CBSE Class 7 Science Advanced Physics Electricity MCQs Set 03

Answer: (c) Chemical effect of electric current

Explanation:
1. In electrotyping a metal is deposited on a mould by passing current through a solution.
2. This is done by electrolysis, which is the chemical effect of current.
3. The answer is (c).

Teacher's Note:
1. Electroplating and electrotyping both use the chemical effect.
2. Heating effect is used in heaters and fuses.

Answer: (a) 2.5 V

Explanation:
1. When identical cells are joined in parallel, the emf of the combination is the same as the emf of one cell.
2. So the emf is 2.5 V. (Parallel joining increases the life of the battery, not the emf.)

Teacher's Note:
1. Series adds emf: 3 × 2.5 = 7.5 V would be for series.
2. Parallel keeps the emf the same.

Answer: (c) 10

Explanation:
1. 1 unit = 1 kWh, so 2 units = 2 kWh = 2000 Wh.
2. Power = 200 W = 0.2 kW.
3. Time = energy ÷ power = 2 ÷ 0.2 = 10 h: option (c).

Teacher's Note:
1. Energy (kWh) = power (kW) × time (h).
2. Convert 200 W to 0.2 kW first.

Answer: (c) Mercury

Explanation:
1. Impurities in zinc form tiny cells on its surface, which wastes the zinc (local action).
2. Coating (amalgamating) the zinc with mercury prevents this.
3. The answer is mercury.

Teacher's Note:
1. Local action means zinc is used up even when no current is drawn.
2. Amalgamated zinc is the remedy.

Answer: (b) A → d; B → a; C → b; D → c

Explanation:
1. The porous pot of a Leclanche cell holds carbon and manganese dioxide: A → d.
2. The positive terminal of a dry cell is a carbon rod with a brass cap: B → a.
3. A depolarizer converts hydrogen into water: C → b.
4. An electric fuse works on the heating effect of current: D → c.
5. This is option (b).

Teacher's Note:
1. Match the one you are sure about first, such as the fuse.
2. Option (d) uses (b) twice, so it cannot be right.

Answer: (c) BDCA

Explanation:
1. Note the power and the hours of use from the data: (B).
2. Calculate the total energy in Wh for the month: (D).
3. Recall that 1 unit = 1 kWh = 1000 Wh: (C).
4. Divide the total Wh by 1000 to get the units: (A).
5. The order is B, D, C, A: option (c).

Teacher's Note:
1. Data first, then energy, then the unit conversion.
2. The conversion to units is the last step.

Answer: (c) Both negative and positive charges, but the number of negatively-charged particles is more than the number of positively-charged particles.

Explanation:
1. Every body has both positive and negative charges.
2. Rubbing with fur moves electrons onto the ebonite, so it has more negative particles than positive ones.
3. The answer is (c).

Teacher's Note:
1. A body is neutral when both are equal.
2. A negative body has extra electrons.

Answer: (a) kWh

Explanation:
1. Joule is the SI unit but too small for household use.
2. The practical unit is the kilowatt-hour (kWh), also called a unit.
3. The answer is kWh.

Teacher's Note:
1. Electricity bills are in units (kWh).
2. Dyne is a unit of force.

Answer: (d) All the above

Explanation:
1. Heat produced depends on the resistance of the wire, which depends on its length, thickness and material.
2. It also depends on the current and the time.
3. So all the given factors matter: (d).

Teacher's Note:
1. Resistance depends on length, area and material.
2. Longer and thinner wires heat more.

Answer: (b) ACBD

Explanation:
1. Take the rods: (A).
2. Charge them by rubbing: (C).
3. Bring them near each other: (B).
4. Observe the attraction and conclude: (D).
5. The order is A, C, B, D: option (b).

Teacher's Note:
1. The rods must be charged before they are brought together.
2. The conclusion is the last step.

Answer: (c) Mechanical energy into electrical energy

Explanation:
1. In a dynamo a coil is rotated in a magnetic field by a mechanical force.
2. This produces electric current.
3. So it converts mechanical energy into electrical energy: (c).

Teacher's Note:
1. Motor and generator do opposite jobs.
2. Chemical to electrical is a cell.

Answer: (d) All the above

Explanation:
1. In the figure B and C lie one after the other on the same wire, so they are in series: (a) is true.
2. This B–C branch and bulb A are joined across the same two points, so the pair B, C is in parallel with A: (b) is true.
3. B and C are in series, so if B fuses the path is broken and C goes out: (c) is true.
4. All three statements are correct: (d).

Teacher's Note:
1. Series means one path; parallel means separate paths between the same two points.
2. A break in a series branch stops the current in all bulbs of that branch.

Answer: (c) Copper

Explanation:
1. Metals have free electrons and conduct well.
2. Pure water and pure alcohol are poor conductors and cork is an insulator.
3. The answer is copper.

Teacher's Note:
1. Metals are good conductors.
2. Pure water conducts poorly; salty water conducts.

Answer: (a) Magnetic effect of electric current

Explanation:
1. An electric bell uses an electromagnet to pull an iron strip.
2. An electromagnet works on the magnetic effect of current.
3. The answer is (a).

