# Electricity

## Exemplar Problems

#### Part 1

Question 19. A child has drawn the electric circuit to study Ohm’s law as shown in following figure. His teacher told that the circuit diagram needs correction. Study the circuit diagram and redraw it after making all corrections.

Answer: The ammeter is connected in parallel and voltmeter is connected in series. While voltmeter is always connected in parallel in a circuit. Thus, the correct diagram is as follows.

Question 20. Three 2 Ω resistors, A, B and C, are connected as shown in this figure. Each of them dissipates energy and can withstand a maximum power of 18W without melting. Find the maximum current that can flow through the three resistors?

Answer: Given, Power = 18W and R = 2Ω

Since,P=I^2 R

⇒18W=I^2 2Ω

⇒I^2=(18W)/(2Ω)=(9W)/(Ω)

Thus,I=3A

Since, there is no division of electric current when resistors are connected in series, thus electric current, through the resistor of 2Ω = 3A

Now, since two resistors of 2Ω each are connected in parallel, thus total current flows in the circuit

= sum of electric current flows from each of the resistors.

Thus, Electric current flows from each of the resistors of 2Ω connected in parallel = (3A) /2 = 1.5 A

Thus, electric current flows through the resistors of 2Ω connected in series = 3A

Electric current flows from each of the resistors of 2Ω connected in parallel = 1.5A

Question 21. Should the resistance of an ammeter be low or high? Give reason.

Answer: Ideally the resistance of an ammeter should be zero, but it is not possible, therefore, resistance of an ammeter is almost negligible. If it is high or low, it will affect the electric current flowing through the circuit.

Question 22. Draw a circuit diagram of an electric circuit containing a cell, a key, an ammeter, a resistor of 2 Ω in series with a combination of two resistors (4 Ω each) in parallel and a voltmeter across the parallel combination. Will the potential difference across the 2 Ω resistor be the same as that across the parallel combination of 4Ω resistors? Give reason.

The total effective resistance because of the parallel connection of two resistors of 4Ω:

Let the total effective resistance because of parallel connection = R

Thus,1/R=1/(4Ω)+1/(4Ω)= 2/(4Ω)= 1/(2Ω)

Therefore,R=2Ω

The potential difference divided in the electric circuit when resistors are connected in series. In the given condition, the total effective resistance of the parallel connection is equal to 2Ω. Therefore two resistors of 2Ω each are connected in series.

Therefore, the potential difference across the 2Ω resistor be the same as that across the parallel combination of 4Ω resistor.

Question 23. How does use of a fuse wire protect electrical appliances?

Answer: A fuse wire has a low melting point. During a surge in voltage across a circuit, the fuse wire melts and thus breaks the circuit. This helps in saving the electrical appliance from damage.

Question 24. What is electrical resistivity? In a series electrical circuit comprising a resistor made up of a metallic wire, the ammeter reads 5 A. The reading of the ammeter decreases to half when the length of the wire is doubled. Why?

Answer: The resistivity is the property of conductor that opposes the flow of electric current through the material of conductor.

Since, resistance is directly proportional to the length of conductor.

And potential difference is the product of resistance and electric current. Thus, if at constant potential difference increase in resistance will decrease the electric current.

That’s why the reading of ammeter decreases when length of the conductor is doubled because increase in resistance of the conductor and consequently because of decrease in electric current.