Frank Brothers Solutions for ICSE Class 9 Physics Chapter 8.2 Electricity And Magnetism Current Electricity

Page No: 312

Question 1. Define electric current.
Answer: The flow of electrons in a particular direction in a conductor is called an electric current.
In simple words: Electric current is like water flowing through a pipe, but instead of water, tiny particles called electrons are moving through a wire in one direction.

📝 Teacher's Note: Use the analogy of a one-way street for electrons. Remind students that while electrons flow from negative to positive, the "conventional" current is marked from positive to negative.

🎯 Exam Tip: To get full marks, always include the phrase "in a particular direction" to distinguish current from random electron movement.

Question 2. What is an electric cell?
Answer: An electric cell is the source of electric current in which chemical energy changes to electrical energy.
In simple words: A cell is like a small chemical factory that creates electricity by using up the energy stored in the chemicals inside it.

📝 Teacher's Note: Show a standard battery (cell) in class and explain that it has two terminals. Mention that "chemical to electrical" is the energy conversion happening inside.

🎯 Exam Tip: The key phrase to remember is the conversion of "chemical energy to electrical energy."

Question 3. What is the charge on an electron?
Answer: The charge on an electron is \( -1.6 \times 10^{-19} \text{ C} \).
In simple words: An electron carries a very, very tiny amount of negative electricity, so small that it takes trillions of them to do any real work.

📝 Teacher's Note: Explain that 'C' stands for Coulomb, the unit of charge. This is a fundamental constant that students should memorize.

🎯 Exam Tip: Do not forget the negative sign (\( - \)) and the unit 'C' (Coulomb) in your answer.

Question 4. What are the constituents of an electric cell?
Answer: The constituents of cell are two electrodes in the form of conducting rods immersed in the solution called electrolyte.
In simple words: A basic cell is made of two metal rods (electrodes) sitting in a special liquid (electrolyte) that allows electricity to move.

📝 Teacher's Note: You can demonstrate a simple lemon cell in class to show the two different metal pins (electrodes) and the lemon juice (electrolyte).

🎯 Exam Tip: Use the specific terms "electrodes" and "electrolyte" for a scientific answer.

Question 5. State the S.I. units of electric current, potential difference, and resistance.
Answer:
(i) S. I unit of electric current is Ampere
(ii) S.I unit of potential difference is volt.
(iii) S.I unit of resistance is ohm.
In simple words: We measure current in Amps, the "push" (voltage) in Volts, and how much a wire blocks the flow in Ohms.

📝 Teacher's Note: Create a matching table on the board. Ohm (\( \Omega \)), Ampere (A), and Volt (V) are the basic building blocks of circuit math.

🎯 Exam Tip: If you use symbols, make sure they are correct: A for Ampere, V for Volt, and \( \Omega \) for Ohm.

Question 6. Calculate the number of electrons flowing in 1 second for a current of 1A.
Answer: \( I = 1\text{A} \)
\( T = 1\text{s}. \)
\( I = Q/t = ne/t \)
\( \implies n = I \cdot t/e = 1 \cdot 1/(1.6 \times 10^{-19}) = 6.25 \times 10^{18} \) electrons.
In simple words: When one Ampere of current flows, it means more than 6 quintillion electrons are passing by every single second!

📝 Teacher's Note: This calculation helps students realize just how many electrons are involved in even a small current. The value \( 6.25 \times 10^{18} \) is a very common result to memorize.

🎯 Exam Tip: Show the step-by-step formula \( n = \frac{I \cdot t}{e} \) to ensure you get marks even if you make a calculation error.

Question 7. If a charge of 0.7 C flows for 7 seconds, find the electric current.
Answer: \( I = Q/t = 0.7 / 7 = 0.1 \text{ Ampere} \).
In simple words: Dividing the total amount of electricity by the time it took to flow gives us the current, which is 0.1 Amps.

📝 Teacher's Note: Use the "Current Triangle" (Q on top, I and t at bottom) to help students remember the formula \( I = Q/t \).

🎯 Exam Tip: Always include the final unit "Ampere" or "A" to avoid losing half a mark.

Question 8. What is the use of a rheostat?
Answer: Rheostat is used to control the current in the circuit.
In simple words: A rheostat is like a volume knob on a radio; you turn it to increase or decrease how much electricity is flowing.

📝 Teacher's Note: Explain that it works by changing the length of the wire that the electricity has to travel through, which changes the resistance.

🎯 Exam Tip: Mention that it "varies the resistance" to control the current.

Page No : 313

Question 9. Define a rheostat in terms of resistance.
Answer: Rheostat is the variable name of electrical resistance.
In simple words: It is just a resistor whose strength can be adjusted by the user.

📝 Teacher's Note: Use the term "Variable Resistor" as a synonym for Rheostat. It helps students understand its function directly from its name.

🎯 Exam Tip: If asked for the symbol, remember it's a zigzag with an arrow over or through it.

