Current Electricity

The differences between d.c. and a.c are as follows —

The differences are —

The diagram below shows closed and open circuits:

The differences between a primary and a secondary cell are —

The symbol and function of the components are as follows —

(i) Key

Symbol:

Function:
A key is used to put the current on and off in a circuit.

(ii) Cell

Symbol:

Function:
A cell acts as a source of direct current for the circuit.

(iii) Rheostat

Symbol:

Function:
It controls the flow of current in a circuit.

(iv) Ammeter

Symbol:

Function:
An ammeter is an instrument used to measures the magnitude of current flowing in a circuit.

(v) Voltmeter

Symbol:

Function:
A voltmeter is used to measure the potential difference between two points of a circuit.

The diagram below shows the direction of flow of current marked by red arrows and the labelled parts — A, B, C, D, E, and F

The name and function of each part is as follows —

A → Ammeter.

An ammeter is an instrument used to measure the magnitude of current flowing in a circuit.

B → Cell.

It acts as a source of direct current for the circuit.

C → key.

It is used to put the current on and off in the circuit.

D → Load.

An appliance which is connected in a circuit. It may be a resistance (e.g., bulb, heater etc.) or a combination of different electrical components.

E → Voltmeter.

A voltmeter is used to measure the potential difference between two points of a circuit.

F → Rheostat.

A rheostat is a device by which resistance in a circuit can be varied continuously.

(a) The circuit with ammeter between the terminals A and C with polarity marked is shown below:

(b) The direction of flow of current in the completed circuit with ammeter (polarity marked) is shown below:

(c) Rh is the rheostat. It controls the flow of current in a circuit.

provide current in a circuit

Reason — The cell or a battery is a source of direct current (d.c.). It provides current as a result of irreversible reaction.

electrical, chemical

Reason — During charging of a secondary cell, the electrical energy changes into chemical energy and gets stored in the cell.

irreversible, reversible

Reason — Primary cells provide current as a result of irreversible chemical reaction and secondary cells provide current as a result of reversible reaction.

none of the above

Reason — Primary cell is capable of giving a weak current only and secondary cells provides a low as well as high current.

The internal resistance of primary cells is high and that of secondary cells is low.

normal to the direction of

Reason — Current is the rate of flow of charge across a cross-section normal to the direction of the flow of current.

electrons

Reason — In metals, current flows due to the movement of electrons.

opposite

Reason — Conventionally, the direction of current is taken positive in the direction of flow of positive charge. Therefore, conventionally, the current will be negative in the direction of flow of electrons. Hence, current is in a direction opposite to the direction of motion of electrons.

Reason — A battery is formed by joining a number of cells together in series (i.e., cathode of one cell connected to the anode of another cell).

rheostat

Reason — A rheostat is a device by which resistance in a circuit can be varied continuously. It is used to adjust the magnitude of current in a circuit by changing the length of the resistance wire included in the circuit.

variable resistance

Reason — A rheostat is a device by which resistance in a circuit can be varied continuously. It is used to adjust the magnitude of current in a circuit by changing the length of the resistance wire included in the circuit.

ammeter

Reason — An ammeter is an instrument used to measure the magnitude of current flowing in a circuit.

galvanometer

Reason — A galvanometer is used to either detect the presence of a weak current or to only know the direction of flow of current in a circuit. It does not measure the magnitude of current in a circuit.

no free, large number of free

Reason — Insulators have no free electrons and offer very high resistance and conductors have large number of free electrons and they offer very small resistance to the flow of electrons.

silk

Reason — Silk does not conduct electricity, hence is an insulator.

insulator

Reason — The presence of insulator in the path makes a circuit incomplete and current does not flow.

Pure water

Reason — Pure water is a good insulator as it has a low concentration of ions so it does not conduct electricity very well. On the other hand, the human body, aluminum, and mercury are not good insulators. The human body contains electrolytes and can conduct electricity. Aluminum is a metal and is highly conductive. Mercury is a liquid metal and also conducts electricity well.

all of these

Reason — Connecting wires are typically made of substances, such as copper or aluminum, that possess highly conducting materials, negligible resistance, and a large number of free electrons. These characteristics allow for efficient flow of electric current through the wires.

option 4

Reason — In circuit 4, the key is closed, completing the circuit. In the other circuits the key (K) is left open, so the circuits are incomplete. For an electric circuit to function, every component must conduct current. If the circuit is broken at any point, the circuit remains open and no current flows.

