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VII - SCIENCE - ELECTRICITY - EVALUATION - QUESTION - ANSWERS
VII
ELECTRICITY
EVALUATION
I. Choose the correct answers
1. In the circuit diagram below, 10 units of electric charge move past point x every second What is the current in the circuit
a) 10 A
b) 1 A
c) 10 V
d) 1 V
Ans: a) 10 A
2. In the circuit shown, which switches (L,M or N) must be closed to light up the bulb?
a) switch L only
b) switch M only
c) Switch M and N only
d) either switch L or switches M and N
Ans: d) either switch L or switches M and N
3. Small amounts of electrical current are measured in milliampere (mA). How many milliamperes are there in 0.25 A ?
a) 2.5 mA
b) 25 mA
c) 250 mA
d) 2500 mA
Ans: c) 250 mA
4. In which of the following circuits are the bulbs connected in series?
Ans: b
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: 6.248 X1018
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: siements/metre
III. True or False – If False give the correct answer
1. Electron flow is in the same direction to conventional current flow.
Ans: False. 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. The fuse wire melts whenever there is overload of the current in the wire.
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. 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
IV. Match the following
Ans:
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.
Option:
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R is NOT the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true. E. Both A and R are false
Ans: A. 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 carriers.
A. If both A and R are true and the R is the correct explanation of A.
B. If both A and R are true but R is not a correct explanation of A.
C. If A is true and R is false.
D. If both A and R are false.
Ans: A. Both A and R are true and 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.
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.
The device used to generate electricity is Electric cell.
4. Define fuse.
Electric fuse is a safety device which is used in household wiring and in many appliances, which melts whenever there is overload of the current in the wire.
5. Name some devices that run using heat effect of electric current.
Electric bulbs, geysers, iron boxes, and immersible water heaters works based on heating effect. These appliances have heating coils of high resistance.
6. Name a few insulators.
Ceramics,Rubber, Glass, Plastic, Porcelain, Wood etc.
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.
Electric Current is the rate of flow of electrons in a conductor. The SI Unit of electric current is the Ampere.
2. Differentiate parallel and serial circuits.
3. Define electrical conductivity.
Electrical conductivity or specific conductance measures a material’s ability to conduct an electric current.
IX. Long Answer
1. Explain the construction and working of a 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.
a) The diaphragm is attached to spring that is fixed to the earpiece.
b) When a current flows through the wires, the soft – iron bar becomes an electromagnet.
c) The diaphragm becomes attracted to the electromagnet.
d) 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.
Generation of heat due to electric current is known as the heating effect of electricity.
When an electric current passes through a wire, the electrical energy is converted to hear.
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.
3. Explain the construction and working of a dry cell.
Primary cell – simply Dry cell:
A dry cell is a type of chemical cell commonly used in the common form batteries for many electrical appliances.
It was developed in 1887 by Yei Sakizo of Japan.
The dry cell is not really dry in nature but the quantity of water in it is very small, as the electrolyte is in the form of a paste.
Electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity.
A dry cell is a portable form of a leclanche cell.
It consists of a 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 and zinc chloride. The ammonium chloride acts as an electrolyte. The purpose of zinc chloride is to maintain the moistness of the paste being highly hygroscopic. The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as a positive electrode or cathode. It is surrounded by a closely packed mixture of charcoal and manganese dioxide (MnO2 ) in a muslin bag. Here MnO2 acts as a 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 a leclanche cell.
Dry cells are normally used in small devices such as remote control for T.V., torch, camera and toys.
X. Higher Order Question
A student made a circuit by using an electric cell, a switch, a torch bulb (fitted in the bulb holder) and copper connecting wires. When he turned on the switch, the torch bulb did not glow at all. The student checked the circuit and found that all the wire connections were tight.
What could be the possible reason for the torch bulb not glowing even when the circuit appears to be complete?
Possible reasons are:
The battery may be exhausted and does not have sufficient energy to generate electricity.
The bulb may be fused.
Connections may be loose.
Connecting wires may be cut inside.
