Chapter 3: Current Electricity
Solved textbook questions with step-by-step explanations.
Grade 9 Q&A: Chapter 3: Current Electricity
Welcome to the Questions and Answers section for Grade 9 Science, Chapter 3: "Current Electricity." This chapter introduces fundamental concepts of electricity, including potential difference, electric current, Ohm's law, resistance, and how components behave in series and parallel circuits. We will also explore domestic electrical wiring and safety precautions.
Important Questions and Answers
Q1: What is meant by potential difference?
Answer: Potential difference (V) between two points is the work done in moving a unit positive charge from one point to another. It is measured in Volts (V). Mathematically, V = Work / Charge.
Q2: Define electric current.
Answer: Electric current (I) is the flow of electric charge (usually electrons) through a conductor per unit time. It is measured in Amperes (A). Mathematically, I = Q / t, where Q is the charge and t is the time.
Q3: What is the unit of electric charge?
Answer: The SI unit of electric charge is the Coulomb (C). One Coulomb is approximately equal to the charge of 6.24 x 10^18 electrons.
Q4: State Ohm's Law.
Answer: Ohm's Law states that if the physical state (like temperature, length, area of cross-section) of a conductor remains constant, the electric current (I) flowing through it is directly proportional to the potential difference (V) across its ends. Mathematically, V ∝ I, or V = IR, where R is the constant of proportionality called resistance.
Q5: What is resistance? What is its SI unit?
Answer: Resistance (R) is the property of a conductor that opposes the flow of electric current through it. It is the ratio of the potential difference across the conductor to the current flowing through it (R = V/I). The SI unit of resistance is the Ohm (Ω).
Q6: Define one Ohm.
Answer: One Ohm (1 Ω) is the resistance of a conductor such that when a potential difference of one Volt (1 V) is applied across its ends, a current of one Ampere (1 A) flows through it.
Q7: What is resistivity? What is its SI unit?
Answer: Resistivity (ρ) is a fundamental property of a material that measures how strongly it resists electric current. It is defined as the resistance of a conductor of that material having unit length and unit cross-sectional area. The SI unit of resistivity is Ohm-meter (Ω·m).
Q8: On what factors does the resistance of a conductor depend?
Answer: The resistance (R) of a conductor depends on:
- Length (l): R ∝ l (Resistance increases with length).
- Area of cross-section (A): R ∝ 1/A (Resistance decreases with increasing area).
- Material of the conductor: Depends on its resistivity (ρ).
- Temperature: Resistance usually increases with temperature for most conductors.
The relationship is given by R = ρ (l/A).
Q9: Differentiate between conductors and insulators.
Answer:
- Conductors: Materials that allow electric current to flow through them easily because they have many free electrons. Examples: Metals like copper, silver, aluminum.
- Insulators: Materials that strongly resist the flow of electric current because they have very few free electrons. Examples: Rubber, glass, plastic, wood.
Q10: What is an electric circuit?
Answer: An electric circuit is a continuous and closed path along which electric current can flow. It typically consists of a source of electricity (like a cell or battery), connecting wires, a switch, and one or more electrical components (like bulbs or resistors).
Q11: Draw the symbols for the following circuit components: Electric cell, Battery, Open switch, Closed switch, Resistor, Voltmeter, Ammeter.
Answer: (Standard symbols should be visualized or described accurately)
- Electric Cell: består av en lång och en kort parallell linje (+/-).
- Battery: A combination of two or more cells in series.
- Open Switch (Plug Key): A break in the line with two terminals, often shown as a lever lifted off a contact.
- Closed Switch (Plug Key): A continuous line representing the closed switch, or a lever making contact.
- Resistor: A zigzag line.
- Voltmeter: A circle with a 'V' inside, connected in parallel.
- Ammeter: A circle with an 'A' inside, connected in series.
Q12: What is the function of a voltmeter and how is it connected in a circuit?
Answer: A voltmeter measures the potential difference (voltage) between two points in a circuit. It is always connected in **parallel** across the component or points where the potential difference is to be measured. It has a very high internal resistance to draw minimal current from the circuit.
Q13: What is the function of an ammeter and how is it connected in a circuit?
Answer: An ammeter measures the electric current flowing through a circuit element. It is always connected in **series** with the component through which the current is to be measured. It has a very low internal resistance so that it doesn't significantly affect the current being measured.
Q14: What happens to the total resistance when resistors are connected in series?
Answer: When resistors are connected in series, the total or equivalent resistance (Rs) is the sum of the individual resistances. Rs = R1 + R2 + R3 + ... The total resistance increases and is always greater than the largest individual resistance in the series combination.
