Class-8 General Science Notes (Gaba Books)- Unit-12 -The Current Electricity - SLO-Based Short and Brief Questions and Answers

Class-8 General Science Notes (Gaba Books)

Unit-12 -The Current Electricity

SLO-Based Short and Brief Questions and Answers

Q1. What is electricity? Describe its importance in our daily life.

Ans.

Electricity is a form of energy that results from the existence of charged particles.

Electricity plays a very important role in our daily life. It lights, heats, and cools our home and performs several daily routine jobs for us. Televisions, radio, cinema projector videos, computers, telephones, telegraphs, refrigerators, microwave ovens, juicers, grinders, blenders, etc. run on electricity. It illuminates our streets, helps to advertise business, and operates electric trains. Thus, most of the charm of modern life would be no more if there were no electricity. In all these cases, electricity moves through the electric wires. Electricity in motion is called current electricity.

Q2. What is an electric current? Define Ampere.

Ans.

The quantity of electricity that flows in one second across a cross-section of the wire. It is represented by the symbol 'I'

Mathematically:

If current 'I' flows in a wire for a time 't,' the quantity of electricity or charge 'Q' transferred: is represented as:

Q = It or

I = Q/t

The S.I unit of the electric current 'I' is the ampere (A), of charge Q is Coulomb (C) and time 't' is second (s).

Ampere:

if 1 Coulomb of charge flows through a conductor in 1 second, the amount of current will be 1 ampere.

Q3. What is the name of the device which measures the electric current?

Ans.

An ammeter is a device that measures the quantity of an electric current (electricity) in amperes.

Q4. What is a conventional current?

Ans

The flow of current from the positive end to the negative end in the wire is called conventional current. In fact, in a metallic wire it is the electrons which drift in the direction from the negative end to the positive end of the wire. We call this current as the electronic current.

Q5. What is potential difference?

Ans.

It is the difference of electrical potential between two points to make an electric current flow continuously is known as potential difference or Voltage. The S.I unit of potential difference is volt (V). It is measured with a voltmeter.

Q6. Briefly explain the potential difference in dry cells.

Ans.

A dry cell is a device that can make charges move continuously in a wire by creating a potential difference. A dry cell has two terminals-one marked positive (+) and the other marked negative (-). The positive terminal is at a higher potential than the negative terminal. When these terminals are connected by a metal wire, conventional current flows from the positive terminal to the negative terminal in the wire. Note that electrons flow from the negative terminal to the positive terminal in the wire. Example. A torch bulb is connected to the wire as shown in Fig, it lights up because moving electrons have energy called electric energy.

Q.7 Write the name of some portable sources of electric current.

Ans.

Some of the common portable sources of electric current are:

1. Voltaic cell 2. Daniel cell

3. Dry cell 4. Storage battery

5. Solar cells 6. Generator or dynamo

Q8. What is a voltaic cell? Explain briefly its construction and working.

Ans.

Voltaic cell (Galvanic cell) The first cell that could make an electric current flow in a wire was made by two Italian scientists named Luigi Galvani and Alessandro Volta 1799. It is also called a simple cell. It consisted of a copper plate and a zinc plate dipped in dilute sulphuric acid taken in a glass vessel.

Explanation:

When the two plates were connected by a copper wire with a bulb in between, the bulb glowed showing the flow of electric current. It was found that electrons flowed in the wire from the zinc to the copper plate.

The copper and zinc plates are called the electrodes of the cell. Zinc is the negative electrode or anode as it supplies the electrons and copper is the positive electrode or cathode. Dilute sulphuric acid is the electrolyte. The copper electrode is at a higher potential than the zinc electrode. Therefore, conventional current flows from the copper electrode to the zinc electrode in the wire. The electronic current flows in the opposite direction. The potential difference between the two electrodes is 1.5 V. This potential difference is called the electromotive force or emf of the cell.)

Q9. What is Daniel cell? Explain briefly its construction and working.

Ans.

This cell was invented by J.F. Daniel in 1836. A Daniel cell consists of a copper vessel containing copper sulphate solution. A porous pot filled with dilute sulphuric acid or zinc sulphate is kept in the copper vessel. The copper vessel serves as the cathode and a zinc rod, dipped into the acid in the porous pot, serves as the anode. If the cathode and anode are connected by a metal wire, electrons flow in the wire from the zinc rod anode) to the copper vessel (cathode).

