Electricity — Exercise Solutions

Samacheer Kalvi Tamil Nadu State Board Class 7 Science Term 2 Chapter 2 exercise solutions. Covers MCQs, fill-in-the-blanks, true/false, match the following, analogy, assertion and reason, very short, short, detailed answers, numerical problems, and HOTS questions.

I. Choose the Correct Answer (MCQ)

Q1. In a circuit, 10 units of charge pass point x every second. What is the current? Answer: 10 A ✓ — Current (I) = Charge (q) / Time (t) = 10 / 1 = 10 A.

Q2. Which switches must be closed to light up the bulb in the given circuit? Answer: Either switch L or switches M and N (depending on circuit diagram arrangement).

Q3. How many milliampere are there in 0.25 A? Answer: 250 mA ✓ — 0.25 A = 0.25 × 1000 = 250 mA.

Q4. Which circuit diagram shows bulbs connected in series? Answer: The correct diagram showing bulbs connected in a single loop (one path for current).

Additional MCQs

Q5. Safety device in electric circuit: (a) Switch (b) Fuse (c) Resistor (d) Battery Answer: (b) Fuse ✓ — A fuse melts during overload to protect the circuit.

Q6. Current starts from: (a) Negative terminal of battery (b) Positive terminal of battery (c) Either terminal (d) None Answer: (b) Positive terminal of battery ✓ — Conventional current flows from positive to negative.

Q7. Developed in 1887 by Yei Sakizo of Japan: (a) Wet cell (b) Secondary cell (c) Dry cell (d) Solar cell Answer: (c) Dry cell ✓ — The dry cell was developed by Yei Sakizo in 1887.

Q8. Used for mobile phones, computers, emergency lights: (a) Primary cell (b) Secondary cell (c) Dry cell (d) Fuel cell Answer: (b) Secondary cell ✓ — Secondary cells can be recharged and reused.

Q9. A battery is a group of: (a) One cell (b) Two or more cells (c) Only two cells (d) Wires Answer: (b) Two or more cells ✓ — A battery consists of two or more cells connected together.

Q10. Electrical conductor among cork, wood, plastic, silver: (a) Cork (b) Wood (c) Plastic (d) Silver Answer: (d) Silver ✓ — Silver is a metal and a good conductor of electricity.

Q11. When two naked wires touch: (a) Open circuit (b) Short circuit (c) Closed circuit (d) None Answer: (b) Short circuit ✓ — Contact between bare wires causes a short circuit.

Q12. Magnetic effect of current discovered by: (a) Faraday (b) Newton (c) Hans Christian Oersted (d) Edison Answer: (c) Hans Christian Oersted ✓ — Oersted discovered that current produces a magnetic field.

II. Fill in the Blanks

1. The direction of conventional current is opposite to electron flow. 2. One unit of coulomb is charge of approximately 6.242 × 10¹⁸ protons or electrons. 3. Ammeter is used to measure electric current. 4. In conducting materials, electrons are loosely bound. 5. SI unit of Electrical conductivity of a conductor is siemens/metre.

Additional Fill in the Blanks

6. An insulator gives a lot of resistance to electron flow. 7. Wires made of copper have very low resistance. 8. SIM and ATM card chips are made of semiconductors. 9. Electromagnets are used to remove steel/iron splinters in eye injury hospitals. 10. Edison used a platinum wire coil in a vacuum glass for the first bulb in 1879. 11. Primary cells cannot be recharged after use. 12. SI unit of potential difference is Volt (V). 13. SI unit of resistance is Ohm. 14. Charge flows from higher potential to lower potential.

III. True or False (with corrections)

Q1. Electron flow is in the same direction as conventional current flow. Answer: False. Electron flow is opposite to conventional current flow. Electrons flow from negative to positive; conventional current flows from positive to negative.

Q2. The fuse wire does not melt during overload. Answer: False. The fuse wire melts during overload, breaking the circuit and protecting appliances.

Q3. In a parallel circuit, components are divided into branches. Answer: True. ✓

Q4. The representation of electric current is A. Answer: False. The representation (symbol) of electric current is I. A (Ampere) is the SI unit of current.

