Current Electricity

Question 0

Define electric current

Answer 0

The rate of flow of charge is known as electric current.

Question 1

Define steady current

Answer 1

Electric current that does not depend on time is known as steady current.

Question 2

Define junction

Answer 2

Junction is any point in the circuit where the current can split.

Question 3

State Kirchhoff’s first law

Answer 3

Aka current law or junction law

According the Kirchhoff’s first law, the algebraic sum of currents at any junction is equal to zero.

Question 4

Give the sign conventions for Kirchhoff’s laws

Answer 4

Current towards junction— positive
Direction of tracing same as that of conventional current flow then potential difference across resistance is considered negative

Question 5

State Kirchhoff’s second law

Answer 5

According to Kirchhoff’s second law (aka voltage law or loop theorem) in a closed loop of electrical network, the algebraic sum of potential difference for all the components plus the algebraic sum of all E.M.F.s is equal to zero.

Question 6

Define electrical network

Answer 6

Am electrical network is the combination of various electric circuit elements (components) and sources of E.M.F connected in complicated manner.

Question 7

Explain how Kirchhoff’s laws are in accordance with the laws of conservation of charge and energy

Answer 7

1st law: no accumulation (gain) or loss of charge at junction
2nd law: ΣiR —> energy spent in sending a unit charge around the circuit
ΣΕ —> total energy supplied by sources of emf
(Unit charge)

Question 8

What is the balance condition for Wheatstone’s network?

Answer 8

R1/R2 = R3/R4

Question 9

Proof - balance condition for Wheatstone’s network

Answer 9

1. Use Kirchhoff’s second law to two loops

2. Ohms law

Question 10

Why are the are branches AC and BD (G and E) called conjugate arms?

Answer 10

If cell or galvanometer are interchanged, same balance condition is obtained.

Question 11

Length of wire used in metre-bridge is

Answer 11

1m

Question 12

Construction of meter-bridge

Answer 12

Pg 215

Wire, meter scale, copper strips

Question 13

Expression for unknown resistance of a Wheatstone’s meter-bridge

Answer 13

X= R(lx/(100-lx))

Question 14

Errors that may arise while using a meter-bridge

Answer 14

Non-uniform wire
Contact resistances
Ends of wire may not coincide with 0 and 100 cm mark on meter scale.

Question 15

Precautions while using a meter-bridge

Answer 15

Wire used must be uniform
Null point is obtained as close to the centre of wire as possible.
The experiment should be repeated by interchanging the positions of X and R to minimize and error due to contact resistance.

Question 16

What is done in Kelvin’s method?

Answer 16

To find resistance of galvanometer. Galvanometer itself connected in one gap and resistance in other.
Equal deflection method.

Question 17

What is a potentiometer?

Answer 17

A potentiometer is an instrument used to measure an unknown emf by comparison with a known emf.

Question 18

Construction of potentiometer

Answer 18

Pg 218

Question 19

Principle of potentiometer

Answer 19

The potential difference between any two points of the potentiometer wire is directly proportional to the length of the wire between those two points.
(Fall in potential per unit length of the wire i.e. Potential gradient is constant)

Question 20

Define potential gradient of a potentiometer

Answer 20

The fall in potential per unit length of wire.

Question 21

Expression for potential gradient

Answer 21

Vap/l = ρ*l/A

Question 22

What the uses of a potentiometer?

Answer 22

To measure the emf of a cell
To compare emfs of two cells
To determine internal resistance of a cell

Question 23

Precautions to use potentiometer

Answer 23

The emf of the battery must be greater than emfs to be compared.
Positive terminal of E1 or E2 must be connected to that end of potentiometer wire where positive terminal of a battery is connected.
Potentiometer wire must be uniform.
Resistance of potentiometer wire must be high.

Question 24

Expression for internal resistance of a cell

Answer 24

r= R((l1-l2)/l2)

Question 25

List the advantages of potentiometer over a voltmeter.

Answer 25

Voltmeter used to measure terminal P.D. Of cell while potentiometer is used to measure small terminal P.D. as well as emf of the cell.
Accuracy of potentiometer can be easily increased by increasing the length of wire.
Small P.D. Can be measured accurately with the help of a potentiometer. Resistance of voltmeter is high but not infinity to act as an ideal voltmeter.
Internal resistance of a cell can be measured with the help of a potentiometer.

Question 26

What are the disadvantages of a potentiometer?

Answer 26

Voltmeter is a direct reading instrument while potentiometer is not so.
Voltmeter is portable while potentiometer is not portable.