Q15: What are the characteristics of a series circuit?
Answer:
- The current is the same through each resistor (I = I1 = I2 = I3 = ...).
- The total potential difference across the combination is the sum of the potential differences across individual resistors (V = V1 + V2 + V3 + ...).
- The equivalent resistance is the sum of individual resistances (Rs = R1 + R2 + R3 + ...).
- If one component in the series breaks or is removed, the entire circuit becomes open, and current stops flowing through all components.
Q16: What happens to the total resistance when resistors are connected in parallel?
Answer: When resistors are connected in parallel, the reciprocal of the equivalent resistance (Rp) is the sum of the reciprocals of the individual resistances. 1/Rp = 1/R1 + 1/R2 + 1/R3 + ... The total resistance decreases and is always less than the smallest individual resistance in the parallel combination.
Q17: What are the characteristics of a parallel circuit?
Answer:
- The potential difference (voltage) across each resistor connected in parallel is the same and equal to the applied voltage (V = V1 = V2 = V3 = ...).
- The total current flowing into the parallel combination is the sum of the currents flowing through the individual branches (I = I1 + I2 + I3 + ...).
- The reciprocal of the equivalent resistance is the sum of the reciprocals of individual resistances (1/Rp = 1/R1 + 1/R2 + 1/R3 + ...).
- If one component in a parallel branch breaks or is removed, the current continues to flow through the other parallel branches.
Q18: Why is the series arrangement not used for domestic circuits?
Answer: Domestic circuits do not use series arrangements primarily because:
- Failure Dependency: If one appliance fails (e.g., a bulb burns out), the entire circuit breaks, and all other appliances stop working.
- Voltage Sharing: The total voltage from the mains supply gets divided among all appliances. Each appliance may not receive its required operating voltage, leading to poor performance or failure to operate.
- Independent Operation: It's not possible to switch individual appliances on or off independently; all are either on or off together.
Q19: Why are domestic appliances connected in parallel?
Answer: Domestic appliances are connected in parallel because:
- Constant Voltage: Each appliance receives the full mains voltage (e.g., 220V), ensuring proper operation according to its specifications.
- Independent Operation: Each appliance can be switched on or off independently using its own switch without affecting the operation of other appliances.
- Failure Isolation: If one appliance fails, it does not affect the functioning of other appliances connected in parallel.
- Lower Total Resistance: Adding more appliances in parallel decreases the total resistance of the circuit, allowing sufficient current to be drawn for each appliance (within safety limits).
Q20: What are the three types of wires used in domestic electrical connections? What are their conventional colours?
Answer: The three types of wires are:
- Live wire (or Phase wire): Carries the current at high potential (e.g., 220V). Conventionally **Red** in colour.
- Neutral wire:** Completes the circuit and is at or near zero potential. Conventionally **Black** in colour.
- Earth wire:** A safety wire connected to the ground, usually connected to the metal casing of appliances. Conventionally **Green** in colour.
Q21: What is the function of an earth wire? Why is it necessary?
Answer: The earth wire provides a safety path for electric current to flow directly to the ground in case the live wire accidentally touches the metallic body of an electrical appliance due to insulation failure. This prevents the user from getting an electric shock upon touching the appliance, as the large current flowing to the earth through the low-resistance path blows the fuse or trips the circuit breaker, disconnecting the power supply.
Q22: What is a fuse? How does it work?
Answer: A fuse is a safety device containing a short piece of wire made of an alloy with a specific low melting point (like tin-lead alloy). It is connected in series with the live wire of a circuit. When the current flowing through the circuit exceeds a predetermined safe value (the fuse rating) due to overloading or a short circuit, the fuse wire heats up rapidly, melts, and breaks the circuit. This interrupts the flow of current, protecting the appliance and the wiring from damage due to excessive current.
Q23: What is meant by overloading?
Answer: Overloading occurs when too many electrical appliances, or appliances with high power ratings, are connected to a single circuit or socket simultaneously. This causes the total current drawn from the supply to exceed the safe current-carrying capacity of the circuit wiring or the rating of the fuse/circuit breaker. Overloading can lead to overheating of wires, damage to appliances, and potentially cause a fire.
Q24: What is a short circuit?
Answer: A short circuit occurs when the live wire comes into direct contact with the neutral wire (or sometimes the earth wire) without any significant resistance (like an appliance) in between. This usually happens due to damaged insulation on the wires. This creates a path of very low resistance, causing an extremely large current to flow from the source. This large current generates intense heat, which can damage the wiring, cause sparks, and potentially start a fire. Fuses or circuit breakers are designed to quickly interrupt the circuit in case of a short circuit.