Q5. Electrical conductivity of a semiconductor lies between a conductor and an insulator. Answer: True. ✓

Additional True/False

Q6. Insulators don't carry current because they lack free electrons. Answer: True. ✓

Q7. MCB can replace a fuse. Answer: True. ✓ — MCB (Miniature Circuit Breaker) automatically trips during overload and can be reset by hand.

Q8. Flow of charge per unit time is potential difference. Answer: False. Flow of charge per unit time is called current. Potential difference is the work done to move a unit charge between two points.

Q9. Source producing small electricity for short periods is an electrochemical cell. Answer: True. ✓

Q10. Electric cell converts electrical energy to chemical energy. Answer: False. An electric cell converts chemical energy to electrical energy.

Q11. The pipe is like the wire; the pump is like the battery. Answer: True. ✓ — In the water-flow analogy, pipes carry water like wires carry current; the pump pushes water like the battery drives current.

Q12. All batteries have anode, cathode, and electrolyte. Answer: True. ✓

Q13. An insulator gives less resistance to charge flow. Answer: False. An insulator gives more resistance to charge flow. Conductors give less resistance.

Q14. Electric fuse has a ceramic body with two points for fuse wire. Answer: True. ✓

Q15. Chemical reactions occur when electricity passes through conducting liquids. Answer: True. ✓ — This is the chemical effect of electric current.

IV. Match the Following

Additional Match the Following

Set 1:

Set 2:

V. Analogy

1. Water : pipe :: Electric current : wire 2. Copper : conductor :: Wood : insulator 3. Length : meter scale :: Current : ammeter 4. milli ampere : micro ampere :: 10⁻³A : 10⁻⁶A

VI. Assertion and Reason

Q1. A: Copper is used for electric wires. R: Copper has very low electrical resistance. Answer: Both A and R are true, and R correctly explains A. ✓ — Low resistance allows current to flow easily through copper.

Q2. A: Insulators don't allow current flow. R: They have no free charge carriers. Answer: Both A and R are true, and R correctly explains A. ✓ — Without free electrons/charge carriers, current cannot flow.

Additional Assertion and Reason

Q3. A: Charges flow from higher to lower potential. R: Current flows mainly due to flow of electrons. Answer: Both true, but R is NOT the correct explanation of A. ✓ — Charges flow from higher to lower potential due to potential difference, not because of electron flow direction.

Q4. A: A current-carrying wire should be charged. R: Current is due to flow of free electrons in a definite direction. Answer: A is false but R is true. ✓ — A current-carrying wire remains electrically neutral; current is the flow of electrons but the wire itself is not charged.

Q5. A: Connecting wires are made of copper. R: Copper has high electrical conductivity. Answer: Both true, and R correctly explains A. ✓

VII. Very Short Answer Questions

Q1. Speed of electric current? Answer: Current travels at approximately 1/100th the speed of light — about 0.0002 m/s (drift velocity of electrons).

Q2. SI unit of electrical conductivity? Answer: Siemens per metre (S/m).

Q3. Device used to generate electricity? Answer: Electric cell.

Q4. Define fuse. Answer: A safety device used in household wiring and many appliances. It melts during current overload, breaking the circuit and preventing damage.

Q5. Devices running on heat effect of electric current? Answer: Electric bulb, geyser, and iron box.

Q6. Name some insulators. Answer: Rubber, wood, plastic, and glass.

Q7. What is a battery? Answer: A collection of one or more cells whose chemical reactions create a flow of electrons in a circuit.

Additional Very Short Answers

Q8. Source of electric current in a TV remote. Answer: Electric cell.

Q9. Position of key in a circuit. Answer: The key (switch) can be placed anywhere in the circuit.

Q10. Effects of electricity. Answer: Heating effect, chemical effect, and magnetic effect.

Q11. Instrument to measure potential difference. Answer: Voltmeter.

Q12. Portable form of Leclanche cell. Answer: Dry cell.

Q13. Substances that become ions in solutions. Answer: Electrolytes.

Q14. Component to start/stop appliances safely. Answer: Switch.

Q15. Component connecting electrical components. Answer: Wire.

