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## ELECTRIC CURRENT AND CIRCUIT

Electric current is expressed by the amount of charge flowing through a particular area in unit time. In other words, it is the rate of flow of electric charges. In circuits using metallic wires, electrons constitute the flow of charges. Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.
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
SI unit of electric charge = coulomb (C)= 6 X 1018 electrons
Electric Current is also showed by Ampere (A)
1 A = 1 C/1s or flow of one coulomb charge per second.
Instrument Measuring Electric Current is called Ammeter. It is always connected in series in a circuit
ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
Electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other –
Potential difference (V) between two points = Work done (W)/Charge (Q)
V= W/Q
The SI unit of electric potential difference is volt (V),
The potential difference is measured by means of an instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.
OHM’S LAW
Georg Simon Ohm (1787–1854) 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. This is called Ohm’s law. In other words –
or V/I = constant = R
or V = IR
R is a constant for the given metallic wire at a given temperature and is called its resistance. It is the property of a conductor to resist the flow of charges.
As per Ohm’s law the resistance of the conductor depends
(i) on its length,
(ii) on its area of cross-section, and
(iii) on the nature of its material.
The SI unit of resistivity is Ω m. It is a characteristic property of the material. The metals and alloys have very low resistivity in the range of 10–8 Ω m to 10–6 Ω m. They are good conductors of electricity. Insulators like rubber and glass have resistivity of the order of 1012 to 1017 Ω m. Both the resistance and resistivity of a material vary with temperature. Resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc. Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminium are generally used for electrical transmission lines.
HEATING EFFECT OF ELECTRIC CURRENT
If the electric circuit is purely resistive, that is, a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current. This effect is utilised in devices such as electric heater, electric iron etc.
Applying Ohm’s law, we get
H = I² x Rt
This is known as Joule’s law of heating. The law implies that heat produced in a resistor is
(i) directly proportional to the square of current for a given resistance,
(ii) directly proportional to resistance for a given current, and
(iii) directly proportional to the time for which the current flows through the resistor.
Practical Applications of Heating Effect of Electric Current
1. Electric Heaters
2. Bulb's Filaments
3. Electric Fuse
4. Electric Kettle
5. Sandwich Maker
6. Toaster
7. Electric Iron

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QUIZ
http://www.mcwdn.org/Physics/ElectricQuiz.html

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ELECTRICAL CIRCUIT- FLOW OF ELECTRONS

## FUNCTION OF RHEOSTAT

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## PRACTICE     QUESTIONS FOR   CBSE

1. Define electric potential difference?
2. Why is an ammeter connected in series in a circuit?
3. Why is a voltmeter connected in parallel in a circuit?
4. State the law that governs the strength of the current passing through a metallic conductor when a p.d is applied across its end. Illustrate this law graphically?
5. State the law which governs the amount of heat produced in a metallic conductor when current is passed through it for a given time. Express this law mathematically?
6. Define resistance. What are the factors on which it depends?
7. A copper wire of resitivity P is stretched to reduce its diameter to half its previous value. What is the new resistivity?
8. Define the S I unit of electric current and potential difference.
9. Derive expressions for equivalent resistance of resistors connected in
a. series combination
b. parallel combination
10. What is an electric fuse? Explain its function.
11. Explain, how the heating effect of electric current is made use in the functioning of an electric bulb?
12. State the relation between the commercial and SI unit of electric energy.
13. Write the advantages of connecting electrical appliances in parallel and disadvantages of connecting them in series in a household circuit.
14. Why is tungsten used almost exclusively for making the filaments of electric lamps?
15. A piece of wire is redrawn by pulling it until its length is doubled .Compare the new resistance with the original value.
16. An electric geyser has rating 2000 w, 220 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 the geyser?
17. Three resistors each of resistance 10 ohm are connected, in turn, to obtain a. minimum resistance b. Maximum resistance. Compute
a) The effective resistance in each case
b) The ratio of minimum to maximum resistance so obtained.
18. Two resistors of resistance 2 ohms and 4 ohms are, in turn connected
a) In series
b) In parallel to a given battery for same interval.
Compute the ratio of the total quantity of heat produced in the combination in the two cases.
19. Two metallic wires A and B are connected in parallel. Wire A has length L and radius R wire B has a length 2L and radius 2r. Compute the ratio of the total resistance of parallel combination and resistance of wire A.
20. In a house three bulb of 100w each are lighted for 4 hours daily and six tube lights of 20w each are lighted for 5 hours daily and a refrigerator of 400w is worked for 10 hours daily for a month of 30 days. Calculate the electricity bill if the cost of one unit is Rs 4.00.
21. Three resistors of 4 ohms, 6 ohms and 12 ohms are connected in parallel. The combination of above resistors is connected in series to a resistance of 2 ohms and then to a battery of 6 volts. Draw a circuit diagram and calculate
a) Current in main circuit
b) Current flowing through each of the resistors in parallel
c) P.d and the power used by the 2 ohm resistor.
22. Two lamps, one rated 100 W at 220 V and the other 60W at 220 V, are connected in Parallel to a 220 volt supply. What current is drawn from the supply line?

## ELECTRICITY:PRACTICE QUESTIONS-QUIZ

ELECTRICITY
HOTS Questions
Q.1   What is represented by joule/coulomb?
Q.2   Why are copper wires used as connecting wires?
Q. 3 A wire of resistivity d is stretched to double its length. What is its new resistivity?
Q4 What is the resistance of connecting wire?
Q.5   How will you join three resistances, each of 20 ohm so that the effective resistance is 30 ohm?
Q.6 What happens to the current in a circuit if its resistance is doubled?
Q.7 What happens to the resistance of a circuit if the current through it is doubled?
Q.8 How does the resistance of a wire depend upon its radius?
Q.9 Two wires are of the same length, same radius, but one of them is of copper and the other is of iron. Which will have more resistance?
Q.10 Given a resistors each of resistors R. How will you combine them to get the
( i )  maximum and (ii) minimum effective resistance? What is the ratio of the maximum to minimum resistance?
Q.11 A wire of length L and resistance R is stretched so that its length its doubled. How will its (a) Resistance change (b) Resistivity change?
Q.12 A household uses the following electric appliances:       (i) Refrigerator of rating 400 W for ten hours each day.
(ii)
Two electric fans of rating 80 W each for twelve hours each day.
(iii)
Six electric tubes of rating 18 W each for 6 hours each day.
Calculate the electricity bill of the household for the month of June if the cost       per unit of electric energy is Rs. 3.00.
Q.13 Ammeter burns out when connected in parallel.