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Selina Concise Physics Class 10 ICSE Solutions Current Electricity

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Selina Concise Physics Class 10 ICSE Solutions Current Electricity

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 10 Physics Chapter 8 Current Electricity. You can download the Selina Concise Physics ICSE Solutions for Class 10 with Free PDF download option. Selina Publishers Concise Physics for Class 10 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina ICSE Solutions for Class 10 Physics Chapter 8 Current Electricity

Exercise 8(A)

Solution 1.

Current is defined as the rate of flow of charge.
I=Q/t
Its S.I. unit is Ampere.

Solution 2.

Electric potential at a point is defined as the amount of work done in bringing a unit positive charge from infinity to that point. Its unit is the volt.

Solution 3.

The potential difference between two points is equal to the work done in moving a unit positive charge from one point to the other.
It’s S.I. unit is Volt.

Solution 4.

One volt is the potential difference between two points in an electric circuit when 1 joule of work is done to move charge of 1 coulomb from one point to other.

Solution 7.

In a metal, the charges responsible for the flow of current are the free electrons. The direction of flow of current is conventionally taken opposite to the direction of motion of electrons.

Solution 8.

It states that electric current flowing through a metallic wire is directly proportional to the potential difference V across its ends provided its temperature remains the same. This is called Ohm’s law.
V = IR
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Solution 11.
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Solution 12.

Ohmic Resistor: An ohmic resistor is a resistor that obeys Ohm’s law. For example: all metallic conductors (such as silver, aluminium, copper, iron etc.)
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Solution 13.
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Solution 14.

  1. Ohmic resistor obeys ohm’s law i.e., V/I is constant for all values of V or I; whereas Non-ohmic resistor does not obey ohm’s law i.e., V/I is not same for all values of V or I.
  2. In Ohmic resistor, V-I graph is linear in nature whereas in non-ohmic resistor, V-I graph is non-linear in nature.

Solution 16.
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In the above graph, T1 > T2. The straight line A is steeper than the line B, which leads us to conclude that the resistance of conductor is more at high temperature T1 than at low temperature T2. Thus, we can say that resistance of a conductor increases with the increase in temperature.

Solution 17.
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Solution 18.

Resistance of a wire is directly proportional to the length of the wire.
R ∝ l
The resistance of a conductor depends on the number of collisions which the electrons suffer with the fixed positive ions while moving from one end to the other end of the conductor. Obviously the number of collisions will be more in a longer conductor as compared to a shorter conductor. Therefore, a longer conductor offers more resistance.

Solution 19.

With the increase in temperature of conductor, both the random motion of electrons and the amplitude of vibration of fixed positive ions increase. As a result, the number of collisions increases. Hence, the resistance of a conductor increases with the increase in its temperature.
The resistance of filament of a bulb is more when it is glowing (i.e., when it is at a high temperature) as compared to when it is not glowing (i.e., when it is cold).

Solution 20.

Iron wire will have more resistance than copper wire of the same length and same radius because resistivity of iron is more than that of copper.

Solution 21.

  1. Resistance of a wire is directly proportional to the length of the wire means with the increase in length resistance also increases.
    R ∝ l
  2. Resistance of a wire is inversely proportional to the area of cross-section of the wire. If area of cross-section of the wire is more, then resistance will be less and vice versa.
    R ∝ 1/A
  3. Resistance increases with the increase in temperature since with increase in temperature the number of collisions increases.
  4. Resistance depends on the nature of conductor because different substances have different concentration of free electrons.Substances such as silver, copper etc. offer less resistance and are called good conductors; but substances such as rubber, glass etc. offer very high resistance and are called insulators.

Solution 22.

The resistivity of a material is the resistance of a wire of that material of unit length and unit area of cross-section.
Its S.I. unit is ohm metre.

Solution 23.
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Solution 24.

Metal < Semiconductor < Insulator

Solution 26.

Manganin

Solution 28.

‘Copper or Aluminium’ is used as a material for making connection wires because the resistivity of these materials is very small, and thus, wires made of these materials possess negligible resistance.
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Solution 30.

Manganin is used for making the standard resistor because its resistivity is quite large and the effect of change in temperature on their resistance is negligible.

Solution 31.

Generally fuse wire is made of an alloy of lead and tin because its resistivity is high and melting point is low.

Solution 32.

