Current Electricity

Current Electricity

Q1. Rewrite the following statements by choosing the correct option:-

1.      The S.I. unit of potential difference is _____________
  1. Ampere
  2. Volt
  3. Ohm
  4. Joule

2.      Electric current is measured with the help of a device called ________________
a.      an ammeter
b.      a voltmeter
c.       a galvanometer
d.      a calorimeter

3.      A substance which does not allow charges to pass through it easily is called as ___________
  1. metal
  2. conductor
  3. insulator
  4. semiconductor

4.      If a current of 0.1A is passed through a wire of resistance 20 ohm, the potential difference across the wire is _____________
  1. 2 ohm
  2. 20 volt
  3. 10 volt
  4. 2 volt

5. From the following substance __________________ substance is used as insulation.
a.       iron
b.      copper
c.       gold
d.      plastic

Q2. Consider the relation between column (I) and (II) and fill in the column (IV) to match the column (III):
No.
Column I
Column II
Column III
Column IV
i.
ii.
iii.
iv
Copper
1 Volt
Electric charge
-ve terminal
Conductor
1 J/ 1s
Coulomb
Cathode
Rubber
1 ampere
Resistance
+ve terminal
Insulator
1 C / 1s
Ohm
Anode

Q3. Fill in the blanks with proper terms from the bracket:
(ampere, smaller, greater, Ohms, Coulombs, random, computer)

1.      The equivalent resistance of a parallel combination is smaller than each of the individual resistance.
2.      Diode does not obey Ohm’s law.
3.      S.I. unit of electric current is ampere.
4.      In conductors electrons are always in the state of random motion.
5.      Superconductors are used in computer.

Q4. State whether the following statements are true or false.

i. Diode obeys Ohm’s law
Answer. False: Diode does not obey ohm’s law.

i.      Ohm’s law is a relation between the current and potential difference in circuit.
Answer. True.

ii.      Resistance in series arrangement is used to decrease resistance of circuit.
Answer. False: Resistance in series arrangement is used to increase resistance of the circuit.

iii.      A conducting wire offers resistance to flow of electrons.
Answer. True.

iv.      In a super conductor as temperature decreases resistance of that material increases.
Answer. False: In a super conductor as temperature decreases resistance of that material decreases.

Q5. Find odd one out:

i. Copper, Silver, Mica, Graphite.
Answer. Mica – It is an insulator while the rest are conductors.

ii. Ammeter, Voltmeter, Galvanometer, Thermometer.
Answer. Thermometer – it is used to measure temperature of a body while the rest are connected in electric circuit to detect and measure electric current and potential difference.

iii. Germanium, Silicon, Porcelain, Gallium.
Answer. Porcelain – is an insulator while the rest are semiconductors.

Q6. Give scientific reasons.

Textual questions

1. Kitchenware material is coated with copper.
Answer.
1.      Kitchenware materials are generally made of stainless steel.
2.      They are coated with a thin layer of copper in order to protect them from rusting (corrosion) and also to improve their appearance.

2. Potential difference of source of current (i.e. cell) is less than e.m.f. OR
            The P.D. is always less than he e.m.f. of the cell.
Answer.
1.      In e.m.f. the cell has to spend energy to overcome its external resistance as well as the internal resistance.
2.      In potential difference, the cell has to spend energy to overcome only its external resistance.
3.      As energy cell has some internal resistance, potential different of a source of current (i.e. cell) is less than e.m.f.

3. The plates of heating devices such as a toaster, an electric iron etc. are made of an alloy rather than a pure metal.
Answer.
1.      A pure metal like copper has low resistance due to which it produces less heat.
2.      But alloys like nichrome have a very high resistance, thus the heat produced by it is more.
3.      Hence, in heating devices like a toaster, an electric iron etc., alloys are used instead of pure metals.

