Electricity Worksheet-22
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Two students perform experiments on series and parallel combinations of two given resistor R1 and R2 and plot the following V – I graphs,
Which of the graphs is (are) correctly labelled in terms of the words 'series' and 'parallel' ? Justify your answer.
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You are given three resistors of 10 Ω , 10 Ω and 20 Ω; a battery of emf 2.5 V a key, an ammeter and a votmeter. Draw a circuit diagram showing the correct connections of all given components such that the voltmeter gives a reading of 2.0 V
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Two electric circuits I and II are shown in the Fig. In circuit I, the key K is closed whereas in circuit II, the key is open. Compare the currents in the circuit I and circuit II.
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An electric geyser has the rating 2000 W, 200 V marked on it What should be the minimum rating (in whole number) of a fuse wire that may be required for safe use with this geyser ?
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An electrician puts a fuse of rating 5 A in that part of domestic electrical circuit in which electrical heater of rating 1.5 kW, 220 V is operating. What is likely to happen in this case and why ? What change, if any, needs to be made ?
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The electric power consumed by a device may be calculated by using either of the two expressions P = I2R or P = V2/R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. How can this different dependence of P on R in these expressions be explained ?
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A cylinder of a material is 10 cm long and has a cross-section of 2 cm2. If its resistance along the length be 20 Ω, what will be its resistivity in number and units.
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Why is tungsten metal selected for making filaments of incandescent lamps ?
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A resistance of 10 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 this circle ?
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Why is much less heat generated in long electric cables than in filaments of electric bulbs ?
Answer:
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The resistance (Rs) in series combination of two resistors is more than the resistance (Rp) in parallel combination of the same two resistors, i.e., Rs > Rp. As I = V/R, for a given pd (V), Is < IP. In both the graphs, for a given pd (V) across each combination, Is < Ip. Hence, both the graphs are properly labelled.
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The circuit diagram showing the correct connections is shown in the fig. Since the two resistors, each of 10 Ω are in parallel combination, their equivalent resistance is given by
or Rp = 5 Ω
Since Rp and 20 Ω are in series combination, the total resistance in the circuit is given by
Current in the circuit,
pd across 20 Ω resistor
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In circuit I, since K is closed, R1 and R2 are in parallel combination. Their equivalent resistance
Is given by
Current in the circuit,
In circuit II, since K is open, the resistance R2 is out of circuit and as such the net resistance in the circuit is R1.
Current in the circuit,
Thus,
Since (R1 + R2) > R2, I1 > I2.
Thus current in circuit I is more than the current in circuit II.
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P = 2000 W, V = 200 V
As P = VI,
Thus, minimum current rating (in whole number) of the fuse wire = 10 A
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P = 1.5kW= 1500W, V = 200 V.
P = VI,
When 6.81 A current flows through the fuse wire of rating 5A, it melts and the circuit breaks. Because the current flowing through the fuse, 6.81 A is more than its rating, i.e. 5 A.
In this case, a fuse of rating higher than 6.81 A need to be used.
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Here both the expressions are correct and are used to calculate electric power. The expression P = I2R is used to calculate electric power when the current flowing through the circuit is constant. This is the case when the electric devices are connected in series.
The expression P = V2/R is used to calculate electric power when the pd (V) across the circuit is constant. This is the case when the electric devices are connected in parallel.
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ohm cm.
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Because it has high melting point.
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Each of the two semicircles between the two ends of a diameter has a resistance of 10 Ω /2 = 5 Ω. These two resistance are in parallel between the two ends of the diameter. Thus, the effective resistance = 5 Ω/2 = 2.5 Ω
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Electric cables are made of thick copper which have much less resistance as compared to thin tungsten filament of electric bulbs.