A wire made of tungsten is used for filament of electric bulb because it has a high melting point and high resistivity.

A nichrome wire is used as a heating element for a room heater because the resistivity of nichrome is high and increase in its value with increase in temperature is high.

Solution 33.

A superconductor is a substance of zero resistance at a very low temperature. Example: Mercury at 4.2 K.

Solution 34.

Superconductor

Solution 1 (MCQ).

Nichrome is an ohmic resistance.
Hint: Substances that obey Ohm’s law are called Ohmic resistors.

Solution 2 (MCQ).

For carbon, resistance decreases with increase in temperature.
Hint: For semiconductors such as carbon and silicon, the resistance and resistivity decreases with the increase in temperature.

Numericals

Solution 1.
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Solution 9.
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Solution 11.
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Solution 12.
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Exercise 8(B)

Solution 1.

e.m.f.: When no current is drawn from a cell, the potential difference between the terminals of the cell is called its electro-motive force (or e.m.f.).
Terminal voltage: When current is drawn from a cell, the potential difference between the electrodes of the cell is called its terminal voltage.
Internal Resistance: The resistance offered by the electrolyte inside the cell to the flow of electric current through it is called the internal resistance of the cell.

Solution 2.

e.m.f. of cellTerminal voltage of cell
1. It is measured by the amount of  work done in moving a unit positive charge in the complete circuit inside and outside the cell.1. It is measured by the amount of work done in moving a unit positive charge in the circuit outside the cell.
2. It is the characteristic of the cell i.e., it does not depend on the amount of current drawn from the cell2. It depends on the amount of current drawn from the cell. More the current is drawn from the cell, less is the terminal voltage.
3. It is equal to the terminal voltage when cell is not in use, while greater than the terminal voltage when cell is in use.3. It is equal to the emf of cell when cell is not in use, while less than the emf when cell is in use.

 

Solution 3.

Internal resistance of a cell depends upon the following factors:

  1. The surface area of the electrodes: Larger the surface area of the electrodes, less is the internal resistance.
  2. The distance between the electrodes: More the distance between the electrodes, greater is the internal resistance.

Solution 4.

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Solution 5.

(a) Terminal voltage is less than the emf : Terminal Voltage < e.m.f.
(b) e.m.f. is equal to the terminal voltage when no current is drawn.

Solution 6.

When the electric cell is in a closed circuit the current flows through the circuit. There is a fall of potential across the internal resistance of the cell. So, the p.d. across the terminals in a closed circuit is less than the p.d. across the terminals in an open circuit by an amount equal to the potential drop across the internal resistance of the cell.

Solution 7.
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Solution 8.
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Solution 9.
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Solution 10.

(a) series
(b) parallel
(c) parallel
(d) series

Solution 11.

For the same change in I, change in V is less for the straight line A than for the straight line B (i.e., the straight line A is less steeper than B), so the straight line A represents small resistance, while the straight line B represents more resistance. In parallel combination, the resistance decreases while in series combination, the resistance increases. So A represents the parallel combination.

Solution 1 (MCQ).

In series combination of resistances, current is same in each resistance.
Hint: In a series combination, the current has a single path for its flow. Hence, the same current passes through each resistor.

Solution 2 (MCQ).

In parallel combination of resistances, P.D. is same across each resistance.
Hint: In parallel combination, the ends of each resistor are connected to the ends of the same source of potential. Thus, the potential difference across each resistance is same and is equal to the potential difference across the terminals of the source (or battery).

Solution 3 (MCQ).
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Numericals

Solution 1.
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Solution 2.
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Solution 3.
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Solution 4.
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Solution 5.
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Solution 6.
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Solution 7.
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Solution 8.
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Solution 9.
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Solution 10.
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Solution 11.
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Solution 12.
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Solution 13.
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Solution 14.
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Solution 15.
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Solution 16.
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Solution 17.
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Solution 18.
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Solution 19.
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Solution 20.
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Solution 21.
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Solution 23.
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Solution 24.
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Solution 25.
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Solution 26.
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Solution 27.
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Solution 28.
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Solution 29.
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Solution 30.
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Exercise 8(C)

Solution 1.
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Solution 2.
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Solution 3.
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Solution 4.

The S.I. unit of electrical energy is joule.
1Wh = 3600 J

Solution 5.

The power of an appliance is 100 W. It means that 100 J of electrical energy is consumed by the appliance in 1 second.

