CBSE Class 7 Science Advanced Physics Electricity MCQs Set 01

Answer: (d) either P is charged, or Q is charged. If both are charged, they are opposite in nature.

Explanation:
1. Two uncharged bodies exert no electric force, so (a) is wrong.
2. Identical charges repel, so (b) is wrong.
3. Attraction can occur between two oppositely charged bodies, and also between a charged body and an uncharged one (by induction).
4. So at least one body is charged, and if both are charged they carry opposite charges. This is (d), which covers every possible case; (c) leaves out the charged-uncharged case.

Teacher's Note:
1. Attraction alone does not prove that both bodies are charged.
2. Always think about the induction case when one body is neutral.

Answer: (d) All the above

Explanation:
1. Charging by conduction passes charge from the charged body to the neutral one.
2. Negative charge (electrons) moves from Y to X, so X gains negative charge and Y loses negative charge.
3. Charge is conserved, so what X gains equals what Y loses. All three statements are true: (d).

Teacher's Note:
1. Charge is never created or destroyed; it only moves from one body to another.
2. Here the electrons are the particles that travel.

Answer: (c) An electric motor

Explanation:
1. A cell and a battery change chemical energy into electrical energy.
2. A generator changes mechanical energy into electrical energy.
3. An electric motor does the opposite: it uses electrical energy to produce motion. So (c) is not a source.

Teacher's Note:
1. Motor and generator are reverse devices of each other.
2. A source gives out electrical energy; a consumer takes it in.

Answer: (c) It can be easily portable from place to place.

Explanation:
1. The Leclanché cell has a liquid electrolyte (ammonium chloride solution) in a glass jar, so (a) is true.
2. Zinc is its negative plate and carbon its positive plate, so (b) is true.
3. Dry cell is its modified form, so (d) is true.
4. A liquid cell in a glass jar spills and breaks, so it is not easily portable; (c) is false.

Teacher's Note:
1. The dry cell was developed precisely to make the cell portable.
2. In a false-statement question, pick the one that contradicts the facts.

Answer: (c) Mercury oxide

Explanation:
1. A depolariser is an oxidising agent that turns the hydrogen on the plate into water.
2. Potassium dichromate, copper sulphate and manganese dioxide are all used for this.
3. Mercury oxide is not on the list, so the answer is (c).

Teacher's Note:
1. Remember the three standard depolarisers by name.
2. Do not confuse mercury oxide with the mercury coating on zinc, which stops local action.

Answer: (c) Fire extinguisher

Explanation:
1. A washing machine, a TV and a water cooler all run on electricity.
2. A fire extinguisher contains chemicals and a compressed gas and needs no electric supply.
3. So (c) is the answer.

Teacher's Note:
1. An electrical appliance needs a power supply to work.
2. Look for the odd one out.

Answer: (b) 4.5 V

Explanation:
1. There are two branches, and each branch has three 1.5 V cells in series.
2. E of one branch = 1.5 + 1.5 + 1.5 = 4.5 V.
3. The two identical branches are in parallel, so the total e.m.f. is the same as one branch: 4.5 V.

Teacher's Note:
1. Add the e.m.f. inside each series branch first.
2. Identical branches in parallel do not add up.

Answer: (b) Switches may be used with wet hands.

Explanation:
1. Using ISI-marked appliances, keeping fingers and wires out of sockets and testing with a tester are all safe practices.
2. Water conducts electricity, so touching switches with wet hands can give a shock.
3. Hence (b) is the false statement.

Teacher's Note:
1. Never touch electrical fittings with wet hands.
2. These safety rules are repeated in the Notes of this chapter.

Answer: (c) Electric generator

Explanation:
1. Electromagnets, electric motors and telegraph instruments all use the magnetic field produced by a current.
2. A generator changes mechanical motion into electricity and does not depend on the magnetic effect of a current.
3. So (c) is the answer.

Teacher's Note:
1. A generator works by turning a coil in a magnetic field; it makes current rather than using one.
2. Motors use current to give motion; generators work the other way round.

Answer: (a) Telephone

Explanation:
1. Purification of metals, electroplating and electrotyping all depend on electrolysis, a chemical effect of current.
2. A telephone does not use that principle, so (a) is correct.

Teacher's Note:
1. Electrolysis is behind all three of the other options.
2. Link each application to its effect before choosing.

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

Explanation:
1. An electric heater uses the heating effect.
2. An electric motor and an electric bell both use magnets produced by current.
3. So the answer is (d).

Teacher's Note:
1. Heater means heat; bell and motor mean magnet.
2. Pick the group that matches the effect.

Answer: (c) Both A and B are true

Explanation:
1. A cell turns chemical energy into electrical energy, so A is true.
2. Two or more cells joined together form a battery, so B is true.
3. The answer is (c).

