NEET Study Notes for Current Electricity- Electric Current is defined as the rate of charge flow through any cross-section. The direction of the movement of the positive charge is considered as the direction of the electric current. It is a scalar quantity and the S.I Unit is Ampere. Some of the important topics from this section include Flow of Electric Current, Resistance, Kircchoffs Law, Potentiometer.

• The weightage of Current Electricity is NEET Syllabus is around 8%. As per the previous year’s analysis NEET Question Paper consist of at least 2 questions from Current Electricity.  Check Detailed NEET Physics Syllabus 
• With a high weightage, candidates should lay more emphasis on this topic as 3-4 questions can be expected to be asked in NEET Question Paper.

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Important Topics in Current Electricity

What is Electric Current?

Electric Current is defined as the flow of charge through a cross-section. This can be defined mathematically as:

\(I=\frac{dq}{dt}\)

• The direction of positive charge flow is considered to be the direction of electric current, which has a scalar quantity. S.I Unit- Ampere (A). 1 Ampere =6.25 × 1018 electrons/sec
• In conductors, current carriers are called electrons, (valence e- s)electrolyte ions, semiconductor (electrons and holes), and positive ions /electrons in gases,
• Charge of electron= 1.6 × 10–19c

Electric Current through metallic conductor

Electric Current Flow in metal is due to flow of charged particles. In a metallic conductor, current flows due to ions or other charged particles that are freely available for movement through the conductor.

We know that I= Q/T

Where I =current, Q-charge and T= time

In a metallic conductor, protons are bounded and so we can conclude that the freely available charged particles that remain are electron. Therefore electric current through metallic conductor is due to the movement of electrons. .

Must Read : NEET Study Notes for Work, Energy, and Power

Drift Velocity

When two ends of a conductor are connected to terminals of a battery, an electric field from positive terminal to negative terminal is built up in the conductor, The free electrons et accelerated due to as they experience a force opposite to the electric field direction. However, the collision with ions of solid interrupts this process of acceleration. The average time during which electron is accelerated is called mean free time or mean relaxation time.

These free electrons acquire a small velocity towards the positive end of conductor, in addition to its random motion. This velocity is Drift Velocity

\(v_d=-\frac{e E_\tau}{m}\)

Where, e- charge

m= mass of electron

E= electric field established in the conductor and average relaxation time

The negative sign is because of the directions of and for electrons are opposite.

\(E=\frac{V}{l}\)

Mobility

It is defined as drift velocity per unit electric field. Mobility is denoted by µ.

\(\mu=\frac{V_d}{E}\)

S.I Unit- m2/volt-sec

Ohms Law and Electrical Resistance

When potential difference is applied across the ends of a conductor, current( I) is set in the conductor. Ohms Law states that “ Keeping the given physical conditions such as temperature, mechanical strain, etc constant, the current (I) is produced in the conductor is directly proportional to the potential difference (V) applied across the conductor

.e., \(I∝V\) or I=KV ... (1)

Where, K is a constant of proportionality (conductance)

Alternatively V= RI ... (2)

Where constant R is called resistance

Thus from the two equations above, R= 1/K

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Ohmic (Linear Conductor)- if a substance adheres to Ohm’s law, then a linear relationship between V and I is established. Such substances are known as ohmic substances.

Non-Ohmic (Non-linear conductor)- Substances that do not follow Ohm’s law are known as non ohmic substances. Some examples of non-ohmic conductors are Diode valve, triode valve, electrolyte, thermistors.

V-I slope of a conductor provides resistance

slope=tanθ= V/1

S.I. Unit of resistance R is volt/ampere = ohm (Ω)

Electrical Resistance

When Potential difference is applied across the ends of the conductor, free e-s flows towards the positive end of the conductor. While flowing , they collide with ions/atoms which obstructs their motion. This net obstruction by the conductor to the flow of current is called resistance. This depends upon the size, geometry, temperature and nature of the conductor.