Question 10. What is the function of a switch in a circuit?
Answer: Switch is used to put the current on and off in the circuit.
In simple words: A switch is like a bridge. When it's closed, the electricity can cross; when it's open, the path is broken and the electricity stops.

📝 Teacher's Note: Point out that in physics, an "Open" switch means OFF (broken path), and a "Closed" switch means ON (complete path). This is the opposite of how we think about doors!

🎯 Exam Tip: Use the terms "making or breaking the circuit" for a more formal answer.

Question 11. Define the following circuit components and provide their symbols: (i) Cell, (ii) Ammeter, (iii) Voltmeter, (iv) Key, (v) Battery, (vi) Rheostat.
Answer:
(i) Cell - Cell provide an electric current in the circuit.
(ii) Ammeter - It is a device to measure an electric current in the circuit.
(iii) Voltmeter - It is used to measure the potential difference between two points in the circuit.
(iv) Key - It is used to put the current on and off in the circuit
(v) Battery - It is used to charge the cell. (Multiple cells connected
(vi) Rheostat - it is used to control the electric current in the circuit
In simple words: These are the standard icons we use to draw circuit diagrams on paper, just like a map uses symbols for roads and houses.

📝 Teacher's Note: Have students practice drawing these symbols. Emphasize that an Ammeter is always connected in "Series" and a Voltmeter in "Parallel."

🎯 Exam Tip: Make sure you can identify each symbol by sight, as "Identify the component" is a frequent exam question.

Question 12. If a current of 1.2 A flows for 3 seconds, find the total charge.
Answer: \( I = Q/t \text{ So, } Q = I \cdot t = 1.2 \times 3.0 = 3.6 \text{ C} \).
In simple words: Multiplying the flow rate (current) by the time gives us the total amount of electricity (charge) that moved through the wire.

📝 Teacher's Note: This is a direct rearrangement of the current formula \( Q = I \cdot t \). Check that students use the correct decimal multiplication.

🎯 Exam Tip: Show the multiplication clearly and use the unit 'C' (Coulomb) for the final result.

Question 13. Identify the components in the given circuit diagram labeled A through F.
Answer: A – is a voltmeter to measure the potential difference, B is an electric resistance to control the current in the circuit , C is the ammeter to measure the magnitude of an electric current, D is cell to provide electric current in circuit, E is an electric key to on and off the circuit, F is the rheostat to control the current in circuit.
In simple words: In a complete circuit, we use A and C to measure "push" and "flow," D to provide energy, and B, E, and F to manage how the electricity behaves.

📝 Teacher's Note: This question tests the student's ability to recall both the name and the function of standard circuit components.

🎯 Exam Tip: Match the instrument name to its function precisely (Ammeter \( \rightarrow \) Current, Voltmeter \( \rightarrow \) Potential Difference).

Question 14. What does the slope of a V-I graph represent?
Answer: The slope of the graph represents that with current flowing through a conductor is directly proportional to the potential difference applied the resistance of conductor is constant.
In simple words: If you draw a graph with voltage and current, the steepness of the line tells you the resistance of the wire. A straight line means the resistance isn't changing.

📝 Teacher's Note: This is the visual representation of Ohm's Law. A straight line through the origin proves that \( V \propto I \).

🎯 Exam Tip: The slope of a \( V \) vs \( I \) graph (where \( V \) is on Y-axis) is equal to the Resistance (\( R \)).

Question 15. Define potential difference between two conductors.
Answer: Potential difference between two conductors is defined as the amount of work done in moving the unit positive charge from one conductor to another through the wire.
In simple words: It is the amount of "work" or energy needed to push a single unit of electricity from one point to another. It's like the electrical "pressure."

📝 Teacher's Note: Use the water tank analogy: potential difference is like the difference in water level between two tanks that causes water to flow through a connecting pipe.

🎯 Exam Tip: Use the formula \( V = W/Q \) to support your definition. The keyword "work done" is essential.

Question 16. Is electric current a scalar or vector quantity?
Answer: Yes, electric current is a scalar quantity.
In simple words: Even though electricity flows in a direction, it follows simple math rules (addition) rather than complex direction-based math (vectors).

📝 Teacher's Note: This is a slightly tricky concept because current has direction but doesn't follow vector addition laws. Stick to "scalar" for school-level physics.

🎯 Exam Tip: If asked why, you can state it does not obey the laws of vector addition.

Question 17. On what factors does the electric resistance of a wire depend?
Answer: The electric resistance of the wire depends on the following factors:

• The length of the wire.
• The area of cross-section of the wire.
• The temperature of the wire.
• The material of the wire.

In simple words: Resistance is like friction for electricity. It changes based on how long the wire is, how thick it is, what it's made of, and how hot it gets.

📝 Teacher's Note: Use the "Pipe Analogy": a long pipe (length) is harder to push water through; a fat pipe (cross-section) is easier.

🎯 Exam Tip: Memorize these four factors as they are the standard answer for any "factors of resistance" question.

Question 18. What is the S.I. unit of resistance?
Answer: The S.I unit of resistance is ohm.
In simple words: We measure how much a material fights against the flow of electricity in units called "Ohms."

