## Chapter 4.1 - Ohm's Law and Electrical Circuits Exercise 173

Question 1
What do you mean by a cell?
Solution 1
An electric cell is a device in which a constant difference in potential is maintained between the two conductors by a chemical reaction. Thus, a cell can be used as a source of electrons or current. In a cell, chemical energy changes into electrical energy when it is in use.
Question 2
Define one ohm.
Solution 2
Resistance of a conductor is said to be 1 ohm, if 1 ampere current flows through it, when the potential difference across its ends is 1 volt.

Question 3
Does Ohm's law hold good for semiconductors and electrolytic solutions?
Solution 3
No, Ohm's law does not hold good for semiconductors and electrolytic solutions.
Question 4
State the factors on which the resistance of a conductor depends.
Solution 4
Factors on which the resistance of a conductor depends are:
(i) Nature of conductor: different materials have different concentration of free electrons and therefore resistance of a conductor depends on its material.
(ii) Length of conductor: Resistance of a conductor is directly proportional to the length of a conductor.
(iii) Area of cross-section of a conductor: Resistance of a conductor is inversely proportional to the area of cross-section of the uniform wire.
(iv) Temperature of conductor: In general for metallic conductors, higher the temperature larger is the resistance.

Question 5
What do you mean by an equivalent resistance?
Solution 5
An equivalent resistor is that resistor which when replace any combination of resistors the current through the circuit is not altered or changed.
Question 6
Solution 6

Question 7
Solution 7
The slope of the graph (dV/dI) gives the value of resistance.
Question 8
Define resistivity of a metal.
Solution 8
The resistivity of a material is the resistance of a wire of that material of unit length and unit area of cross-section.

Question 9
What are the SI units of specific resistance?
Solution 9
Question 10
What do you mean by conductance of a conductor?
Solution 10
Conductance of a conductor is the reciprocal of resistance of that conductor. Its unit is mho.

Question 11
Name the following substances:
(i) Showing low resistivity,
(ii) Showing very high resistivity,
(iii) Showing moderate resistivity.
Solution 11
(i) Metals e.g. copper
(ii) Alloys e.g. Constantan
(iii) Semiconductors e.g. Germanium
Question 12
What is the difference between a cell and a battery?
Solution 12

Question 13
Solution 13
Question 14
What do you understand by an electric circuit?
Solution 14
An electrical circuit is a continuous path comprising of conducting wires and resistances between the terminals of a battery, along which an electric current is set up. It is represented by drawing a circuit diagram.

Question 15
Name any four components of an electric circuit. Write their symbols also.
Solution 15
Question 16
What do you mean by a circuit diagram?
Solution 16
A diagram indicating how various components in an electrical circuit have been connected using symbols for those components is a circuit diagram.

Question 17
What is wrong with the circuit diagram (Fig. 34)? Carefully check the symbols and assign proper positive and negative signs to them.
Solution 17
In the given diagram, ammeter is connected in parallel and voltmeter in series, which is wrong. Also the terminals of the two devices are wrongly connected to the battery.
Correct diagram:
Question 18
State Ohm's law. Is it always true?
Solution 18
According to Ohm's law, the current flowing in a conductor is directly proportional to the potential difference across its ends provided the physical conditions and temperature of conductor remains constant.
No, it is not always true. E.g. Diode valve, junction diode etc. do not obey Ohm's law.

Question 19
What is electrical resistance? Write its SI unit.
Solution 19
The obstruction offered to the flow of current by the wire is called its electrical resistance. It is a kind of friction between the free electrons and the atoms of the conductor along which they flow.
Its SI unit is 'ohm'.
Question 20
Name the physical quantity that represents the ratio of potential difference and current.
Solution 20
The physical quantity is 'resistance'.

## Chapter 4.1 - Ohm's Law and Electrical Circuits Exercise 174

Question 1
State the law which relates the current in a conductor to the potential difference across its ends.
Solution 1
Ohm's law relates the current in a conductor to the potential difference across its ends.
According to Ohm's law, the current flowing in a conductor is directly proportional to the potential difference across its ends provided the physical conditions and temperature of conductor remains constant.
Question 2
What happens to the resistance if the conductor is made thinner?
Solution 2
If the conductor is made thinner, electrons get smaller area of cross-section to flow and it therefore, offers a large resistance.

Question 3
What kind of plot would you except when current (I) is plotted against potential difference (V) for a conductor, at constant temperature?
Solution 3
At constant temperature, if current is plotted against potential difference, the plot will be a straight line making some slope with the x-axis as shown in figure below:
Question 4
What is an ammeter? How is it connected in a circuit?
Solution 4
The instrument used to measure electric current is called an ammeter. It has a low resistance and is always connected in series.

