Chapter 6 – Current Electricity

Book Name	: Madhyamik Physical Science And Environment
Subject	: Physical Science
Class	: 10 (Madhyamik)
Publisher	: Oriental Book Company  Pvt. Ltd
Chapter Name	: Current Electricity

Very short Answer Type Questions (VSA)

Question 1

What is electric current?

Answer

Electric current is the rate of flow of electric charge through any cross-section of a conductor per unit time.

Question 2

Mention the factor that causes the flow of charge through a conductor.

Answer

Electric potential difference is the factor which causes the flow of charge through a conductor.

Question 3

What is potential difference?

Answer

Electric Potential difference between two points is the work done per unit charge in moving a unit positive charge from one point to the other.

Question 4

What is the unit of current ?

Answer

Ampere (A).

Question 5

State Ohm’s law.

Answer

At constant temperature and other physical conditions, the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor. i.e I  ∝ V (or, V=IR)

Question 6

State SI unit of resistance.

Answer

Question 7

Name the physical quantity whose unit is volt/ampere.

Answer

Question 8

State SI unit of resistivity.

Answer

Ohm-meter

Question 9

What is electric power?

Answer

Electric power is the amount of electrical energy consumed in a circuit per unit time.

Question 10

State SI unit of electric power.

Answer

Watt (W).

Question 11

What is commercial unit of electrical energy?

Answer

kWh (kilowatt hour)

Question 12

What is the direction of conventional current?

Answer

The direction of conventional current is opposite to the direction of flow of electron or from positive to negative terminal of the circuit.

Question 13

What is the difference between kilowatt and kilowatt-hour?

Answer

Kilowatt is the bigger unit of power in SI system where as kilowatt hour is the commercial unit of electrical energy.

Question 14

Name the material used for making the filament of an electric bulb.

Answer

Tungsten.

Question 15

What is meant by CFL?

Answer

Compact Fluoroscent Lamp.

Question 16

What do you mean by LED?

Answer

Light Emitting Diode.

Question 17

Define electromagnetic induction.

Answer

The process by which a change in the magnetic field in a conductor induces a current in another conductor.

Question 18

What is the frequency of direct current?

Answer

Zero, Direct current has no frequency.

Question 19

What is the value of frequency of an A.C. current in India?

Answer

50 Hz is the value of frequency of an A.C current in India.

Question 20

What is an electric motor?

Answer

Electric motor is an electro-mechanical machine which converts electrical energy into mechanical energy.

Question 21

What is an electric generator?

Answer

A electric generator is a machine which converts mechanical energy into electrical energy.

Question 22

What is electric fuse?

Answer

An electric fuse is a device which limits the current in an electric circuit.

Question 23

State one advantage of AC over DC?

Answer

A.C can be transmitted to distant places with little loss of electric power compare to D.C.

Question 24

Out of three wires live, neutral, and earth, which one goes through ON/OFF switch?

Answer

Live wirei isconnect with ON/OFF switch.

Question 25

In domestic electric circuit, mention the potential difference between live wire and neutral wire and frequency of AC.

Answer

The potential difference supplied in domestic electrical circuit is 220 V A.C of 50 Hz frequency.

Short Answer Type Questions

Question 1

Define electric currents, State and define its SI unit.

Answer

Electric current is the flow of electric charge through a conductor, such as a wire.

The SI unit of electric current is the ampere (A), which is defined as the flow of one coulomb of electric charge per second.

Question 2

What is an electric circuit?

Answer

An electric circuit is a system or network of electrical components that are interconnected and designed to conduct electricity. It consists of one or more energy sources, such as batteries or generators, which supply the electrical energy to the circuit, and various electrical components, such as resistors, capacitors, etc.

Question 3

What is meant by electric resistance of a conductor?

Answer

The electric resistance of a conductor is the ratio of the potential difference across the ends of a conductor to the current flowing through the conductor.

Question 4

What is commercial unit of electrical energy? Express it in joule.

Answer

The commercial unit of electrical energy is kWh.