The reactions that take place are as follows.

the outer circuit, electrons flow from zinc rod to copper vessel.

Q10. What is dry cell? Explain briefly its construction and working.

Ans

The problem with earlier cells was that they contained liquids and were cumbersome to carry. This shortcoming was overcome with the invention of the dry cell, which is now commonly used in torches and electronic appliances. Dry cell was invented by French scientist G. Leclanché in 1866.

The dry cell consists of a zinc container, which serves as the negative terminal. The electrolyte is a paste of ammonium chloride. A carbon rod with a brass cap serves as the positive electrode. It is surrounded by a paste of powdered manganese dioxide and carbon articles as shown in the figure. In the dry cell also, hydrogen is formed in the chemical reactions that take place in the cell. The manganese dioxide reacts with the hydrogen formed and prevents it from collecting on the zinc electrode.

A seal at the top prevents the chemicals from drying out. The cell is dry from the outside, but the chemicals must be kept moist, otherwise it will not work. The electrons flow in a wire Connecting the two terminals from the zinc to the carbon electrode.

Q11. Write some advantages and disadvantages of dry cell.

Ans.

Advantages of Dry Cell:

1. It is compact in size.

2. It is light and easy to carry from place to place.

3. It is convenient to use.

Disadvantages of Dry Cell:

1. It produces small currents for a relatively short period.

2. It cannot be reused when it is discharged.

Q12. What are the primary and secondary of a storage battery?

Ans.

Primary cells are not rechargeable. They cannot be reused when they are discharged. They have to be replaced with new cells. Such cells are called primary cells.

Secondary cell can be recharged and used again and again after recharging. Such rechargeable cells are called secondary cells, storage cells or accumulators.

Q13. Name the types of accumulators (i.e. secondary cells or storage cell).

Ans.

There are two types of accumulators:

1. Lead-acid accumulator.

2. Nickel-cadmium alkaline accumulator.

Q14. Explain briefly a lead-acid accumulator.

Ans.

A lead acid accumulator is the common type of storage cell. It consists of several plates of lead dipped in H2SO4. The H2SO4 acts as an electrolyte. A grid of lead packed with lead dioxide (PbO2) acts as an anode. Each cell consists of number of cathode and anode plates. One cell gives a voltage of about 2 volts. Six cells, joined in series in car battery, as shown in figure.

Q.15 Write some uses of storage batteries.

Ans.

Storage batteries are extensively used in cars, buses, trucks etc. and other vehicles for ignition and lighting.
They are also used for illuminating railway carriages.
They are also used in submarines, radio transmitters and satellites.

Q16. Write the advantages and disadvantages of storage batteries

Ans.

Advantages of storage batteries

1. They can be recharged a number of times.

2. They produce a large current for a long duration.

Disadvantages of storage batteries

1. They are quite heavy.

2. They require good maintenance.

3. They are expensive.

Q17. What are electrochemical cells called?

Ans.

Voltaic cell, Daniel cell, dry cell and accumulators are called electrochemical cells because, in these cells, chemical energy is converted into electrical energy.

Q18. Describe briefly the Solar cells. Write its uses.

Ans.

Solar cells are also used as a source of electric current. A simple solar cell consists of a wafer of silicon in which some arsenic atoms have been embedded. This wafer is coated with a thin layer of silicon and boron. As the sunlight strikes the silicon wafer, a small electric current is produced. To obtain a sizeable current, a very large number of solar cells is connected together to form a solar panel. Current can be transmitted from the wafer through the wires.

Solar cells are used in watches, calculators, transistors and satellites.

Solar furnaces, solar heaters and solar cookers also make use of solar cells. These cells are also used in remote parts of the country for generating electricity.

Q19. What is a generator or dynamo? Describe briefly its working.

Ans.

A Generator or dynamo is a device for producing electric currents for use in home and industry. It converts mechanical energy into electrical energy.

A Generator or dynamo in power houses and dams consists of a magnet moving rapidly within a coil of copper wire. The current is drawn from the two ends of the coil. The greater the speed of rotation the greater the current produced.

