Selina Concise Class 9 Physics Solution Chapter No. 9- ‘Current Electricity’ For ICSE Board Students.
Exercise-A Solution:
1.) Solution:
Ans:
Cell is the DC source while mains is the AC source.
2.) Solution:
Ans:
AC/ alternating current:
- The direction of AC is continuously changing with respect to time.
- In India the frequency of AC current used is 50Hz.
- AC is suitable for transferring over longer distances.
- And the magnitude of the AC current changes with respect to the time.
For example:
Current coming from AC generator and mains is the AC current.
DC/ direct current:
- DC current flows only in one direction through the circuit.
- The frequency of direct current DC is zero.
- As there is energy loss while transferring DC current hence it cannot be transferred over longer distances.
- And DC current has constant magnitude.
For example:
The current coming from cell or battery is the DC current.
3.) Solution:
Ans:
An electric cell is the device which converts chemical energy into electrical energy when connected in the circuit and also acts as DC source in the circuit.
4.) Solution:
Ans:
When current is drawn from a cell in the circuit then chemical energy is converted into the electrical energy.
5.) Solution:
Ans:
Two electrodes and an electrolyte in a vessel are the main constituents of the cell.
6.) Solution:
Ans:
Cells are of two kinds:
- Primary cell and
- Secondary cell or accumulators
Simple voltaic cell, Daniel cell, dry cell are the examples of primary cell.
Lead accumulator, Ni-Fe accumulator, Li-H battery are the secondary cells.
7.) Solution:
Ans:
- Primary cells are the cells which provides current which is the result of irreversible chemical reaction. When the entire chemical energy of primary cell is converted into electrical energy then this type of cells are discarded.
- And hence primary cells are the use and throw cells and cannot be recharged further.
For example:
Simple voltaic cell, Daniel cell, dry cell are the primary cell.
8.) Solution:
Ans:
- Secondary cell are also called as accumulators. Secondary cell provides current which is the result of chemical reaction.
- But, the chemical reaction occurring in the secondary cell is the reversible reaction so that they can be further recharged and used again and again.
- Secondary cells are having low resistance.
For example:
Lead accumulator, Ni-Fe accumulator, Li-H battery are the examples of secondary cells.
9.) Solution:
Ans:
Primary cells:
- In primary cell the reaction occurring is the irreversible chemical reaction.
- When the current is drawn from the primary cell then only chemical energy is converted into electrical energy.
- Primary cells cannot be recharged further as there is irreversible chemical reaction.
- Primary cells has high internal resistance.
- Primary cells has capacity of giving weak current only.
- Primary cells are light and cheaper.
For example:
Simple voltaic cell, Daniel cell, dry cell are the primary cells.
Secondary cell/accumulator:
- In secondary cell the chemical reaction occurring is the irreversible chemical reaction.
- When current is drawn from secondary cell it converts chemical energy into electrical energy. While when current is passed through it, it converts electrical energy into chemical energy.
- As there is reversible chemical reaction is occurring in the secondary cells, it is recharged for further use.
- Secondary cell has low internal resistance.
- Secondary cells provides low as well as high current.
- Secondary cells are heavy and also expensive.
For example:
Lead accumulator, Ni-Fe accumulator, Li-H battery are the secondary cells.
10.) Solution:
Ans:
- The rate of flow of charge is the current and it passing normally across the cross section normal to the direction of flow of current.
Current = Charge/Time
I= Q/t
The SI unit of current is ampere.
- The current flowing across a cross section is said to be 1A when there is 1C of charge is flowing per second.
Thus, 1A= 1C/1S
11.) Solution:
Ans:
The electron is negatively charged particle having charge on it -1.6*10-19C.
12.) Solution:
Ans:
We know that,
Current flowing through a conductor is the rate of flow of charge per second.
Here, n electrons flow through a cross section of conductor in time t and charge on an electron is e.
Hence, total charge on n electrons will be ne.
Thus, current flowing through the circuit is given by,
Current= Charge/Time
I= ne/t
This is the expression for the current flowing in a conductor.
13.) Solution:
Ans:
Rheostat is the electrical instrument which is used to control the current in an electric circuit.
14.) Solution:
Ans:
In the given electrical circuit the parts with their names and functions are as given below.
- A-shows ammeter, which is used for measuring current flowing through the circuit.
- B- shows cell, which is acts direct current source for the given electrical circuit.
- C-shows key, which is used to on or off the current flowing through the circuit.