1 kWh = 1000 W × 1 hr

= 1000 W × 3600s

= 36 × 10⁵ Ws

= 3.6 × 10⁶ J.

Question 5

Should the heating element of an electric iron be made of iron, silver or nichrome wire ?

Answer

The heating element of an electric iron should be made of nichrome wire rather than iron or silver wire. Nichrome is an alloy of nickel, chromium, and sometimes other elements, and it has a high electrical resistance, which makes it an ideal material for use in heating elements.

Question 6

State the rule to determine the direction of force experienced by a current-carrying conductor in a magnetic field.

Answer

Fleming’s left hand is used to determine the direction of force experienced by a current-carrying conductor in a magnetic field.

The rule states that if the thumb, the first finger and the middle finger of the left hand be held mutually perpendicular to each other in such a way that the first finger points to the direction of the magnetic field and the second finger to that of the current then the numb will indicate the direction of motion of the conductor.

Question 7

State Fleming’s left-hand rule.

Answer

If the thumb, the first finger, and the middle finger of the left hand be held mutually perpendicular to each other in such a way that the first finger points in the direction of the magnetic field and the second finger to that of the current, then the thumb will indicate the direction at the motion of the conductor.

Question 8

State Faraday’s laws of electromagnetic induction.

Answer

Faraday’s law of electromagnetic induction :

(i) Whenever there is a change in magnetic flux linked with a coil, an emf is induced, which lasts as long as the change in magnetic flux is linked with the coil changes.

(ii) The magnitude of the induced emf is directly proportional to the rate of change magnitude flux linked with the coil.

Question 9

State Len’s law. Discuss that the law is consequence of conservation of energy.

Answer

Lenz’s law : The direction of induced emf or current always tends to oppose the very cause which produces it.

Lenz’s law is based on law of conservation of energy. It shows that the mechanical energy spent in doing work against the opposing force experienced by the moving magnet transformed into the elecetrical energy, which is the cause of flow current in the coil.

Question 10

Discuss the advantages of AC over DC.

Answer

Advantages of AC over DC:

1. The cost of generation of AC is less than that of DC.
2. The magnitude of AC can be increased or decreased using a transformer whereas it difficult in DC.
3. AC can be easily converted into DC using a rectifier.

Question 11

Discuss the basic idea about AC dynamo.

Answer

An AC generator converts mechanical energy into electrical energy in the form of AC.

It consists of four main parts:

1. Armature: Armature consists of a large number of insulated copper wire wounded over a soft iron core.
2. Field magnet: A strong permanent or electromagnet with cylindrical pole acts as a field magnet which generates uniform magnetic field perpendicular to the axis of rotation of the coil in between which the armature coil rotates.
3. Slip ring: The ends of the armature coil are connected to brass slip rings, which rotates along with the armature.
4. Brushes: Two carbon brushes are pressed against the slip rings, which carry the current produced from the generator to the external circuit.

Question 12

Discuss the basic idea about D.C. dynamo.

Answer

A D.C generator converts mechanical energy into electrical energy in the form of D.C.

The main parts of D.C generator are:

1. Armature coil: A large number of insulated copper wire is wound on an iron core of the armature.
2. Field magnet: Armature is placed between two pole pieces of a strong magnet. The pole piece is made concave to provide a radial field.
3. Split-ring commutator: Two split rings from the commutator, to which the ends of the armature coil are connected.
4. Brushes: Two carbon brushes press against the commutators.
5. Load: The output is connected to the load.

Question 13

Explain in brief about different components of a domestic electric circuits.

Answer

Some components of an electric circuit are discussed below:

1. Switches: It is an ON-OFF device which is used to control the flow of current in a circuit. It is connected to live wires.
2. Socket: A socket is a component used in the electric circuit in which a plug is inserted.
3. Wires: It is the path through which electricity is brought to our homes from the power generating station.
4. Fuse: A fuse is a device which limits the current in an electric circuit. It is an arrangement for protecting the house wiring from damage due to the passage of excess current.
5. Main switch: It is a double pole switch having an iron covering. The cover is connected to the earth wire. It is used to switch off the supply to the entire house.