Given,

charge (Q) = 0.5 C,

time (t) = 5 s

current (I) = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$\text I = \dfrac{0.5}{5} \\[0.5em] \Rightarrow \text I = 0.1 \text { A}$

Hence, current = 0.1 A

Given,

current (I) = 1.5 A

time (t) = 2 s

charge (Q) = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$1.5 = \dfrac{\text Q}{2} \\[0.5em] \Rightarrow \text Q = 1.5 \times 2 \\[0.5em] \Rightarrow \text Q = 3 \text { C}$

Hence, amount of charge that passes through the conductor = 3 C

Given,

Time = 0.8 s

charge = 24 C

current = ?

$\text I = \dfrac{\text Q}{\text t}$

Substituting the values we get,

$\text I = \dfrac{24}{0.8} \\[0.5em] \Rightarrow \text I = 30 \text { A}$

Hence, the current in the coil = 30 A

Primary cell — These cells provide current as a result of irreversible chemical reaction. The cells are discarded after use when the entire chemical energy in them has been converted into electrical energy. Thus, these are 'use and throw' types of cells and cannot be recharged.

Depending upon the material of electrodes and electrolyte, we have different types of primary cells e.g., simple voltaic cell, dry cell, etc.

Secondary cells or accumulators also provide current as a result of a chemical reaction. In these cells, the chemical reaction is reversible and so they can be recharged after use. e.g., Lead accumulator.

Current is the rate of flow of charge across a cross-section normal to the direction of flow of current.

The S.I. unit of current is coulomb per second which is called ampere. It is denoted by the symbol A.

Conductors — The substances which allow the current to flow through them easily are called conductors. They have large number of free electrons and they offer a very small resistance to the flow of current.

Examples — impure water (or acidulated water) and mercury are conductors of electricity.

Insulators — The substances which do not allow current to flow through them are called insulators. They have almost no electrons and offer a very high resistance to the flow of current.

Example — cotton, rubber.

The differences between a conductor and an insulator of electricity are as follows —

For an electric circuit to be closed, each component of it must pass current through it, i.e., it should be conducting

When a strong direct current is needed, either we join a number of cells together in series (cathode of one cell connected to the anode of another cell), to form a battery or we use a storage cell.

Manganin is an alloy which is used for preparing wires in a resistance box. It is used because of its property that its resistance does not vary much with temperature.

A galvanometer is used to know the direction of flow of current in a circuit. Current can enter or leave through any of its terminals, therefore it has just an 'O' mark at the middle and no +/- signs at its terminals.
On the other hand, Ammeter and Voltmeter measure the current and voltage in the circuit, respectively. Therefore, the + sign marked at the left hand corner indicates that current must enter through this terminal.

An appliance which is connected in a circuit is called a load. It may be just a resistance or a combinations of different electrical components. Examples : bulb, refrigerator, microwave etc.

The cell or a battery is a source of direct current (d.c.).

The mains in our house or an a.c. generator are the source of alternating current (a.c.).

An electric cell is a device that converts chemical energy into electrical energy when it sends current in a circuit. It consists of a vessel containing two conducting rods, called the electrodes, at some separation, placed or immersed in a solution (or jelly), called the electrolyte.

When current is drawn from a cell, chemical energy changes to electrical energy.

A cell consists of two electrodes and an electrolyte placed in a vessel.

The two kinds of cells with examples are —

1. Primary cell
   Example — Daniel cell
2. Secondary cells or accumulators
   Example — Ni-Fe (or alkali) accumulator

The charge on an electron is -1.6 x 10^-19 coulomb.

Current in conductor I = $\dfrac{\text Q}{\text t}$ = $\dfrac{\text {ne}}{\text t}$

Rheostat.

A key is used to put the current on or off in a circuit.

The conductors of electricity are — copper wire, acidulated water, human body and mercury.

We know that, like charges repel and unlike charges attract, therefore to create an excess or deficit of electrons at a point, some work is to be done in moving the charges (or electrons) against the forces between them. The force between the two charges is zero when they are at infinite separation.

Hence, quantitatively, potential at a point is measured in terms of work done in bringing a charge q from infinity to that point. If work W^' is done in bringing a charge q from infinity to a point, then potential at that point is:

$\text V = \dfrac{\text W'}{\text q}$

Hence, the potential difference between two conductors is measured in terms of work done in transferring the charge from one conductor to the other, through a metallic wire.

Potential difference is a scalar quantity.