📝 Teacher's Note: Named after Georg Simon Ohm. The symbol used is the Greek letter Omega (\( \Omega \)).

🎯 Exam Tip: Be careful to distinguish between the name (ohm) and the symbol (\( \Omega \)).

Question 19. How does connecting bulbs in series and parallel affect the total resistance?
Answer:
(i) If another bulb is connected in series then the resistance of the wire will increase.
(ii) If another bulb is connected in parallel then resistance will decrease.
In simple words: Adding bulbs in a single line (series) is like making a longer, harder obstacle course. Adding them side-by-side (parallel) is like opening more lanes on a highway, making it easier for traffic to flow.

📝 Teacher's Note: Explain that in series, the resistances add up directly (\( R_s = R_1 + R_2 \)). In parallel, the reciprocal of total resistance is the sum of reciprocals (\( 1/R_p = 1/R_1 + 1/R_2 \)).

🎯 Exam Tip: Remember: Series = more resistance; Parallel = less resistance.

Question 20. State the mathematical formula for Ohm's Law.
Answer: \( V = IR \).
In simple words: This formula connects "push" (Voltage), "flow" (Current), and "friction" (Resistance). It tells us that more push makes more flow, unless the friction blocks it.

📝 Teacher's Note: This is the single most important equation in electricity. Draw the \( V-I-R \) triangle to help students solve for any of the three variables.

🎯 Exam Tip: Make sure you define the symbols: \( V \) is Potential Difference, \( I \) is Current, and \( R \) is Resistance.

Question 21. Define a resistance of 2 ohms.
Answer: The resistance of the wire is 2 ohms if a current of 1 ampere flows through it when the potential difference across it is 2 volt.
In simple words: If you use a 2-volt "push" and exactly 1 Amp of electricity flows through, it means the wire has 2 Ohms of resistance.

📝 Teacher's Note: Unit definitions always come from the main formula. Since \( R = V/I \), then \( 2 \Omega = 2\text{V} / 1\text{A} \).

🎯 Exam Tip: To define any unit value, use 1 unit for the other variables in the denominator.

Question 22. If Voltage is 14V and Resistance is 28 Ohms, calculate the current.
Answer: The current \( I = V/R = 14/28 = 0.5 \text{ Ampere} \).
In simple words: Since resistance is twice as high as the voltage push, the resulting flow is cut down to half an Amp.

📝 Teacher's Note: This is a simple application of Ohm's Law. Check that students place \( V \) in the numerator.

🎯 Exam Tip: Show the division \( 14/28 = 1/2 = 0.5 \) to avoid mental math mistakes.

Question 23. Explain how length, cross-section, and temperature affect resistance.
Answer: The factors on which resistance of the wire depends are:

• The length of the wire, resistance is directly proportional to the length of wire.
• The cross-section of the wire, resistance is inversely proportional to the cross-section of the wire.
• The temperature of the wire, resistance of wire is directly proportional to the temperature of the wire.
• The material of the wire (good conductors possess less resistance.)

In simple words: Long wires have more "friction" (resistance). Fat wires have less. Hotter wires usually have more.

📝 Teacher's Note: Use the mathematical relationships: \( R \propto L \), \( R \propto 1/A \), and \( R \propto T \) (for most metals). This helps prepare students for more complex physics later.

🎯 Exam Tip: "Directly proportional" means as one goes up, the other goes up. "Inversely proportional" means as one goes up, the other goes down.

Question 24. Calculate work done if Voltage is 6V and Charge is 3C.
Answer: \( W = V \cdot Q = 6 \cdot 3 = 18 \text{ Joule} \).
In simple words: The energy spent (work) is found by multiplying the "push" by the amount of electricity moved. Here, it is 18 Joules.

📝 Teacher's Note: This comes from the definition of Potential Difference \( V = W/Q \). Rearranging it gives \( W = V \cdot Q \).

🎯 Exam Tip: Always use 'Joule' or 'J' as the unit for Work Done.

Question 25. Define electrical resistance.
Answer: The resistance of the conductor is the property due to which it opposes the flow of current in it.
In simple words: Resistance is like the "difficulty" a wire gives to the electricity trying to pass through it.

📝 Teacher's Note: Relate this to a narrow doorway: the narrower the door, the harder it is for people (electrons) to get through. This difficulty is resistance.

🎯 Exam Tip: The keyword "opposes" or "opposition" is essential for this definition.

Question 26. Define 1 volt of potential difference.
Answer: The potential difference between two points is 1 volt if the work done in transferring 1 coulomb of charge from one point to another point is 1 joule.
In simple words: If it takes exactly 1 Joule of energy to move 1 Coulomb of electricity, the "push" between those two spots is exactly 1 Volt.

📝 Teacher's Note: This is a common unit definition. It maps \( 1\text{V} = 1\text{J} / 1\text{C} \).

🎯 Exam Tip: Mention both "1 Joule" and "1 Coulomb" to define "1 Volt" completely.