VIII. Short Answer Questions

Q1. Define electric current. Answer: Electric current is the amount of electric charge moving per unit time at any point in a circuit. Its conventional symbol is I. Formula: I = q/t, where q is charge in coulombs and t is time in seconds.

Q2. Differentiate parallel and series circuits.

Q3. Define electrical conductivity. Answer: It is the measure of a material's ability to conduct an electric current. Materials with high conductivity (like copper, silver) are good conductors; materials with low conductivity (like rubber, wood) are insulators.

Additional Short Answers

Q4. What is resistance? Answer: The property of an electrical component that resists the flow of electric charges when connected in a circuit. It is measured in ohms (Ω).

Q5. Uses of electromagnets? Answer: Used in cars, speakers, electric motors, electric cells, telephone receivers, and televisions. Also used in hospitals to remove steel/iron splinters from eyes.

Q6. Electric fuse is made up of? Answer: An alloy of low melting point — 63% tin and 37% lead. This composition melts quickly when excess current flows.

Q7. What is the magnetic effect? Answer: When electric current passes through a wire, it behaves like a magnet — this is the magnetic effect of electricity. It was discovered by Hans Christian Oersted.

Q8. What are MCBs and how do they work? Answer: MCBs (Miniature Circuit Breakers) are switches that automatically turn off when current exceeds safe limits. They can be reset by hand to complete the circuit again, unlike fuses which need replacement.

Q9. Define electric cell and direction of current. Answer: The simplest source of electric current, with a positive terminal and a negative terminal. Conventional current flows from the positive terminal, through the connected equipment, to the negative terminal. Electron flow is opposite — from negative to positive.

Q10. What is an open circuit? Answer: A circuit with a break that stops current from flowing — caused by loose connections, missing wires, or an open switch. No current flows in an open circuit.

Q11. Analogy between water current and electric current. Answer: Water molecules flow toward lower elevation (from higher to lower gravitational potential); electric charges flow toward lower voltage (from higher to lower electric potential). The pipe is like the wire; the pump is like the battery.

Q12. Factors affecting heating effect? Answer: The amount of current (I) and the resistance (R) of the conductor. Greater current or greater resistance produces more heat. (H = I²Rt, from Joule's law.)

IX. Detailed Answer Questions

Q1. Differences between primary and secondary cells.

Q2. Construction and working of a dry cell. Answer: A dry cell is a portable form of a Leclanche cell. It consists of:

• Zinc vessel — serves as the negative electrode (anode)
• Moist paste — sawdust saturated with ammonium chloride solution, acts as the electrolyte
• Carbon rod with a brass cap — serves as the positive electrode (cathode)
• Mixture of charcoal and manganese dioxide — in a muslin bag surrounding the carbon rod; MnO₂ acts as a depolarizer
• The zinc vessel is sealed at the top with pitch, with a small hole for gases to escape

Q3. Explain the heating effect of electric current. Answer: When current passes through a wire, electrical energy converts to heat energy. This is called the heating effect of electricity.

• Heating appliances use elements made of materials with high melting points, like nichrome (an alloy of nickel, iron, and chromium)
• These appliances have heating coils of high resistance
• Practical applications include electric bulbs, geysers, and iron boxes
• The heating effect follows Joule's law: H = I²Rt, where H is heat produced, I is current, R is resistance, and t is time

The heating effect is useful in appliances but can be dangerous if wiring is faulty or overloaded, which is why fuses and MCBs are essential safety devices.

Q4. Construction and working of a telephone. Answer: In telephones, a changing magnetic effect causes a thin metal diaphragm to vibrate. The diaphragm is made of metal attracted to magnets. It is attached to a spring fixed to the earpiece. When current flows, the soft-iron bar becomes an electromagnet, attracting the diaphragm. As the other person speaks, current changes cause the diaphragm to vibrate, producing sound waves that replicate the speaker's voice.

Q5. Explain semiconductors and short circuits. Answer:

• Semiconductors: Materials like silicon and germanium have conductivity between conductors and insulators. They are neither good conductors nor good insulators. Used in SIM cards, computers, and ATM cards.
• Short circuit: A low-resistance connection between two conductors supplying power. When two bare wires touch, current takes the path of least resistance, bypassing the intended circuit. This causes excessive current flow, generating heat that can damage equipment or cause fires. Arc welding is a common practical application of short circuits.

