What is the function of an earth wire? Why is it necessary to earth metallic appliances

Q. No 18: What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Ans: The metallic body of electric appliances is connected to the earth wire so that any leakage of electric current is transferred to the ground. This prevents severe electric shock to the user. That is why earthing of the electrical appliances is necessary.

When does an electric short circuit occur

Q. No 17: When does an electric short circuit occur?

Ans:  If the insulation of the wires used in the circuit is damaged or the appliance used is faulty due to which the live wire and the neutral wire comes in direct contact as a result current in the circuit rises and the short circuit occurs. 

Explain the underlying principle and working of an electric generator by drawing a labelled diagram

Q. No 16:  Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Ans: An electric generator converts mechanical energy into electrical energy.

The principle behind the electric motor is based on Fleming’s right hand rule.

when a coil of insulated copper wire is forced to rotate inside a magnetic field then electric current is induced. The following figure shows circuit diagram of a simple AC generator. If axle X is rotated clockwise, then the length PQ moves upwards and the length RS moves downwards.

Since the lengths PQ and RS are moving in a magnetic field, a current will be induced due to electromagnetic induction. Length PQ is moving upwards and the magnetic field acts from left to right. Hence, according to Fleming’s right hand rule, the direction of induced current will be from P to Q. Similarly, the direction of induced current in the length RS will be from R to S. The direction of current in the coil is PQRS. Hence, the galvanometer shows a deflection in a particular direction. After half a rotation, length PQ starts moving down whereas length RS starts moving upward. The direction of the induced current in the coil gets reversed as SRQP. As the direction of current gets reversed after each half rotation, the produced current is called an alternating current (AC). To get a unidirectional current, instead of two slip rings, two split rings are used, as shown in the following figure. In this arrangement, brush A always remains in contact with the length of the coil that is moving up whereas brush B always remains in contact with the length that is moving down. The split rings C and D act as a commutator.

 

The direction of current induced in the coil will be PQRS for the first half and SRQP in the second half of the rotation. Therefore a unidirectional current is produced from the generator called DC generator

State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current

Q. No 15:  State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

Ans: (i) Right hand thumb rule

(ii) Fleming’s left hand rule

(iii) Fleming’s right hand rule 

Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B

Q. No 14: Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Ans:  If the current in the coil A is changed then some current will definitely be induced in the coil B. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This process is known electromagnetic induction. 

Name some devices in which electric motors are used

Q. No 12: Name some devices in which electric motors are used?

Ans: Electric motors are used in:

Water pumps

Electric fans

Electric mixers

Washing machines e.t.c

Draw a labelled diagram of an electric motor. Explain its principle and working

Q. No 11: Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

Ans: An electric motor is a device which converts electrical energy into mechanical energy. The principle behind the electric motor is based on Fleming’s left hand rule.

Working: When a current is allowed to flow through the coil PQRS the coil starts rotating anti-clockwise. This happens because a downward force acts on length PQ and at the same time, an upward force acts on length RS. As a result, the coil rotates anti-clockwise.

Current in the length PQ flows from P to Q and the magnetic field acts from left to right, normal to length PQ. Therefore, according to Fleming’s left hand rule, a downward force acts on the length PQ. Similarly at an upward force acts on the length RS. These two forces cause the coil to rotate anti-clockwise.

When coil complete half rotation, the position of PQ and RS interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. therefore the direction of current in the coil PQRS gets reversed. Now the current flows through the coil in the direction SRQP.

The direction of current through the coil PQRS reverse after every half rotation. As a result, the coil rotates in same direction. The split rings help to reverse the direction of current in the circuit. These are called the commutator.

Imagine that you are sitting in a chamber with your back to one wall

Q. No 10: Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Ans:

The direction of magnetic field is given by Fleming’s left hand rule. The direction of electric current will be from front wall to back wall as it is opposite to the direction of electron. Now since the direction of force is towards the right side. Hence by applying Fleming’s left hand rule, we can conclude that the magnetic field inside the chamber is in downward direction.

When is the force experienced by a current-carrying conductor placed in a magnetic field largest

Q. No 9: When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

Ans: The force experienced by a current-carrying conductor is largest when the directions of magnetic field and electric current are perpendicular to each other.

How does a solenoid behave like a magnet

Q. No 8: How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.

Ans:

Solenoid is coil having n number of turns of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is passed through it. The magnetic field produced by it is similar to the magnetic field of a bar magnet. The field lines produced in a current-carrying solenoid is shown in the following figure.

When the north pole of a bar magnet is brought near to the end connected to the negative terminal of the battery, then the solenoid repels the bar magnet. It means the end of solenoid which is connected to the negative terminal of the battery behaves as north pole as like poles repel each other similarly the other and behaves as a south pole.

List three sources of magnetic fields

Q. No 7: List three sources of magnetic fields.

Ans: Three sources of magnetic fields are:

(a) Permanent magnet

(b) Electromagnet

(c) Current-carrying conductor

State whether the following statements are true or false

Q. No 6:  State whether the following statements are true or false.

(a) An electric motor converts mechanical energy into electrical energy. FALSE: An electric motor is a device which converts electrical energy into mechanical energy.

