Electricity Online Test 7th Science Lesson 8 Questions in English

Explanation
In 1882, when it was sun set in the west that miracle happened in New York city. When Thomas
Alva Edison gently pushed the switch on 14,000 bulbs in 9,000 houses suddenly got lighted up. It
was the greatest invention to mankind. From then the world was under the light even in the night.

Explanation
Many countries began using electricity for domestic purposes. Seventeen years after the New York,
in 1899 electricity first came to India. The Calcutta Electric Supply Corporation Limited
commissioned the first thermal power plant in India on 17 April 1899

Explanation
Around 1900s, a thermal power station was set up at Basin Bridge in Madras city and power was
distributed to the government press, general hospital, electric tramways and certain residential
areas in Madras. Today electricity is a common household commodity.

Explanation
The centre of the atom is called the nucleus. The nucleus consists of protons and neutrons. Protons are positively charged. Neutrons have no charge. Negatively charged electrons revolve around the nucleus in circular orbits. Electricity is a form of energy that is associated with electric charges that exists inside the atom.

Explanation
Comb your dry hair. Immediately after combing the dry hair, bring the comb closer to the bits of paper. what will you observe? When you are getting up from the plastic chair, the nylon shirt seems to be stuck to the chair and make crackling sound. What is the reason for the creation of the sound? A balloon sticks to wall without any adhesive after rubbing on your hand. Do you know the reason for all? In all the above activities, when a body is rubbed against some other body become charged

Explanation
Electric charge is measured in a unit called coulomb. One unit of coulomb is charge of approximately 6.242×10^18 protons or electrons. Electrical charges are generally denoted by the letter `q`.

Explanation
The flow of electric charges constitutes an electric current. For an electrical appliance to work, electric current must flow through it. An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is ‘I’.

Explanation
The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second.
I = q / t
Where I ⇒ current (in Ampere – A) q ⇒ charge (in coulomb – c) t ⇒ time taken (in seconds – s)

Explanation
Charge (q) = 30 coulomb
Time (t) = 2 min x 60s = 120 s
Current I = q/t = 30C/120s = 0.25 A

Explanation
Before the discovery of electrons, scientists believed that an electric current consisted of moving positive charges. This movement of positive charges is called conventional current

Explanation
After the electrons were discovered, it was known that electron flow actually takes place from the negative terminal to the positive terminal of the battery. This movement is known as electron flow. Conventional current is in the direction opposite to electron flow.

Explanation
Electric current is measured using a device called ammeter. The terminals of an ammeter are marked with + and – sign. An ammeter must be connected in series in a circuit.

Explanation
Instruments used to measure smaller currents, in the milli ampere or micro ampere range, are
designated as milli ammeters or micro ammeters.

Explanation
Given that the current flows through the circuit is 0.002A
We know that 1 A = 106 µA
0.002A = 0.002 × 106 µA
= 2 × 10-3 × 106 µA
= 2 × 103 µA
0.002A = 2000 µA

Explanation
Electrical charges need energy to push them along a circuit. Water always flows from higher to
lower ground. Similarly, an electric charge always flows from a point at higher potential to a point
at lower potential. An electric current can flow only when there is a potential difference (V) or P.D.
The potential difference between any two points in the circuit is the amount of energy needed to
move one unit of electric charge from one point to the other.

Explanation
The term mentioned in the board volt is the measurement for the electric potential difference. The
SI unit of potential difference is volt (V). potential difference between two points is measured by
using a device called voltmeter.

Explanation
An electrical component resists or hinders the flow of electric charges, when it is connected in a
circuit. In a circuit component, the resistance to the flow of charge is similar to how a narrow
channel resists the flow of water. The higher the resistance in a component, the higher the
potential difference needed to move electric charge through the component. We can express
resistance as a ratio. Resistance of a component is the ratio of the potential difference across it to
the current flowing through it
V = I/ R

Explanation
Resistance of a component is the ratio of the potential difference across it to the current flowing
through it. The S.I unit of resistance is ohm. Greater the ratio of V to I, the greater is the resistance.

Explanation
Electrical conductivity or specific conductance is the measure of a material’s ability to conduct an
electric current. It is commonly represented by the Greek letter σ (sigma). The S.I Unit of electrical
conductivity is Siemens/meter(S/m).

Explanation
Electrical resistivity (also known as specific electrical resistance, or volume resistivity) is a
fundamental property of a material that quantifies how strongly that material opposes the flow of
electric current. The SI unit of electrical resistivity is the ohm-metre (Ω.m).