4. A Parallel combination of resistances decreases effective resistance of the circuit. OR
            A parallel combination is used to decrease the resistance of a circuit.
Answer.
1.      In a parallel combination, the equivalent resistance is smaller than each of the individual resistances.
2.      Therefore, a parallel combination is used to decrease the resistance of a circuit.   

5. Metals are good conductors. OR Metals are good conductors of electricity. OR
An electric current can flow through silver wire.
Answer.
1.      A metal contains large number of free electrons.
2.      When a metallic wire is connected between the two terminals (electrodes) of a cell, the negatively charged free electrons flow from a point at a lower potential to a point at a higher potential.
3.      This motion of electrons in one direction constitutes an electric current.
4.      Hence, metals are good conductors of electricity.



Non – textual questions.
1. Electric force and gravitational force are analogous (comparable).
Answer.
1.      The electric force (F) between two charged particles is given by the formula 

2.      The gravitational force between any two objects in the universe is given by the formula 
3.      Since both formulae are identical, the electric force and gravitational force are analogous.

2. Plastic is an insulator.
Ans.
  1. Substances like plastic possess practically no free electrons since the electrons are tightly bound to the parent nucleus.
  2. Due to this the charges cannot pass easily through plastic.
  3. Hence, plastic is an insulator.

3. Gallium, germanium and silicon are semiconductors.
Answer.
1. Substances like gallium, germanium and silicon are insulators.
2. However under certain specific and particular condition, they behave like conductors.
3. Thus, they are referred as “Semiconductors”.

4. Potential difference produces a current in the circuit.
Answer.
  1. The difference between the electric levels of the two electrodes of a cell is called potential difference.
  2. The chemical reactions taking place within the cell are responsible for the generation of potential difference between the electrodes.
  3. The potential difference sets the electrons in motion and produces a current in the circuit.

5. Diode is used as a rectifier.
Answer.
  1. A diode is a non – Ohmic conductor. It behaves like a valve and allows the flow of current only in one direction (forward bias) but not in the opposite direction (reverse bias).
  2. Due to this property, a diode is used as a rectifier. 

6.  Superconductors are useful in transferring electricity from one place to another.
Answer.
  1. Superconductors are materials that have no resistance and hence current passes through them without loss of energy.
  2. Thus, they are useful for transferring electricity from one place to another.

7. Thermistors are used for temperature measurement and in temperature control circuits.
Answer.
  1. Thermistors are non – ohmic conductors.
  2. Their resistance decreases as temperature increases.
  3. hence, they are used for temperature measurements and in temperature control circuits.
8. A series combination is used to increase the resistance of a circuit.
Answer.
1.      In a series combination, the equivalent resistance is greater than each of the individual resistances.
2.      Therefore, a series combination is used to increase the resistance of a circuit.

9. A thick wire has a low resistance.
Answer.
1.      The motion of free electrons constitutes an electric current in conductors.
2.      If the metal wire is thick (bigger cross – sectional area), the number of free electrons increases and the space available for their flow too increases.
3.      Hence, thick wires offer less opposition to the motion of free electrons and consequently have less resistance.

10. The I.V. graph of a diode is not a straight line.
Answer.
1.      A diode is a semi conducting device which does not obey Ohm’s Law i.e. it is a non – ohmic conductor.
2.      The resistance of a diode is not constant. It is low when the current flows in one direction but it is high if the current flows in the opposite direction.
3.      Hence, since the value of the resistance keeps on changing, the I.V. graph of a diode is not linear (straight line)


Q7. Distinguish between: give any two points.

Textual Questions

1. Voltmeter and Ammeter
Voltmeter
Ammeter
  1. It is a device used to measure the potential difference between two points.
  2. It is connected in parallel with the cell.
  3. It has a very high resistance.
  1. It is a device used to measure the current flowing through the circuit.
  2. It is connected in series with the cell.
  3. It has a very low resistance.