- D-shows load, it is only a resistance to the circuit or it may be combination of different electrical components.
- E-shows voltmeter, which is used for measuring potential difference between any two points of the circuit.
- F-shows rheostat, which is used to control the current flowing through the circuit.
16.) Solution:
a) key
b) cell
c) rheostat
d) ammeter
e) voltmeter
Ans:
The following figure shows the symbols of above given electrical components.
a) key:
Key or switch in an electric circuit is used to put current on or off in an electric circuit.
b) cell:
Cell acts as the direct current source for the given electrical circuit.
c) rheostat:
Rheostat is used to control the electric current flowing through the electrical circuit.
d) ammeter:
The ammeter is used to measure the electric current flowing through the circuit and it is connected in series with the circuit.
e) voltmeter:
Voltmeter is used to measure the potential difference or voltage between any two points of the electrical circuit and it is always connected in parallel with the electrical circuit.
17.) Solution:
Ans:
a) The following figure shows the ammeter is inserted between the terminals A and C.
b) The following diagram indicates the polarity at terminals of ammeter and also indicates the direction of flow of current.
c) Rh is the rheostat which is used to control the current flowing through the circuit.
18.) Solution:
Ans:
Conductors:
- Conductors are the materials which allows electric current to pass through them easily.
- For example:
Metals and impure water are the best examples of conductors.
Insulators:
- Insulators are the materials which does not allows current to pass through them.
- For example:
Wood and rubber are the best examples of insulators.
19.) Solution:
Ans:
Following are the conductors of electricity from the given list:
Copper wire, acidulated water, human body and mercury.
20.) Solution:
Ans:
Conductors:
- Conductors are those which allows current to pass through them easily.
- Conductors have large number of free electrons and they offers very low resistance to the flow of current.
Insulators:
- Insulators are those which does not allows current to pass through them.
- Insulators have very small number of free electrons and they offers very high resistance to the flow of current.
21.) Solution:
Ans:
Closed circuit:
- Closed circuit is that circuit whose each part is made from conductor and when plug key is on the circuit become completed and allows current to flow through it then it is called as closed circuit.
- The following figure shows the closed circuit diagram.
Open circuit:
- Open circuit is that circuit which does not allow current through it which may due to broken of circuit or plug key is not on or the components may used are not made from conductor.
The following figure shows the open circuit diagram.
22.) Solution:
Ans:
The main condition for the circuit to be a closed circuit is that every component used in making the circuit must be made of conducting material.
Multiple choice type:
1.) A cell is used to
Ans: b) provide current in a circuit
2.) The unit of current is
Ans: a) ampere
3.) The insulator of electricity is
Ans: d) silk
Numerical:
1.) Solution:
Ans:
Given that,
Charge Q= 0.5C
Time t= 5s
We know that,
Current= charge/time
I= 0.5/5= 0.1A
Thus, current flowing is 0.1A.
2.) Solution:
Ans:
Given that,
Current I = 1.5A
Time t= 2s
We know that,
Current= charge/time
Charge= current*time
Charge= 1.5*2= 3C
Thus, the charge flowing is 3coulomb.
3.) Solution:
Ans:
Given that,
Charge Q= 24 C
Time t= 0.8 s
We know that,
Current= charge/time
I= 24/0.8= 240/8= 30 A
Thus, the current flowing through the coil will be 30 A.
Exercise-B Solution:
1.) Solution:
Ans:
From figure we can say that, when both the conductors are joined by metal wires
a) electron flow is from A to B and
b) The flow of current is in reverse direction of flow of electrons means flow of current is in direction from B to A.
2.) Solution:
Ans:
If there is difference in potentials of two conductors connected then the direction of flow of current is always from high potential to low potential.
3.) Solution:
Ans:
- The potential difference between two conductors is measured in terms of work done in transferring charge from one conductor to the another through the metallic wire connected.
- Thus, we can define the potential difference between two conductors as it is the amount of work done in transferring the unit positive charge from one conductor to the another conductor.
- Let, W be the work done in moving a test charge Q from one conductor to other then potential difference between those two conductors is given by,
V1 – V2 = W/q
4.) Solution:
Ans:
Potential difference:
The potential difference between two conductors is nothing but the amount of work done in moving a unit positive charge from one conductor to another conductor.
5.) Solution:
Ans:
- The SI unit of potential difference is volt and it is represented by V.
