Chapter wise important questions and notes pdf Electricity

Chapter wise important questions and notes pdf Electricity

sms (smart studies ) November 18, 2020

Chapter wise important questions and notes

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Chapter wise important questions

1.Electrical resistivity of a given metallic wire depends upon:

(a) Its length 

(b) Its thickness 

(c) Its shape

(d) Nature of the materials

Ans.(d)Nature of the material

Explanation:The resistivity of a material is constant for a particular temperature at a constant temperature. Resistivity of material does not depend on length, thickness and shape of the material. It only depends on the temperature.Resistivity:It is the amount of resistance offered by a conductor of unit length and unit area of cross-section. SI unit of resistivity is ohm m 

2.The instrument used for measuring electric current is:

 (a)galvanometer 

 (b)ammeter  

(c)voltmeter 

(d) potentio meter 

Ans.(b) ammeter 

Explanation: Ammeter is a device used for measuring electric current in amperes. 

Related Theory:An ammeter is a low resistance instrument, connected in series in a given electric circuit and does not consume any current from an electric circuit. 

Potentio meter is used to measure the internal resistance of a cell, to compare the e.m.f. of two cells and potential difference across a resister. 

Galvano meter is used for detecting and indicating an electric current.

 Voltmeter is used for measuring electrical potential difference between two points in an electric circuit.

 3.Define resistance. Give its S.I. unit.[CBSE 2019] 

Ans.Resistance is the property of a conductor to oppose the flow of charges through it. SI unit of resistance is Ohm

Related Theory: It is due to the opposition encountered by the electrons as the electrons are restrained by the attractive force of the atoms and also due to the collisions with other electrons and with the atoms. 

The resistance of a conductor is said to be 1

4.Define the term electrical resistivity of a material.[CBSE 2019] 

Ans.Electrical resistivity is defined as the electrical resistance of a conductor having cross-sectional area 1 m2 and length 1m. 

Related Theory:

 SI unit of electrical resistivity is ohm m (Wm) 

 It is a characteristic property of the material and varies with temperature.

Metals and alloys have very low resistivity but insulators have very high resistivity.

 

 5.What will happen to the resistivity of a wire of length L if it is cut into three parts? 

Ans.Resistivity of the wire will not change even when the wire is cut into three parts as resistivity is a characteristic of the material of the conductor and does not depend on the physical dimensions of the conductor. 

6.The potential difference across the wire having fixed resistance is tripled. By how much does the electric power increase?

 Ans.The electric power will increase by nine times when the potential difference across the wire having fixed resistance is tripled. According to Ohm’s law, potential difference V is proportional to current, I. Therefore, when V is made 3 times, I will increase 3 times. As Power P = VI, therefore, Power will increase by 9 times. 

7.Should the resistance of an ammeter be low or high? Give reason.[CBSE 2014]

 Ans.An ideal ammeter is one which has zero resistance. But that is not possible. Therefore, the resistance of an ammeter should be as close to zero as possible. If it is non-zero and substantial, it will affect the current flowing through the circuit. This is because an ammeter is connected in series in the circuit for the measurement of electric current.

 8. (A) What is the resistance of a conductor? 

(B) What happens to the electrical resistance when mercury is cooled to   4.12 K a? 

(C) What name is given to this phenomenon? 

Ans.

(A)The property of a conductor due to which it opposes the flow of current through it is called resistance. The resistance of a conductor is numerically equal to the ratio of potential difference across its ends to the current flowing through it. 

(B)When mercury is cooled to 4.12 K, the electrical resistance of mercury disappears completely and becomes zero and the mercury becomes super conductor

(C)This phenomenon of loss of electrical resistance of a substance on cooling it to an extremely low temperature is known as superconductivity.

9.What would you suggest to a student if while performing an experiment he finds that the pointer needle of the ammeter and voltmeter do not coincide with the zero marks on the scales when circuit is open? No extra ammeter/voltmeter is available in the laboratory. [CBSE 2019] 

Ans.As pointer of both the ammeter and voltmeter do not coincide with the zero marks on the scales when circuit is open, it indicates zero error in both the instruments. 

This zero error should be subtracted from the readings taken when circuit is closed.To get correct readings using these instruments, first we should find out the Least counts of both the instruments, i.e., the minimum value which can be accurately measured. 

 Zero error = Initial reading (in open circuit) = Pointer reading (in open circuit) 

X Least count To get the actual reading, subtract zero error or the initial reading of the open circuit to the reading when you perform the experiment.

 Actual reading = Final reading - Initial reading (zero error)

10.justify the following statements: 

(A)Tungsten is used exclusively for filaments of electric lamps.

 (B)Series arrangement is not used for domestic circuits. 

(C)Copper and aluminum wires are usually employed for electricity transmission.[CBSE Marking Scheme 2019]

Ans.(A)Due to high melting point/high resistance. 

(B)In series arrangement, same current will flow through all the appliances which is not required as every appliance needs current of different values. / If one component fails, the circuit is broken and none of the components works. 

(C)Good conductors of electricity/ Have low value of resistivity/ Less loss during transmission.(any one

 

11.Why is parallel arrangement used in domestic wiring?[CBSE 2010] 

Ans.Parallel arrangement is used in domestic wiring due to the following reasons:•Each device will have the same voltage which is equal to the voltage of the supply.•If two or more devices are used at the same time, then each appliance will be able to draw the required current.•If one of the devices fails, then the other keeps working.

12.What would you suggest to a student if while performing an experiment he finds that the pointerȰneedle of the ammeter and voltmeter do not coincide with the zero marks on the scales when circuit is open? No extra ammeterȰvoltmeter is available in the laboratory. [CBSE 2019] 

Ans.As pointer of both the ammeter and voltmeter do not coincide with the zero marks on the scales when circuit is open, it indicates zero error in both the instruments. This zero error should be subtracted from the readings taken when circuit is closed.To get correct readings using these instruments, first we should find out the Least counts of both the instruments, i.e., the minimum value which can be accurately measured.  

Zero error = Initial reading (in open circuit) = Pointer reading (in open circuit) 

X Least count To get the actual reading, subtract zero error or the initial reading of the open circuit to the reading when you perform the experiment.

 Actual reading = Final reading - Initial reading (zero error).

13.What is Joule's heating effect? How can it be demonstrated experimentally? List its four applications in daily life. 

Ans.Joule's heating effect: When an electric current is passed through a high resistance metallic wire, like nichrome wire, the resistance wire becomes very hot and produces heat. This effect is known as heating effect of current or Joule’s heating effect.

Joule’s law of heating states that the heat H produced in a conductor of resistance R due to current flowing through it for time t is H = I2Rt 

A simple experiment to demonstrate heating effect of current is that if we switch on the bulb for a long period of time then it will become hot. This shows that when electric current flows through a metallic conductor, heat is produced in it.

Applications of Joule's heating effect in daily life are: 

(1)Electric fuse is a safety circuit device works on this principle.Electric fuse in the electric circuit melts when large current flows in the circuit. 

(2)Electric iron, electric heater and water heater etc. work on the principle of heating effect of current.

  

(3)Electric bulb glows when electric current flows through the filament of the bulb.

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