Chapter Review:

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• A continuous and closed path of an electric current is called an
  electric circuit.
• 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.
• The SI unit of electric charge is coulomb (C).
• An electron possesses a negative charge of 1.6 × 10^-19 C.
• The electric current is expressed by a unit called ampere (A).
• 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.
• Ammeter is an instrument used to measure the electric current in a
  circuit.It is always connected in series in a circuit.
• The electric current flowingthrough 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..
• A conductor having some appreciable resistance is called a resistor.
• The SI unit of resistivity is Ω m.
• 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.
• The unit of power is watt (W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V.
• The commercial unit of electrical energy is kilowatt hour (kWh).
  1 kW h = 3,600,000 J = 3.6 × 10⁶J.

Text-book Questions

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Page no. 200 

Q1. What does an electric circuit mean?

Ans: A continuous and closed path of an electric current is called an electric circuit. In which various electric components are arranged in series or parrallel. 

Q2. Define the unit of current.

Ans: S.I unit of current is Ampere, which denoted by Letter 'A'. Ampere is defined as "When one coulomb of charge flows in one second it is callled one Ampere of current".

Q3. Calculate the number of electrons constituting one coulomb of charge.

Ans: One electron possesses a charge of 1.6 × 10^-19 C, 

Therefore, Charge on 1 electron = 1.6 × 10^-19 C

Total Charge = 1 coulomb = 1C (given) 

The number of elecrons = ? 

Total Charge = number of electrons × Charge on 1 electron

1C = number of electrons × 1.6 × 10^-19 C

The number of electrons constituting one coulomb of charge is 6 × 10¹⁸

Page no. 202 

Q1. Name a device that helps to maintain a potential difference across a conductor.

Ans: Cell or Battery is a device that helps to maintain a potential difference across a conductor. 

Q2. What is meant by saying that the potential difference between two points is 1 V?

Ans: If 1 J of work is required to move a charge of amount 1 C from one point to another then it is said that the potential difference between the two point is 1 V.  

Q3. How much energy is given to each coulomb of charge passing through a 6 V battery?

Ans: The energy given to each coulomb of charge is equal to the amount of work done to move it. 

V = W/Q

Page no. 209 

Q1. On what factors does the resistance of a conductor depend?

Q2. Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?

Q3. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

Q4. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

Q5. Use the data in Table 12.2 to answer the following –

(a) Which among iron and mercury is a better conductor?

(b) Which material is the best conductor? 

Page no. 213

Q1. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.

Q2. Redraw the circuit of Question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter? 

Page no. 216

Q1. Judge the equivalent resistance when the following are connected in parallel – (a) 1 Ω and 106 Ω, (b) 1 Ω and 103 Ω, and 106 Ω.

Q2. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?

Q3. What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

Q4. How can three resistors of resistances 2 Ω, 3 Ω, and 6 Ω be connected to give a total resistance of (a) 4 Ω, (b) 1 Ω?

Q5. What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?

Page no. 218 

Q1. Why does the cord of an electric heater not glow while the heating element does?

Q2. Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.

Q3. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

Page no. 220

Q1. What determines the rate at which energy is delivered by a current?

Q2. An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h

NCERT Solutions Exercise

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Q1. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R′, then the ratio R/R′ is –

(a) 1/25

(b) 1/5

(c) 5

(d) 25

Ans: (d) 25

Q2. Which of the following terms does not represent electrical power in a circuit?

(a) I 2R

(b) IR2

(c) VI

(d) V2/R

Ans: (b) IR2

P = VI = I2R = V2/R

Q3. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be –

(a) 100 W

(b) 75 W

(c) 50 W

(d) 25 W

Ans: (d) 25 W

Q4. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be –

(a) 1:2

(b) 2:1

(c) 1:4

(d) 4:1

Ans: (c) 1:4

Q5. How is a voltmeter connected in the circuit to measure the potential difference between two points?

Ans: Voltmeter is always connected in parallel.

Q6. A copper wire has diameter 0.5 mm and resistivity of 1.6 × 10–8 Ω m. What will be the length of this wire to make its resistance 10 Ω? How much does the resistance change if the diameter is doubled?

Ans: 

Q7. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –

I (amperes) 0.5 1.0 2.0 3.0 4.0

V (volts) 1.6 3.4 6.7 10.2 13.2

Plot a graph between V and I and calculate the resistance of that resistor.

Q8. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

Q9. A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω , 0.5 Ω and 12 Ω, respectively. How much current would flow through the 12 Ω resistor?

Q10. How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?

Q11. Show how you would connect three resistors, each of resistance 6 Ω, so that the combination has a resistance of (i) 9 Ω, (ii) 4 Ω.

Q12. Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?

Q13. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 Ω resistance, which may be used separately, in series, or in parallel. What are the currents in the three cases?

Q14. Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors

Q15. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?

Q16. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?

Q17. An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.

Q18. Explain the following.

(a) Why is the tungsten used almost exclusively for filament of electric lamps?

(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?

(c) Why is the series arrangement not used for domestic circuits?

(d) How does the resistance of a wire vary with its area of cross-section?

(e) Why are copper and aluminium wires usually employed for electricity transmission?

Chapter-12. Electricity

Q1: What is an electric circuit?

Ans: A continuous and closed path of an electric current is called an electric circuit.

Q2: Name the instrument which measures electric current in a circuit.

Ans: Ammeter.

Q3: In which order, an ammeter is connected in a circuit?

Ans: In series.

Q4: In which direction does the electric current flow in a circuit?

Ans: the electric current flows in the circuit from positive terminal of the cell to negative terminal of the cell.

Q5: Name the device which produces potential difference in a wire?

Ans: Cell or battery.

Q6: What is potential difference?

Ans: The electrons move only if there is a difference of electric pressure in a conducting metallic wire. This difference of electric pressure is called potential difference.

Q7: Two bulbs of 100 W and 25 W are connected in series to 200 V-AC mains. Which bulb glows brightly? Explain your observation.

Q8: Why does the element of a room heater become red hot but the lead wires remain cold?

Ans:  The element of a room heater has high resistance. According to jule’s law of heating , increasing in resistance cause increasing in heat. While the device wire has not so highly resistance.  

Q9: Calculate the number of electrons constituting one coulomb of charge.

Ans: 6 x10¹⁸ electrons.

Q10: State the Ohm’s Law.

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. This is called Ohm’s law.

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