7th Std Science Term 2 Solution in English | Lesson.2 Electricity

Lesson 2. Electricity

Lesson 2. Electricity

I . Choose the correct answer.

1. In the circuit diagram below, 10 units of electric charge move past point x every second What is the current in the circuit ————————

1. 10 A
2. 1 A
3. 10 V
4. 1 V

Ans : Kelvin

2. In the circuit shown, which switches (L,M or N) must be closed to light up the bulb?

1. switch L only
2. switch M only
3. Switch M and N only
4. either switch L or switches M and N

Ans : expands

3. Small amounts of electrical current are measured in milliampere (mA). How many milliampere are there in 0.25 A ?

1. 2.5 mA
2. 25 mA
3. 250 mA
4. 2500 mA

Ans : 37°C

4. In which of the following circuits are the bulb connected in series?

Ans : d

II. Fill in the blanks:

1. The direction of conventional current is_______________ to electron flow.

Ans : Opposite

2. One unit of coulomb is charge of approximately _______________ protons or electrons.

Ans : 6.242 x 10¹⁸

3. _______________ is used to measure the electric current

Ans : Ammeter

4. In conducting materials electrons are _______________  bounded with atoms.

Ans : Loosely

5. S.I. unit of Electrical conductivity of a conductor is ______________

Ans : Siemens/metre (s/m)

III. Match the following:

IV. True or False – If false give the correct answer:

1. Electron flow is in the same direction to conventional current flow.

Ans : False.

Cu. An : Electron flow is in the opposite direction to conventional current flow.

2. The fuse wire does not melts whenever there is overload in the wiring.

Ans : False.

Cu. An. : The fuse wire melts whenever there is overload in the writing.

3. In a parallel circuit, the electric components are divided into branches.

Ans : True.

4. The representation of the electric current is A.

Ans : False.

Cu. An. : The representation of the electric current is I.

5. The electrical conductivity of the semiconductor is in between a conductor and an insulator.

Ans : True.

V. Analogy:

1. Water : pipe : : Electric current : __________

Ans: Wire

2. Copper : conductor : : Wood : __________

Ans : Insulator

3. Length : metre scale : : Current : __________

Ans : Ammeter

4. Milli ampere : micro ampere : : 10-3A : ______________

Ans : 10-6A

VI. Assertion and Reason:

1. Assertion (A) : Copper is used to make electric wires.

Reason (R) : Copper has very low electrical resistance.

1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
3. (A) is true but (R) is false.
4. (A) is false but (R) is true.
5. Both (A) and (R) are false.

Ans : Both (A) and (R) are true and (R) is the correct explanation of (A).

2. Assertion (A) : Insulators do not allow the flow of current through themselves.

Reason (R) : They have no free charge carries.

1. If both (A) and (R) are true and the (R) is the correct explanation of (A).
2. If both (A) and (R) are true but (R) is not a correct explanation of (A).
3. If (A) is true and (R) is false.
4. If both (A) and (R) are false.

Ans : If both (A) and (R) are true and the (R) is the correct explanation of (A).

VII. Very short answer :

1. What is the speed of electric current?

Current travels at the speed of 1/100th times the speed of light 0.0002 m/s.

2. What is the S.I unit of electrical conductivity?

The SI unit of electrical conductivity is Siemens / metres (s/m).

3. Name the device used to generate electricity.

Electric cell.

4. Define fuse.

Electric fuse is a safety device which is used in household wiring and in many appliances.

5. Name some devices that run using heat effect of electric current

• Electric bulb
• geyser
• Iron box.

6. Name few insulators.

• Rubber
• wood
• plastic
• glass.

7. What is a battery?:

Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit.

VIII. Short Answer :

1. Define an electric current.

An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is I.

2. Differentiate parallel and serial circuits.

3. Define electrical conductivity.

Electrical conductivity or specific conductance is the measure of a material’s ability to conduct an electric current.

IX. Long Answer :

1. Explain flie construction and working of an Telephone.

In telephones, a changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate. The diaphragm is made up a metal that can be attracted to magnets.

• The diaphragm is attached to spring that is fixed to the earpiece.
• When a current flows through the wires, the soft – iron bar becomes an electromagnet.
• The diaphragm becomes attracted to the electromagnet.
• As the person on the other end of the line speaks, his voice cause the current in the circuit to change. This causes the diaphragm in the earpiece to vibrate, producing sound.

2. Explain the heating effect of electric current

• When an electric current passes through a wire, the electrical energy is converted to heat.
• In heating appliances, the heating element is made up of materials with high melting point. An example of such a material is nichrome (an alloy of nickel, iron and chromium).
• The heating effect of electric current has many practical applications.
• The electric bulb, geyser, iron box, immersible water heater are based on this effect.
• These appliances have heating coils of high resistance.
• Generation of heat due to electric current is known as the heating effect of electricity.

3. Explain the construction and working of a dry cell.

• A dry cell is a portable form of a leclanche cell.
• It consists of zinc vessel which acts as a negative electrode or anode.
• The vessel contains a moist paste of saw dust saturated with a solution of ammonium chloride an chloride.
• The ammonium chloride acts as an electrolyte.
• The purpose of zinc chloride is to maintain the moistness of the paste being highly gyroscopic.
• The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as positive electrode or cathode.
• It is surrounded by a closely packed mixture of charcoal and manganese dioxide (Mn0₂) in a muslin bag.
• Here Mn0₂ acts as depolarizer. The zinc vessel is sealed at the top with pitch or shellac.
• A small hole is provided in it to allow the gases formed by the chemical action to escape.
• The chemical action inside the cell is the same as in avalanche cell.

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