Question 14

What precautions are to be taken in using electricity?

Answer

Precaution is taken in using electricity:

1. Insulation of the appliance should be properly done.
2. The electric appliance should not be touched with a wet hand.
3. Each appliance should have proper earthing.
4. The appliance should be provided with a proper fuse.

Question 15

What are the differences between AC and DC?

Answer

Alternating current (AC)	Direct current (DC)
(i) Its magnitude changes with time.	(i) Its magnitude remains constant.
(ii) It can not be used in electrolysis.	(ii) It can be used in electrolysis.

Long Answer Type Questions

Question 1

State Ohm’s law. Explain it and define resistance from it.

Answer

Ohm’s law : At constant temperature and other physical conditions, the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor.

I  ∝ V

or,      V=IR

Resistance: The resistance of a conductor is the ratio of the potential difference across the ends of the conductor to the current flowing through it.

Question 2

What are the differences between the potential difference and the emf of a cell?

Answer

Electromotive force (EMF)	Potential difference
(i) It is the difference of potential between two electrodes of a cell or battery in an open circuit.	(i) It is the difference of potential between two electrodes of a cell or battery in a closed circuit.
(ii) It does not depend upon the resistance of the circuit.	(ii) It depends on the resistance of the circuit.
(iii) It is the cause.	(iii) It is the effect.

Question 3

What is Joule’s heating effect? List applications of Joule’s heating effect in daily life.

Answer

Definition: Joule heating (also referred to as resistive or ohmic heating) describes the process where electrical energy is converted into heat as it flows through a resistance.

Application: There are many applications of the heating effect of electric current in our daily life. Electric lamps, irons, heaters etc. are some of them.

Question 4

What are resistance and resistivity? Give their units.

Answer

Resistance: The ratio of the potential difference across the ends of a conductor to the current flowing through the conductor is called the resistance of the conductor. Its unit is Ohm( Ω)

Resistivity: The resistivity of a conductor is the resistance of the conductor of unit length and unit area of the cross-section at a constant temperature. Its unit is Ohm-meter (Ω-m).

Question 5

Describe series and parallel combinations of three resistances.

Answer

When we connect resistors together, we can connect them in different ways to create different electrical circuits. The two most common ways to connect resistors are in series and in parallel.

Series Combination of Three Resistances: In a series combination, resistors are connected one after the other, so that the current flows through each resistor in turn. When three resistors are connected in series, their total resistance is the sum of their individual resistances. That is,

Total resistance = R₁ + R₂ + R₃

Parallel Combination of Three Resistances: In a parallel combination, resistors are connected side by side so that the voltage across each resistor is the same, but the current is divided among them. When three resistors are connected in parallel, their total resistance is calculated using the following formula:

1/Total resistance = 1/R₁ + 1/R₂ + 1/R₃

Question 6

State Fleming’s left-hand rule. Describe Barlow’s wheel experiment.

Answer

Fleming’s left-hand rule: It states that if you extend your thumb, forefinger, and middle finger of your left hand so that they are all perpendicular to each other, and the forefinger points in the direction of the magnetic field, and the middle finger points in the direction of the current in the conductor, then your thumb will point in the direction of the force on the conductor.

Barlow’s wheel experiment is a classical demonstration of the interaction between electricity and magnetism. It was first performed by Peter Barlow in 1822 to demonstrate the principles of electromagnetic induction.

The experiment involves a copper disk, known as Barlow’s wheel, which is mounted on an axle and rotated by a belt connected to a power source. The wheel has a series of copper spokes extending radially from its centre, and a magnet is placed near the rim of the wheel so that it can exert a force on the spokes as they pass by.

When an electric current is passed through the spokes of the wheel, it creates a magnetic field around them. As the spokes pass by the magnet, they experience a force that causes the wheel to rotate. The rotation of the wheel can be observed and measured, and it demonstrates the principle of electromagnetic induction, which is the generation of an electric current in a conductor due to a changing magnetic field.

Question 7

Describe an electric motor.

