Download Q.1 how is electric current expressed? ans. electric current is

Q.1 how is electric current expressed?
ans. electric current is expressed as the amount of charged flowing through a
particular area in unit time.
Q.2 what does a switch do?
ans. a switch makes a conducting link between the cell/battery and the resistor(s).
Q.3 what is an electric circuit?
ans. a continuous and closed path of an electric current is called an electric circuit.
Q.4 what constitute the flow of electric charges?
ans. electrons
Q.5 why is direction of electric current taken from positive to negative, even if the
electrons actually move from negative to positive terminal?
ans. -electrons were not known at the time when the phenomenon of electricity was
first observed.
-so electric current was considered to be the flow of positive charges and the
direction of flow of positive charges was taken to be the direction of electric current.
- conventionally, in an electric circuit the direction of electric current is taken
as opposite to the flow of electrons, which are negative charges.
Q.6 how is current measured?
ans. if a net charge Q, flows across any cross-section of a conductor in time t, then the
current I, through the cross-section is
I = Q/t
Q.7 what is the SI unit of electric charge?
ans. coulomb(C)
Q.8 what is how many electrons constitute 1 C of charge?
ans. 6 X 1018
Q.9 what is the charge of 1 electron?
ans. 1.6 X 10-19 C
Q.10 what is the SI unit of electric current? after whom is it named?
ans. SI unit of electric current is ampere(A), named after the French scientist,
Andre-Marie Ampere.
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Q.11 what is 1 ampere equal to?
ans. 1 ampere is constituted by the flow of one coulomb of charge per second, i.e.,
1A = 1C / 1s.
Q.12
ans.
express milliampere and microampere in terms of ampere.
1 milliampere (1 mA) = 10-3 A
1 microampere(1 µA) = 10-6 A
Q.13
ans.
what does ammeter measure?
ammeter measures electric current in a circuit.
Q.14 what makes the electric charge to flow?
ans. creation of potential difference on one side, using a cell or a battery, makes the
electric charge to flow to the other side.
Q.15 how does a cell produce an electric current?
ans. when the cell is connected to a conducting circuit element, the potential
difference sets the charges in motion in the conductor and produces electric current.
Q.16 define potential difference.
ans. potential difference between two points in an electric circuit carrying some
current is the work done to move a unit charge from one point to the otherV = W/Q
Q.17 what is the SI unit of electric potential difference, and after whom is it named?
ans. SI unit of electric potential difference is volt(V), named after Alessandro
Volta, an Italian physicist.
Q.18 what is 1 V of voltage?
ans. 1 volt is the potential difference between two points in a current carrying
conductor when 1 joule of work is done to move a charge of 1 coulomb from one
point to the other.
1 V = 1 J/C
Q.19
ans.
what is voltmeter?
voltmeter is an instrument used to measure the potential difference.
Q.20 how is voltmeter connected in a circuit?
ans. the voltmeter is always connected in parallel across the points between which
the potential difference is to be measured.
Q.21
ans.
what do you mean by circuit diagram?
pictorial representation of electric circuit is called circuit diagram.
Q.22 what is Ohm’s law? who found it and when?
ans. the electric current flowing through a metallic wire is directly proportional to
the potential difference V, across its ends provided its temperature remains the same.
V <is proportional to> I
or
V = IR
it was discovered in 1827 by a German Physicist Georg Simon Ohm.
Q.23 what is resistance?
ans. resistance is the property of a conductor to resist the flow of charges through
it.
Q.24 what is the SI unit of resistance?
ans. ohm (Ω)
Q.25 what do you mean by 1 Ω?
ans. if the potential difference across the two ends of a conductor is 1 V and the
current through it is 1 A, then the resistance R, of the conductor is 1 Ω.
i.e.,
Q.26
ans.
1Ω=1V/1A
what is the relationship between current through a resistor and its resistance?
the current through a resistor is inversely proportional to its resistance.
Q. 27 what happens to the current if the resistance is doubled?
ans. if the resistance is doubled the current gets halved.
Q.28 what is variable resistor?
ans. variable resistor is a component used to regulate current without changing the
voltage source.
Q.29 what is rheostat?
ans. rheostat is a device used to change the resistance in the circuit without
changing the voltage source.
Q.30 what do you mean by a good conductor of electricity?
ans. a component of a given size that offers a low resistance is called a good
conductor of electricity.
Q.31
ans.
what is a resistor?
a conductor having some appreciable resistance is called a resistor.
Q.32
what happens to the ammeter reading when the length of the wire is doubled?
Why?
ans. the ammeter reading decreases to one-half when the length of the wire is
doubled. this is because when the length is doubled, the resistance gets doubled. and
since resistance is inversely proportional to current, the current becomes half.
Q.33
ans.
on what factors does the resistance of a conductor depend?
the resistance of a conductor depends:
1) on its length
2) on its area of cross-section
3) on the nature of its material
Q.34 how does resistance depend on length and area of cross section of a metallic
conductor?
ans. the resistance of a uniform metallic conductor is directly proportional to its
length(l) and inversely proportional to the area of cross-section (A).
Q.35 define resistivity of a material.
ans. resistivity of a conductor is the resistance offered 1 unit of length of the
conductor at a given temperature.
Q.36 what is the resistivity range of good conductors and insulators?
ans. -the good conductors have a very low resistivity of range 10-8 Ωm to 10-6 Ωm.
-the insulators have resistivity of the order of 1012 to 1017 Ω m.
Q.37 which among metals, alloys, glass and rubber are good conductors and which
are insulators?
ans. metals and alloys are good conductors of electricity while rubber and glass are
insulators.