The principle on which generators work was discovered by Michael Faraday in 1931. He showed that a current is produced (or induced) in a coil of wire when a magnet is moved in and out of the coil. Alternately, the magnet can be kept stationary and the coil moved. Thus, he concluded that electric current is induced in a coil as long as there is relative motion between the coil and the magnet kept close to it. This is known as the principle of electromagnetic induction.

Q20. Set up an experiment to test the principle of electromagnetic induction.

Ans.

Apparatus: Insulated copper wire, a bar magnet, and a galvanometer

Procedure:

Made a coil of copper wire by winding closely 20-25 turns of the wire around a cylindrical object such as gum tube. Remove the tube and connect the two ends of the wire to the galvanometer. Made sure that magnet can go in and out of the coil. Quickly inserted the magnet in the coil. Noticed that the needle deflects, showing that a current is induced in the coil, as well as the magnet is moving through it. As soon as the magnet stops moving, the current turns to zero. Now if quickly take the net out of the coil, you will notice that the needle again deflects, but this time in the opposite direction. This shows that when the magnet moves in the opposite direction, the current is also induced in the opposite direction. If you rapidly move the magnet in and out of the coil, the induced current rapidly changes directions.

Q21. How does a generator generate electricity?

Ans.

Generator uses the principle of electromagnetic induction to generate electricity. It consists of a rectangular coil having a large number of turns, free to rotate it a horizontal axis within the poles of a powerful U-shaped magnet. The coil is connected to two slip rings against which carbon brushes press lightly. When the coil rotated, electric current is generated. The current can be supplied through wires connected e brushes.

Q22. How does a bicycle dynamo generate electricity?

The bicycle dynamo is attached to the wheel. The movement of the wheel is used to move the coil. When the coil rotates, an electric current is generated. The current can be supplied through wires connected to brushes.

Q23. Write the name of the means of large-scale commercial production of electricity.

Ans.

Following are the common means for the large-scale production of electricity:

Thermal power Plant.
Hydroelectric power plant
Nuclear power plant

Q24. Describe briefly the Thermal power station.

Ans.

In this type of electric power station, steam is created by heating water with the help of coal, oil, gas, waste materials, etc. is the source of mechanical energy that runs turbines. These running turbines run the generator. In Pakistan, such thermal power plants work in Hyderabad, Jamshoro, Multan, Quetta etc. This method of generating electricity is expensive. Burning coal, oil, gas, and waste material also increases air pollution in these cities.

Q26. Describe briefly the nuclear power plant.

Ans.

This type of electric power plant works like a thermal electric power plant. However, in such plants, the nuclear fuel like uranium or plutonium is used to generate electricity. In such a method, heat energy is produced by nuclear fission. In this process, energy is released from the nuclei of the atoms when they are broken. In our country, such a Nuclear Power Plant is working in Hub near Karachi. It is also known as 'KANNUP' (Karachi Nuclear Power Plant). Chashma Nuclear Power Plant has also been built to meet the demand of electricity in our country.

Q27. Describe briefly the hydroelectric power plant.

Ans.

In hydroelectric power plants flowing water from dams is used as a source of mechanical energy. Water is stored in a lake or reservoir at a height behind the dam. When water is allowed to fall from a height, it gains kinetic energy. The kinetic energy of the moving water flowing through the tunnels is used to turn the blades of turbines. These are attached to the lower end of the axle of the electric generator. This generator produces electricity which is supplied to the consumers through power lines. This type of plant is low-cost and produces negligible pollution. In Pakistan, such types of power stations are working at Tarbela, Warsak, Mangla, and Ghazi Brotha.

Q28. What are conductors and insulators?

Differentiate between conductors and insulators.

Ans,

Conductors: Substances that allow electricity to flow through them are called conductors. e.g. Silver, gold, aluminium, brass, steel, water, acids/base/salts, human body, etc

Insulator: Substances that do not allow electricity to flow through them are called insulators. e.g. Cotten, rubber, plastic, mica, wood, paper, glass, dry air, etc.

Q29. What is an electric circuit? How it works?

Ans.

The conducting path through the bulb, wires, and cell is called an electrical circuit.

A cell or battery can make electrons move through a conducting material, such as a piece of copper wire or some other metal, between its two terminals. Then a chemical reaction inside the battery will push electrons out of the negative (-) terminal and route to the positive (+) terminal.

The cell in the picture below is being used to light up a torch bulb. As the electrons pass through the thin wire or filament inside the bulb, they make it heat up so that it glows.