Teacher's Note:
1. The coil becomes a magnet when current flows.
2. The bell rings by repeated attraction.

Answer: (c) 4.5 V

Explanation:
1. In series the emfs add up.
2. Total emf = 1.5 + 1.5 + 1.5 = 4.5 V.
3. The answer is 4.5 V.

Teacher's Note:
1. Series adds the emf of the cells.
2. Parallel would give 1.5 V.

Answer: (a) 120

Explanation:
1. Energy per day = 2000 W × 2 h = 4000 Wh = 4 kWh.
2. April has 30 days: 4 × 30 = 120 kWh.
3. 1 kWh = 1 unit, so the consumption is 120 units.

Teacher's Note:
1. April has 30 days.
2. 1 unit = 1 kWh.

Answer: (b) Calcium carbonate

Explanation:
1. A depolarizer removes the hydrogen bubbles that collect on the positive plate by oxidizing it to water.
2. Potassium dichromate, copper sulphate and manganese dioxide are all used in cells for this.
3. Calcium carbonate is not used: option (b).

Teacher's Note:
1. Look for the compound that does not give oxygen or react with hydrogen.
2. MnO₂ is used in the Leclanche cell.

Answer: (b) A → c; B → d; C → a; D → b

Explanation:
1. A charge detector is an electroscope: A → c.
2. An electric bell has an electromagnet: B → d.
3. A switch makes or breaks a circuit: C → a.
4. A three-pin socket has an earth connector: D → b.
5. This is option (b).

Teacher's Note:
1. An electroscope detects charge.
2. Match the obvious pair, switch with make or break, first.

Answer: (d) All the above

Explanation:
1. Only electrons move. The positively charged y attracts electrons from x.
2. So x loses negative charge: (a) is true. y gains negative charge: (b) is true.
3. The charge gained by y equals the charge lost by x: (c) is true.
4. All three are correct: (d).

Teacher's Note:
1. Charge is conserved, so what one body loses the other gains.
2. In solids only electrons move.

Answer: (b) CAEBD

Explanation:
1. Current makes the coil a magnet that pulls the iron strip: (C).
2. The hammer moves with the strip and strikes the gong: (A).
3. The strip loses contact with the screw and the circuit opens: (E).
4. The coil stops being a magnet and the strip springs back: (B).
5. The circuit closes again and the process repeats: (D).
6. The order is C, A, E, B, D: option (b).

Teacher's Note:
1. The bell rings because the circuit keeps making and breaking.
2. Follow one cycle: pull, strike, break, return, repeat.

Answer: (a) 3.6 × 10⁶

Explanation:
1. 1 kWh = 1000 W × 3600 s = 3,600,000 J.
2. = 3.6 × 10⁶ J: option (a).

Teacher's Note:
1. 1 h = 3600 s and 1 kW = 1000 W.
2. Watt-second is a joule.

Answer: (b) Electroplating

Explanation:
1. Depositing a thin layer of a costly metal on a cheaper one with electric current is electroplating.
2. The answer is electroplating.

Teacher's Note:
1. Electrotyping copies a pattern, plating covers an object.
2. Plating uses the chemical effect of current.

Answer: (b) BCDA

Explanation:
1. Take the compass: (B).
2. Wind the wire and connect the cell and switch: (C).
3. Close and open the switch and watch the needle: (D).
4. Conclude that a current-carrying wire acts as a magnet: (A).
5. The order is B, C, D, A: option (b).

Teacher's Note:
1. Equipment first, observation next, conclusion last.
2. Oersted's experiment works this way.

Answer: (c) Electrical energy into mechanical energy

Explanation:
1. A motor uses current in a magnetic field to produce rotation.
2. So it converts electrical energy into mechanical energy: (c).

Teacher's Note:
1. A motor is the opposite of a generator.
2. Fans and mixers use motors.

Answer: (a) Two cells are connected in parallel.

Explanation:
1. In the figure the two cells are connected side by side between the same two points, so they are in parallel: (a) is true.
2. Bulb A is directly across the cells, while B and C are in series on another branch, so A glows brighter than B and C: (b) is false.
3. B and C are in series, so if either fuses the other goes out: (c) and (d) are false.
4. The answer is (a).

Teacher's Note:
1. Cells joined side by side are in parallel.
2. Bulbs one after another are in series.

Answer: (d) Glass

Explanation:
1. Gold, copper and aluminium are metals and good conductors.
2. Glass is an insulator, a bad conductor.
3. The answer is glass.

Teacher's Note:
1. Insulators have no free electrons.
2. Glass, rubber and plastic are insulators.

Answer: (b)

Explanation:
1. A cell is drawn as a long thin line (positive plate) and a short thick line (negative plate) between two wires.
2. Option (a) is a coil, option (c) has two equal plates, and option (d) has an arrow through it, which shows a variable device.
3. Option (b) has the long and short plates marked + and −: (b).

Teacher's Note:
1. Long line is positive, short line is negative.
2. The + and − marks help to identify the cell symbol.

Answer: (d) 6 V

Explanation:
1. In series the emfs add: 4 × 1.5 V = 6 V.
2. The answer is 6 V.