Teacher's Note:
1. Both statements are basic definitions.
2. Learn them as a pair.

Answer: (a) Only A is true

Explanation:
1. The body has excess electrons, so these flow to the ground.
2. After the excess electrons have gone, the body is neutral, not positive.
3. So A is true and B is false: (a).

Teacher's Note:
1. Earthing removes the extra charge and leaves the body neutral.
2. A body becomes positive only if it loses more electrons than it had in excess.

Answer: (c) Both heat and magnetic field

Explanation:
1. A current heats the wire (heating effect).
2. It also creates a magnetic field around it (magnetic effect).
3. So it produces both: (c).

Teacher's Note:
1. Heating effect and magnetic effect always go together.
2. Both can be shown by simple activities.

Answer: (a) electrical energy into mechanical energy.

Explanation:
1. A motor takes in electrical energy and makes its shaft turn.
2. So electrical energy is changed into mechanical energy: (a).

Teacher's Note:
1. A generator does the opposite job.
2. Fans and mixers are common motor examples.

Answer: (c) alcohol

Explanation:
1. Gold and copper are metals and conduct well.
2. A living plant has water and salts and conducts to some extent.
3. Pure alcohol does not conduct, so it is a bad conductor: (c).

Teacher's Note:
1. Metals are the best conductors.
2. Liquids like pure alcohol and distilled water are poor conductors.

Answer: (b)

Explanation:
1. (a) shows one cell, (c) is a circle with V and stands for a voltmeter, and (d) is an open tap key.
2. (b) shows several cells joined one after the other, which is the symbol of a battery.

Teacher's Note:
1. A battery is drawn as cells in series.
2. Learn the symbols from the table in the Notes.

Answer: (a) volt

Explanation:
1. Second is the unit of time, metre of length and m s⁻¹ of speed.
2. Electromotive force is measured in volt.

Teacher's Note:
1. E.m.f. and potential difference share the same unit.
2. The unit is named after Alessandro Volta.

Answer: (a) magnetic compass

Explanation:
1. A current makes a compass needle turn because of its magnetic effect.
2. A cell is a source and an inverter converts supply; a voltmeter measures voltage.
3. So the device that can show the presence of current is the magnetic compass.

Teacher's Note:
1. The deflecting needle is a simple current detector.
2. This is the idea behind the galvanoscope.

Answer: (d) ACDBE

Explanation:
1. First the coil becomes a magnet and pulls the strip (A).
2. The hammer moves and strikes the gong (C).
3. The strip leaves the screw and the circuit opens (D).
4. The coil loses its magnetism and the strip springs back (B).
5. The circuit closes again and everything repeats (E).
6. The order is ACDBE.

Teacher's Note:
1. Follow the strip: pulled, hits gong, breaks circuit, released, repeats.
2. The ringing is continuous because the sequence repeats.

Answer: (b) ACBD

Explanation:
1. Take the rods (A).
2. Charge them by rubbing with silk (C).
3. Bring them close (B).
4. Observe the repulsion (D).
5. The order is ACBD.

Teacher's Note:
1. Experiments follow the pattern: set up, treat, bring together, observe.
2. Charging must come before bringing the rods near.

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

Explanation:
1. Every body contains both positive and negative particles.
2. A body is positively charged when it has lost some negative particles, so positive particles outnumber the negative ones.
3. This is option (c).

Teacher's Note:
1. No body has only one kind of charge.
2. Charging only changes the balance between the two.

Answer: (d) All the above

Explanation:
1. Rubbing two bodies together charges them by friction.
2. Touching a charged body charges by conduction.
3. A charged body brought near charges by induction.
4. So all three methods work: (d).

Teacher's Note:
1. Learn all three methods by name.
2. Induction needs no contact.

Answer: (a) A generator

Explanation:
1. A motor, an electromagnet and a bulb all take in electrical energy.
2. A generator produces electrical energy, so it is the one that is not a consumer: (a).

Teacher's Note:
1. Generator gives, the others take.
2. Think of the direction of energy conversion.

Answer: (d) (d)

Explanation:
1. Statements (a), (b) and (c) match the standard symbol: a long thin line for the positive terminal and a short thick line for the negative terminal.
2. A cell gives only a small amount of energy, so calling it a major source is wrong.
3. Statement (d) is false.

Teacher's Note:
1. Big supplies come from generators in power plants.
2. Cells are used in small devices.

Answer: (a) 1.5

Explanation:
1. Identical cells in parallel give the e.m.f. of a single cell.
2. So the total is 1.5 V.