Resistivity

In a given conductor with uniform cross-section A and length l , resistance R is directly proportional to length l and inversely proportional to cross-sectional area A

i.e., \(R∝\frac{l}{a}\) or \(R=\frac{\rho l}{a}\) or \(\rho=\frac{RA}{l}\)

where p is called specific resistance or electrical resistivity

Also,

S.I. unit of resistivity = Ohm (Ω)

Conductivity

It can be defined as the reciprocal of resistivity

i.e. \(\sigma=\frac{1}{\rho}\)

S.I unit of conductivity Ohm-1m-1 or mho/m

Carbon Resistors

Metals like nichrome, brass, platinum, and tungsten and alloys are used to produce resistance. Since most of these metals have low electrical resistance in comparison to carbon resistors, producing high resistance is difficult as it will make the resistor bulky.

We know that resistance is directly proportional to the product of length and resistivity of the resistor.

Resistance α( length×resistivity)

Carbon resistors produce a high accuracy value of resistance, thus they are mostly used to calibrate the resistance. Other advantages of carbon resistors are cost efficient, compact . Thus carbon resistors are preferable than metal wires. Carbon resistors are also available in large quantities.

Also Read

• NEET Study Notes for Kinematics

• NEET Study Notes for Thermodynamics

Color Coding for Carbon Resistors

This helps in indicating the value of resistors by a system of color-coding. For a fixed moulded composition resistor, four-color bands are printed on one end of the outer casing. This can be demonstrated below:

In order to read color bands, read left to right from end that contains bands closest to it.

• First and second color bands represent resistance value of first and second significant digits respectively.
• Third color band represents the number of digits that follow the second digit. If the third band is gold or silver, a multiplying factor of 0.1 or 0.01 is represented.
• Fourth band indicates the tolerance of the manufacturer. It represents the precision with which resistor is formed.
• In case of absence of the fourth band, tolerance is assumed to be ±20%

The standard value of color codes for carbon resistors is tabulated below:

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Series and Parallel Combination of Resistors

Resistance in Series

When resistances are joined end to end in such a way that current flows through each resistor and potential difference is applied across the combination, the conductor is said to be connected in series.

Here, the equivalent resistance

(Req)s= R1+R2+...+Rn

wherein the equivalent resistance of same resistance connected in a series is greater than the greatest individual resistance.

Potential division rule in series combination-

\(V_1 = \frac{V R_1}{R_1 + R_2} ;\)

\(V_2 = \frac{V R_2}{R_1 + R_2}\)

Resistance in Parallel

The resistors are said to be connected in parallel, if the potential difference that exists across all resistors is the same. Here the equivalent resistance is

Equivalent resistance in a parallel combination is always less than the least individual resistance.

\(\frac{1}{(R_eq)_p}=\frac{1}{R_1}+\frac{1}{R_2}+....+\frac{1}{R_n}\)

Current division rule in parallel combination

\(I_1 = \frac{I R_2}{R_1+R_2}\) ; \(I_2 = \frac{I R_1}{R_1+R_2}\)

How to determine whether the resistance is in parallel or series?

In order to determine whether the resistance, in a combination of resistor is in series or parallel then observe the current flow and potential difference. If the same current flows through two series then it is series, whereas if the potential difference is same then resistance is said to be in parallel. The potential difference across combinations of resistors is the same and equal to the applied potential difference.

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Electromotive Force

Electromagnetic force is defined as the energy supplied a battery/cell per coulomb (Q) of charge passing through it. When there is no current flowing through the circuit, the magnitude of emf is equal to the potential difference (V) across the cell terminals.

e= E/Q

Where e= emf or emf (V), W=energy (Joules), Q= charge ( Coulombs. Emf as well and the potential difference is measured in terms of V (Volts)

Example: What is the potential difference in a cell when it is connected to a 9 ohm load with cell emf = 2 volts and internal resistance is 1 ohm?

Answer: Given emf=2

External resistance= 9 ohm

Internal resistance= 1 ohm

We know, that R = External resistance + Internal resistance

Therefore 9+1= 10 ohm

Since I= V/r

I= 2/10 = 0.2 Ampere

Now, e = V + Ir

V= 2-0.2

2= V+ (0.2)1

V= 1.8 Volts

Combination of Cells

Cells can be connected either in series, parallel or a combination of both.