(b) An electric generator works on the principle of electromagnetic induction.

TRUE:

(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.

 TRUE

(d) A wire with a green insulation is usually the live wire of an electric supply.

FALSE

Ans:

(a) False

An electric motor converts electrical energy into mechanical energy.

(b) True

A generator is an electric device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.

(c) True

A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel lines.

(d) False

Live wire has red insulation cover, whereas earth wire has green insulation colour in the domestic circuits.

At the time of short circuit, the current in the circuit

Q. No 5: At the time of short circuit, the current in the circuit

(a) reduces substantially

(b) does not change

(c) increases heavily

(d) vary continuously.

 Ans: The option (c) is correct. At the time of short circuiting then the amount of current that is flowing in the circuit increases. 

The essential difference between an AC generator and a DC generator is that

Q. No 4: The essential difference between an AC generator and a DC generator is that

(a) AC generator has an electromagnet while a DC generator has permanent magnet.

(b) DC generator will generate a higher voltage.

(c) AC generator will generate a higher voltage.

(d) AC generator has slip rings while the DC generator has a commutator. 

Ans: The option (d) is correct that an AC generator has slip rings while a DC generator has commutator.

The device used for producing electric current is called a

Q. No 3: The device used for producing electric current is called a

(a) Generator, (b) galvanometer, (c) ammeter and (d) motor

Ans: (a) Electric current is produced by electric generator which converts mechanical energy into electricity. 

The phenomenon of electromagnetic induction is (a) the process of charging a body

Q. No 2:  The phenomenon of electromagnetic induction is

(a)  the process of charging a body

(b)  the process of generating magnetic field due to a current passing through a coil

(c)  producing induced current in a coil due to relative motion between a magnet and the coil

(d)  the process of rotating a coil of an electric motor

Ans: The option (c) is correct that is when a straight coil and a magnet are moved relative to each other then a current is induced in the coil. 

Which of the following correctly describes the magnetic field near a long straight wire

Q. No 1: Which of the following correctly describes the magnetic field near a long straight wire?

(a) The field consists of straight lines perpendicular to the wire

(b) The field consists of straight lines parallel to the wire

(c) The field consists of radial lines originating from the wire

(d) The field consists of concentric circles centred on the wire Ans:

Ans: According to right hand thumb rule the magnetic field lines, produced around a straight current-carrying conductor, are concentric circles. 

LAWS OF CHEMICAL COMBINATIONS

LAWS OF CHEMICAL COMBINATIONS:

Law of Conservation of Mass

It states that matter can neither be created nor destroyed.

Law of Definite Proportions

It stated that a given compound always contains exactly the same proportion of elements by weight.
Example : Oxygen and hydrogen in water always bear ratio of 16:2 or 8:1 by mass

Law of Multiple Proportions

According to law of multiple proportion , if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.
Example

Hydrogen + Oxygen → Water

2g               16g             18g

Hydrogen + Oxygen → Hydrogen Peroxide

2g              32g                           34g

Here, the masses of oxygen (i.e. 16 g and 32 g) which combine with a fixed mass of hydrogen (2g) bear a simple ratio, i.e. 16:32 or 1: 2.

Gay Lussac’s Law of Gaseous Volumes

When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.
Example

Hydrogen + Oxygen → Water

100 mL        50 mL      100 mL

Thus, the volumes of hydrogen and oxygen which combine together (i.e. 100 mL and 50 mL) bear a simple ratio of 2:1

Avogadro Law

Avogadro proposed that equal volumes of gases at the same temperature and pressure should contain equal number of molecules
Example .

Hydrogen + Oxygen → Water

we see that two volumes of hydrogen combine with one volume of oxygen to give two volumes of water without leaving any unreacted oxygen.

rules in rounding off numbers

Rules in rounding off numbers

(1) If the rightmost digit to be removed is more than 5, the preceding number is increased by one.

Example 1.386 If we have to remove 6, we have to round it to 1.39

(2) If the rightmost digit to be removed is less than 5, the preceding number is not changed. 

Example, 4.334 if 4 is to be removed, then the result is rounded upto 4.33.

(3) If the rightmost digit to be removed is 5, then the preceding number is not changed if it is an even number but it is increased by one if it is an odd number. 

Example, if 6.35 is to be rounded by removing 5, we have to increase 3 to 4 giving 6.4 as the result. However, if 6.25 is to be rounded off it is rounded off to 6.2.

significant figures addition subtraction multiplication and division

Significant Figures

1. Zeros at the end or right of a number are significant provided they are on the right side of the decimal point. Otherwise, the zeros are not significant.
For example, 0.200 g has three significant figures.But100 has only one significant figure.

2. Zeros preceding to first non-zero digit are not significant. Such zero indicates the position of decimal point.
Example:
0.03 has one significant figure
0.0052 has two significant figures.