Teacher's Note:
1. Parallel does not add the voltage.
2. Series does add it: 3 × 1.5 = 4.5 V.

Answer: (a) series

Explanation:
1. In a series circuit the energy of the battery is shared by all bulbs, so adding a bulb makes each one dimmer.
2. In a parallel circuit every bulb is on the battery directly, so brightness stays the same.
3. The bulbs are in series.

Teacher's Note:
1. More bulbs in series means a dimmer glow.
2. In parallel the brightness does not change.

Answer: (c) 5

Explanation:
1. 1 unit = 1 kW h = 1000 W h.
2. Time = energy ÷ power = 1000 W h ÷ 200 W = 5 h.

Teacher's Note:
1. Always convert 1 unit to 1000 W h.
2. A 100 W bulb takes 10 h and a 200 W bulb half of that.

Answer: (d) All the above

Explanation:
1. The resistance of the wire depends on its length, its thickness (cross-section) and the material it is made of.
2. The heat produced depends on all of these, so the answer is (d).

Teacher's Note:
1. Remember: length, thickness and material.
2. The current and time also matter.

Answer: (d) All the above

Explanation:
1. Statement (a) is true: it is a magnet made by a current.
2. Statement (b) is true: the bell coil is an electromagnet.
3. Statement (c) is true: cranes use electromagnets to lift heavy iron.
4. All are true: (d).

Teacher's Note:
1. List the uses of electromagnets for exams.
2. All three statements come straight from the Notes.

Answer: (a) charging by conduction

Explanation:
1. The charge passes by touching, which is conduction.
2. Friction needs rubbing and induction needs no contact.
3. The method is charging by conduction.

Teacher's Note:
1. Contact means conduction.
2. The new charge has the same sign as the charging body.

Answer: (d) All the above

Explanation:
1. An iron, a heater and a bulb all have a wire that heats up when current flows through it.
2. So all of them use the heating effect: (d).

Teacher's Note:
1. Iron, heater and bulb: heat is the main working principle.
2. Bell and motor use the magnetic effect instead.

Answer: (c) Both A and B are true

Explanation:
1. In the voltaic cell the zinc plate is the negative electrode and the copper plate is the positive electrode, so A is true.
2. The electrolyte is dilute sulphuric acid, so B is true.
3. The answer is (c).

Teacher's Note:
1. Many textbooks call zinc the cathode and copper the anode, but negative and positive electrode is the clearer wording.
2. Zinc negative, copper positive: remember it as Z for zero-side.

Answer: (a) only A is true

Explanation:
1. A positive body has a shortage of electrons.
2. Electrons flow in from the ground and fill the shortage, so the body becomes neutral (A is true).
3. The flow stops once it is neutral, so it does not become negative (B is false).
4. Answer: (a).

Teacher's Note:
1. The ground supplies or takes electrons until the body is neutral.
2. Earthing never gives the opposite charge.

Answer: (b) magnetic effect of electric current.

Explanation:
1. The current produces a magnetic field around the conductor.
2. This field acts on the needle and turns it.
3. So the cause is the magnetic effect of the current.

Teacher's Note:
1. Oersted discovered this effect.
2. Heat and chemical effects cannot turn a needle.

Answer: (a) mechanical energy into electrical energy.

Explanation:
1. A generator is turned by water, steam or wind, which gives it mechanical energy.
2. It changes this into electrical energy: (a).

Teacher's Note:
1. Generator: mechanical to electrical.
2. Motor: electrical to mechanical.

Answer: (a) impure water

Explanation:
1. Glass, ebonite and wood are insulators.
2. Impure water contains dissolved salts and conducts electricity, so it is the good conductor among these: (a).

Teacher's Note:
1. Never touch electrical fittings with wet hands, because impure water conducts.
2. Pure (distilled) water conducts very poorly.

Answer: (a) (a)

Explanation:
1. Symbol (b) is a circle with a coil, (c) is a pair of brackets and (d) is a cell with + and − signs.
2. The wavy S-shaped line between two wires is the fuse symbol: (a).

Teacher's Note:
1. The fuse wire is shown as a thin wavy line.
2. Compare with the table of symbols in the Notes.

Answer: (b) kW h

Explanation:
1. The joule is the SI unit and the watt hour is too small for billing.
2. Electricity bills use the kilowatt hour (kW h), so the answer is (b).

Teacher's Note:
1. 1 kW h is called one unit.
2. 1 kW h = 3.6 × 10⁶ J.

Answer: (a) parallel combination.

Explanation:
1. In parallel every bulb gets the full supply and works independently.
2. If one fails, the others continue to work.
3. So houses use parallel combination.

Teacher's Note:
1. Parallel wiring avoids interruptions.
2. Series wiring would put all lights off when one fails.