Answer

An electric motor is a device that converts electrical energy into mechanical energy. It works on the principle of electromagnetic induction, where a magnetic field is generated by passing an electric current through a coil of wire, which then interacts with a magnetic field to produce rotational motion.

The basic components of an electric motor are a stator (stationary part) and a rotor (rotating part). The stator consists of a frame or housing, and one or more coils of wire usually arranged in a circular pattern around the rotor. The rotor consists of a shaft and a series of magnets or magnetic poles, which interact with the stator’s magnetic field to produce rotational motion.

When an electric current is passed through the coil of wire in the stator, it creates a magnetic field that interacts with the magnets in the rotor. This interaction creates a force that causes the rotor to rotate, and the rotational motion is transferred to the motor’s mechanical load.

Question 8

What is electromagnetic induction? State Faraday’s laws about electromagnetic inductions.

Answer

Electromagnetic Induction: The phenomena of production of induced current in a closed circuit or coil due to the change in magnetic field linked with it is called electromagnetic induction.

Faraday’s laws of electromagnetic induction :-

• Whenever there is change in magnetic flux linked with a coil, an emf is induced, which lasts as long as the change in magnetic flux linked with the coil last.
• The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linked with the coil.

Question 9

Describe in brief about domestic electric circuit.

Answer

Figure gives a schematic diagram of one of the common domestic circuits. In each separate circuit, different appliances can be connected across the live and neutral wires. Each appliance has a separate switch to ‘ON’/‘OFF’ the flow of current through it. In order that each appliance has equal potential differences, they are connected parallel to each other. The electric fuse is an important component of all domestic circuits. A fuse in a circuit prevents damage to the appliances and the circuit due to overloading.

Question 10

Describe an A.C. generator.

Answer

An AC generator, also known as an alternator, is a device that converts mechanical energy into electrical energy in the form of an alternating current (AC). It works on the principle of electromagnetic induction, where a changing magnetic field induces an electric current in a conductor.

It consists of four main parts:

1. Armature: Armature consists of a large number of insulated copper wire wound over a soft iron core.
2. Field magnet: A strong permanent or electromagnet with a cylindrical pole acts as a field magnet which generates a uniform magnetic field perpendicular to the axis of rotation of the coil in between which the armature coil rotates.
3. Slip ring: The ends of the armature coil are connected to brass slip rings, which rotate along with the armature.
4. Brushes: Two carbon brushes are pressed against the slip rings, which carry the current produced from the generator to the external circuit.

Numerical Questions

Question 1

Calculate the amount of charge that would flow in 2h through an element of an electric bulb drawing a current of 0.25 A.

Answer

Question 2

1.6 × 10^-19 C charge flows through a conductor in 10^-10s, Find the current flowing through the conductor.

Answer

Given that the charge Q = 1.6 × 10^-19 C and the time t = 10^-10 s, we can substitute these values into the formula to get:

I = Q/t

= (1.6 × 10^-19 C)/(10^-10 s)

= 1.6 × 10^-9 A

Therefore, the current flowing through the conductor is 1.6 × 10^-9 A.

Question 3

How many electrons pass through a conductor in 0.2s, if the current passing through it is 0.5A?

Answer

n = It / q

where n is the number of electrons, I is the current, t is the time, and q is the charge on a single electron (which is approximately 1.6 x 10^-19 Coulombs).

Given that the current I = 0.5 A and the time t = 0.2 s, we can substitute these values into the formula to get:

n = (0.5 A) × (0.2 s) / (1.6 x 10^-19 C)

n = 6.25 × 10¹⁸ electrons

Therefore, approximately 6.25 x 10^18 electrons would pass through the conductor in 0.2 seconds, if the current passing through it is 0.5 A.

Question 4

A heater of resistance 50Ω is connected to a 220 V line. How much current will this heater draw?

Answer

Given that the resistance of the heater is R = 50 Ω and the voltage across it is V = 220 V, we can substitute these values into the formula to get:

I = V / R

I = (220 V) / (50 Ω) = 4.4 A

Therefore, the heater will draw a current of 4.4 A from the 220 V line.