Q25: Mention two precautions to be taken while using electricity.
Answer: (Any two of the following or similar valid precautions)
- Never touch electrical switches, sockets, or appliances with wet hands, as water increases the risk of electric shock.
- Ensure that electrical appliances with metallic bodies are properly earthed to prevent shocks in case of internal faults.
- Do not insert fingers or metallic objects into electrical sockets. Use safety covers for unused sockets, especially if there are children.
- Replace frayed or damaged wires and broken plugs immediately. Do not use appliances with damaged cords.
- Avoid overloading electrical circuits by plugging too many high-power appliances into a single socket or extension cord.
- In case of an electrical fire, do not use water to extinguish it (especially if the power is still on); use a fire extinguisher suitable for electrical fires (e.g., CO2 or dry powder) or sand, after switching off the main supply if possible.
Q26: If three resistors R1, R2, and R3 are connected in series, what is the formula for the equivalent resistance Rs?
Answer: The equivalent resistance Rs for resistors connected in series is the sum of the individual resistances:
Rs = R1 + R2 + R3
Q27: If three resistors R1, R2, and R3 are connected in parallel, what is the formula for the equivalent resistance Rp?
Answer: The equivalent resistance Rp for resistors connected in parallel is given by the formula where the reciprocal of the equivalent resistance is the sum of the reciprocals of the individual resistances:
1/Rp = 1/R1 + 1/R2 + 1/R3
Q28: How does the resistance of most conductors change with temperature?
Answer: The resistance of most conductors (like metals such as copper, aluminum, silver) **increases** as their temperature increases.
Q29: What are non-ohmic conductors? Give an example.
Answer: Non-ohmic conductors are materials or devices that do not obey Ohm's Law. This means the ratio of voltage (V) across them to the current (I) flowing through them (V/I) is not constant; their resistance changes with the applied voltage or current, or temperature. Examples include semiconductor diodes, transistors, thermistors, and filament lamps (whose resistance increases significantly as they heat up).
Q30: Calculate the resistance of a conductor if a current of 0.5 A flows through it when a potential difference of 2 V is applied across its ends.
Answer: Given: Current (I) = 0.5 A, Potential Difference (V) = 2 V.
According to Ohm's Law, V = IR.
Therefore, Resistance (R) = V / I.
R = 2 V / 0.5 A = 4 Ω.
The resistance of the conductor is 4 Ω.
Exercise Solutions
Q1: The accompanying figure shows some electrical appliances connected in a circuit in a house. Answer the following questions.
(Note: Assuming a standard diagram showing parallel connection of appliances like TV, lamp, fan to mains via meter and switch.)
- By which method are the appliances connected?
Answer: The appliances (TV, lamp, fan) are connected in parallel to the mains supply. - What must be the potential difference across individual appliances?
Answer: Since the appliances are connected in parallel to the mains supply, the potential difference across each individual appliance must be the same and equal to the voltage of the mains supply (e.g., typically 220-240 V in India). - Will the current passing through each appliance be the same? Justify your answer.
Answer: No, the current passing through each appliance will likely not be the same. Different appliances have different power ratings and therefore different resistances. According to Ohm's Law (I = V/R), since the voltage (V) across each is the same, the current (I) drawn by each appliance will be inversely proportional to its resistance (R). Appliances with lower resistance (higher power rating) will draw more current. - Why are the domestic appliances connected in this way?
Answer: Domestic appliances are connected in parallel for several reasons:- Each appliance receives the full mains voltage required for its proper operation.
- Each appliance can be operated independently using its own switch.
- If one appliance fails or is switched off, the others continue to work unaffected.
- If the T.V. stops working, will the other appliances also stop working? Explain your answer.
Answer: No, if the T.V. stops working, the other appliances (lamp, fan) will continue to work. This is because they are connected in parallel. In a parallel circuit, each appliance forms a separate branch. A break or fault in one branch (like the T.V. stopping) does not interrupt the flow of current to the other branches.
Q2: The following figure shows the symbols for components used in the accompanying electrical circuit. Place them at proper places and complete the circuit.
(Note: Requires drawing the completed circuit diagram based on provided symbols: cell, variable resistance, ammeter, voltmeter, resistance, closed plug key.)
Answer: The completed circuit diagram should show the following connections:
- The **cell**, **closed plug key**, **ammeter**, **fixed resistance**, and **variable resistance (rheostat)** are all connected in **series** to form the main loop.
- The **voltmeter** is connected in **parallel** across the terminals of the **fixed resistance**.