Explanation

Explanation
Water flowing through pipes is pretty good mechanical system that is a lot like an electrical circuit.
This mechanical system consists of a pump pushing water through a closed pipe. Imagine that the
electrical current is similar to the water flowing through the pipe. The following parts of the two
systems are related:
 The pipe is like the wire in the electric circuit and the pump is like the battery
 The pressure generated by the pump drives water through the pipe.
 The pressure is like the voltage generated by the battery which drives electrons through the
electric circuit
 Suppose, there are some dust and rust that plug up the pipe and slow the flow of water,
creating a pressure difference from one end to the other end of the pipe. In similar way, the
resistance in the electric circuit resists the flow of electrons and creates a voltage drop from
one end to the other. Energy loss is shown in the form of heat across the resistor

Explanation
The sources which produce the small amount of electricity for shorter periods of time is called as
electric cell or electro chemical cells. Electric cell converts chemical energy into electrical energy.
In addition to electro chemical, we use electro thermal source for generating electricity for large
scale use. It has two terminals. When electric cells are used, a chemical reaction takes place inside
the cells which produces charge in the cell.

Explanation
The dry cell commonly used in torches is an example of a primary cell. It cannot be recharged after
use.

Explanation
Secondary cells are used in automobiles and generators. The chemical reaction in them can be
reversed, hence they can be recharged. Lithium cylindrical cells, button cells and alkaline cells are
the other types that are in use.

Explanation

Explanation

Explanation
A dry cell is a type of chemical cell commonly used in the common form batteries for many
electrical appliances. It is a convenient source of electricity available in portable and compact
form. It was developed in 1887 by Yei Sakizo of Japan.

Explanation
A dry cell is a portable form of a leclanche cell. It consists of zinc vessel which acts as a negative
electrode or anode. The vessel contains a moist paste of saw dust saturated with a solution of
ammonium chloride and zinc chloride. The ammonium chloride acts as an electrolyte.

Explanation
The purpose of zinc chloride is to maintain the moistness of the paste being highly hygroscopic.
The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as positive
electrode or cathode. It is surrounded by a closely packed mixture of charcoal and manganese
dioxide (MnO2) in a muslin bag. Here MnO2 acts as depolarizer. The zinc vessel is sealed at the top
with pitch or shellac. A small hole is provided in it to allow the gases formed by the chemical action
to escape. The chemical action inside the cell is the same as in leclanche cell.

Explanation
The dry cell is not really dry in nature but the quantity of water in it is very small, as the electrolyte
is in the form of a paste. In other cells, the electrolyte is usually a solution.

Explanation
Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons
in a circuit. All batteries are made up of three basic components: an anode (the ‘+’ side), a cathode
(the ‘–’ side), and some kind of electrolyte. Electrolyte is a substance that chemically reacts with
the anode and cathode.

Explanation
One fateful day in 1780, Italian physicist, physician, biologist, and philosopher, Luigi Galvani, was
dissecting a frog attached to a brass hook. As he touched the frog’s leg with an iron scapel, the leg
twitched. Galvani theorized that the energy came from the leg itself, but his fellow scientist,
Alessandro Volta, believed otherwise.

Explanation
The invention of the modern battery is often attributed to Alessandro Volta. It actually started with
a surprising accident involving the dissection of a frog.

Explanation
In a cell, the longer line denotes the positive (+) terminal and the short line denotes the negative (-
) terminal. We shall use these symbols to show components in the circuits we draw. Such diagrams
are called circuit diagrams.

Explanation
Two kinds of circuits can be made with two bulbs and a cell. In this experiment we shall make one
of them and study it. Look at the circuit with two bulbs, and a cell and a switch given here (Figure).
It is clear from the circuit diagram, that the two bulbs are connected one after the other. The circuit
diagram shows the sequence of the bulbs and cell, not their real position. The way in which the
bulbs have been connected in this circuit is called series connection.

Explanation
Figure – shows a circuit in which two bulbs are connected in different places. This is a second type
of circuit. Two bulbs in this circuit are said to be connected in parallel and such circuits are called
parallel circuits.

Explanation
We know that all materials are made up of the basic building block, the ‘atom’. An atom, in turn,
contains electrically charged particles. Many of these particles are fixed to the atoms but in
conductors (such as all metals) there are lots of particles that are not held to any particular atom
but are free to wander around randomly in the metal. These are called ‘free charge’.