2. Resistance in series and Resistances in Parallel combination.
Resistance in Series
Resistances in Parallel
  1. 1. If a number of resistances are connected in such a way that the same current flows through each resistance, then the arrangement is called ‘Resistances in Series’.
  2. The voltage across each resistance is different.
  3. This combination is used to increase the resultant resistance of the circuit.
  4. This combination decreases the current in the circuit.
  1. If a number of resistances are connected between two common points such that the potential difference across each is the same, then that arrangement is called ‘Resistances in Parallel’.
  2. The current across each resistance is different.
  3. This combination is used to decrease the resistance of the circuit.
  4. This combination increases the current in the circuit.


3. E.M.F. and P.D.
E.M.F.
P.D.
  1. It is the work done by the source (cell) in raising a unit positive charge from its low potential and end to its high potential unit.
  2. It is greater than the P.D. of the cell.
  1. It is the energy spent by a cell to send a unit positive charge across the external circuit.
  2. It is less than the E.M.F. of the cell.

Non – Textual Questions

1. Static electricity and Current electricity
Static electricity
Current electricity
  1. The physical phenomena caused due to charges at rest are called static electricity.
  2. Both the ends of the object are at the same electric potential.
  3. This phenomenon is observed in insulators.
  1. The physical effects caused due to charges in motion are caused current electricity.
  2. Both the ends of the conductor are at different electric potentials.
  3. This phenomenon is observed only in conductors.

2. Conductors and Insulators.
Conductors
Insulators
  1. Substances that allow charges to flow freely are called conductors.
  2. They contain a large number of free electrons.
  3. Generally metals are conductors.
  1. Substances that do not allow charges to flow freely are called insulators.
  2. They contain no free electrons.
  3. Generally non – metals are insulators.

3. Anode and Cathode
Anode
Cathode
  1. The positively charged electrode in an electrolytic cell is known as anode.
  2. Negative ions (anions) are attracted to it.
  1. The negatively charged electrode in an electrolytic cell is known as cathode.
  2. Positive ions (cations) are attracted to it.

4. Ohmic Conductors and Non – Ohmic conductors.
Ohmic Conductors
Non – Ohmic Conductors
  1. Materials that obey Ohm’s Law are called ‘Ohmic Conductors’.
  2. For e.g. Copper
  3. It has a ‘Linear’ current voltage relationship.
  4. The value of the resistance (R) is constant.
  1. Materials that do not obey Ohm’s Law are called ‘Non – Ohmic Conductors.
  2. For. e.g. Diodes
  3. It has a non – linear current voltage relationship.
  4. The value of the resistance is variable.

5. Potential difference and electric current.
Potential Difference (P.D.)
Electric current
  1. The difference between the electric levels of the terminals of a conductor is known as potential difference.
  2. Its S.I. unit is ‘Volt’.
  1. The flow of electric charge is called electric current.
  2. its S.I. unit is ‘Ampere’.
6. Equivalent Resistance in Series and Equivalent Resistance in Parallel.
Equivalent Resistance in Series
Equivalent Resistance in Parallel
  1. The equivalent resistance of a series combination is equal to the sum of the individual resistances.
  2. The equivalent resistance in series is greater than the individual resistances.
1. The reciprocal of the equivalent resistance in parallel is equal to the sum of the reciprocals of the individual resistances.
The equivalent resistance in parallel is smaller than the individual resistances.

7. Flow of electric current through a metal and flow of electric current through an electrolyte.
Flow of current through a metal
Flow of current through an electrolyte
  1. The electric current flows due to the motion of free electrons.
  2. The flow of chare is unidirectional.
  3. No chemical change occurs in the substance carrying the current.
  1. The electric current flows due to the motion of ions.
  2. The flow of charge is bi – directional.
  3. Chemical change occurs in the substance carrying the current.

Q8. Draw and label the diagram:
1. Atoms of hydrogen and lithium.