Series combination of Cells

In a series combination of cells

EAB = E1 + E2 + ... + En

where E= overall emf of the battery

Also, equivalent internal resistance of the cell is RAB = r1 + r2 + ....... + rn

Parallel Combination of Cells

In parallel combination of cells, equivalent emf

Wherein the equivalent internal resistance is

Kirchhoff’s Laws

Gustav Kirchhoff, a German physicist developed two laws commonly known as Kirchhoff’s voltage and current law/ These help in the calculation of electrical resistance in a complex network and current flow in a network of different streams.

Kirchhoffs Current Law/ Kirchhoffs Junction Law

According to Kirchhoff's current law, the total current entering the junction must be equal to the current leaving it. Thus the current entering and leaving junction has to be null. This can be expressed as

I1+I2+I3-I4-I5= 0

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Kirchoff's Voltage Law/Kirchoff's Loop Rule

According to Kirchoff's Voltage Law, the voltage around a loop is equal to the sum of every voltage drop in the close network of the same loop and equals to zero.

Applications of Kirchoff’s Laws

Kirchoff’s Laws can be used to find:

• Value of current, voltage = and internal resistance in DC circuits.
• Unknown resistance in the circuit
• Wheatstone Bridge is also an important application of Kirchoff’s Laws. Analysis of mesh and node is made possible by Kirchoff’s Laws

Potentiometer

Potentiometer is defined as a three-terminal resistor that has either sliding or rotating contract that forms an adjustable voltage divider.

Workings of a Potentiometer

The working of potentiometer depends on the potential across any portion of wire that is directly proportional to the length of wire where cross sectional area is uniform and current flow is constant

V= IR

where V= voltage

R= total resistance

I= current

Applications of Potentiometer

• Audio Control- Linear and rotary potentiometers can be used for changing loudness and audio-related signals by controlling audio equipment.
• Television- Potentiometer is also used in television to control picture brightness,color response, and contrast.
• Motion control- Potentiometers are used as position feedback devices known as a servomechanism in order to create a close-loop control.
• Transducers- Since Potentiometers emit large output signals, they can be used in designing displacement transducers
• Comparision of emfs of cells 
• To find internal resistance of a cell  

NEET Current Electricity Sample FAQs

Question: What is not based on the heating effect of current?

1. Electric Heater
2. Electric Iron
3. Microwave
4. Electric Bulb (with filament)

Answer: Microwave

Microwave oven uses microwaves. In this case, the commonly used radio wave frequency is close to 2,500 megahertz. While electric heater, iron, and electric wave work on the principle of the Joule heating effect, These electric devices convert electric current into heat.

Question: What is not a conductor of electricity?

1. Distilled water
2. Saltwater
3. Lime Juice
4. Vinegar

Answer: Distilled water

Most of the liquids that are good conductors of electricity are solutions made of acid, bases and salts. Distilled water is free of salts and thus it is a poor conductor of electricity.

Question: What are superconductors?

1. Conduct electricity at high temperature
2. Offer high resistance to flow of current
3. Offer no resistance to flow of current
4. Conduct electricity at high-temperature

Answer: offer no resistance to the flow of current

Superconductors are conductors that have zero resistance at very low temperatures. This enables electricity to flow through them rapidly. An example of a superconductor is Mercury and lead. Mercury below 4.2 Kelvin, Lead below 7.25 Kelvin to act like superconductors

Question: What is the work done in moving a unit charge between two points in an electric circuit?

1. Current
2. Potential
3. Power
4. Resistance

Answer: Potential

Potential or Electric Potential is defined as the work done in moving a unit charge between two points in the electric circuit. Measured in volts, it depends upon the charge of the object that experiences an electric field.

Question: In which of the following fields of science, Kirchhoff's laws are applicable?

1. Atomic Structure
2. Organic Chemistry
3. Electrical circuits
4. Optics

Answer: Electrical Circuits

Kirchhoff's laws are applicable to electrical circuits. Kirchhoff's loop rule also referred to as the voltage law or second law explains that the sum of all potential differences nearby a loop is zero. This is so since energy cannot enter or leave a closed circuit.

Question. What is Kirchhoff's law a consequence of?

1. Conservation of Electric Charge
2. Conservation of electron mass
3. Conservation of electron momentum
4. Conservation of electrical energy

Answer: Conservation of electric charge

Kirchhoffs Junction Law (also referred to as Kirchhoff's First Law) is a consequence of electrical charge as it states that the total current that enters through a junction is equal to the current out of the junction.

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