- Thus, 1V = 1J/1C
- Hence, the potential difference between the two points or conductors is said to be one volt when the amount of work done in moving 1 coulomb of charge from one conductor to other conductor is one joule.
6.) Solution:
Ans:
The potential difference between the two conductors is one volt, means the amount of work done in moving one coulomb of charge from one conductor to another conductor is one joule.
7.) Solution:
Ans:
- The opposition to the flow of electrons i.e. current by the conductor is called as resistance of that conductor.
- The material of the wire, length of the wire, area of cross section of the wire and the temperature of the wire affects the resistance of the conductor.
8.) Solution:
Ans:
- When there is no cell connected across the metal wire then there will be large number of free electrons are present in the wire in random direction.
- But, when we have given a potential to the metal wire thought the cell then electrons get started to move from negative terminal of the cell to the positive terminal of the cell through the metal wire.
- When electrons moves towards the positive terminal of cell during their movement, they collides with the fixed ions and also with the other free electrons. Due to this collision the speed of electrons decreases and their direction of motion also changes.
- This process of collision continues due to which the free electrons not get moved directly towards the positive terminal of the cell in bulk but they get drifted towards the positive terminal of the cell. Thus, we can say that in this way a metal wire offers resistance to the flow of current through it.
- The following figure a) shows how electrons are randomly oriented in absence of potential difference applied.
- And the figure b) shows how electrons are drifted towards the positive terminal of the cell and how metal wire offers the resistance to the flow of current through it.
9.) Solution:
Ans:
- The SI unit of resistance is ohm.
- Thus, 1ohm= 1V/1A
- Hence, we can define the resistance of a conductor as, it is the resistance of a conductor when one ampere of current is flowing through it when there will be one volt of potential difference is applied between its two ends.
10.) Solution:
Ans:
Ohm’s law:
According to Ohm’s law, when there is potential difference of V across the two ends of conductor and I be the current flowing through it then the resistance given by the conductor to the flow of current is the ratio of potential difference across the conductor and the electric current flowing through it.
Thus, from Ohm’s law we can write as
R= V/I
Or V= I*R
11.) Solution:
Ans:
The relation between potential difference V, current I and the resistance R is given by the statement of Ohm’s law which is stated as
V= I*R
12.) Solution:
Ans:
The resistance of a wire is one ohm means the resistance of wire is said to be one ohm only when current of one ampere is flowing through it if potential difference of one volt is applied across its ends.
13.) Solution:
Ans:
Let I be the current flowing when potential difference of V is applied and resistance will be R.
Now, if the resistance of the conductor is doubled and potential difference is kept constant then what will be the current.
Let us see,
V= I*R
Thus, I= V/R
Resistance is doubled, R=2R
Hence, I’= V/2R= 1/2(V/R) = I/2
Thus, the current flowing in a conductor become halved.
14.) Solution:
Ans:
The following are the factors on which resistance of the conductor depends:
a) Material of the conductor used:
- The different materials has different number of free electrons and the difference atomic arrangements due to which the materials having same length, same area of cross section but made of different materials may have different resistance.
- Thus, resistance of the conductor depends on the material used.
- Good conductors are those which are having high free electrons and offers less resistance to the flow of current.
b) The length of the wire:
- If the length of the wire used is more then for electrons there will be more efforts have to take complete the path of the circuit and hence there will be more collisions.
- Due to more collisions there will be more resistance offered to the flow of current.
- Hence, long wire has more resistance than the short wire.
c) The area of cross section of wire:
- In case of thick wire there will be more cross sectional area is available for free electrons to move than the thin wire.
- Hence, thick wire offers less resistance than the thin wire used.
d) The temperature of the wire:
- If the temperature of the wire is increased then the ions inside the wire vibrating more due to which number of collisions increases and hence resistance of the conductor also increases.
- Thus, the increase in temperature of the conductor increases the resistance of the conductor.
15.) Solution:
Ans:
a)
We know that, the resistance of the wire is directly proportional to the length of the conductor.
Hence, if the length of the conductor is doubled then the resistance of the wire also doubles or become twice.
b)
We know that, the resistance of the conductor is inversely proportional to the area of cross section of the conductor.
For the wire, area = π*r2
If the radius is doubled then area of the wire becomes = π*4*r2
But, resistance is inversely proportional to the area of cross section.
Hence, we can say that, if the radius of the wire is doubled then it’s resistance become one fourth.