Solution 6.

The S.I. unit of electrical power is Watt.

Solution 7.

(i) The household unit of electricity is kilowatt-hour (kWh).
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 1 kW when it is used for one hour.
(ii) The voltage of the electricity that is generally supplied to a house is 220 Volt.

Solution 8.

(i) Electrical power is measured in kW and
(ii) Electrical energy is measured in kWh.

Solution 9.

One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 1 kW when it is used for one hour.
Its value in SI unit is 1kWh = 3.6 x 106J

Solution 10.

Kilowatt is the unit of electrical power whereas kilowatt-hour is the unit of electrical energy.

Solution 11.
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Solution 12.

An electrical appliance such as electric bulb, geyser etc. is rated with power (P) and voltage (V) which is known as its power rating. For example: If an electric bulb is rated as 50W-220V, it means that when the bulb is lighted on a 220 V supply, it consumes 50 W electrical power.
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Solution 13.

It means that if the bulb is lighted on a 250 V supply, it consumes 100 W electrical power (which means 100J of electrical energy is converted in the filament of bulb into the light and heat energy in 1 second).

Solution 14.
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Solution 15.
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Solution 16.

When current is passed in a wire, the heat produced in it depends on the three factors:

  1. on the amount of current passing through the wire,
  2. on the resistance of wire and
  3. on the time for which current is passed in the wire.
  • Dependence of heat produced on the current in wire: The amount of heat H produced in the wire is directly proportional to the square of current I passing through the wire,  i.e., H ∝ I2
  • Dependence of heat produced on the resistance of wire: The amount of heat H produced in the wire is directly proportional to the resistance R of the wire, i.e., H ∝ R
  • Dependence of heat produced on the time: The amount of heat H produced in the wire is directly proportional to the time t for which current is passed in the wire, i.e., H ∝ t

 

Solution 1 (MCQ).
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Solution 2 (MCQ).
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Numericals

Solution 1.
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Solution 2.
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Solution 3.
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Solution 4.
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Solution 5.
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Solution 6.
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Solution 7.
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Solution 8.
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Solution 9.
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Solution 10.
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When one lamp is connected across the mains, it draws 0.25 A current, while if two lamps are connected in series across the mains, current through each bulb becomes
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(i.e., current is halved), hence heating (= I2Rt) in each bulb becomes one-fourth, so each bulb appears less bright.

Solution 11.
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Solution 12.
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Solution 13.
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Solution 14.
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Solution 15.
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Solution 16.
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Solution 17.
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Solution 18.
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Solution 19.
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Solution 20.
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More Resources for Selina Concise Class 10 ICSE Solutions

How series and parallel circuits are different?

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How series and parallel circuits are different?

Series and Parallel Circuits

Electrical circuit can be connected in two basic ways, in series or in parallel.

In a series circuit, all the components are connected one after the other in one single path.

  1. Figure shows a series circuit where three bulbs, L1 , L2 and L3 are connected to a switch and a cell.
    Series and parallel circuits 1
  2. In a parallel circuit, all the components are connected with their corresponding ends joined together at common points to form separate and parallel paths. These paths are called branches.
  3. Figure shows a parallel circuit, where the three bulbs, L1 , L2 and L3 and their respective switches are connected with their ends joined at two common points before the ends are connected to a cell.
    Series and parallel circuits
  4. The brightness of each bulb in a series circuit is equally the same since the same current flows through each bulb.
  5. The brightness of the bulbs in a parallel circuit is brighter than those in a series circuit with the same number of bulbs.This is because the bulbs in the parallel circuit draw as much current as a single bulb.
  6. Household wiring circuits operating devices such as a lamp, air conditioner, water heater, fan and washing machine are connected in parallel to the mains supply. Current flow from the mains supply to these circuits are controlled by a fuse box.
  7. In the fuse box or a consumer unit, a circuit breaker is connected in series to each circuit.
  8. There is a main switch in the fuse box which is connected in series to the mains supply.
  9. Therefore, in case of an emergency or repair work, the connection between the mains supply and the household wiring circuits can be broken by turning off the main switch.

People also ask

Difference between Series and Parallel Circuits

Table gives the summary of the comparison between a series circuit and a parallel circuit.

