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 parallel 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 negativeterminal of the cell.
Ans. True
4.      Silver and copper are good conductors.
True
5.      Resistivity of pure metal 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. For eg. Tungsten (33800C)

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 the wire.
iii.                Copper being more ductile, it is used in making thin 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 a flow of current through it is called resistance.
2.      The SI unit of resistance is Ohm ( )
3.      The resistance of a conductor depends on its length and area of cross section.
1.      The 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.       The resistivity of a conductor does not depend on its length and area of cross section.

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.