Q30. Write the essential Components of an Electric Circuit.

Ans.

The essential components or elements of an electric circuit are given below:

Electric sources (cell, battery etc.)
Electric appliances (a device for using the electric current).
Conductor (for providing a path for the flow of current).
Switch or a key (for opening and closing the circuit).

Q31. What is an Open and Closed Circuit?

Ans.

Open Circuit:

A circuit where the path of the flow of current is incomplete or "opened" at some point is called an open circuit. In an open circuit, no current is drawn from the cell. In a circuit, if the switch is opened or the wire is cut, the flow current is stopped, and we say that the circuit is open or incomplete.

Closed Circuit:

A circuit where the path of the flow of current is complete or "closed" is called a closed circuit. In a closed circuit, current is drawn from the cell.

If the switch is closed, a current flows the flow of current is indicated by the glowing of an electric bulb included in the circuit. Thus, if a current is drawn from the cell, the circuit is said to be closed or complete.

Q32. Write some symbols of Circuit Components.

Ans.

The conventional symbols for some common elements of electric circuits are given below.

Q33. What is a Circuit Diagram?

Ans.

A circuit diagram indicates the arrangement of different circuit elements with their conventional symbols.

Some of the most commonly used circuit diagrams are given below:

Q34. What is an electric fuse? Why is fuse a necessary component of any electric circuit?

Ans.

An electric fuse is just a short piece of wire 'low melting point. It is made of an alloy 'tin and lead because they have very low melting points. It is connected in series in an electrical appliance or in the mains. When e the current exceeds a particular value (due to defective wiring or short circuit), the fuse melts and the circuit is broken. Safety fuses, fuses prevent the accidental flow of large of current, which may damage electrical appliances or cause a fire.

Q35. Why earthing or grounding an electrical appliance important?

Ans.

Earthing or grounding an electrical appliance is as important as the provision of a safety fuse. All electrical appliances are usually provided with an earth or ground wire which is connected to the frame of the appliances as shown in Fig. In case of contact of the metal casing of the appliance with a live wire, the current passes through the earth wire and goes to the earth. In this way, a dangerous current is removed from the casing that can harm users.

Q36. Briefly some hazards of electricity.

Ans.

Electricity could be dangerous and may cause electric shock, death, accident and fire, if certain precautions and safety measures are not observed.

Some of the causes which make electricity hazardous are:

1. Loose connections

2. Defective and broken switches

3. Improper wiring

4. Poor insulation of wiring

5. Over loading

6. Improper earthing

Q37. What Safety Measures should we take while handling electricity?

Ans.

While handling any electrical circuit or any electrical appliance, the following precautions must be observed.

The wires used in a circuit should be of good quality, proper amperage, and insulation.
All connections at plugs, switches, sockets, and junctions must be covered with insulating tape.
Defective and damaged plugs, switches, and sockets must be immediately replaced.
Do not insert your fingers in the socket fixed on the walls of your room.
The tools used for repairing such as testers, screwdrivers, pliers etc. should have proper insulation.
The main switch should be immediately switched off in case of fire or short circuit. All appliances must be properly earthed.
Always use a three-pin plug.
Use a safety fuse of proper rating and material.

Q38. What is electric power? Write and define the unit of power.

Ans.

The electrical energy can be converted into heat, light, sound and mechanical energy in the household appliances. The rate at which the electrical energy is transferred from one form to another is called electric power.

Power is measured in watts. One watt (w) is the power of a machine or device where energy is being used up or produced at a rate of one joule per second. (1 watt = 1 joule/sec).

Q39. How can we calculate the cost of electricity in our homes?

Ans.

Electric power is calculated using the following formula:

Power (P) in watts = Current (1) in amperes x Voltage (V) in volts

P = IV

How much electricity a particular appliance will use will depend upon its power (rate of Using energy) and the length of time that is used. The unit in which this electricity usage is measured is the Kilowatt hour-which means using one kilowatt (kW) of power for one hour. A kilowatt is 1000 W. So, a kWh of electricity would also be used if we had an appliance of one watt running for 1000 hours, one of 500 W running for 2 hours etc. If we use an appliance of 1000 watt power for 1 hour, the electrical energy used up is

If we use an appliance of 100 watt for 10 hour, the electrical energy used up is

The kilowatt-hour is the electrical unit which is used by the electricity corporation to charge our consumption of electricity or the total amount of units (kWh) of electricity we have used.

