Test: Current Electricity - 1

Kirchhoff’s loop rule is based on the principle of conservation of energy. The work done in transporting a charge in a closed loop is zero. The algebraic sum ( since potential differences can be both positive and negative) of potential differences around any closed loop is always zero.

The balance point of the Wheatstone’s bridge is determined by the ratio of the resistances. The change in the emf of the external battery will have no effect on the balance point.

According to Ohm’s law, V = IR. Therefore R= V/I

We want the Voltage difference V_AB.
let A and B be open and not connected to any thing.
There is a current that flows from cell of larger emf to the cell of small emf.
Call that current as  I Amperes.

1.25 V - 0.75 V - I * R2 - I * R1 = 0

I = 0.5 / (R1+R2)
V_AB =  -0.75 - I * R2 =  - 0.75 - 0.5 * R2 / (R1+R2)

= - (0.75 R1 + 1.25 R2) / (R1+R2)

= - ().75 * 1 + 1.25 * 1) / (1 + 1)  volts

=  - 1 volts

The flow of electrons is termed electronic current. Electrons flow from the negative terminal to the positive.

The connecting metal strips include a small resistance in the circuit called the end resistance.On reversing the current, the end resistance tends to get cancelled out.

Power P= V²/R​
If length reduced 10% then new resistance of filament will be R′.
R′=R−10% of R
R′=0.9R
Now new power of heater is P2​
P_2​= V²​​/R′ = V²/0.9R​​=1.1P
% increase power=11%

According to Ohm’s law, V=I R. Therefore R=V/I

Ohm’s law states I is proportional to V.This holds good at steady temperatures and for the flow of constant current.

Potentiometer is a long wire of uniform cross section made of manganin.

1. Newton is SI Unit of Force
2. Joule is SI Unit of Work Done
3. Coulomb is SI Unit of Charge
Hence, the unit of current is Ampere.

No current passes through the Galvanometer in the bridge balance condition.Therefore, the resistance between B and D is infinite.

• Both potentiometer and voltmeter are devices to measure potential difference.
• EMF is the terminal p.d between the electrodes of a cell in open circuit, i.e., when no current is drawn from it.
• Potentiometer measures the potential difference using null deflection method, where no current is drawn from the cell; whereas voltmeter needs a small current to show deflection.
• So, accurate measurement of p.d is done using a potentiometer.

• An electric field exists in the electrolyte between the positive and negative terminals of the battery.
• In the external circuit, the current flows from the positive electrode to the negative electrode.
• To maintain continuity, in the electrolyte, the current (positive charges) flow from the negative electrode (lower potential) to the positive electrode (higher potential).
• Work done by the source in taking unit positive charge from lower to higher potential is called electromotive force.

The electrons in a conductor have random velocities and when an electric field is applied, they suffer repeated collisions and in the process move with a small average velocity, opposite to the direction of the field. This is equivalent to positive charge flowing in the direction of the field.

Potentiometer measures the potential difference using null deflection method, where no current is drawn from the cell; whereas voltmeter needs a small current to show deflection. So, accurate measurement of p.d is done using a potentiometer.

Meter bridge is the application of wheatstone’s bridge. A Wheatstone bridge is an electrical network which is used to calculate the value of unknown resistance.

• Copper is a conductor and we know that for conductors, resistance is directly proprtional to temperature. Therefore on decreasing temperature resistance also decreases.
• Whereas, germanium is a semiconductor and for semiconductors, resistance is inversely proportional to temperature.
• So on decreasing temperature resistance increases.

Power is rate of doing work.P=W/t.The unit of power is J/s which is also called watt.

If the current due to the auxillary battery is decreased, the potential gradient will be decreases, so the balancing length increases.Thus null point will move to fifth wire.

With a known resistance in one of the gaps, the meter bridge is used to determine the value of unknown resistance by the formula. 

Kirchhoff’s junction rule is based on the principle of conservation of charge. For steady currents, there can be no accumulation of charge in a junction. The currents entering a junction bring charge to the junction and the currents leaving a junction carry away the charge brought to the junction.

Ammeter is a device used to measure current. Since it has to allow the complete current flowing in the circuit through it, it has to be connected in series. For this reason, ammeters have very low values of resistances so that they do not add to the value of resistance connected in the circuit.

• A potentiometer is considered to be sensitive if the potential gradient dV/dl is low.
• Such a potentiometer can measure very small changes in potential difference.
• Increasing the length of the potentiometer wire decreases the potential gradient. Its sensitivity increases. Increasing potential gradient decreases the sensitivity.
• Increasing the emf of the primary cell and by decreasing the length, potential gradient increases.

Current flows in a conductor due to the flow of negatively charged electrons. However, the direction of conventional current is taken to be opposite to the direction of flow of electrons. It can therefore be considered as the direction in which positive charges move.

Kirchhoff's second law, also known as the Kirchhoff's voltage law (KVL) states that the sum of all voltages around a closed loop in any circuit must be equal to zero. This again is a consequence of conservation of energy.

Potential gradient Is given by V/l. If V and l are constant, the potential gradient also remains constant. The change in the radius will cause a change in the current. This does not change the potential gradient.

60/40 = l/100-l, = R/8

so R=12

now changed position 8 is proportional to l and 12 to (100-l)

8/12 = l/100-l

l = 40ohm

The balance point will not change . For balancing of wheatstone bridge four arms of resistors are responsible. Interchange the position of galvanometer and cell does not affect balancing of wheatstone bridge.