THE ELECTRIC SPARK

Ch. No. 4. THE ELECTRIC SPARK

Q1. Choose the correct alternative and rewrite the following:

1.     1mA = ___________ A

a.     10³A

b.    10 ^{– 3} A

c.      10⁶A

d.     10 ^{– 6} A

2.     To increase the effective resistance in a circuit, the resistors are connected in _________

a.     Series

b.     Parallel

c.      Both ways

d.     None of these

3.     1 kilowatt hr = ____________ Joules.

a.     4.6 x 10⁶ Joule

b.    3.6 x 10⁶ Joule

c.      30.6 x 10⁶ Joule

d.     3.6 x 10⁵ Joule

4.     If a P.D. of 12 V is applied across a 3Ω resistor, then the current passing through it is _______

a.     36A

b.    4A

c.      0.25A

d.     15A

Q2. State True or False. If false correct it.

1.     The SI unit of charge is volt.

False: The SI unit of charge is the coulomb and the SI unit of P.D. is Volt.

2.     A voltmeter is always connected in series with the device.

False: A voltmeter is always connected in series with the device.

3.     The conventional direction of flow of current is from positive terminal to negative terminal of the cell.

Ans. True

4.     Silver and copper are good conductors.

True

5.     Resistivity of pure meal is more than alloys.

False: Resistivity of pure metals is less than that of alloys.

6.     The electric bulb consists of the filament whose melting point is low.

False: The electric bulb consists of the filament whose melting point is high.

Q3. Match the column.

I	II	III
Heat generated	Is used to reduce effective resistance in a circuit	V = IR
Resistance in parallel	Proportional to the square of current
Resistivity	Gives relation between V and I
Ohm’s law	Depends on the material of the conductor

Ans.

I	II	III
Heat generated	Proportional to the square of current
Resistance in parallel	Is used to reduce effective resistance in a circuit
Resistivity	Depends on the material of the conductor
Ohm’s law	Gives relation between V and I

Q4. Give scientific reasons.

1.     The material used for fuse has low melting point.

Ans.

i.                 A fuse is used to protect a circuit and the appliances connected in the circuit by stopping the flow of an excess electric current. For this, a fuse is connected in series in the circuit.

ii.               When the current in the circuit passes through the fuse, its temperature increases. When the current exceeds the specified value, the fuse must melt to break the circuit. For this, the material used for a fuse has low melting point.

2.     Wood and glass are good insulators.

Ans.

i.                 When a current flows through a conductor, the free electrons in the atoms move from one end of the conductor to the other.

ii.               Certain materials have less free electrons in their atoms and the current does not easily flow through the material.

iii.              There are no such free electrons in wood and glass. Hence they cannot conduct electricity. Hence, they are good insulators.

3.     The melting point of filament of a bulb is very high.

Ans.

i.                 The bulb begins to glow only when the filament is heated to a high temperature, and it becomes incandescent (bright) without melting.

ii.               This happens only when the material of the filament has a high melting point.

iii.              Hence, the material used in the filament of an electric bulb must have a high melting point.

4.     Connecting wires in a circuit are made of copper and aluminium.

Ans.

i.                 Copper and aluminium offer a low resistance to the flow of current and hence they are good conductors of electricity.

ii.               Copper and aluminium are highly ductile and hence can be used for preparing this wire.

iii.              Copper being more ductile, it is used in making thing wires, and aluminium is used for making thicker wires.

iv.              Thus, they are suitable for making wires used in electrical circuit.

Q5. Define

1.     1 volt: The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to another.

2.     1 ampere: 1 coulomb of charge passing through a cross – section of a conductor in 1 second is one ampere. It is the S.I. unit of an electric current.

3.     1 ohm: If one ampere current flows through the conductor, and 1 volt potential difference is applied across it, then its resistance is 1 ohm.

4.     Potential: Electric potential is the electrical level.

5.     Resistivity: The resistivity of a conductor is defined as the resistance of a conductor of unit length and the unit area of the cross – section.

6.     Electric power: Electric power is the rate at which electric energy is consumed. It is the electrical work done per unit time.

Q6. Differentiate between.

1.     Resistances in series and parallel.

Resistance in series

Resistance in parallel

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 effective resistance is a series combination is greater than the individual resistances. 3.     This combination is used to increase resistance in a 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 effective resistance of the combination is less than the individual resistances. 3.     This combination is used to decrease resistance in the circuit. 4.     This combination increases the current in the circuit.