A metal wire has free electrons which move in a random manner in the absence of any cell connected across it.

When the ends of the wire are connected to a cell, the electrons start moving from the negative terminal of cell to it's positive terminal through the metal wire.

During their movement, they collide with the fixed positive ions and other free electrons of the wire due to which their speed decreases and direction of motion changes.

After each collision, they again accelerate towards the positive terminal and suffer collision with other positive ions and free electrons again. This process continues.

As a result, the electrons do not move in bulk with increasing speeds from one end to the other but they drift towards the positive terminal. This is how a wire offers resistance to the flow of electrons (or current) through it.

The factors on which the resistance of a wire depend are —

1. The length of the wire — The number of collisions suffered by the moving electrons will be more if they have to travel a longer distance in a wire, therefore a long wire offers more resistance than a short wire (i.e., resistance of a wire is directly proportional to the length of the wire).
2. The area of cross section of wire — In a thick wire, electrons get a larger area of cross section to flow as compared to a thin wire, therefore, a thick wire offers less resistance (i.e., resistance of wire is inversely proportional to the area of cross section of the wire).
3. The temperature of the wire — If the temperature of the wire increases, ions in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of the wire increases with the increase in it's temperature.

A to B

Reason — Conductor A is negatively charged while conductor B is positively charged. Electrons flow from the negatively charged conductor A to positively charged conductor B.

lower, higher

Reason — A conductor having an excess of electrons is said to be at negative (lower) potential while a conductor having a deficit of electrons is at positive (higher) potential.

zero

Reason — The force between the two charges is zero when they are at an infinite separation.

volt

Reason — Potential difference is measured in joule per coulomb which is named as volt (V).

Both (a) and (b)

Reason — The S.I. unit of potential is joule/coulomb or volt (symbol V).

decreases

Reason — Current (I) is inversely proportional to the resistance (R) hence it decreases.

more

Reason — The resistance of a wire increases with its length because the moving electrons experience more collisions when traveling a longer distance. Hence, resistance of a wire is directly proportional to the length of the wire.

less

Reason — In a thick wire, electrons get a larger area of cross section to flow as compared to a thin wire, therefore, a thick wire offers less resistance (i.e., resistance of wire is inversely proportional to the area of cross section of the wire).

increases

Reason — If the temperature of the wire increases, ions in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of the wire increases with the increase in it's temperature).

1Ω

Reason — As, R = $\dfrac{\text{V}}{\text{I}}$

I = 2A
V = 2 volt

Hence, R = $\dfrac{2}{2}$ = 1Ω

double

Reason — Let initial and final current be I and I_new, initial and final resistance is R and R_new and initial and final potential be V and V_new

From Ohm's law,

Case 1 :

$\text V = \text {IR}$

Case 2 :

$\text V_{\text {new}} = \text I_{\text {new}}\text R_{\text {new}}$

According to question,

$\text V_{\text {new}}=\text V\ \text { and}\ \text { R}<em>{\text {new}}=\dfrac {1}{2}\text R\\[1 em] \Rightarrow \text I</em>{\text {new}}\text R_{\text {new}} = \text {IR} \\[1 em] \Rightarrow \text I_{\text {new}}\times \dfrac{1}{2}\text R = \text {IR} \\[1 em] \Rightarrow \dfrac{\text I_{\text {new}}}{2} = \text I \\[1 em] \Rightarrow \text I_{\text {new}} = 2\text I$

Given,

charge (q) = 1.5 C

work = 9 J

potential difference across the wire

$\text V = \dfrac{\text W}{\text q}$

Substituting the values in the formula, we get,

$\text V = \dfrac{9}{1.5} \\[0.5em] \Rightarrow \text V = 6\ \text V$

Hence, the potential difference across the wire = 6 V.

Given,

V = 12 V

R = 24 ?

I = ?

From Ohm's Law,

V = IR

Substituting the values in the formula, we get,

$12 = \text I \times 24 \\[0.5em] \Rightarrow \text I = \dfrac{12}{24} \\[0.5em] \Rightarrow \text I = 0.5\ \text A$

Hence, the current drawn from the cell = 0.5 A

Given,

I = 1.5 A

Potential difference = 6 V

R = ?

From Ohm's Law,

V = IR

Substituting the values in the formula, we get,

$6 = 1.5 \times \text R \\[0.5em] \Rightarrow \text R = \dfrac{6}{1.5} \\[0.5em] \Rightarrow \text R = 4Ω$

Hence, the resistance of filament of bulb while glowing = 4Ω

Given,

I = 0.2 A

R = 15Ω

Potential difference = ?