2. Simple Voltaic cell.


3. Verification of Ohm’s law.

4. Graph of characteristics of diode and Thermistors.


5. Connection diagram of three resistances in series.

6. Connection diagram of three resistances in parallel.


Q9. Solve the following numerical.

Ex. 4.1
1.      A filament of electric lamp draws a current of 0.5 amperes. The bulb is used for 2 hours. Calculate the amount of charge that flows through the circuit.
2.      The electric current passing through the cross section of a wire is 2 amperes. Find the amount of charge that flows through it for two minute.
3.      A filament of electric bulb draws a current of 2.5 amperes. If the bulb is used for one hour calculate the amount of charge that flows through the circuit.
4.      Find the electric current passing through a cross section of a wire, if 30 coulomb of electric charge flows through it for one minute.
5.      Calculate the electric current passing through a cross section of a wire when a charge of 150 coulomb passes through it for half minute.
6.      The electric current flowing through the filament of an electric bulb is 1.5A. Find the amount of charge flowing through the filament in 3 min 10 seconds.

Ex. 4.2
  1. Two points in an electric field have a potential difference of 10V. How much work needs to be done to move a charge of 5C between these points>\?
  2. How much work needs to be done to move a charge of 2C between two points if the potential difference between the points is 18 volts.
  3. Calculate the work done in moving a charge of 4 coulombs from a point at 220 volts to another point at 230 volts.
  4. Find the work done in moving a charge of 100 coulomb between two points, if the potential difference between the points is 0.5V.
  5. Determine the P.D. if 150 Joule of work is done to carry a charge of 30 coulomb between 2 points.
  6. If 18 joule of work is done to carry a charge of 12 coulomb between two ends of a wire, find the P.D. across the wire.

Ex. 4.3
1.      Calculate the current flowing through 5Ω resistor when a potential difference of 7.5 volt is applied across it.
2.      Calculate the current in circuit if the resistance of a wire is 10 Ω and P.D. across is 5 volt. (Text Book Questions)
3.      Calculate the potential difference ‘V’ across a 7 Ohm resistor carrying a current of 0.2 A. (Text Book Questions)
4.      Calculate the current flowing through 3 Ω resistor when a potential difference of 4.5 volt is applied across it.
5.      A metallic wire of 100 Ω resistance has a P.D. of 2 volts. How much electric current can flow through the wire?
6.      Calculate the potential difference when a current of 500 mA flows through a conductor having a resistance of 25 Ω.
7.      Calculate the Potential difference between the two terminals of a wire of resistance 35 Ohms when a current of 250 mA flows through the wire.
8.      Determine the P.D. which should be applied across a wire of resistance 50 Ohms in order to send a current of 0.02 A through it.


Ex. 4.4
  1. You are given three resistances of 1 Ω, 2 Ω and 3 Ω. What will be their equivalent resistance in series and equivalent resistance in parallel combination?
  2. Resistors of 10 Ohms and 2.5 Ohms are connected in parallel combination and 3 Ohm resistances are connected in series combination with them. Find combined resistance. (Text Book Question)  
  3. Thee resistances of 6 Ohm each are connected in parallel combination. Determine their equivalent resistance. (Text Book Questions)
  4. Resistors of 3 Ohm and 6 Ohm are connected in parallel combination and 5 Ohm resistance is connected in series combination with them. Find combined resistance.
  5. You are provided with three resistances 4 Ω, 5 Ω and 20 Ω. What will be their equivalent resistance in series and in parallel combinations?
  6. Find the equivalent resistance of a parallel combination of resistances 20 Ω and 5 Ω.
  7. You are provided with two resistances of 60 Ω and 40 Ω. What will be their equivalent resistance in series and in parallel combinations?
  8. Two resistances of equal value are connected in parallel. If their resultant resistance is 2.5Ω. Find the value of each.

