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Current electricity study material, Study notes of Physics

Physics

This notes are based on the chapter current electricity. This is in the format of questions and answers, so that it will be helpful for students to understand how the questions will be framed on particular topic.

Typology: Study notes

2022/2023

Available from 08/12/2023

aman-sheikh
aman-sheikh 🇮🇳

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12th Physics. Current Electricity Aman Sir
9. Current Electricity
Electrons
1. Free electrons
2. Bound electrons
Current: Flow of free electrons
Resistance: Opposition to the flow of current
To increase current, resistance decrease by connecting in parallel
To decrease current, resistance increases by connecting in series.
Voltmeter always connected in series.
Ammeter always connected in parallel.
Basic terms:
Junction: Any point in an electric circuit where two or more conductors are joined together is a
junction.
Loop: Any closed conducting path in an electric network is called a loop or mesh.
Branch: A branch is any part of the network that lies between two junctions.
Q. State and explain kirchhoffs current law.
Ans: The algebraic sum of the currents at a junction is zero in an electrical network.
Incoming current is considered as positive and outgoing current is considered as negative.
In the given circuit incoming current are I1, I3and I4.
Outgoing currents are I2, I5and I6.
Thus, I1- I2+ I3+I4-I5-I6= 0
I1+ I3+ I4= I2+I5+ I6
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9. Current Electricity _Electrons

  1. Free electrons
  2. Bound electrons Current: Flow of free electrons Resistance: Opposition to the flow of current To increase current, resistance decrease by connecting in parallel To decrease current, resistance increases by connecting in series. Voltmeter always connected in series. Ammeter always connected in parallel._ Basic terms: Junction: Any point in an electric circuit where two or more conductors are joined together is a junction. Loop: Any closed conducting path in an electric network is called a loop or mesh. Branch: A branch is any part of the network that lies between two junctions. Q. State and explain kirchhoffs current law. Ans: The algebraic sum of the currents at a junction is zero in an electrical network. Incoming current is considered as positive and outgoing current is considered as negative. In the given circuit incoming current are I 1 , I 3 and I4. Outgoing currents are I 2 , I 5 and I 6. Thus, I 1 - I 2 + I 3 +I 4 -I 5 -I 6 = 0 I 1 + I 3 + I 4 = I 2 +I 5 + I 6

Thus the total current flowing towards the junction is equal to the total current flowing away from the junction. Q. State and explain kirchhoffs voltage law. Ans: The algebraic sum of the potential differences (products of current and resistance) and the electromotive forces (emfs) in a closed loop is zero. While tracing the current in loops, if direction of conventional current is same then the voltage across resistance is negative otherwise positive. If tracing the circuit through the cell or battery through negative terminal then the emf is considered positive. Otherwise negative. For the figure given applying KVL in loop ABFGA -I 1 R 1 -I 3 R 5 -I 1 R 3 +ε 1 = 0 ∴ε 1 = I 1 R 1 + I 3 R 5 + I 1 R 3 For the loop BCDFB applying KVL Q. Write a short note on Wheatstone Bridge. Ans: The Wheatstone Bridge was developed by Charles Wheatstone to measure the values of unknown resistances. Wheatstone’s bridge is generally used to measure resistances in the range from tens of ohm to hundreds of ohms. Four resistances P, Q, R and S are connected to form a quadrilateral ABCD as shown in the Fig. A battery of emf ε along with a key is connected between the points A and C such that point A is at higher potential with respect to the point C. A galvanometer of internal resistance G is connected between points B and D.

Q. Explain the determination of unknown resistance by the use of meter bridge. Ans:  Metre bridge consists of a wire of uniform cross section and one metre in length, stretched on a metre scale which is fixed on a wooden table.  Ends of the wire are fixed below two L shaped metallic strips. A single metallic strip separates the two L shaped strips leaving two gaps.  An unknown resistance X is connected in the left gap and a resistance box is connected in the other gap.  One terminal of a galvanometer is connected to the terminal C on the central strip, while the other terminal of the galvanometer carries the jockey (J).  A cell of emf ε along with a key and a rheostat are connected between the points A and B.  The galvanometer shows no deflection when the jockey is at the balance point. using the conditions for the balance, we get where RAD and RDB are resistance of the parts AD and DB of the wire resistance of the wire. If l is length of the wire, ρ its specific resistance, and A its area of cross section then Knowing R, lx and lR , the value of the unknown resistance can be determined. Q. Write applications of wheatstone meter bridge. Ans: • The Wheatstone bridge is used for measuring the values of very low resistance precisely.

  • We can also measure the quantities such as galvanometer resistance, capacitance, inductance and impedance using a Wheatstone bridge. Q. What is potentiometer? Ans: potentiometer is a device which is used to measure the potential difference. Potentiometer does not draw any current from the circuit thus considered as ideal voltmeter. Q. Explain potentiometer principal. Ans: A potentiometer consists of a long wire AB of length L and resistance R having uniform cross sectional area A. A cell of emf ε having internal resistance r is connected across AB as shown in the fig. When the circuit is switched on, current I passes through the wire. Q. Define potential gradient. Ans: potential difference per unit length of wire is known as potential gradient. Q. Explain use of potentiometer to control emf of two cells. Ans:

Q. Describe with the help of a neat circuit diagram how you will determine the internal resistance of a cell by using a potentiometer. Derive the necessary formula. Ans:

motion sensor. Q. What are advantages and disadvantages of potentiometer over voltmeter? Ans: Advantages: i)Potentiometer is more sensitive than a voltmeter. ii) A potentiometer can be used to measure a potential difference as well as an emf of cell. iii) Measurement of potential difference or emf is very accurate in the case of a potentiometer. Disadvantage : Potentiometer is not portable and direct measurement of potential difference or emf is not possible. Q. Explain the working of galvanometer. Ans: A galvanometer is a device used to detect weak electric currents in a circuit. It has a coil pivoted (or suspended) between concave pole faces of a strong laminated horse shoe magnet. When an electric current passes through the coil, it deflects. The deflection is proportional to the current passing through the coil. The deflection of the coil can be read with the help of a pointer attached to it. Q. Write the steps to convert MCG into ammeter. Ans: To convert an MCG into an ammeter, the modifications necessary are

  1. Its effective current capacity must be increased to the desired higher value.
  1. Its effective resistance must be decreased. The finite resistance G of the galvanometer when connected in series, decreases the current through the resistance R which is actually to be measured. In ideal case, an ammeter should have zero resistance.
  2. It must be protected from the possible damages, which are likely due to the passage of an excess electric current to be passed. This can be achieved by connecting a small value of resistance in parallel with galvanometer. Since S and G are parallel, ∴ GIg = S Is Current range is given as n = I/Ig Q. Write the steps to convert MCG into voltmeter. Ans :To convert an MCG into a Voltmeter the modifications necessary are:
  3. Its voltage measuring capacity must be increased to the desired higher value.
  4. Its effective resistance must be increased, and
  5. It must be protected from the possible damages, which are likely due to excess applied potential difference. All these requirements can be fulfilled, if we connect a resistance of suitable high value (X) in series with the given MCG.