Which law can you prove with the help of the above circuit?
Answer: **Ohm's Law** can be proved using this circuit. By adjusting the variable resistance, the current (I) measured by the ammeter changes. The corresponding potential difference (V) across the fixed resistor is measured by the voltmeter. Plotting V against I yields a straight line through the origin, confirming V ∝ I, which is Ohm's Law. The slope of the graph gives the value of the fixed resistance (R = V/I).
Q3: Umesh has two bulbs having resistances of 15 Ω and 30 Ω. He wants to connect them in a circuit, but if he connects them one at a time the filament gets burnt. Answer the following.
- Which method should he use to connect the bulbs?
Answer: Umesh should connect the bulbs in **series**. Connecting them individually burns the filament, indicating the source voltage is too high for either bulb alone. Connecting in series increases the total resistance (Rs = 15 Ω + 30 Ω = 45 Ω), reducing the overall current (I = V/Rs). More importantly, the source voltage gets divided across the two bulbs (V1 + V2 = V), reducing the voltage across each bulb and preventing them from burning out. - What are the characteristics of this way of connecting the bulbs depending on the answer of A above?
Answer: Characteristics of connecting the bulbs in series:- Same Current: The same current flows through both bulbs.
- Voltage Division: The total voltage of the source is divided between the two bulbs (V1 across 15Ω, V2 across 30Ω, where V1+V2 = V_source). The voltage across each bulb is proportional to its resistance (V2 will be twice V1).
- Increased Total Resistance: The total resistance is 45 Ω, higher than either individual resistance.
- Reduced Brightness: Both bulbs will likely glow dimmer than if connected to a suitable voltage individually, due to reduced current and shared voltage.
- Circuit Dependency: If one bulb's filament breaks, the circuit becomes open, and the other bulb will also turn off.
Q4: The following table shows current in Amperes and potential difference in Volts.
| Sr. No. | V (Volt) | I (Ampere) |
|---|---|---|
| 1 | 4 | 9 |
| 2 | 5 | 11.25 |
| 3 | 6 | 13.5 |
- Find the average resistance.
Answer: Calculate resistance R = V/I for each reading:- R1 = 4 V / 9 A ≈ 0.444 Ω
- R2 = 5 V / 11.25 A = 500 / 1125 = 20 / 45 = 4/9 ≈ 0.444 Ω
- R3 = 6 V / 13.5 A = 60 / 135 = 12 / 27 = 4/9 ≈ 0.444 Ω
Average Resistance = (4/9 + 4/9 + 4/9) / 3 = (12/9) / 3 = 4/9 Ω ≈ **0.444 Ω**. - What will be the nature of the graph between the current and potential difference? (Do not draw a graph).
Answer: Since the resistance (V/I ratio = 4/9 Ω) is constant, the graph of potential difference (V) versus current (I) will be a **straight line passing through the origin**. - Which law will the graph prove? Explain the law.
Answer: The graph will prove **Ohm's Law**.
**Ohm's Law states that** provided the physical conditions (like temperature) of a conductor remain unchanged, the electric current (I) flowing through it is directly proportional to the potential difference (V) applied across its ends. Mathematically, V ∝ I, or V = IR, where R is the constant resistance. The straight-line graph through the origin confirms this direct proportionality.
Q5: Match the pairs
Group ‘A’
- Free electrons
- Current
- Resistivity
- Resistance
Group ‘B’
- V/I
- Increases the resistance in the circuit
- Weakly attached
- VA/LI
Answer:
- 1. Free electrons --- **c. Weakly attached**
- 2. Current --- **(No direct match; option 'b' is context-dependent or potentially flawed in the question)**
- 3. Resistivity --- **d. VA/LI** (Since ρ = RA/L and R=V/I, ρ = (V/I)A/L = VA/LI)
- 4. Resistance --- **a. V/I** (Definition from Ohm's Law)
(Note: Option 'b' likely refers to a component like a rheostat used to control current by changing resistance, or perhaps the effect of temperature increase due to current, but doesn't define current itself.)
Q6: The resistance of a conductor of length x is r. If its area of cross-section is a, what is its resistivity? What is the unit of resistivity?
Answer:
The formula relating resistance (R), resistivity (ρ), length (l), and area of cross-section (A) is: R = ρ (l/A).
Given: R = r, l = x, A = a.
Substituting these into the formula: r = ρ (x/a).
To find the resistivity (ρ), rearrange the equation:
ρ = (r * a) / x
The resistivity is **ρ = ra/x**.
The SI unit of resistivity is **Ohm-meter (Ω·m)**.
