CBSE Class 12 Physics Current Electricity Worksheet Set B

SHORT ANSWER QUESTIONS 

Question. Three identical resistors, each of resistance R when connected in series with a d c source dissipate power X. If the resistors are connected in parallel to the same d c source, how much power will be dissipated?
Answer. P = V²/R
R effective in series is 3R
R effective in parallel is R/3
Hence power = 9X

Question. A battery of emf e and internal resistance r is connected to a variable external resistance R. Find the value of R for which current in the circuit is maximum, terminal potential difference across the battery is maximum also find the maximum value of current and terminal voltage in each case
Answer. 𝐼 = 𝐸 /(𝑅+𝑟)
Hence to get maximum current R = 0
I = E / r
V= E -Ir
For maximum value of V , I must be zero and for that R = infinity
Hence V = E

Question. The resistance of a tungsten filament at 150⁰C is 133 Ω. What will be its resistance at 5000C? Given the temperature coefficient of tungsten is 0.0045/⁰C.
Answer. 𝑅 = 𝑅₀ +𝛼𝑅₀ 𝑥 𝛥𝑇

Question. A battery of emf 2V and internal resistance 0.1Ω is being charged by a current of 5A. What will be the direction of current inside the battery? What is the potential difference between the terminals of the battery?
Answer. During charging a battery the current inside the battery flows from the positive to the negative terminal of the battery, so the terminal potential difference is greater than the emf
V = E +Ir = 2+5 x 0.1
V = 2.5V

1. An electrical appliances (geysers) uses a lot of electric energy when it is operated. The electrical energy consumed is dependent on the time for which a specific appliance of fixed power rating is used. The commercial unit of electric energy is KWh (I unit). Different electric appliances have different power consumption which is mentioned in the device clearly.

Question. A bulb is rated 60W, 220V.It signifies
(i) It consumes 60J in one second
(ii) It consumes 220J in one second
(iii) It has 60W energy
(iv) None of the above
Answer. A

Question. Which of the following does not represent electric power?
(i) VI
(ii) I²R
(iii) V²/R
(iv) I²V
Answer. D

Question. The electric bulbs P and Q having resistance ratio 1:2 will consume power in the ratio when V is constant
(i) 1:2
(ii) 2:1
(iii) 1:4
(iv) 4:1
Answer. B

Question. An electric heater consumes 1.5KW power. If it is used every day for 2 hours, then the electric energy used in January is
(i) 90unit
(ii) 92unit
(iii) 93unit
(iv) 95 unit
Answer. C

2. A single cell provides a feeble current. In order to get higher current in a circuit we use combination of cells . Cells can be joined in series and in parallel and in a mixed way. Two cells are said to be connected in series when negative of one cells in connected to the positive of the other cell and so on .Cells are said to be connected in parallel when positive terminal of each cell is connected to one point and negative terminal of each cell is connected to another point .In mixed grouping of cells certain number of cells are joined in series and all such rows are connected in parallel with each other

Question. To draw the maximum current from a combination of cells, how should the cells be Grouped
(i) Parallel
(ii) Series
(iii) mixed combination
(iv) Depends upon the relative value of internal and external resistance
Answer. D

Question. The total emf of the combination of cells, when n identical cells each of emf E are connected in parallel
(i) E
(ii) nE
(iii) E/n
(iv) 2nE
Answer. A

Question. Two cells out of n number of cells each of internal resistance r are connected in series , then total resistance of the cells is
(i) nr
(ii) nr-2r
(iii) nr -4r
(iv) r
Answer. A

Question. Two identical but non-ideal batteries are connected in parallel. Consider the following statements
I. The equivalent emf is smaller than either of the two emfs
II. The equivalent internal resistance is smaller than the resistance of the two internal resistance
(i) Both I and II are correct
(ii) I is correct and II is not correct
(iii) II is correct but I is wrong
(v) Both I and II are wrong
Answer. C

More question-

Very Short Answer

Question. What is the advantage of the potentiometer?
Answer. The potentiometer has the advantage that it draws no current from the voltage source being measured.

Question. What are the two types of resistors used in domestic use?
Answer. The two types of resistors used in domestic use are: wire bound resistors and carbon resistors.

Question. What is internal resistance?
Answer. The electrolyte through which a current flow has a finite resistance is called the internal resistance.

Question. How many electrodes cell have?
Answer. A cell has two electrodes are: positive and negative.

Question. What is galvanometer?
Answer. Galvanometer is an instrument for detecting and measuring small electric currents.

Short Answer

Question. How atmospheric electricity arise?
Answer. Atmospheric electricity arises due to the separation of electric charges. Atmospheric ions created by cosmic rays and natural radioactivity move in the electric field, so a very small current flows through the atmosphere, even away from thunderstorms.

Question. Why are cells connected in series and parallel?
Answer. When external resistance is negligible in comparison to the internal resistance, then the cells are connected in parallel to get maximum current. Cells are connected in
series when they are joined end to end so that the same quantity of electricity must flow through each cell.

Question. Why resistors are made up of carbon?
Answer. A carbon composition resistor is their ability to withstand high energy pulses. When the current flows through the resistor, the entire carbon composition body conducts the energy. The wire wound resistor has a much smaller volume of the wire to conduct.