16.) Solution:
Ans:
When bulb glows the resistance of the filament of bulb will increase because there is increase in temperature of the filament when bulb glows and hence resistance of the filament increases.
17.) Solution:
Volt, coulomb, ampere, ohm.
Ans:
- volt is the SI unit of potential difference.
- coulomb is the SI unit of charge.
- ampere is the SI unit of current.
- ohm is the SI unit of resistance.
Multiple choice type:
1.) Current in a circuit flows:
Ans: a) in a direction from high potential to low potential
2.) The unit of potential difference is
Ans: b) volt
3.) On increasing the resistance in a circuit, the current in it
Ans: a) decreases
Numerical:
1.) Solution:
Ans:
We know that,
Potential difference is given by,
Potential difference= work done/ charge
V= 9/1.5= 6V
Thus, the potential difference across the wire will be 6V.
2.) Solution:
Ans:
Given that,
Potential difference applied= 12V
Resistance of the filament= 24 ohm
Hence, the current drawn from the cell is given by,
I= V/R
I= 12/24= 0.5 A
Thus, the current drawn from the cell will be 0.5A.
3.) Solution:
Ans:
Given that,
Current I= 1.5A
Potential difference V = 6V
Then, according to Ohm’s law resistance of the conductor is given by,
R=V/I
R= 6/1.5= 4 ohm
Thus, the resistance of the filament of bulb while glowing will be 4 ohm.
4.) Solution:
Ans:
Given that,
Current I= 0.2 Ao
Resistance of wire R= 15 ohm
We know that, according to Ohm’s law the potential difference across the ends of the wire is given by,
V = I*R
V= 0.2*15= 3 V
Thus, the potential difference across the ends of the wire will be 3 V.
Exercise-C Solution:
1.) Solution:
Ans:
The efficient use of energy means the reduction in cost and amount of energy used in order to provide us the different products and also services.
2.) Solution:
Ans:
Energy can be saved in following ways:
- We have to use the energy efficient sources at home.
- We have to use the light energy coming from sun for various purposes like for producing electricity, for cooking food.
- We have to use the vehicles which are riding on electricity not on the fuels so we can save the fuels and also avoid the pollution.
- We have to use electricity only when we have a need and after use we have to close all the plugs and buttons.
- We have to plant more trees and reduce deforestation.
- We have use water in small amount when we need and we have to store it for future purposes.
- In this way, we can save lot of energy in our daily life also.
3.) Solution:
Ans:
- The proper insulation of home maintains the comfortable temperature inside the house.
- Because of this proper insulation, we can reduce cost of heating devices in winter whereas cooling devices in summer.
- Thus, we can maintain the temperature which is comfortable to us in our house.
4.) Solution:
Ans:
LED that is light emitting diode are the devices which are most efficient for lighting purpose.
5.) Solution:
Ans:
- The modern energy efficient appliances such as refrigerators, freezes, ovens, stoves, dishwashers, dryers etc are now using significantly less amount of energy than the older appliances.
- And now a days appliances are star rated according to their efficient use of electricity. Higher is the star rating means higher is the efficiency of the appliances we are using.
- In this way, the modern eco-friendly technologies are more energy efficient and also less polluting.
6.) Solution:
Ans:
Following are the ways of efficient use of energy:
- The use of fluorescent and LED lights or natural skylights instead of traditional incandescent light bulbs helps in reducing the same amount of energy required for the illumination.
- By using LED bulbs we can reduce the consumption of energy drastically.
- LED bulbs are also helpful in reducing the global warming and by using LEDs there will be reduction in the harmful effects of mercury used in the fluorescent lights.
- In this way, LEDs are mostly energy efficient for lighting purpose.
- In industries we can save sufficient amount of energy in order to attaining a high temperature by burning less fuel also only by using advance boilers and furnaces in the industry.
- Such type of technologies are more energy efficient and also less polluting.
7.) Solution:
Ans:
- The following social initiatives must be taken for the sensitive use of energy:
- We have to create awareness amongst the people and we have to encourage them for using the available resources judicially in order to decrease the demand for electricity as more possible.
- We can improve the public awareness through mass media and by increasing the children’s participation in campaign and eco friendly activities.
- The non governmental organizations i.e. NGOs can be used in order to create social awareness about sensitive use of the energy resources.
- In this way, through social initiatives we make sensitive use of energy.
Multiple choice type:
1.) The most non-polluting and efficient lighting device is
Ans: b) LED
2.) IEA is the short form of
Ans: c) International Energy Agency