 Series circuit Parallel circuit
 1. Same current at all points
Series and parallel circuits 2
In a series circuit, the current has only one path to flow. The current leaving and returning to the source is not lost. Therefore, in a series circuit, the current is the same at all points, where I = I1 = I2 = I3.		 1. Different current at different paths
Series and parallel circuits 4
In a parallel circuit, the current has more than one path to flow. The current from the source splits into separate branches. Therefore, in a parallel circuit, the current leaving and returning to the source is the sum of the currents in the separate paths, where I = I1 = I2 + I3 = I4.
 2. Different potential differences at different points
Series and parallel circuits 3
In a series circuit, the potential difference (p.d.) will drop from the highest point (at P, p.d.= 3 V) after crossing each component in the circuit (at Q it drops to 1.5 V and at R it drops to 0 V). The total potential difference across all the components is shared among them. Therefore, in a series circuit, the total potential difference across all the components is the sum of the potential differences across the individual components, V = V1 + V2		 2. Same potential difference at the same junction
Series and parallel circuits 5
In a parallel circuit, the potential drop across each component in the circuit is the same as they share the same two points, P and Q. The potential difference across a component (3 V) is the potential difference of any other component connected in between. Therefore, the potential difference across the separate branches of a parallel circuit are the same, where V = V1 = V2.
 3. When a bulb in a series circuit has blown up, the other bulbs would not be able to light up. 3. When a bulb in a parallel circuit has blown up, the other bulbs would still be able to light up.

Application of Series and Parallel Circuits in the Real World

Application of the Principles of Current, Potential Difference and Resistance in Series and Parallel Circuits

  1. Most of the electric components in our household appliances are connected in parallel circuits. There are some which are connected in combinations of series and parallel circuits.
  2. Figure shows a hairdryer with two switches, A and B.
    Application of Series and Parallel Circuits 1
  3. Figure shows the circuit diagram of the hairdryer. The fan and the resistor, R are connected in series, while the fan and the heating element are connected in parallel.
    Application of Series and Parallel Circuits 2
  4. This is an example of an application of combining series and parallel circuits together.
  5. When the main switch is closed, the fan is switched on and the air blown out from the hairdryer is cold. Switch A is used to control the heating element. When it is closed, the heating element is turned on and the air blown out is hot.
  6. The speed of the fan can be controlled by connecting a resistor, R in series with the fan. The speed of the fan is in slow mode when the main switch is closed.
  7. When switch B is closed, the current bypasses resistor R and flows straight to the fan. This will increase the voltage across the fan and the speed of the fan can be increased.

Series and Parallel Circuits Experiment

Aim: To identify series and parallel circuits.
Materials: Eight bulbs, dry cells, connecting wires
Apparatus: Eight bulb holders, battery holders, five plug-key switches
Method:

Series and Parallel Circuits Experiment
Series and Parallel Circuits Experiment 1

  1. All the electrical circuits as shown in Table are set up.
  2. The circuits are switched on and the brightness of the bulbs are observed and compared.
  3. In each circuit, one of the bulbs is removed and what happens to the other bulb is observed.

Discussion:

  1. The circuits in 1 and 4 are connected in series. These circuits have only one path for the charges to flow from one terminal to another terminal of the battery.
  2. The circuits in 2 and 3 are connected in parallel. These circuits have more than one continuous path or branch for the charges to flow from one terminal to another terminal of the battery.
  3. The bulbs in the parallel circuits light up brighter as compared to the bulbs in the series circuits. The effective resistance in a parallel circuit is much smaller and the current that passes through each bulb is larger.
  4. When one of the bulbs is removed from a series circuit, the other bulb does not light up. This shows that
    e series circuit is broken when one of the bulbs is removed and current cannot continue to pass through the circuit.
  5. When one of the bulbs is removed from a parallel circuit, the other bulb still lights up. This shows that the broken circuit in one branch will not affect the circuit in other branches.

Comparison of Current and Potential Difference in Series and Parallel Circuits Experiment

Aim: To compare the current, I and potential difference, V in series and parallel circuits.
Materials: Two bulbs, dry cells, connecting wires
Apparatus: Two bulb holders, battery holder, three voltmeters, three ammeters
Method:
Series and Parallel Circuits Experiment 2

  1. The electrical circuit is set up as shown in above Figure.
  2. The brightness of the bulbs in the circuit is observed. The readings of the ammeters and voltmeters are recorded in Table 1.
  3. One of the bulbs is removed and what happens to the other bulb in the circuit is observed. The readings of the ammeters and voltmeters are recorded in Table 2.
  4. The bulbs, batteries, ammeters and voltmeters are now connected as shown in Figure.
    Series and Parallel Circuits Experiment 3
  5. The brightness of the bulbs is observed. The readings of the ammeters and voltmeters are recorded in Table 1.
  6. One of the bulbs is removed and what happens to the other bulb is observed. The new readings of the ammeters and voltmeters are recorded in Table 2.