Ex. 4.5
  1. 4 Ohm and 2 Ohm resistors are connected in parallel across the terminals of cell of e.m.f. 1.5V and internal resistance 2/3 ohm. Find the total current in circuit. (Text Book Questions)
  2. 7.2 Ohm and 4.8 Ohm resistors are connected in series across the terminals of cell of e.m.f. 6V and internal resistance ½ ohm. Find the total current in the circuit.
  3. A cell of e.m.f 12V and internal resistance 0.25 Ohm is connected in an electrical circuit shown below. Find the total current flowing in the circuit. 
  4. Two resistors of 15 Ω and 12.5 Ω are connected in series across the terminals of a cell of e.m.f. 1.5 V and internal resistance 2.5 Ω. Find the current passing through the circuit.
  5. The e.m.f. of a voltaic cell is 1.5 V with an internal resistance of 0.25 Ω. If the external resistance of the circuit is 1.25 Ω, find the current.


Q10. Define the following.
Textual Questions
1.      Ampere: - It is the unit of electric current. If a charge of one coulomb passes across any section of a conductor in one second, the current in the conductor is said to be one ampere. Therefore 1 ampere = 1 coulomb / 1 second.
2.      Volt: - It is the SI unit of potential difference. The potential difference is one volt, if one joule of work is done in moving a charge of one coulomb. Therefore 1Volt = 1joule/ 1coulomb.
3.      Ohm: - It is the unit of resistance. The resistance of the conductor is one ohm, if a potential difference of one volt applied between its ends causes a current of one ampere to flow through it.  Therefore 1 Ohm = 1 volt / 1 ampere.
4.      Potential Difference: - The difference between the electric potential of the two terminals of a wire is called potential difference.
5.      E.M. F: - The electromotive force (E.M.F) of a source is the work done by the source in raising a unit positive charge from its lower potential end to the higher potential end. OR E.M.F of the source is equal to the work done by the source in circulating unit positive charge round the complete circuit.

Non – Textual Questions
1.      Resistances in Series: - If a number of resistances are connected in such a way that the same current flows through each resistance, then such an arrangement is called resistances in series.

2.      Static Electricity: - The physical phenomena produced by charges at rest are called static electricity.
3.      Current electricity: - The physical effects due to changes in motion is called current electricity.
4.      Conductors: - Substances that allow charges to pass freely through them are called conductors.
5.      Insulators: - Substances that do not allow charges to pass freely through them are called insulators.
6.      Semi - conductors: - Insulators that allow electricity to flow through them under specific conditions are called semi – conductors.
7.      Electrolytes: - Solutions that are good conductors of electricity are called electrolytes.
8.      Electrolysis: - The conduction of the electricity through electrolyte is called electrolysis.
9.      Coulomb: - It is the SI unit of electric charge. If two like point charges of equal magnitudes are placed in a vacuum at a distance of 1 metre from each other an if they repel each other with a force of 9 X 109N, then each charge is called 1 coulomb.
10.  Resistance in Parallel: - If a number of resistances are connected between two common points in such a way that the potential difference across each resistance is the same, then such an arrangement is called resistances in parallel.

11.  Ohmic conductor: - Materials that obey Ohm’s Law are called Ohmic conductors. Their graph of electric current against voltage is a straight line. The value of the resistance is a constant. E.g. Copper, Aluminium, silver etc.
12.  Non Ohmic conductor: - Materials that do not obey Ohm’s Law are called non – Ohmic conductors. Their graph of electric current against voltage is a non – linear graph. The value of resistance is not constant. E.g. Diodes, Thermistors.




Q12. Discuss how the electric current is set up when charges are in motion.
Answer.
1.      Metals (conductors) contain a large number of free electrons called ‘conduction electrons’. They are in a state of random motion.
2.      When the wire is connected between the two terminals of the cell, the negatively charges free electrons move from a point at a lower potential to a point at a higher potential.
3.      This motion of free electrons in one direction sets up an electric current in the wire.
4.      The direction of the conventional current is assumed to be from the positive terminal to the negative terminal of the cell.

