |
| Switch (plug key) | open gap / closed line | makes or breaks circuit |
| Resistor | rectangle | restricts current |
| Rheostat | resistor with arrow | continuously variable R |
| Ammeter (A) | circle with A — series | measures current |
| Voltmeter (V) | circle with V — parallel | measures potential difference |
| Bulb | circle with cross | converts electrical energy to light & heat |
Worked Example 4 — Potential Difference from Work
Q. How much work is done in moving a charge of 2 C across two points having a potential difference of 12 V?
Solution. From \(V = W/Q\), \(W = V \times Q = 12 \times 2 = 24\) J.
Worked Example 5 — Energy Supplied by a Battery
Q. A battery of 9 V drives 500 C of charge through a conductor. Calculate the energy supplied.
Solution. Energy = \(V \times Q = 9 \times 500 = 4500\) J = 4.5 kJ.
Activity 11.1 — A Simple Electric CircuitL3 Apply
Aim: To assemble a simple circuit with a cell, bulb, switch and ammeter, and observe the current.
Materials: 1.5 V dry cells (×2), a small torch bulb (1.5 V), connecting wires, a switch (plug key), an ammeter (0–1 A).
Procedure:
- Connect the + terminal of the battery to one terminal of the switch using a wire.
- From the other terminal of the switch, run a wire through the ammeter to one terminal of the bulb.
- Connect the other terminal of the bulb back to the − terminal of the battery.
- Close the switch and note the ammeter reading.
- Reverse the ammeter connections — what happens to the pointer?
Predict: If you add a second cell in series with the first, will the ammeter reading increase, decrease or stay the same?
With one cell the bulb glows dimly and the ammeter shows a small current, say 0.18 A. Adding a second cell in series doubles the potential difference from 1.5 V to 3.0 V, so (for the same bulb resistance) the current roughly doubles and the bulb glows brighter. Reversing the ammeter terminals makes the pointer try to go backward (below zero) — this is why the + terminal of an ammeter must always be connected to the side that leads to the + terminal of the cell.
Competency-Based Questions
Priya sets up a circuit in her school laboratory with a 6 V battery, a resistor, an ammeter and a switch. When she closes the switch, the ammeter reads 0.30 A. She lets the current flow for 5 minutes.
Q1. Define electric current and give its SI unit. L1 Remember
Electric current is the rate of flow of electric charge across a cross-section of a conductor, \(I = Q/t\). SI unit is ampere (A); 1 A = 1 C/s.
Q2. (Numerical) Calculate the total charge that flows through the ammeter during those 5 minutes. L3 Apply
\(t = 5 \times 60 = 300\) s. \(Q = I\,t = 0.30 \times 300 = 90\) C.
Q3. (MCQ) The potential difference across the terminals of Priya's battery is 6 V. The work done in carrying 2 C from the − to the + terminal through the external circuit is L2 Understand
(c) \(W = VQ = 6 \times 2 = 12\) J.
Q4. (Short answer) Why is an ammeter connected in series but a voltmeter in parallel? L2 Understand
The ammeter reads the current through a component, so the same current must flow through it — series connection ensures this. The voltmeter reads potential difference across the two ends of a component, which is the same only when it is placed across those two points — i.e., in parallel. Also, an ammeter is low-resistance and a voltmeter is high-resistance, which matches these connections.
Q5. (HOT) If 3.2 × 10¹⁹ electrons cross a section of a wire in 2 s, calculate the current in the wire. L4 Analyse
Charge \(Q = n e = 3.2 \times 10^{19} \times 1.6 \times 10^{-19} = 5.12\) C. Current \(I = Q/t = 5.12/2 = 2.56\) A.
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): Conventional current flows from the positive terminal of the cell to the negative terminal through the external circuit.
Reason (R): Electrons actually flow from the positive terminal to the negative terminal.
(C) — A is true but R is false. Electrons flow from the negative terminal to the positive terminal; conventional current is taken opposite to the electron drift.
Assertion (A): A voltmeter is always connected in parallel with the component across which voltage is to be measured.
Reason (R): An ideal voltmeter has very high resistance so that it draws negligible current from the circuit.
(B) — Both are true but R does not directly explain why the voltmeter is in parallel. The parallel connection is required because potential difference is defined between two points; R is a design requirement to avoid disturbing the circuit.
Assertion (A): 1 ampere is equal to 1 coulomb per second.
Reason (R): Electric current is defined as charge flowing per unit time.
(A) — Both true and R correctly explains A. The unit ampere follows from the defining equation \(I = Q/t\).
Frequently Asked Questions — Charge, Current & Potential Difference
What is charge, current & potential difference in Class 10 Science (CBSE board)?
Charge, Current & Potential Difference is a key topic in NCERT Class 10 Science Chapter 11 — Electricity. It explains electric charge, current, potential difference and simple electric circuits. Core ideas covered include electric charge, electric current, ampere, potential difference. 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 electric charge important in NCERT Class 10 Science?
Electric charge is important in NCERT Class 10 Science because it forms the foundation for understanding charge, current & potential difference in Chapter 11 — Electricity. Without a clear idea of electric charge, students cannot answer higher-order CBSE board questions involving electric current, ampere, potential difference. Board papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link electric charge to real-life situations. Building clarity here pays off directly in board marks.
How is charge, current & potential difference tested in the Class 10 Science CBSE board exam?
The CBSE Class 10 Science board exam tests charge, current & potential difference 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 electric charge, electric current, ampere 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 charge, current & potential difference in Class 10 Science?
The key terms to remember for charge, current & potential difference in NCERT Class 10 Science Chapter 11 are: electric charge, electric current, ampere, potential difference, volt, electric circuit. 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 Charge, Current & Potential Difference included in the Class 10 Science syllabus for 2025–26 CBSE board exam?
Yes, Charge, Current & Potential Difference is a part of the NCERT Class 10 Science syllabus (2025–26) prescribed by CBSE. It falls under Chapter 11 — Electricity — and is examined in the annual board paper. The current syllabus retains the full treatment of electric charge, electric current, ampere 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 charge, current & potential difference for the CBSE Class 10 Science board exam?
Prepare charge, current & potential difference for the CBSE Class 10 Science board exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of electric charge, electric current, ampere. 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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