Question. What do you mean by the series and parallel resistors?
Answer. Series resistors: Resistors are in series when they are connected head-to-tail and there are no other wires branching off from the nodes between components. Parallel resistors: When two resistors are in parallel, the equivalent resistance is the product of the two resistors divided by their sum. When both resistors are the same value, the equivalent parallel resistance is exactly half of the original resistance.

Question. What do you mean by ohmic loss?
Answer. The voltage drop across the cell during passage of current due to the internal resistance of the cell is called ohmic loss. Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to the flow of electrons through the electrically conductive fuel cell components.

Long Answer

Question. Write short note on Kirchhoff’s rule?
Answer. The two rules are called Kirchhoff’s rule, they are very useful for the analysis of the electric circuits. The two rules are:
1. Junction rule: Kirchhoff's junction rule states that at any junction or the node in an electrical circuit, the sum of the currents flowing into that junction is equal to the sum of the currents flowing out of that junction. In a circuit, sometimes we have the places called junctions, where several wires come together because charge is conserved and current measures the rate at which charges are flowing, the total current coming into to a junction must equal the total current coming out the other side of the junction, just as it did in the light bulb.
2. Loop rule: Kirchhoff's loop rule states that the sum of all the electric potential differences around a loop is zero. It is also known as Kirchhoff's voltage law or Kirchhoff's second law. In a circuit there are charges moving through these potential differences, so another way to say the rule is that when a charge goes around a complete loop, returning to its starting point, its potential energy must be the same. a circuit loop is a closed conducting path so no energy is lost.

Question. Write short note on Wheatstone bridge?
Answer. Wheatstone bridge is a simple circuit for measuring an unknown resistance by connecting it so as to form a quadrilateral with three known resistances and applying a voltage between a pair of opposite corners. It is in a shape of a bridge, the bridge is a galvanometer. It is named after Sir Charles Wheatstone who has popularized this bridge. Wheatstone bridge helps in finding the unknown resistance value of a resistor. Wheatstone bridge is also known as post office box. Wheatstone bridge applications are used to sense electrical and mechanical quantities, light measurement using photo resistive device. The Wheatstone bridge is used for measuring the very low resistance values precisely. Wheatstone bridge along with operational amplifier is used to measure the physical parameters like temperature, strain, light, etc. The Wheatstone bridge method is more accurate than the other methods of measuring resistances because it uses null method. So, the internal resistance of the cell and resistance of galvanometer do not affect the null point. Wheatstone Bridge is also the most sensitive when the bridge is balanced the resistances which are present in the Wheatstone bridge, i.e. all the four resistances are equal in quantity, that is the ratio of the resistances are 1:1. Then, the Galvanometer that is present in between the bridges will show zero deflection.

Question. Difference between potentiometer and voltmeter?
Answer. 
1. Potentiometer measures the unknown electromotive force very accurately whereas voltmeter measures the unknown electromotive force approximately.
2. Potentiometer has a high sensitivity whereas voltmeter has a low sensitivity.
3. Potentiometer is based on zero deflection method whereas voltmeter is based on half deflection method.
4. Potentiometer used for various applications like the measurement of the internal resistance of the cell, comparison of electromotive force etc. whereas voltmeter is only used to measure electromotive force or unknown potential difference.
5. In potentiometer while measuring the electromotive force resistance of potentiometer becomes infinite whereas in voltmeter while measuring the electromotive force resistance of the potentiometer becomes very high but measurable.

Question. Write short note on meter bridge?
Answer. Meter bridge Meter Bridge is an instrument that is used to find the unknown resistance of a coil or any other material. It is an instrument that works on the principle of a Wheatstone bridge. A meter bridge is used in finding the unknown resistance of a conductor as that of in a Wheatstone bridge. It is also known as slide wire bridge. It consists of a one metre long wire of uniform cross sectional area, fixed on a wooden block. The two gaps are formed on it by using thick metal strips in order to make the Wheatstone’s bridge. The terminal B between the gaps is used to connect galvanometer and jockey. A meter bridge is an electrical instrument which is used to determine the resistance of a given conductor. It can be also used to calculate the effective series and parallel equivalent of resistances. The material of wire used in meter bridge is made of manganin due to their high specific resistance or resistivity. Thick copper strips are used in meter bridge because copper is a good conductor of electricity. The shifting of zero of the scale at different points as well as the stray resistance gives rise to the end error in meter bridge wire. This error arise due to the non-uniformity of the meter wire end corrections can be estimated by including known resistances P1 and Q1 in the two ends and finding the null point.

Question. Difference between emf and voltage?
Answer. 
1. Electromotive force produced voltage inside the electric sources whereas voltage is the difference of the electrical potential between two points.
2. Electromotive force is measured with Emf meter whereas voltage is measured by voltmeter.
3. Emf induced in electric, gravitational or magnetic field whereas voltage only induced in electric field.
4. In emf the work performed in moving a charge from one pole to another in a source whereas in voltage work performed in moving charge form one pole to another through a wire.
5. In emf constant intensity with greater magnitude whereas in voltage non-constant intensity lower than EMF.
6. Emf is E = I * (R + r) whereas voltage is V = I* R.