Question 5

Four resistors of 2 Ω each are joined end to end to form a square ABCD. Calculate the equivalent resistance of the combination between any two adjacent corners.

Answer

The equivalent resistance of the series combination of BC, CD, and AD can be calculated by simply adding the resistances:

R_eq = BC + CD + AD = 2 Ω + 2 Ω + 2 Ω = 6 Ω

Now, we can use the formula for resistors in parallel to find the equivalent resistance between corners A and B:

1/R_eq = 1/2 + 1/6 = 4/6 Ω

R_eq = 3/2 Ω = 1.5 Ω

Question 6

A copper wire having resistance R is cut into four equal parts. (i) Find the resistance of each part in terms of the original resistance of the wire and (ii) find the resistance of the combination if these four parts are joined to parallel.

Answer

(i) When a copper wire of resistance R is cut into four equal parts, each part will have a resistance of R/4. This is because the resistance of a wire is directly proportional to its length, and when the wire is cut into four equal parts, each part will have one-fourth of the length and therefore one-fourth of the resistance.

(ii) When the four parts are joined in parallel, the equivalent resistance of the combination can be calculated using the formula for resistors in parallel:

1/R_eq = 1/R₁ + 1/R₂ + 1/R₃ + 1/R₄

1/R_eq = 1/(R/4) + 1/(R/4) + 1/(R/4) + 1/(R/4)

1/R_eq = 4/R + 4/R + 4/R + 4/R

1/R_eq = 16/R

Therefore, the equivalent resistance of the four parts when joined in parallel is R/16, which is the same as the resistance of each individual part.

Question 7

An electric lamp of resistance 100 Ω draws a current of 0.5 A. Find the line voltage.

Answer

We can use Ohm’s law to find the line voltage:

V = I × R

where V is the line voltage, I is the current, and R is the resistance of the electric lamp.

Substituting the given values, we get:

V = 0.5 A × 100 Ω

Simplifying the expression, we get:

V = 50V

Therefore, the line voltage is 50V.

Fill in the blanks

Question

1. Like charges ____ each other.
2. SI unit of electric potential is ____.  
3. Electric fuse is a ____ device.
4. Fleming’s left-hand rule is also called ____ rule.
5. An electric generator converts ____ energy into electrical energy.

Answer

1. Like charges repel each other.
2. The SI unit of electric potential is the volt (V).
3. An electric fuse is a safety device.
4. Fleming’s left-hand rule is also called the motor rule.
5. An electric generator converts mechanical energy into electrical energy.

State whether true or false

Question

1. Electric charge is quantized. 
2. SI unit of power is joule. 
3. A.C. is more dangerous than D.C. 
4. Live wire is also called phase wire.
5. CFL lamps are cheaper than incandescent bulbs.

Answer

1. True
2. False (SI unit of power is watt)
3. True
4. True
5. False (CFL is more expensive)

Match the following

Answer

Choose the correct answer

Question 1

Which of the following is an ohmic resistance?

1. diode valve
2. Junction diode
3. carbon-arc lamp
4. nichrome

Answer

(d) Nichrome

Explanation:

Nichrome is an ohmic resistance because its resistance remains constant regardless of the voltage applied to it. Nichrome is a type of resistance wire commonly used in heating elements, such as in electric heaters, to convert electrical energy into heat.

Question 2

For which of the following substances, resistance decreases with an increase in temperature?

1. copper
2. mercury
3. carbon
4. platinum

Answer

(c) Carbon

Question 3

In parallel combination of resistances –

1. A p.d. is the same across each resistance 
2. total resistance is increased
3. current is the same in each resistance
4. all above are true

Answer

(a) A p.d. is same across each resistance

Question 4

The main fuse is connected in

1. livewire
2. neutral wire
3. both the live and earth wires
4. both the earth and the neutral wire

Answer

(a) live wire

Question 5

The electric meter in a house record –

1. charge
2. current
3. energy
4. power

Answer

(c) energy