Observation:

Series and Parallel Circuits Experiment 4
Table 1
Series and Parallel Circuits Experiment 4
Table 2

Discussion:

  1. In the series circuit, the current that passes through each bulb is the same. The potential difference across each bulb is also the same as the bulbs are similar. The sum of the potential difference across each bulb is the same as the potential difference across the battery.
  2. In the parallel circuit, the bulbs are connected parallel to the battery and share the same two points, therefore the potential difference across each bulb is the same as the potential difference across the battery. The total current that flows in the circuit is the sum of the current that passes through each bulb in separate paths.
  3. In the series circuit, the circuit will break off if one of the bulbs is removed. The ammeter reading is zero as no current passes through it. The potential difference across each bulb is also zero as no current passes through it.
  4. In the parallel circuit, only the path in which the bulb has been removed will break off. The current can still flow through the other path. The potential difference across the bulb is still the same as the potential difference across the battery.
  5. The bulbs in the parallel circuit light up brighter as compared to the bulbs in the series circuit. This is because in a parallel circuit, the potential difference across each bulb is very much higher as compared to the potential difference of each bulb in a series circuit. A bulb that lights up brighter indicates that the current that passes through it is larger.

 

How do you calculate the total resistance of a parallel circuit?

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How do you calculate the total resistance of a parallel circuit?

The Effective Resistance of Resistors Connected in Parallel

  1. There are three important characteristics in a parallel circuit:
    (a) The potential difference is the same across each resistor.
    (b) The current that passes through each resistor is inversely proportional to the resistance of the resistor.
    (c) The total current in the circuit equals to the sum of the currents passing through the resistors in its parallel branches.
    parallel combination of resistance 1
  2. When two or more resistances are connected between two common points so that the same potential difference is applied across each of them, they are said to be connected is parallel.
    How do you calculate the total resistance of a parallel circuit 1When such a combination of resistance is connected to a battery, all the resistances have the same potential difference across their ends.
  3. Derivation of mathematical expression of parallel combination:
    Let, V be the potential difference across the two common points A and B. Then, from Ohm’s law
    Current passing through R1,     I1 = V/R1             … (i)
    Current passing through R2,     I2 = V/R2           … (ii)
    Current passing through R3,     I3 = V/R3          … (iii)
  4. If R is the equivalent resistance, then from Ohm’s law, the total current flowing through the circuit is given by,
    I = V/R                               … (iv)
    and I = I1 + I2 + I3           … (v)
  5. Substituting the values of I, I1, I2 and I3 in Eq. (v),
    \( \frac{\text{V}}{\text{R}}=\frac{\text{V}}{{{\text{R}}_{\text{1}}}}+\frac{\text{V}}{{{\text{R}}_{\text{2}}}}+\frac{\text{V}}{{{\text{R}}_{\text{3}}}}\text{ }……..\text{ (vi)} \)
  6. Cancelling common V term, one gets
    \( \frac{\text{1}}{\text{R}}=\frac{\text{1}}{{{\text{R}}_{\text{1}}}}+\frac{\text{1}}{{{\text{R}}_{2}}}+\frac{\text{1}}{{{\text{R}}_{3}}} \)
    The equivalent resistance of a parallel combination of resistance is less than each of all the individual resistances.
  7. The equivalent circuit is shown in Figure.parallel combination of resistance 2

Important results about parallel combination:

  1.  Total current through the circuit is equal to the sum of the currents flowing through it.
  2.  In a parallel combination of resistors the voltage (or potential difference) across each resistor is the same and is equal to the applied voltage i.e. V1 = V2 = V3 = V.
  3.  Current flowing through each resistor is inversely proportional to its resistances, thus higher the resistance of a resistors, lower will be the current flowing through it.