Q.38
ans.
which among metals and alloys have higher resistivity?
alloys
Q.39 why are alloys commonly used in electrical heating devices.
ans. alloys do not oxidise(burn) readily at high temperatures. for this reason they
are commonly used in electrical heating devices.
Q.40 which substance is used in filaments of bulb and why?
ans. tungsten is used almost exclusively for filaments of electric bulbs because it
has a very high melting point (3380°C ).
Q.41 why are copper and aluminium generally used for electrical transmission
lines?
ans. 1) they are good conductors of electricity.
2) they are cheaper.
3) they have a high melting point.
Q.42
ans.
what are the two types of connections of resistors?
1) series
2) parallel
Q.43 in which type of connection will the ammeter reading (current) remain
constant?
ans. series.
Q. 44 what is the equivalent resistance in series?
ans. the total potential difference across a combination of resistors in series is equal
to the sum of potential difference across the individual resistors.
i.e.,
V = V1 + V2 + V3 +...
Q.45 derive the formula to find out equivalent resistance in parallel and series
circuit.
ans. ( refer to ncert textbook – for series pg 211/2 and for parallel pg 214.)
Q.46 what are the disadvantages of series connections?
ans. 1) since the current is constant throughout the series connection, it is
obviously impracticable to connect resistors of different resistance, because they need
currents of widely different values to operate properly.
2) when one component fails the circuit is broken and none of the components
works.
Q. 47 why does an electrician spend a lot of time in repairing ‘fairy lights’ ?
ans. in fairy lights, the lights are connected in series. so if even one light is
damaged, the circuit breaks and it becomes very difficult to know which light is
damaged unless each light is tested separately. so the electrician takes a lot of time in
locating and replacing the ‘dead’ bulb.
Q.48 what are the advantages of parallel connection?
ans. 1) for each appliance of different resistance, the current varies, and so all the
appliances operate properly.
2) even if one appliance is damaged, the circuit is not broken and the other
appliances work properly.
Q. 49 where does the electrical energy go in an electric circuit?
ans. electrical energy is converted into light energy, mechanical energy, etc. or lost
as heat energy in the surrounding.
Q. 50 what do you mean by heating effect of electric current?
ans.
when the electric circuit is purely resistive, that is , a configuration of resistors
only connected to a battery, the source of energy continually gets dissipated entirely
in the form of heat. this is known as the heating effect of electric current.
Q.51
ans.
what do you mean by power input in a circuit?
power input in a circuit refers to the work done by the circuit in a unit time.
what do you mean by Joule’s law of heating?
Joule’s law of heating implies that heat produced in a resistor is
1) directly proportional to the square of current for a given resistance
2) directly proportional to resistance for a given current, and
3) directly proportional to the time for which the current flows through the
resistor.
i.e., H=I2Rt
Q.52
ans.
Q. 53 according to Joule’s law, H=I2Rt ,R is proportional to H. but in practical
situations for a known, voltage, we take R as inversely proportional to H ,i.e.,
H=V2t/R. why?
ans. -Joule’s law is applicable only when current(I) is constant.
`
- but in practical situation, where mainly parallel circuits are used, the voltage
remains constant.
- so by using the formula I=V/R, substituting I in Joule’s law, we get H=
V2t/R.
Q.54 give some practical applications of Joule’s heating.
ans. 1) the electric laundry iron, electric toaster, electric oven, electric kettle and
electric heater are some of the familiar devices based on Joule’s heating.
2) the electric heating is also used to produce light, as in an electric bulb.
3) the fuse in the electric circuit melts when the current is larger(as the heat
increses) , thus breaking the circuit and protecting the electrical appliances.
Q. 55 why are bulbs filled with chemically inactive nitrogen and argon gases?
ans. the heat emitted by the bulbs to produce light is very high. so to thermally
isolate the filaments, chemically inactive nitrogen and argon gases are filled to
prolong the life of filament.
Q. 56 how does a fuse protect a device?
ans. -the fuse is kept in series with a device(s).
-it consists of a piece of wire made of a metal or an alloy of appropriate
melting point, e.g., aluminium, copper, iron, lead, etc.
- if the current is larget than the specified value flow through the circuit, the
temperature of the fuse wire increases, and it melts, thus breaking the circuit. this
protects the device from damage.
Q.57
ans.
how is fuse rated for domestic purpose?
the fuses used for domestic purposes are rated as 1A, 2A, 3A, 5A, 10A, etc.
Q. 58 give an example to show how a fuse of suitable range is taken.
ans. for an electric iron which consumes 1 kW electric power when operated at
220V, a current of (1000/220) A ,i.e., 4.54 A will flow in the circuit. in this case, a 5A
fuse must be used.
Q .59 define electric power in terms of energy consumption.
ans. electric power is the rate at which electric energy is dissipated or consumed in
an electric circuit
Q.60 how does power change with resistance?
ans. -when the voltage is constant, power is inversely proportional to resistance,
i.e., P=V2/R.
-when the current is constant, power is directly proportional to resistance, i.e.,
P=I2R.
Q.61
ans.
what is the SI unit of electric power?
watt(W)
Q.62 what do you mean by 1 W of power.
ans.
1W is the power consumed by a device that carries 1A of current when
operated at a potential difference of 1V.
i.e., 1W = 1V A.
Q.63 what is the commercial unit of electric energy?
ans. kilowatt hour(kW h) , also known as ‘unit’.
Q.64 what is 1 W h of energy?
ans. 1 watt hour(1 W h) is the energy consumed when 1 watt power is used for 1
hour.
Q.65 express 1 kW h in the SI unit of energy.
ans.
1 kW h = 1000 watt x 3600 second
= 3.6 x 106 watt second
= 3.6 x 106 joule (J).