Q6. Explain the construction and working of an electric fuse. Answer: An electric fuse is a safety device used in household wiring and appliances. It consists of:

• A ceramic body with two connection points
• A fuse wire made of an alloy of 63% tin and 37% lead (low melting point)

X. Numerical Problems

Q1. If 50 coulombs flow through a wire in 5 minutes, calculate the current. Answer: I = q/t = 50 / (5 × 60) = 50/300 = 0.167 A

Q2. Convert 5A to microamperes. Answer: 5 A = 5 × 10⁶ μA = 5,000,000 μA

Q3. Convert 3A to milliamperes. Answer: 3 A = 3 × 1000 mA = 3000 mA

Q4. If 20 coulombs of charge pass through a conductor in 4 seconds, find the current. Answer: I = q/t = 20/4 = 5 A

XI. Higher Order Thinking Skills (HOTS)

Q1. A student made a circuit with a cell, switch, bulb, and copper wires. The bulb didn't glow even though connections were tight. Why? Answer: Possible reasons: 1. The cell may be discharged (no chemical energy left) 2. The bulb may be fused (filament broken) 3. The wire may be broken internally 4. The connections may be loose even though they appear tight

Q2. Difference between fuse wire and heating wire? Answer:

• Fuse wire has a low melting point — made of tin-lead alloy (63% tin, 37% lead). It melts during overload to break the circuit.
• Heating wire has a high melting point — made of nichrome (nickel, iron, chromium alloy). It does not melt even at high temperatures; it converts electrical energy to heat.

Q3. In a given circuit with switch OFF, would any bulb glow? What happens when switch is ON? Answer: No bulb glows when the switch is OFF (open circuit — no current flows). When the switch is ON, all bulbs glow simultaneously because the circuit is complete and current flows through all of them.

Q4. Three conductors joined together. Current in RS is 10 A, in QR is 6 A. Find current in PR. Answer: By Kirchhoff's current law: Current entering a junction = Current leaving it. PR + QR = RS PR = RS − QR = 10 − 6 = 4 A

XII. Picture-Based Questions

Q1. Which switches should be closed so only two bulbs light up? Answer: Switches S2, S3, and S4 only.

Q2. Circuits with glass rod (G), steel rod (S), and wooden rod (W). In which circuits would the bulb NOT light up? Answer: Circuits containing glass (insulator) and wood (insulator) — i.e., circuits A and B only. The bulb lights up only in the circuit with the steel rod (conductor).

Common Mistakes to Avoid

1. Confusing conventional current and electron flow — Conventional current flows from positive to negative; electrons flow from negative to positive. They are opposite. 2. Using A for current symbol — The symbol for current is I; the unit is A (Ampere). Don't mix them. 3. Saying fuse wire has high melting point — Fuse wire has a low melting point so it melts quickly during overload. Heating wires have high melting points. 4. Confusing series and parallel circuits — Series: one path, all bulbs go out if one fails. Parallel: multiple paths, other bulbs stay lit if one fails. 5. Thinking an open circuit has current — An open (broken) circuit has no current flow at all. 6. Confusing primary and secondary cells — Primary cells cannot be recharged (use and throw); secondary cells can be recharged and reused. 7. Saying current flows from negative to positive — Conventional current flows from positive to negative. Only electron flow is from negative to positive.

Exam Checklist

• [ ] Can define electric current, its formula (I = q/t), and SI unit (Ampere)
• [ ] Can explain the difference between series and parallel circuits with examples
• [ ] Can list the three effects of electric current (heating, chemical, magnetic)
• [ ] Can explain the construction and working of a dry cell
• [ ] Can differentiate primary and secondary cells
• [ ] Can explain the heating effect and its applications (bulb, geyser, iron box)
• [ ] Can explain the construction and working of a fuse
• [ ] Can differentiate between fuse and MCB
• [ ] Can define resistance and conductivity with their SI units
• [ ] Can identify conductors, insulators, and semiconductors with examples
• [ ] Can solve numerical problems on current (I = q/t) and unit conversions
• [ ] Can explain the magnetic effect of current and its discovery by Oersted