3. When numbers are written in scientific notation, the number of digits between 1 and 10 gives the number of significant figures
Examples:
4.01×10² has three significant figures,
8.256 × 10^–3 has four significant figures.

4. Zeros between two non-zero digits are significant. Thus, 2.005 has four significant figures.

Addition and Subtraction of Significant Figures

The result cannot have more digits to the right of the decimal point than either of the original numbers.
Here, 18.0 has only one digit after the decimal point and the result should be reported only up to one digit after the decimal point which is 31.1.

Addition and Subtraction of Significant Figures

Multiplication and Division of Significant Figures

In these operations, the result must be reported with no more significant figures as are there in the measurement with the few significant figures.
2.5×1.25 = 3.125
Since 2.5 has two significant figures, the result should not have more than two significant figures, thus, it is 3.1

how to do scientific notation

How to do scientific notation ?

Scientific notation is exponential notation in which any number can be represented in the form N × 10ⁿ where n is an exponent having positive or negative values and N can vary between 1 to 10.

Example

we can write 232.508 as 2.32508 x10² in scientific notation.

Base Si units name symbol and definitions

Base SI Units quantity	Name	Sym bol	Definition
Length	metre	m	The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983)
Mass	kilogram	kg	The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889)
Time	second	S	The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. (1967)
Electric current	Ampere	A	The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10–7 newton per metre of length. (1948)
Temperature	Kelvin	K	The kelvin, is the fraction 1/273.16 of the thermodynamic dynamic temperature of the triple point of water. (1967)
Amount of substance	Mole	mol	The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon - 12. (1971)
Luminous intensity	Candela	cd	The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. (1979)

definition of one mole

Definition of one mole 

1 mol : The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is “mol.

physical and chemical properties of matter

Physical properties of matter

Physical properties are those properties which can be measured or observed without changing the identity or the composition of the substance.
Examples: colour, odour, melting point,boiling point, density etc

Chemical properties of the matter 

Chemical properties are those properties which require a chemical changes to occur
Examples: acidity or basicity, combustibility etc.

Classification of matter mixtures pure substances

Classification of matter

Classification of matter

Matter can be classified as mixtures or pure substances.

Mixture : A mixture contains two or more substances present in it in any ratio. These substances are called its components.
For example, sugar solution in water, air, tea etc.

Mixtures are two types

heterogeneous mixtures : In homogeneous mixture, components completely mix with each other and its composition is uniform throughout.
Examples Sugar solution, air
Heterogeneous mixtures: In heterogeneous mixture, compositions are not uniform throughout and sometimes the different components can be observed.
Example: mixtures of salt and sugar



Pure substances. Pure substance have fixed composition. Constituents components of pure substances cannot be separated by simple physical methods.
Examples Copper, silver, gold, water, glucose etc

Pure substances are two types

Elements: An element consists of only one type of particles. These particles may be atoms or molecules.
Examples: Sodium, copper, silver, hydrogen, oxygen etc.

Compound: When two or more atoms of different elements combine, the molecule of a compound is obtained. The atoms of different elements are always present in a compound in a fixed and definite ratio and this ratio is characteristic of a particular compound. The properties of a compound are different from those of its constituent elements. Constituent particles can be separated by chemical methods.
The examples of some compounds are water, ammonia, carbon dioxide, sugar etc.

states of matter solids liquids and gases

States of matter solids liquids and gases 

Solid :
Solids have definite volume and definite shape.
Reason: these particles are held very close to each other in an orderly fashion and there is not much freedom of movement.

States of matter solids liquids and gases 

Liquid .
Liquids have definite volume but not the definite shape. They take the shape of the container in which they are placed.
Reason: the particles are close to each other but they can move around
Gas:
Gases have neither definite volume nor definite shape. They completely occupy the container in which they are placed.
Reason: the particles are far apart as compared to those present in solid or liquid states and their movement is easy and fast

What is matter

What is matter? 

Anything which has mass and occupies space is called matter. for example, book, pen, pencil, water, air, all living beings etc. are composed of matter.
Matter can exist in three physical states viz. solid, liquid and gas.

What is the importance of chemistry

What is the importance of chemistry?

1.  Chemistry plays an important role in meeting human needs for food, health care products and other materials
2.  Production of a variety of fertilizers, improved varieties of pesticides and insecticides
3.  Preparation of drugs isolated from plant and animal sources or prepared by synthetic methods.
4.  Design and synthesize new materials having specific magnetic, electric and optical properties.
5. Production of superconducting ceramics, conducting polymers, optical fibres and large scale miniaturization of solid state devices

definition of chemistry

definition of chemistry:

Chemistry is the science of molecules and their transformations and is also called the science of atoms and molecules. Chemistry is the branch of science that studies the composition, properties and interaction of matter.

A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil

Q. No 13: A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

Ans: (i) when magnet is pushed near a bar magnet then it induces current due to electromagnetic induction.

(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire again the current is induced in the coil but this time it is in reverse direction.

(iii) When a bar magnet is held stationary inside the coil then no current is induced that’s why galvanometer will show no deflection.