From Ohm's Law,

V = IR

Substituting the values in the formula, we get,

V = 0.2 x 15 = 3 V

Hence, the potential difference across the ends of the wire = 3 V

The obstruction offered to the flow of current by a conductor is called it's electrical resistance.

According to Ohm's law,

$\text{V} = \text{I}\text{R} \\[0.5em] \Rightarrow \text{I} = \dfrac{\text V}{\text R} \\[1em]$

Let the doubled resistance be 2R. Potential difference remains the same

$\text{I}<em>{\text{new}} = \dfrac{\text V}{2\text R} \\[0.5em] \Rightarrow \text{I}</em>{\text{new}} = \dfrac{\text I}{2}$

∴ If the resistance of conductor is doubled keeping the potential difference across it the same then current flowing in a conductor will be halved.

(a) The direction of flow of electrons is from A to B (i.e., from a low potential to a high potential)

(b) The direction of flow of current when both the conductors are joined by a metal wire is from B to A. (i.e., from a high potential to a low potential)

(a) If the length is doubled, the resistance of the wire doubles as resistance of a wire is directly proportional to the length of the wire.

(b) The resistance of a wire becomes less or one-fourth when the radius of the wire is doubled as resistance of a wire is inversely proportional to the area of cross section of the wire.

The statement 'potential difference between two conductors is 1 volt' means that 1 joule of work is done in transferring 1 coulomb of charge from one conductor to the other.

'The resistance of wire is 1 ohm' means that current of 1 ampere flows through it when the potential difference across it's ends is 1 volt.

(a) Volt (V)

(b) Ampere (A)

A battery is connected in a circuit when a strong current is needed.

The resistance of a wire will increase when it is stretched due to increase in length and decrease in area of cross section.

On increasing the voltage, the current increases and vice versa because voltage (potential difference) is directly proportional to current.

The physical quantities are —

(a) Charge and Current

(b) Potential difference and Resistance

The direction of flow of current is from a body at a higher potential to the one at a lower potential i.e., in direction opposite to the direction of flow of electrons.

Potential difference between two conductors is equal to the work done in transferring a unit positive charge from one conductor to the other conductor.

If work W is done in transferring a test charge q from one conductor to the other, the potential difference between them is

$\text V_1 - \text V_2 = \dfrac{\text W}{\text q}$

Potential difference is a scalar quantity.

The S.I. unit of potential difference is Volt (V) or joule per coulomb.

The potential difference between two points is said to be 1 volt if work done in transferring 1 coulomb of charge from one point to the other point is 1 joule.

S.I. unit of resistance is Ohm (Ω) or volt per ampere.

The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when the potential difference across it's ends is 1 volt.

Ohm's law states that current flowing through a conductor is directly proportional to the potential difference applied across it's ends provided it's temperature is constant. i.e., I ∝ V or V ∝ I or V = IR where R is the resistance of the conductor.

V = IR

The resistance of the filament increases, when a bulb glows as the temperature of the filament increases.

both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because primary cells (like dry cells, alkaline batteries) are non-rechargeable. Once their chemical reaction is completed, they cannot be reused, so they are discarded after use.

Reason (R) is true because in primary cells, the chemical reaction is irreversible, which means the chemicals cannot be restored to their original form by passing a current therefore, the cell cannot be recharged. Hence, reason correctly explains assertion.

both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because electronic current refers to the actual flow of electrons, which move from the negative terminal to the positive terminal of a power source and the rate at which electrons flow defines the magnitude of current.

Reason (R) is true because conventional current is defined as the flow of positive charge, which is taken to be from positive to negative terminal of a cell opposite to the direction of electron flow.

The reason correctly explains the difference between electronic current (actual flow of electrons) and conventional current (assumed flow of positive charge).

assertion is false but reason is true

Explanation

Assertion (A) is false because an ammeter is used to measure current, not potential difference and to measure potential difference, voltmeter is used.

Reason (R) is true because an ammeter is connected in series in a circuit and to not affect the current it is measuring, its resistance should be as low as possible.

assertion is true but reason is false

Explanation

Assertion (A) is true because both cotton and rubber do not conduct electricity under normal conditions since they have almost no free electrons and offer a very high resistance to the flow of current so they are commonly used as insulating materials in electrical applications.