Q13. Obtain an expression for equivalent resistance of three resistances connected in series combination.
Answer. If the number of resistance are connected one after another in such a way that the same current flows through each resistance, then the arrangement is called resistance in series.

1.      LetR1, R2 and R3 be three resistances connected in a series combination and let R be their equivalent resistance.
Let V1, V2 and V3 be the P.D. across resistances R1, R2 and R3  respectively.
Let ‘V’ be the P.D. of the cell.
Let ‘I’ be the current flow through each resistance.

2. According to Ohm’s Law,
            

    

Conclusion
Therefore, equivalent resistance in series is equal to the sum of the individual resistances.

Q14. Obtain an expression for equivalent resistance of three resistances connected in parallel combination.

Answer. If the numbers of resistance are connected between two common points, such that the potential difference across each resistance is the same, then the arrangement is called resistance in parallel.

Three resistances R1, R2 and R3 are connected in parallel between the points A and B. Let R be the equivalent resistance of the parallel combination.
            A Cell E, Key K and the ammeters A are also connected with resistances.
            Let the current passing through R1 be I1, R2 be I2, and R3 be I and that of R be I.

Thus, the reciprocal of the equivalent resistance is equal to the sum of the reciprocal of the individual resistance.

Q15. State Ohm’s law and give its limitations.
Answer. Ohm’s Law: - The current (I) in a conductor is directly proportional to the potential difference (V) between its ends, provided the physical state of the conductor remains the same.
                                                

Non – Textual Questions

Q 16. Explain with the help of diagrams.

1. Explain the construction and working of a simple voltaic cell with the help of a neat and labelled diagram.
Answer.

Construction: -
A simple voltaic cell consists of two metal plates immersed in a glass vessel containing dilute sulphuric acid.
One of the plates is of Zinc which serves as the negative terminal while the other plate is of copper which serves as the positive terminal.
The two plates are connected to a small bulb through a copper wire.

Working: -
1.      When the copper and zinc plates are dipped in dilute sulphuric acid, the zinc and copper plates get negatively and positively charged respectively.
2.      This charging of the plates is due to the chemical reaction taking place within the cell. Hence chemical energy gets converted to electrical energy.
3.      The electrons in the wire move from the zinc plate (-ve) to the copper plate (+ve). Thus as the current flows in the circuit, the bulb begins to glow.

2. Explain how motion of ions towards the respective electrodes kept in a solution of common salt constitutes an electric current.
Answer.
1.      The conduction of electricity through an electrolyte is called ‘electrolysis’.
2.      When Sodium chloride (common salt) is dissolved in water, it dissociates into Na+ and Cl- ions.
3.      When electrodes are connected to a cell, the Cl- ions move towards the anode (+ve terminal) while the Na+ ions move towards the cathode(-ve terminal).
4.      At the cathode, Cl- loses electrons while at the cathode, Na+ gains electrons.


5.      Thus, the motion of the ions towards their respective electrodes constitutes an electric current in a solution of common salt.

3. What is meant by ‘resistance’ of a conductor?
Answer.
1.      When a potential difference is applied between the ends of the conductor, the free electrons move from lower potential end to the higher potential end.
2.      The moving electrons collide with the fixed atoms of the conductor. Due to these collisions, the flow of electrons is slowed down.
3.      Thus there is ‘opposition’ to the flow of current. This opposition is called as the resistance of the conductor. Greater the number of collisions, greater is the resistance of the conductor.

4. State and explain the factors on which the resistance of a conductor depends.
Answer. The resistance of a conductor depends upon the following factors:
1. Material of the conductor: - Conductors of different materials have different resistance.
2. Length (L) of the conductors: - The resistance (R) of a conductor is directly proportional to the length of the conductor.

