Chapter 1 Electricity
Very Short Answer type question
Page No. 5
Question 1. By what other name is the unit of joule/coulomb called ?
The unit of joule/coulomb is commonly referred to as the “volt” (symbol: V). This unit is used to measure electric potential difference or voltage in the International System of Units (SI). One volt is equal to one joule per coulomb (1 V = 1 J/C).

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Question 2 . Which of the following statements correctly defines Volt.
(a) A volt is Joule/Ampere.
(b) A Volt is Joule/Coulomb
A volt (symbol: V) is the unit of measurement for electric potential difference, electric potential, and electromotive force (EMF) in the International System of Units (SI). It represents the amount of electric potential energy per unit charge in an electric circuit.
Formally, one volt is defined as the electric potential difference across a conductor when a current of one ampere (1 A) flows through it and dissipates one watt (1 W) of power. In equation form:
1 V = 1 W / 1 A
In simpler terms, a volt can be thought of as the force that drives electric charge through a conductor in an electrical circuit. It is a fundamental unit of measurement in electricity and plays a crucial role in understanding and quantifying electrical phenomena.

Question 3
(a) What do the letters p.d. stand for ?
(b) Which device is used to measure p.d. ?

(a) The letters “p.d.” stand for “potential difference.” Potential difference refers to the voltage or electric potential difference between two points in an electrical circuit. It represents the electrical energy difference per unit charge between those two points. Potential difference is commonly measured in volts (V).

(b) A device used to measure potential difference (p.d.) is called a voltmeter. A voltmeter is an instrument specifically designed for measuring voltage or potential difference in an electrical circuit. It is typically connected in parallel across the points between which you want to measure the potential difference. When connected correctly, a voltmeter provides a numerical reading in volts, indicating the p.d. between those two points. Voltmeters are essential tools for troubleshooting, testing, and maintaining electrical circuits and devices.

Question 4:
What is meant by saying that the electric potential at a point is 1 volt ?

Saying that the electric potential at a point is 1 volt means that at that point, there is 1 joule of electric potential energy for every 1 coulomb of charge. It’s a measure of the energy per unit charge at that location in an electric field.

Question 5:
How much work is done when one coulomb charge moves against a potential difference of 1 volt ?

When one coulomb of charge moves against a potential difference of 1 volt, 1 joule of work is done.

Question 6:
What is the SI unit of potential difference ?

The SI (International System of Units) unit of potential difference is the “volt,” and it is represented by the symbol “V.”


Question 7:
How much work is done in moving a charge of 2 C across two points having a potential difference of 12 V ?

To calculate the work done (W) in moving a charge of 2 coulombs (2 C) across two points with a potential difference of 12 volts (12 V), you can use the formula for electrical work:
W = Q * V
In this case, you have Q = 2 C and V = 12 V:
W = 2 C * 12 V = 24 joules (J)
So, the work done in moving a charge of 2 coulombs across two points with a potential difference of 12 volts is 24 joules.


Question 8:
What is the unit of electric charge ?

The unit of electric charge is the “coulomb” (C).


Question 9:
Define one coulomb charge.

One coulomb (C) of charge is defined as the amount of electric charge carried by approximately 6.242 x 10^18 elementary charges. An elementary charge is the fundamental charge associated with subatomic particles, such as electrons and protons. Specifically, it is the charge magnitude of about 1.602 x 10^-19 coulombs, and one coulomb is equivalent to the charge of approximately 6.242 x 10^18 elementary charges.


Question 10:
Fill in the following blanks with suitable words :
(a) Potential difference is measured in………….. by using a……………. placed in………… across a component.
(b) Copper is a good………….. Plastic is an……………

(a) volts; voltmeter; parallel
(b) conductor; insulator


Question 11:
What is meant by conductors and insulators ? Give two examples of conductors and two of insulators.

Conductors are materials that allow the easy flow of electric charge. Examples include copper and aluminum.
Insulators are materials that resist the flow of electric charge. Examples include rubber and glass.


Question 12:
Which of the following are conductors and which are insulators ?

Sulphur, Silver, Copper, Cotton, Aluminium, Air, Nichrome, Graphite, Paper, Porcelain, Mercury, Mica, Bakelite, Polythene, Manganin.
Conductors: Silver ,Copper ,Aluminium ,Nichrome, Graphite, Mercury (in liquid form)
Insulators: Sulphur , Cotton, Air, Paper, Porcelain, Mica, Bakelite, Polythene, Manganin


Question 13:
What do you understand by the term “electric potential” ? (or potential) at a point ? What is the unit of electric potential ?