Question 5
What is a voltmeter? How is it connected in a circuit?
Solution 5
The instrument used to measure electric potential or potential difference is called a voltmeter. It has a very high resistance and is always connected in parallel.
Question 6
Does an ammeter have a low or high resistance?
Solution 6
An ammeter has a low resistance.

Question 7
Does a voltmeter have a low or high resistance?
Solution 7
A voltmeter has a high resistance.
Question 8
What is the role of a key in an electric circuit?
Solution 8
A key acts as a switch in an electric circuit. It helps to open or close the circuit as required.

Question 9
What do you mean by a rheostat?
Solution 9
A rheostat is a device used in electric circuit to regulate current without changing the voltage source. It is also called variable resistance.
Question 10
Solution 10

Question 11
Potential difference between two points of a wire carrying 2 ampere current is 0.1 volt. Calculate the resistance between these points.
Solution 11
Question 12
The potential difference between the terminals of an electric iron is 240 V and the current flowing through it is 6 A. What is the resistance of the electric iron?
Solution 12

Question 13
Solution 13
Question 14
Solution 14

Question 15
Solution 15
Question 16
What is the value of a resistance if 20 V drives 2 A through it?
Solution 16

Question 17
Solution 17
Question 18
Solution 18

Question 19
Solution 19
Question 20
Give two uses of conductors.
Solution 20
Two uses of conductors:
(i) Connecting wires are made of conductors like copper.
(ii) Conductors are used as electrolytes in cells

## Chapter 4.1 - Ohm's Law and Electrical Circuits Exercise 175

Question 1
Write the SI units for:
(a) Electric current        (b) Potential difference
(c) Charge.
Solution 1
(a) Ampere
(b) Volt
(c) Coulomb
Question 2
Define potential difference between two points.
Solution 2
The potential difference between two points may be defined as the work done in moving a unit positive charge from one point to the other.

Question 3
Is electric potential a scalar or a vector physical quantity?
Solution 3
Electrical potential is a 'scalar' quantity.
Question 4
Define electric intensity. Write its SI unit. Is it a scalar or a vector physical quantity?
Solution 4
Electric intensity is defined as the force experienced by a unit positive charge when kept at that point.
Its SI unit is newton per coulomb. It is a vector quantity.

Question 5
By what name is the unit joule/coulomb called?
Solution 5
Joule/coulomb is known as 'volt'.
Question 6
How much work is done when 5 C charge moves across a p.d. of 1 V?
Solution 6
Work done = charge x potential difference
Or, W = 5 x 1 = 5 J

Question 7
The work done in moving unit positive charge across two points in an electric circuit is a measure of
(i) Current             (ii) Potential difference
(iii) Resistance            (iv) Power
Solution 7
(ii) potential difference
Question 8
The device used for measuring potential difference is
(i) Potentiometer       (ii) Ammeter
(iii) Galvanometer        (iv) Voltmeter
Solution 8
(iv) voltmeter

Question 9
What are the conditions under which charges can move in a conductor?
Solution 9
Charges move in a definite direction in a conductor when a potential difference is applied across the ends of the conductor.
Question 10
How will you maintain a potential difference between the ends of a conductor?
Solution 10
We can maintain a potential difference between the ends of a conductor by connecting the two ends of the conductor to the two terminals of a battery or cell.

Question 11
What do you mean by electric current?
Solution 11
The rate of movement of charge from one point to another through a section of the conductor is called electric current. Current refers to rate of flow of charges in a conductor.
I = Q/t
Question 12
What is the direction of electric current?
Solution 12
The direction of conventional current is taken as the direction of flow of positive charges.
The direction of electronic current is taken as opposite to the direction of motion of electrons.

Question 13
Name the physical quantity which defines the rate of flow of charge.
Solution 13
Current defines the rate of flow of charges in a conductor.
Question 14
Name the physical quantity which is measured by an ammeter.
Solution 14
Current is measured by an ammeter.

Question 15
How many electrons are present in 1 C?
Solution 15
Question 16
A battery of e.m.f. 15 V and internal resistance 3 ohm is connected to two resistors of resistance 3 ohm and 6 ohm in series. Find:
(i) The current through the battery,
(ii) The p.d. between the terminals of the battery.
Solution 16

Question 17
Solution 17
Question 18
Solution 18
To get a total resistance less than 2 ohm, the given resistors should be connected in parallel because in parallel combination the equivalent resistance is less than the resistance smallest connected resistor.

Question 19
Solution 19
Question 20
A combination consists of three resistors in series. Four similar sets are connected in parallel. If the resistance of each resistor is 2 ohm, find the resistance of the combination.
Solution 20