3. Area of cross section (A): - The resistance (R) of a conductor is inversely proportional to the area of cross section (thickness) of the conductor.
                                                

6. State the laws

i. Coulomb’s Law
Answer. The electric force (F) between two charged particles varies directly as the product of the two charges q1 q2 and inversely as the square of the distance (r) between them.



ii. Ohm’s Law
Answer. The current (I) in a conductor is directly proportional to the potential difference (V) between its ends, provided that physical state of a conductor remains the same.


Non – Textual Questions

Q17. Write short notes on

i. Super conductors
Answer.
1.      Materials which conduct electricity without resistance at very low temperature are called ‘Super conductors’.
2.      The temperature at which the resistance in a super conductor becomes zero is called the ‘Critical Temperature’.
3.      Since super conductors have zero resistance, electric current passes through them without loss of energy.
4.      Super conductor are useful for transmission of electricity over long distances, making faster computers, for frictionless transportation and Nuclear Magnetic Resonance (N.M.R.) technique.
5.      The Dutch scientist H.K. Onnes was among the first to prepare a super conductors in 1911. He found that the resistance of mercury becomes zero at a temperature of -2690C (4.2K).

ii. Flow of current through conductor.
Answer.
1.      Metals (conductors) contain a large number of free electrons called ‘conduction electrons’. They are in a state of random motion.
2.      When the wire is connected between the two terminals of the cell, the negatively charged free electrons move from a point at a lower potential to a point at a higher potential.
3.      This motion of free electrons in one direction sets up an electric current in the wire.
4.      The direction of the conventional current is assumed to be from the positive terminal to the negative terminal of the cell.

iii. Concept of Resistance.
Answer.
1.      When a potential difference is applied between the ends of the conductors, the free electrons move from the lower potential end to the higher potential end.
2.      The moving electrons collide with the fixed atoms of the conductors. Due to these collisions the flow of electrons is slowed down.
3.      Thus there is ‘opposition’ to the flow of current. This opposition is called as the resistance of the conductor.
4.      Greater the number of collisions, greater is the resistance of the conductor.


Q18. Answer in 2 to 3 sentences.

Non – Textual Questions

1. What are the advantages of electricity?
Answer. Electricity is a very important part of our lives and it has many advantages and applications.
1. Domestic applications: - Refrigerator, electric oven, electric geyser, mixer – grinder, computer etc.
2. Industrial uses: - Electric furnaces, motors, special instruments for cutting, etc.
3. Hospitals, banks, offices, private institutions etc. are heavily dependant on electricity.
Electricity can be transmitted over long distances with only a small loss of energy. Also it can be converted into various forms of energy.

2. What are conductors? Give examples.
Answer.
1.      Conductors are substances that allow the charges (electricity) to pass freely.
2.      Metals are good conductors since they contain large number of free electrons.
3.      Some common examples of conductors are: copper, silver, gold, graphite etc.

3. What are semi conductors? Give examples.
Answer.
1.      Semi conductors are certain materials which are insulators.
2.      However under certain conditions (like increase in temperature) they become good conductor of electricity.
3.      Silicon and Germanium are two commonly used semi conductors.

4. What are insulators? Give some examples.
Answer.
1.      Insulators are substances that do not allow the passage of electric charges.
2.      They contain negligible free electrons since the electrons are tightly bound to the nucleus.
3.      Rubber, plastic, glass etc. are insulators.

5. What is diode? What is it used for?
Answer.
1.      A diode is a semi conducting device that does not obey Ohm’s Law.
2.      It is like a valve and it allows passage of current only in one direction. Hence it is used as a ‘Rectifier’.
3.      Its resistance is low for the current in one direction i.e. current can flow while the resistance is high for the current in the opposite direction i.e. current cannot flow.

6. What are Thermistors? What are their uses?
Answer.
1.      Thermistors are a device that does not obey Ohm’s Law.
2.      Unlike metals, some Thermistors are made so that their resistance falls as the temperature increases.
Thermistors are used for measurement of temperature and in temperature control circuits.