Electric potential, or simply “potential,” at a point in an electric field represents the electric potential energy per unit charge at that location. It is measured in volts (V).

Question 14:
(a) State the relation between potential difference, work done and charge moved.
(b) Calculate the work done in moving a charge of 4 coulombs from a point at 220 volts to another point at 230 volts.

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(a) The relation between potential difference (V), work done (W), and charge moved (Q) is given by W = Q * V.

(b) The work done in moving a charge of 4 coulombs from 220 volts to 230 volts is 40 joules.


Question 15:
(a) Name a device that helps to measure the potential difference across a conductor.
(b) How much energy is transferred by a 12 V power supply to each coulomb of charge which it moves around a circuit ?

(a) A voltmeter measures potential difference across a conductor.
(b) A 12 V power supply transfers 12 joules of energy to each coulomb of charge in the circuit.

Question 16: (a) What do you understand by the term “potential difference” ? (b) What is meant by saying that the potential difference between two points is 1 volt ? (c) What is the potential difference between the terminals of a battery if 250 joules of work is required to transfer 20 coulombs of charge from one terminal of battery to the other ? (d) What is a voltmeter ? How is a voltmeter connected in the circuit to measure the potential difference between two points. Explain with the help of a diagram. (e) State whether a voltmeter has a high resistance or a low resistance. Give reason for your answer.

(a) Potential Difference: Potential difference, or voltage, is a term in electricity that means the energy difference between two points due to electric charges.

(b) 1 Volt: Saying the potential difference is 1 volt means it takes 1 joule of energy to move 1 coulomb of charge from one point to another against an electric field.

(c) Potential Difference with Work and Charge: The potential difference (V) between battery terminals can be found using V = W/Q. In your case, with 250 joules of work and 20 coulombs of charge, it’s 12.5 volts.

(d) Voltmeter and Connection: A voltmeter measures voltage between two points in a circuit. Connect it in parallel by attaching one lead to one point and the other lead to the other point.

(e) Voltmeter Resistance: A voltmeter has high resistance to prevent it from affecting the circuit. If it had low resistance, it would draw too much current and disrupt voltage measurement. High resistance helps accuracy.

Question 22:
Three 2 V cells are connected in series and used as a battery in a circuit.
(a) What is the p.d. at the terminals of the battery ?
(b) How many joules of electrical energy does 1 C gain on passing through (i) one cell (ii) all three cells ?

(a) When three 2 V cells are connected in series, their voltages add up. So, the potential difference (p.d.) at the terminals of the battery would be:

P.d. = Voltage of one cell × Number of cells = 2 V × 3 = 6 V

So, the potential difference at the terminals of the battery is 6 volts.

(b) To calculate the electrical energy gained by 1 Coulomb (C) of charge passing through the cells, you can use the formula:

Energy (in joules) = Voltage (in volts) × Charge (in coulombs)

(i) For one cell: Energy = 2 V × 1 C = 2 joules

(ii) For all three cells in series: Energy = 6 V × 1 C = 6 joules

So, (i) 1 Coulomb of charge gains 2 joules of electrical energy when passing through one cell, and (ii) it gains 6 joules of electrical energy when passing through all three cells in series.

Question 23:
The atoms of copper contain electrons and the atoms of rubber also contain electrons. Then why does copper conduct electricity but rubber does not conduct electricity ?

Copper conducts electricity while rubber does not primarily due to their differences in electron mobility and electron arrangement within their atomic structures.

In copper:

  1. Electron Mobility: Copper is a metal, and in metals like copper, the outermost electrons in the atomic structure are not tightly bound to individual atoms. Instead, they form a “sea” of electrons that are relatively free to move throughout the metal lattice. These delocalized electrons are responsible for electrical conductivity because they can carry an electric current.
  2. Conduction Band: In metals, there is a partially filled conduction band in the electronic band structure. Electrons in the conduction band are mobile and can move freely in response to an electric field, allowing the flow of electric current.

In rubber:

  1. Electron Mobility: Rubber is an insulator, and its atomic or molecular structure does not provide free-moving, delocalized electrons like in metals. The electrons in the outermost shells of rubber’s atoms are tightly bound to the nuclei, making them less mobile.
  2. Energy Gap: Insulators like rubber have a significant energy gap between the valence band (where electrons are bound tightly) and the conduction band (where electrons can move freely). This energy gap prevents the electrons from easily moving into the conduction band, even under the influence of an electric field. As a result, rubber cannot conduct electricity.

So, in summary, while both copper and rubber contain electrons, the nature of electron mobility and the presence of a conduction band in metals like copper allow them to conduct electricity, whereas the lack of such characteristics in insulators like rubber prevents them from conducting electricity.

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