Teacher's Note:
1. Series adds up.
2. Three cells would give 4.5 V.

Answer: (b) Local action

Explanation:
1. Mercury covers the impurities on the zinc and stops tiny local cells from forming.
2. This prevents local action. Polarization is prevented by a depolarizer.
3. The answer is local action.

Teacher's Note:
1. Mercury coating = amalgamation.
2. Polarization is a different problem, solved by a depolarizer.

Answer: (d) Both (b) and (c)

Explanation:
1. Telephones use electromagnets in the receiver.
2. MRI scanners use strong electromagnets.
3. Purification of metals uses the chemical effect.
4. So the answer is (d) Both (b) and (c).

Teacher's Note:
1. Purification by electrolysis is a chemical effect.
2. Electromagnets are everywhere in medicine and communication.

Answer: (b) A → r; B → p; C → q

Explanation:
1. A depolarizer converts hydrogen into water: A → r.
2. An MCB (miniature circuit breaker) acts as a fuse: B → p.
3. The main fuse at the kWh meter prevents unauthorized use of current: C → q.
4. This is option (b).

Teacher's Note:
1. MCB means miniature circuit breaker.
2. Option (c) uses (r) twice, so it cannot be right.

Answer: (b) RSPQ

Explanation:
1. Take the compass: (R).
2. Wind the wire and connect the cell and switch: (S).
3. State that the current-carrying wire behaves as a magnet: (P).
4. Close and open the switch to see the needle move: (Q).
5. The order is R, S, P, Q: option (b).

Teacher's Note:
1. Start with the apparatus.
2. This order is slightly different from the Test II version because the statement P comes before the observation here.

Answer: (a) Nichrome

Explanation:
1. Copper and silver are very good conductors, with low resistance.
2. Mercury also conducts fairly well.
3. Nichrome is an alloy with high resistance, used in heating elements.
4. The answer is nichrome.

Teacher's Note:
1. Heater coils are made of nichrome.
2. Alloys usually have higher resistance than pure metals.

Answer: (b) Negative

Explanation:
1. Glass loses electrons to silk and becomes positive.
2. The silk gains these electrons and becomes negatively charged.
3. The answer is negative.

Teacher's Note:
1. Glass rod with silk: glass +, silk −.
2. The two bodies get equal and opposite charges.

Answer: (d) All the above

Explanation:
1. Heat produced H = I²Rt = VIt.
2. So it depends on the resistance, the current and the potential applied.
3. All of the above are correct: (d).

Teacher's Note:
1. More current or resistance gives more heat.
2. The time of heating also matters.

Answer: (d) Both (b) and (c)

Explanation:
1. (a) is false: electric bells use electromagnets.
2. (b) is true: a tester shows whether a wire carries current.
3. (c) is true: this is the magnetic effect of current.
4. So (b) and (c) are correct: (d).

Teacher's Note:
1. Pick the true statements, then match the option that combines them.
2. Oersted found the magnetic field around a current.

Answer: (a) ‘A’ gains positive charge.

Explanation:
1. The negative charge of B repels the electrons in A to the far side.
2. The side of A nearest to B is left with positive charge: induction.
3. So A shows a positive charge near B (option (a)). No electrons move from B to A without contact, so (b) and (c) are wrong.
4. The answer is (a).

Teacher's Note:
1. Bringing close is charging by induction.
2. Charge moves from one body to the other only on contact.

Answer: (a)

Explanation:
1. (a) Three 2 V cells in series: 2 + 2 + 2 = 6 V.
2. (b) Two 2 V cells in series are joined to a third 2 V cell in parallel, which is not a proper battery and gives only about 4 V.
3. (c) and (d) join cells of different emf (3 V and 6 V) in parallel. The effective emf of unequal cells in parallel does not add up to a clean 6 V.
4. Only (a) is a plain series battery of 6 V: option (a).

Teacher's Note:
1. Series cells add their emf.
2. Unequal cells are not joined in parallel in a proper battery.

Answer: (c)

Explanation:
1. In a three-pin socket the earth pin is the top, larger pin; the live pin is on the right and the neutral pin is on the left (as seen from the front).
2. Only the arrangement with the earth connector at the top and the neutral and live connectors at the bottom left and right matches: option (c).

Teacher's Note:
1. Earth pin is at the top.
2. Live and neutral are the two lower pins.

Answer: (a)

Explanation:
1. A bulb glows brighter when the total emf in its circuit is larger, with the same bulb in each case.
2. (a) 2 V and 8 V in series with a 4 V–3 V parallel pair: 2 + 8 + 4 = 14 V.
3. (b) 2 V with a parallel group of 4 V, 1 V and one more cell: about 2 + 4 = 6 V. (c) 2 V + 4 V (parallel group) + 2 V = 8 V.
4. (d) Two branches of 4 V + 2 V and 4 V + 8 V are joined in parallel, which gives at most 12 V.
5. Circuit (a) has the greatest emf, so the bulb is brightest: (a).

Teacher's Note:
1. Add the emf of cells that are in series.
2. In a parallel group take the largest emf, as the book does.