People also ask

Parallel Circuit Problems with Solutions

  1. The three resistors, R1, R2 and R3, in are connected in parallel to the battery as shown in Figure.
    Parallel Circuit Problems with Solutions 2
    Calculate
    (a) the potential difference across each resistor,
    (b) the effective resistance, R of the circuit,
    (c) the current, I, in the circuit,
    (d) the currents, I1, I2 and I3 passing through each resistor.
    Solution:
    (a) Since this is a parallel circuit, the potential difference across each resistor is 6 V, same as the potential difference across the battery, which is 6 V.
    Parallel Circuit Problems with Solutions 1

What is the Ohm’s law?

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What is the Ohm’s law?

  1. The proportional relationship of the potential difference between the ends of an ideal conductor and the current passing through it is known as Ohm’s law.
  2. Ohm’s law states that the current passing through an ideal conductor is directly proportional to the potential difference between its ends, provided that the temperature and other physical factors of the conductor are kept constant.
  3. Definition: According to the Ohm’s law at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across the conductor.
  4. Thus, if I is the current flowing through a conductor and V is the potential difference (or voltage) across the conductor, then according to Ohm’s law.
    I ∝ V (when T is constant)
    \(I=\frac{V}{R}\text{                    }…..\text{ (i)}\)
    where R is a constant called the resistance of the conductor.
    Equation (i) may be written as,
    V = I × R                          …… (ii)
  5. Unit of resistance:
    The SI unit of resistance (R) is ohm. Ohm is denoted by the Greek letter omega (Ω).
    \( \text{From Ohms law,        }R=\frac{V}{I} \)
  6. Now, if, V = 1 volt and I = 1 ampere
    \( \text{Then,                          }R=\frac{\text{1}\,\text{volt}}{\text{1}\,\text{ampere}} \)
    Thus, 1 ohm is defined as the resistance of a conductor which allows a current of 1 ampere to flow through it when a potential difference of 1 volt is maintained across it.
  7. Results of Ohm’s law
    Current flowing through a conductor is directly proportional to the potential difference across the conductor.
    What is the Ohm's law 1
  8. When the potential difference in a circuit is kept constant, the current in inversely proportional to the resistance of the conductor.
    \( I\propto \frac{1}{R} \)
  9. The ratio of potential difference to the current is constant. The value of the constant is equal to the resistance of the conductor (or resistor).
    \( \frac{V}{I}=R \)

People also ask

Ohm’s Law Problems with Solution

  1. When a potential difference of 12 V is applied across a conductor, it is found that the current that passes through the conductor is 0.2 A. What is the resistance of the conductor?
    Solution:
    \( R = \frac{V}{I} \)
    = 12/0.2
    = 60 Ω
  2. What is the potential difference across a light bulb of resistance 5 Ω when the current that passes through it is 0.5 A?
    Solution:
    V = I × R
    = 0.5 × 5 = 2.5 V
  3. A potential difference of 230 V is applied across an electric heater. A current of 5 A is flowing through the heating element of the heater.
    (a) What is the resistance of the heating element?
    (b) Can this electric heater be used in a country where the main supply is 110 V? Explain your answer.
    Solution:
    (a) \( R = \frac{V}{I} \)
    = 230/5
    = 46 Ω
    (b) The heater can still be used but the heating effect will be very much lesser as the current passing through the heater is very much smaller. The current that passes through the heater is:
    \( I = \frac{V}{R} \)
    = 110/46
    = 2.4 A
    This value is smaller than 5 A.
  4. An electric motor which has an internal resistance of 10 Ω can run at a certain speed when a current of 0.5 A passes through it. The speed of the motor increases when a potential difference twice as big as before is applied across it. What is the new current that passes through the motor?
    Solution:
    Potential difference across the motor when a current of 0.5 A is passing through:
    V = IR = 0.5 x 10 = 5 V
    When the potential difference is doubled
    2 x 5 = I x 10
    I = 1 A

What is a superconductor made of?