Explanation
You might have observed the spark in the electric pole located nearby your house. Do you know the
cause of this electric spark? This is due to the short circuiting of electricity along its path. A short
circuit is simply a low resistance connection between the two conductors supplying electrical
power to any circuit. Arc welding is a common example of the practical application of the heating
due to a short circuit.

Explanation
Based on the property of conductance of electricity, substances are classified into two types,
namely, Conductors and Insulators (or) bad conductors of electricity. Electrons of different types of
atoms have different degrees of freedom to move around. With some types of materials, such as
metals, the outermost electrons in the atoms are loosely bound and they chaotically move in the
space between the atoms of that material. Because these virtually unbound electrons are free to
leave their respective atoms and float around in the space between adjacent atoms, they are often
called as free electrons.

Explanation
Conductors are the materials whose atoms have electrons that are loosely bound and are free to
move through the material. A material that is a good conductor gives very little resistance to the
flow of charge (electron) on the application of external voltage. This flow of charge (electron) is
what constitutes an electric current. A good conductor has high electrical conductivity.

Explanation
The chip which are used in SIM Cards, Computers, and ATM cards are made up of semiconductors
namely, silicon and germanium because of their electrical conductivity lies between a conductor
and an insulator.

Explanation
An insulator gives a lot of resistance to the flow of charge (electron). During the drift of the electrons
in an object when an external voltage is applied, collisions occur between the free electrons and
the atoms of the material also affect the movement of charges. These collisions mean that they get
scattered. It is a combination of the number of free electrons and how much they are scattered that
affects how well the metal conducts electricity. The rubber eraser does not allow electric current
to pass through it. So, rubber is a non-conductor of electricity. Rubber is an insulator.

Explanation
Wires made of copper, an electrical conductor, have very low resistance. Copper wires are used to
carry current in households. These wires are in turn enclosed in electrical insulators, or materials
of high electrical resistance. These materials are usually made of flexible plastic.

Explanation
The light of the bulb is thus one of the effects of electricity. There are several other important
effects of electricity. We shall study some of these effects in this chapter. There are 3 main effects
of electricity as,
1. Heating effect
2. Magnetic effect (Magnetism)
3. Chemical effect

Explanation
When electric current passes through a wire, the electrical energy is converted to hear. In heating
appliances, the heating element is made up of materials with high melting point. An example of
such a material is nichrome (an alloy of nickel, iron and chromium). The heating effect of electric
current has many practical applications. The electric bulb, geyser, iron box, immersible water
heater are based on this effect. These appliances have heating coils of high resistance.

Explanation
Electric fuse is a safety device which is used in household wiring and in many appliances. Electric
fuse has a body made of ceramic and two points for connecting the fuse wire. The fuse wire melts
whenever there is overload of the current in the wire. This breaks the circuit and helps in
preventing damage to costly appliances and to the wiring. In electrical devices, a glass fuse is often
used. This is a small glass tube, in which lies the fuse wire.

Explanation
MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a
big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric
supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break
the circuit automatically. One just needs to switch it on to resume the electric supply. Many models
of MCBs have a built-in mechanism by which the electric supply is automatically resumed.

Explanation
The next effect of electric current is Magnetism. In 1819, Hans Christian Oersted discovered the
electricity that has a magnetic effect. The experiment in activity-5 will help you understand the
magnetic effect of electric current.

Explanation
Magnetic effect of electric current has been used in making powerful electromagnets.
Electromagnets are also used to remove splinters of steel or iron in hospitals dealing with eye
injuries. Electro magnets are used in many appliances that we use in our day to day life, namely,
electric bell, cranes and telephone.

Explanation
In telephones, a changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate. The
diaphragm is made up a metal that can be attracted to magnets.
 The diaphragm is attached to spring that is fixed to the earpiece
 When a current-flows through the wires, the soft – iron bar becomes an electromagnet.
 The diaphragm becomes attracted to the electromagnet.
 As the person on the other end of the line speaks, his voice cause the current in the circuit to change. This causes the diaphragm in the earpiece to vibrate, producing sound.

Explanation
Edison was an American Scientist and Industrialist. He invented many instruments like Electric
bulb, electric motor, gramophone and kinetoscope. He was known as for taking the world of
darkness to brightness crossing all the obstacles in life.

Explanation
As a mark of respect to Edison on his death, the light of “Statue of Liberty” in New York was turned
off. Except the road lights of Chicago and Broadway, all the lights in the city were turned off.

Explanation
Thomas Alva Edison invented a commercially viable electric bulb. This was exhibited in 1897. He
used a platinum wire coil in a vacuum glass and discovered the first electric bulb in 1879.