Current Electricity Flashcards
Define the term current and state it’s S.I. unit.
Current is defined as the rate of flow of charge. The S.I. unit of charge is coulomb
Define the term electric potential. State it’s S.I. unit.
the amount of work done per unit charge in bringing a positive test charge from infinity to that point. The S.I. unit of electric potential is volt.
How is the electric potential difference between the two points defined? State it’s S.I. unit.
The potential difference (p.d.) between two points is equal to the work done per unit charge in moving a positive test charge from one point to the other. It’s S.I. unit is volt. It is a scalar quantity.
Explain the statement ‘the potential difference between two points is 1 volt’
if the work done in moving 1 coulomb charge from one point to other is 1 joule
State whether the current is a scalar or vector?
Current is a scalar quantity.
What does the direction of current convey?
mean that thedirection of motion of electrons is opposite to it.
State whether the potential is a scalar or vector?
Potential is a scalar quantity.
What does the positive and negative sign of potential convey?
work has to be done on the positive test charge against the repulsive force due to the positive charge in bringing it from infinity
theattractive force on test charge due to the negative charge does work by itself.
Define the term resistance. State it’s S.I. unit.
Theobstruction offered to the flow of current by the conductor (or wire)is called it’s resistance.
The S.I. unit of resistance isOhm.
Name the particles which are responsible for the flow of current in a metallic wire.
free electrons.
Explain the flow of current in a metallic wire on the basis of movement of the particles
In a metallic wire, the moving charges are the free electrons which constitute the current.
Q (charge) = n × e
and the current in conductor is
I=Q/t=ne/t
State Ohm’s law
According to Ohm’s law,the current flowing in a conductor is directly proportional to the potential difference applied across it’s ends provided that the physical conditions and the temperature of the conductor remain constant.
Name and state the law which relates the potential difference and current in a conductor.
Ohm’s law
Ohm’s law states thatthe current flowing through the conductor is directly proportional to the potential difference across it’s ends provided that the physical conditions and the temperature of the conductor remain constant.
What is the necessary condition for a conductor to obey the ohm’s law
the physical conditions and the temperature should remain constant.
What is an ohmic resistor? Give one example of an ohmic resistor.
The conductors which obey the Ohm’s law are called the ohmic resistors or linear resistances. Examples are all metallic conductors (such as Silver, Aluminium, Copper, Iron etc.)
What are non-ohmic resistors? Give one example
The conductors which do not obey the Ohm’s law are known as the non-ohmic resistors (or non-linear resistances). Examples — LED, solar cell, junction diode, etc.
Give two differences between an ohmic and non-ohmic resistor.
Refer book
How does the resistance of a wire depend on it’s radius?
resistance of a wire is inversely proportional to the square of the radius of the wire.
How does the resistance of a wire depend on it’s length ?
Resistance of a wire is directly proportional to the length of the wire.
R ∝ l
How does the resistance of a metallic wire depend on it’s temperature?
resistance of a conductor increases with an increase in it’s temperature.
Two wires, one of copper and other of iron, are of the same length and same radius. Which will have more resistance? Give reason.
The iron wire will have more resistance.
Iron wire has more resistance as compared to copper because specific resistance of iron is more than that of copper.
Name three factors on which resistance of a given wire depends and state how it is affected by the factors stated by you?
affected by the factors stated by you?
Answer
The three factors on which the resistance of wire depends are —
Dependence on length of the wire — the resistance of a wire is directly proportional to the length of the wire.
R ∝ l
Dependence on the thickness of the wire — the resistance of a wire is inversely proportional to it’s area of cross section (a) normal to the direction of flow of current.
R ∝ 1/a
Dependence on the temperature of the wire — the resistance of conductor increases with an increase in it’s temperature.
Define the term specific resistance and state it’s S.I. unit.
Specific resistance of a material is the resistance of a wire of that material of unit length and unit area of cross section.
The S.I. unit of specific resistance is ohm × metre or (Ω m)
State the order of specific resistance of (i) a metal, (ii) a semiconductor and (iii) an insulator.
The specific resistance is very low for metals, because it allows most of current to pass through it.
The specific resistance is low for semiconductor.
The specific resistance is very high for insulators, as the current won’t pass through it.
Name two factors on which the specific resistance of a wire depends?
Material of the substance — It is a characteristic property of the substance. It is different for different substances.
Temperature of the substance — It increases with the increase in temperature for metals, but it decreases with the increase in temperature for the semiconductor.
Name a substance of which the specific resistance remains almost unchanged by the increase in temperature.
manganin
How does the specific resistance of a semi-conductor change with the increase in temperature?
Decrease
How does (a) resistance a wire depend on it’s (i) length, and (ii) radius?
Resistance of a wire is directly proportional to length of the wire.
Resistance of a wire is inversely proportional to the square of radius of the wire.
How does specific resistance of a wire depend on it’s (i) length, and (ii) radius?
Specific resistance of a wire does not depend on length of the wire as it is a characteristic property of the material of wire.
Specific resistance of a wire does not depend on radius of the wire as it is a characteristic property of the material of wire.
Name the material used for making connection wires. Give reason for your answer.
Copper or aluminium
materials are used for making connection wires because their specific resistance is very low and hence they possess least possible resistance.
Why should a connection wire be thick?
resistance becomes low
Name a material which is used for making the standard resistor. Give a reason.
Standard resistors are made of magnanin, constantan etc. for which the specific resistance is high and the effect of change in temperature on their resistance is negligible.
Name the material used for making fuse wire. Give a reason.
The material used for making fuse wire is an alloy of lead and tin because it’s melting point is low and it’s specific resistance is more than that of copper or aluminium
Name the material used for (i) filament of an electric bulb
a tungsten wire because it has a high melting point
Name the material used for heating element of a room heater.
nichrome because the specific resistance of nichrome is high and it’s resistance increases to a great extent with the increase in temperature.
What is a superconductor ?
A superconductor is a substance of zero resistance (or infinite conductance) at a very low temperature.
Example — Mercury below 4.2 K, lead below 7.25 K and niobium below 9.2 K
Which of the following is an ohmic resistance?
nichrome wire
For which of the following substances, resistance decreases with increase in temperature?
carbon
