Electricity
V.S.A Question (1 mark each)

What is the resistance of an ideal voltmeter and an ammeter?
Ans. Infinite and zero.
What is the value of charge on an electron in coulomb?
Ans. -1.6 x 10-19
Why is resistance more in combination of resistors?
Ans. In series combination of resistors, the effective length of the conducting path increases and as such resistance increases (R ∞l)
Name two special characters of a heater coil?
Ans. A heater coil should have (i) high resistivity (ii) high melting point.
What is the resistance of a connecting wire?
Ans. It is negligible, almost zero.
S.A.Q (2 marks each)
1. What is the difference between a resistor and resistance?
Ans. A resistor is an object where as resistance is its property due to which it obstructs the flow of electrons through it.
Electric current is flowing through a wire from west to east direction. What is the direction of motion of the conductor electrons in the wire?
Ans. From east to west as direction of electric current is opposite to the direction of flow of conduction electrons.
Why are the conductors of electric heating devices such as toasters and electric irons) made of an alloy rather than a pure metal?
Ans. It is because alloy is less affected by the rise in temperature than a pure metal. In other words alloys do not oxidise (or burn) easily at high temperature.
What will be the equivalent resistance R of two resistors R1 and R2 (i) connected in series and (ii) connected in parallel?
Ans. When connected in series
R=R1+R2
When connected in parallel
1/R=1/R1+1/R2 or R=(R1R2) /(R1+R2)
What will happen to current if the voltage across the circuit reduces to half its original value while the resistance of the circuit remains constant?
Ans. Current=Voltage/Resistance
If voltage becomes half its original value, the resistance remains the same, then current in the circuit will be reduced to half its original value.
SAQ (3 marks each)
A wire of resistance 5 ohm is bent in the form of a closed circle. What is the effective resistance between the two points at the ends of any diameter of the circle?
Ans. The length of the wire between two points at the ends of a diameter of a circle is half of the whole wire. The resistance of a conductor is directly proportional to its length.
Thus, the two resistance 5/2 Ω will be in parallel.
Let R be the effective resistance.
1/R=2/5+2/5
1/R=4/5
R=5/4=1.25 Ω
Calculate the effective resistance in the diagram given.

Ans. In this case resistance DA and AB are in series. DC and CB are also in series. But AD and AB together are in parallel to DC and CB together.
Thus, Resistance (DA + AB)=1+1=2Ω
and Resistance (DC + AB)=1+1=2Ω
Let Rp be the effective resistance, then (by parallel formula)
1/Rp=1/2+1/2
=(1+1)/2=2/2
Rp=1Ω
Thus, the combination has a resistance of 1 ohm.
Why does resistance of a conductor increase with increase in temperature?
Ans. When a metallic is heated the atoms in the metal vibrate with greater amplitude and frequency due to increase in temperature the thermal velocities of free electrons also increase. Therefore the numbers of collisions between free electrons and atoms increase. This increases the resistance of the conductor.
Why is it not advisable to handle electrical circuit with wet hands?
Ans. The resistance of dry skin human body is about 50,000Ω. When the skin is wet, the resistance is lowered to about 10,000Ω. If a person with wet hand touches the electrical circuit the current will flow suddenly due to less resistance. Therefore, it is advisable not to touch the electrical circuit with wet hands.

Long Answer Question (5 marks)
A dry cell should be used intermittently and not continuously. Explain why?
Ans. Hydrogen gas in the formed during the working of the dry cell. This gas in the form of bubbles forms an insulated layer around the carbon rod (i.e anode). As a result of this, the resistance of the cell is increased and hence the flow of charges in the cell is opposed. Ultimately, the dry cell stops working if used continuously. However, these bubbles of hydrogen gas are removed slowly by Mno2, which acts depolarizers. The dry cell can work if the bubbles of hydrogen gas around carbon rod are completely removed by Mno2. For this purpose time is required. So the dry cell should be used intermittently to provide sufficient time for the removal of hydrogen gas bubbles by manganese dioxide.

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