Q7: Resistances R1, R2, R3, and R4 are connected as shown in the figure. S1 and S2 are two keys. Discuss the current flowing in the circuit in the following cases.
(Note: Assuming diagram shows R1 and R2 in series, this pair in parallel with R3, and the entire combination in series with R4. S1 is main switch, S2 is switch in R3 branch.)
- Both S1 and S2 are closed.
Answer: When both S1 and S2 are closed, the circuit is complete. Current flows from the source, through S1. It then splits: one part goes through the series combination of R1 and R2, the other part goes through S2 and R3. These currents recombine after the parallel section and flow together through R4 before returning to the source. The total current depends on the total equivalent resistance: Req = [(R1 + R2) * R3 / (R1 + R2 + R3)] + R4. - Both S1 and S2 are open.
Answer: When S1 (the main switch) is open, the circuit is broken at the source. Therefore, **no current** flows anywhere in the circuit, regardless of the state of S2. - S1 is closed but S2 is open.
Answer: When S1 is closed but S2 is open, the main circuit path through S1 is complete. However, the branch containing R3 is open because S2 is open. Therefore, current flows from the source through S1, then only through the series combination of R1 and R2. It bypasses the open branch with R3 and S2. The current then flows through R4 and returns to the source. Effectively, R1, R2, and R4 are connected in series. The equivalent resistance is Req = R1 + R2 + R4.
Q8: Three resistances x1, x2 and x3 are connected in a circuit in different ways. x is the effective resistance. The properties observed for different ways of connecting x1, x2 and x3 are given below. Write the way in which they are connected in each case. (I, V, x are current, potential difference and effective resistance respectively).
- Current I flows through x1, x2 and x3.
Answer: Connected in **Series**. - x is larger than x1, x2 and x3.
Answer: Connected in **Series**. - x is smaller than x1, x2 and x3.
Answer: Connected in **Parallel**. - The potential difference across x1, x2 and x3 is the same.
Answer: Connected in **Parallel**. - x = x1 + x2 + x3
Answer: Connected in **Series**. - x = 1 / (1/x1 + 1/x2 + 1/x3)
Answer: Connected in **Parallel**. (This formula gives x directly, equivalent to 1/x = 1/x1 + 1/x2 + 1/x3).
Q9: Solve the following problems.
- The resistance of a 1 m long nichrome wire is 6 Ω. If we reduce the length of the wire to 70 cm, what will its resistance be? (Answer: 4.2 Ω)
Solution:
Resistance (R) is directly proportional to length (l), assuming constant area and material (R ∝ l).
R1 / l1 = R2 / l2
Given: R1 = 6 Ω, l1 = 1 m = 100 cm, l2 = 70 cm.
R2 = R1 * (l2 / l1) = 6 Ω * (70 cm / 100 cm) = 6 * 0.7 Ω = **4.2 Ω**. - When two resistors are connected in series, their effective resistance is 80 Ω. When they are connected in parallel, their effective resistance is 20 Ω. What are the values of the two resistances? (Answer: 40 Ω, 40 Ω)
Solution:
Let the resistances be R1 and R2.
Series: R1 + R2 = 80 Ω (1)
Parallel: 1/R1 + 1/R2 = 1/20 => (R1 + R2) / (R1 * R2) = 1/20 (2)
Substitute (1) into (2):
80 / (R1 * R2) = 1/20
R1 * R2 = 80 * 20 = 1600 (3)
From (1), R2 = 80 - R1. Substitute into (3):
R1 * (80 - R1) = 1600
80*R1 - R1^2 = 1600
R1^2 - 80*R1 + 1600 = 0
This is a quadratic equation. It factors as (R1 - 40)(R1 - 40) = 0.
So, R1 = 40 Ω.
Then R2 = 80 - R1 = 80 - 40 = 40 Ω.
The two resistances are **40 Ω** and **40 Ω**. - If a charge of 420 C flows through a conducting wire in 5 minutes what is the value of the current? (Answer: 1.4 A)
Solution:
Given: Charge (Q) = 420 C, Time (t) = 5 minutes.
Convert time to seconds: t = 5 min * 60 s/min = 300 s.
Current (I) = Charge (Q) / Time (t)
I = 420 C / 300 s = 42 / 30 A = 14 / 10 A = **1.4 A**.
References
- Maharashtra State Board Science and Technology Standard Nine Textbook (Latest Edition) - Chapter 3: Current Electricity.
- Maharashtra State Board 9th Standard Science Syllabus.
- Balbharati Science and Technology Textbook Part 1.