  1. Most materials have electrical resistance which leads to power loss and heating.
  2. Many electrical applications require materials with very low resistance – the lower the better.
  3. Power transmission lines, electromagnets and computer chips would be revolutionised by resistanceless materials.
  4. In fact, under special conditions, materials with zero resistance can be made. For example, when mercury is cooled to a temperature of 4.15 K, its resistance suddenly dropped to zero. Mercury thus becomes a superconductor. This phenomenon is known as superconductivity.
  5. Figure illustrates superconductivity. The resistance of a superconductor suddenly drops to zero as its temperature decreases below the critical temperature, Tc.
    superconductor
  6. A superconductor conducts electricity without loss of energy. Figure shows a permanent magnet floating above a superconductor. This is an effect of superconductivity that is due to a continuous flow of induced current.
    superconductor 1

Flow Of Current In A Metal

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Flow Of Current In A Metal

Flow of electric current 
Three basic conditions are required for an electric current to flow.
A device used to produce an electric current like cell, battery, or a plug point acting as a source.
A wire made of a metal like copper, silver, or aluminium, which will allow electric current to flow through easily.
An unbroken loop (of the wire) running from one terminal of the source, through various appliances, back to the other terminal of the source.

Flow Of Current In A Metal 1

Precaution: Never connect the two terminals of a cell with a wire without an appliance connected in a circuit. This will cause overheating of the wire and also destroy the cell.

Making a Simple Electric Circuit
When we connect the terminals of a pencil cell (name given to the cell due to its shape) to a bulb using two wires, the bulb glows. This happens because we provide a path for the current to flow. A path for an electric current to flow is called an electric circuit.
In Figure (a), one wire from the pencil cell is connected to the torch bulb, while the other wire is not. The electric circuit is not complete here. In Figure (b), both the wires from the cell are connected to the torch bulb. The electric circuit is complete in this case. Electric current flows only if there is an unbroken path or closed circuit starting from one terminal of the source, through the torch bulb, to the other terminal of the source. Thus, the bulb glows in Figure (b) but not in Figure (a). The circuit in Figure (a) is not complete. Hence, current cannot flow through the circuit and the bulb does not glow. Such a circuit is called an open circuit. The circuit in Figure (b) is complete. Electric current flows through the circuit and, as a result, the bulb glows. Such a circuit is called a closed circuit.

Flow Of Current In A Metal 2

Electric current flows in a particular direction. In an electric circuit, the electric current flows from the positive terminal to the negative terminal of the electric cell. Figure 14.7 shows the direction of flow of electric current in a circuit.

Flow Of Current In A Metal 3
The direction of electric current in the circuit

Activity
Aim: To make a simple circuit (adult supervision required)
Materials needed: Electrical wire about 1 m (from your local electrical shop), pencil cell, small torch bulb (from your local electrical shop), blade, scissors, and sticky tape/ insulation tape (from the local electrical shop)
Method:
1. Cut out two pieces of the wire about 8 inches each, using scissors .
2. Strip the ends of the wire with a blade so that the metal is exposed.
3. You will see the signs’+’ and at the two ends of the pencil cell. These are the positive and negative terminals of the cell. Use sticky tape and attach one end of a wire to the negative terminal of the pencil cell.
4. Attach the other end of the same wire to the side of the bulb. Use a small piece of sticky tape to stick it well. Make sure that the bottom portion of the bulb is left open and also that the wire does not touch it. (You could also get a bulb holder and connect the wire to the two screws as shown in the picture.)
Flow Of Current In A Metal 4
5. Take the second wire and attach one end of it to the positive terminal of the cell.
6. Touch the other end of this wire to the bottom end of the bulb and see what happens.
Observation: When you touch the bottom end of the bulb with the wire, the bulb will glow.
Note: Do not use any source other than a pencil cell for any of the activities given in this book. Using the electrical output at the plug points in your house or school could be extremely dangerous.

Flow of electric current in a metal
Metals show a very different kind of bonding called metallic bonding. According to this bonding, the outermost electrons are not bound to any particular atom, and move freely inside the metal randomly as shown in fig. So, these electrons are free electrons. These free electrons move freely in all the directions. Different electrons move in different directions and with different speeds. So there is no net movement of the electrons in any particular direction. As a result, there is no net flow of current in any particular direction.
Flow Of Current In A Metal 5Fig. Flow of electrons inside a metal wire when no potential is applied across its ends
Flow Of Current In A Metal 6Fig. Flow of electrons inside a metal wire when the two ends of a wire are connected to the two terminals of a battery

Conduction of electricity
We get electricity in our homes through cables and wires. An electric cable consists of a number of metal wires with or without a plastic covering. The metal wires conduct or transmit electricity whereas the plastic covering do not. Materials that conduct electricity are called conductors. Materials that do not conduct electricity are called insulators. For example, metals are conductors of electricity; wood, air, and plastic are insulators.

Electric-cable
Electric cable