Current Electricity Flashcards
Define electric current
The rate of flow of charge is known as electric current.
Define steady current
Electric current that does not depend on time is known as steady current.
Define junction
Junction is any point in the circuit where the current can split.
State Kirchhoff’s first law
Aka current law or junction law
According the Kirchhoff’s first law, the algebraic sum of currents at any junction is equal to zero.
Give the sign conventions for Kirchhoff’s laws
Current towards junction— positive
Direction of tracing same as that of conventional current flow then potential difference across resistance is considered negative
State Kirchhoff’s second law
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.
Define electrical network
Am electrical network is the combination of various electric circuit elements (components) and sources of E.M.F connected in complicated manner.
Explain how Kirchhoff’s laws are in accordance with the laws of conservation of charge and energy
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)
What is the balance condition for Wheatstone’s network?
R1/R2 = R3/R4
Proof - balance condition for Wheatstone’s network
- Use Kirchhoff’s second law to two loops
2. Ohms law
Why are the are branches AC and BD (G and E) called conjugate arms?
If cell or galvanometer are interchanged, same balance condition is obtained.
Length of wire used in metre-bridge is
1m
Construction of meter-bridge
Pg 215
Wire, meter scale, copper strips
Expression for unknown resistance of a Wheatstone’s meter-bridge
X= R(lx/(100-lx))
Errors that may arise while using a meter-bridge
Non-uniform wire
Contact resistances
Ends of wire may not coincide with 0 and 100 cm mark on meter scale.
Precautions while using a meter-bridge
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.
What is done in Kelvin’s method?
To find resistance of galvanometer. Galvanometer itself connected in one gap and resistance in other.
Equal deflection method.
What is a potentiometer?
A potentiometer is an instrument used to measure an unknown emf by comparison with a known emf.
Construction of potentiometer
Pg 218
Principle of potentiometer
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)
Define potential gradient of a potentiometer
The fall in potential per unit length of wire.
Expression for potential gradient
Vap/l = ρ*l/A
What the uses of a potentiometer?
To measure the emf of a cell
To compare emfs of two cells
To determine internal resistance of a cell
Precautions to use potentiometer
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.
Expression for internal resistance of a cell
r= R((l1-l2)/l2)
List the advantages of potentiometer over a voltmeter.
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.
What are the disadvantages of a potentiometer?
Voltmeter is a direct reading instrument while potentiometer is not so.
Voltmeter is portable while potentiometer is not portable.