2.     Conductors and insulators.

Conductors	Insulators
1.     Those substances through which electricity can flow are called conductors. 2.     Electrical resistances of conductors are very low. 3.     They contain large number of free electrons. 4.     Generally metals are conductors. E.g. silver, copper, aluminium	1.     Those substances through which electricity cannot flow are called insulators. 2.     Electrical resistances of insulators are infinitely very high. 3.     They do not contain free electrons. 4.     Generally non – metals are insulators. E.g. wood, rubber, plastic

3.     Resistance and resistivity.

Resistance	Resistivity
1.     The property of the conductor due to which it opposes flow of current through it is called resistance. 2.     The SI unit of resistance is Ohm ( ) 3.     The resistance of a conductor is inversely proportional to its area of cross section. This means that a thick wire has less resistance.	1.     Resistivity of a conductor is the resistance of a conductor of unit length and unit area of cross section. 2.     The SI unit of resistivity is Ohm metre ( m) 3.     Resistivity is independent of the shape and size of the conductor.

4.     High resistance and low resistance.

High resistance	Low resistance
1.     A high resistance indicates a material that hardly allows the movement of electrons. 2.     It is due to the less number of free flowing electrons in the outer most orbit of an element. 3.     Substances with infinitely high electrical resistance are insulators. 4.     High resistance provides low conductivity.	1.     A low resistance indicates a material that readily allows the movement of electrons. 2.     It is due to large number of electrons in the outer most orbit of an element. 3.     Substances with low electrical resistances are good conductors. 4.     Low resistance provides high conductivity.

Q7. State the laws:

1.     Ohm’s law: Ohm’s law states that the electric current flowing in a metallic conductor is directly proportional to the potential difference across its terminals, provided physical conditions of the conductor such as length, area of cross section, temperature and material remain constant.

If I is the current and V is the potential difference across the ends of a conductor then,

R is the resistance which is constant for given conductor. The SI unit of resistance is ohm (W)

2.     Joule’s law: - Joule’s law states that the quantity of heat generated (H) is a conductor of Resistance (R), when a current (I) flows through it for a time (t) is directly proportional to:

i.                 The square of the current.

ii.               The resistance of the conductor, and

iii.              The time for which the current flows.

Q8. Find the following.

a.     Find the expression for the resistors connected in series and parallel.

Ans.  

i.                 Resistors connected in series: - 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.      LetR_1, R₂ and R₃ are three resistances connected in a series combination and let R be their equivalent resistance.

Let V_1, V₂ and V₃ be the P.D. across resistances R_1, 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(R) is equal to the sum of the individual resistances

ii.               Resistors connected in parallel: 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 R_1, 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 I_1, R₂ be I₂, and R₃ be I_3­ and that of R be I.

 Conclusion

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

b.     Find the expression for resistivity of a material.

Ans. Resistance R of a conductor depends on the length ‘  and area of cross section  of the conductor.

Where (rho) is called resistivity of the conductor it is also called as specific resistance.

If we put  and  then

Conclusion

Thus, resistivity of a conductor is defined as the resistance of a conductor of unit length and unit area of cross section.

Q9.  Answer the following:

1.     P and Q are the two wires of same length and different cross sectional areas and made of same metal. Name the property which is same for both the wires and that which is different for both the wires.

Ans.

i.                 The property which is same for both the wires is resistivity. 

ii.               The property which is different for both the wires is resistance.

2.     Resistivity of some material is given below. State which one will be the best conductor.

Material	Copper	Aluminium	Silver	Nickel
Resistivity (Ωm)	1.62 x 10 – 8	2.63 x 10 – 8	1.60 x 10 – 8	6.48 x 10 – 8

Ans. From the above table we find that of all the metals, silver has the lowes resistivity (1.60 x 10 ^{– 8} Ωm), which means that silver offers the least resistance to the flow of current through it. Thus, silver metal is the best conductor of electricity.

3.     If the resistance of wire A is four times the resistance of wire B, find the ratio of their cross sectional areas.

Ans. Let resistance of wire  be  and that of wire B be .

   

  ----------------------equation number (1)

 

  (  is a constant called resistivity)

 

 

 

  --------------------------- from equation number (1)

 

 ratio of cross sectional areas of wires is 1:4.

4.     Two dissimilar bulbs are connected in series, which bulb will be brighter? (Hint: consider the resistance of the bulb).

Ans. When two bulbs are connected in series the first bulb will receive more current. As the bulb has its own resistance, less current will flow to the next bulb. Hence the first bulb will glow more. 


Write the SI unit of the following quantities

Particulars	Units
Charge	The coulomb
Current	The ampere
Potential difference	The volt
Resistance	The ohm
Resistivity	the ohm metre
Energy	The joule
Power	The watt