Reason (R) is false because they have almost no free electrons and offer a very high resistance to the flow of current.

assertion is true but reason is false

Explanation

Assertion (A) is true because in a thick wire, electrons get a larger area of cross-section to flow as compared to a thin wire, therefore, a thick wire offers less resistance.

Reason (R) is false because resistance is inversely proportional to the area of cross-section, not directly proportional since flow of electrons (i.e., current) depends upon the area of cross-section through which they flow so more area implies high current as number of collisions between electrons decreases and vice versa.

both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because conventional current is defined to flow from higher potential to lower potential, even though actual charge carriers (electrons) move in the opposite direction.

Reason (R) is true because electrons are negatively charged, so they move from lower potential to higher potential opposite to the direction of conventional current.

The reason correctly explains why current flows from high to low potential because electrons, which actually move, go the opposite way.

both A and R are true and R is not the correct explanation of A

Explanation

Assertion (A) is true because the number of collisions suffered by the moving electrons will be more if they have to travel a longer distance in a wire, therefore, a long wire offers more resistance than a short wire (i.e., resistance of a wire ∝ length of wire).

Reason (R) is true because if the temperature of wire increases, electrons in it vibrate more violently. As a result, the number of collisions increases and hence the resistance of wire increases (i.e., the resistance of a wire increases with the increase in its temperature).

The assertion talks about wire length, but the reason explains the effect of temperature. So, although both statements are true, the reason does not explain the assertion.

The meaning of efficient use of energy is to reduce the cost and amount of energy used to provide us the various products and services. This results in reduction of (i) cost of energy and (ii) the emission of green house gases.

Example — By properly insulating a home, it is possible to maintain a comfortable temperature inside. It will reduce the cost of heating devices in winter and cooling devices in summer.

A building's location and its surrounding play a key role in regulating its temperature and illumination. Proper placement of windows and skylights and the use of architectural features that reflect light into the building can reduce the need of artificial lighting. White roof systems can save more energy in summers.

Modern energy efficient appliances such as refrigerators, ovens, freezers, dishwashers, dryers etc. make use of significantly less energy than the older appliances. Nowadays appliances are star rated according to their efficient use of electricity.

Hence, modern eco-friendly technologies are more efficient and less polluting.

Listed below are ways for the efficient use of energy —

1. The use of LED (light emitting diode) bulbs for lighting reduces the consumption of energy drastically. It is also helpful in reducing global warming and the harmful effects of mercury used in fluorescent lights.
2. Fuel efficiency in vehicles can be increased by reducing the weight of the vehicle using advanced tyres and computer controlled engines.
3. Modern energy efficient appliances such as refrigerators, ovens, freezers, dishwashers, dryers etc. make use of significantly less energy than the older appliances. Nowadays appliances are star rated according to their efficient use of electricity.

Social initiatives taken to sensitise use of energy are as follows —

1. Public awareness can be improved through mass-media and children's participation in campaigns and eco-club activities.
2. Community involvement will surely be effective in reducing the misuse of electricity.
3. The non-government organizations (NGOs) can be used to create social awareness about the sensitive use of resources.

both (b) and (c)

Reason — An efficient use of energy results in reduction of (i) the cost of energy and, (ii) the emission of greenhouse gases.

LED

Reason — The use of LED (light emitting diode) bulbs for lighting reduces the consumption of energy drastically. It is also helpful in reducing global warming and the harmful effects of mercury used in fluorescent lights.

International Energy Agency

Reason — According to the International Energy Agency (IEA), the improved energy efficiency in buildings, industries and transportation could reduce the world's energy need in 2050 by one-third and thus it can help to control global emission of greenhouse gases.

Light emitting diode

Reason — LED stands for Light emitting diode.

The ways to save energy are —

1. By properly insulating a home, it is possible to maintain a comfortable temperature inside. It will reduce the cost of heating devices in winter and cooling devices in summer.
2. The use of fluorescent and LED lights or natural sky light instead of traditional incandescent light bulbs, reduces the amount of energy required to attain the same level of illumination.

By properly insulating a home, it is possible to maintain a comfortable temperature inside. It will reduce the cost of heating devices in winter and cooling devices in summer.

The use of compact fluorescent lights (CFL) saves 67% energy and they may last 6 to 10 times longer than the usual bulb.

Appliances are star rated according to their efficient use of electricity.

The traditional incandescent light bulbs were the first electric device to be used for home lighting.

LED (Light Emitting Diode)