Q40. What are semiconductors? Write its importance?

Ans.

The materials that are mid-way between conductors and insulators in their ability to pass electricity. They are known as semiconductors. Semiconductors are extremely useful in the field of electronics. Without them, it would not have been possible to produce small transistors, radios, calculators, computers, and several other electronic equipment.

Exercise Q/A

Answer these questions.

1. What is an electric current?

Ans.

The quantity of electricity that flows in one second across a cross-section of the wire. It is represented by the symbol 'I'

Mathematically:

If current 'I' flows in a wire for a time 't,' the quantity of electricity or charge 'Q' transferred: is represented as:

Q = It or

I = Q/t

The S.I unit of the electric current 'I' is the ampere (A), of charge Q is Coulomb (C) and time 't' is second (s).

Ampere:

if 1 Coulomb of charge flows through a conductor in 1 second, the amount of current will be 1 ampere.

2. What is the direction of flow of electrons?

Ans

In a metallic wire, the electrons flow in the direction from the negative end to the positive end of the wire. We call this current as the electronic current.

3. Who designed the first electric cell and when? Describe the construction of this cell.

Ans.

The two Italian scientists named Luigi Galvani and Alessandro Volta in 1799 made the first cell that was named as Voltaic cell (Galvanic cell). It is also called a simple cell.

Construction: It consisted of a copper plate and a zinc plate dipped in dilute sulphuric acid taken in a glass vessel. When the two plates were connected by a copper wire with a bulb in between, the bulb glowed showing the flow of electric current. It was found that electrons flowed in the wire from the zinc to the copper plate.

4. Draw neat labelled diagrams of Voltaic cell, Daniel cell and Dry cell.

Ans.

Voltaic cell (Galvanic cell)

Daniel cell

Dry cell.

5. Write two advantages of a dry cell over a wet cell.

Ans.

1. Dry cells are compact in size.

2. It is light and easy to carry from place to place.

6. What is the main difference between a primary cell and a secondary cell?

Ans.

Primary cells are not rechargeable. They cannot be reused when they are discharged. They have to be replaced with new cells. Such cells are called primary cells.

Secondary cell can be recharged and used again and again after recharging. Such rechargeable cells are called secondary cells, storage cells or accumulators.

7. Define a generator. Explain the principle on which a generator works.

Ans.

A Generator is a device for producing electric currents for use in home and industry. It converts mechanical energy into electrical energy.

A Generator in power houses and dams consists of a magnet moving rapidly within a coil of copper wire. The current is drawn from the two ends of the coil. The greater the speed of rotation the greater the current produced.

8. Write a short note on storage cells and their types.

Ans.

storage (Secondary) cell can be recharged and used again and again after recharging. Such rechargeable cells are called secondary cells, storage cells or accumulators.

There are two types of storage cells (accumulators):

1. Lead-acid accumulator.

2. Nickel-cadmium alkaline accumulator.

9. Define conductors and insulators? Give two examples of each.

Ans,

Conductors: Substances that allow electricity to flow through them are called conductors. e.g. Silver, gold, aluminium, brass, steel, water, acids/base/salts, human body, etc

Insulator: Substances that do not allow electricity to flow through them are called insulators. e.g. Cotten, rubber, plastic, mica, wood, paper, glass, dry air, etc.

10. What is an electric circuit? What is meant by open and closed circuit?

Ans.

The conducting path through the bulb, wires, and cell is called an electrical circuit.

The cell in the picture below is being used to light up a torch bulb. As the electrons pass through the thin wire or filament inside the bulb, they make it heat up so that it glows.

Open Circuit:

Closed Circuit:

A circuit where the path of the flow of current is complete or "closed" is called a closed circuit. In a closed circuit, current is drawn from the cell.

11. What is a fuse and what substance is used to make it? Write four safety measures to be taken while working with live electric lines.

Ans.

Electricity could be dangerous and may cause electric shock, death, accident and fire, if certain precautions and safety measures are not observed.

All connections at plugs, switches, sockets, and junctions must be covered with insulating tape.
Defective and damaged plugs, switches, and sockets must be immediately replaced.
Do not insert your fingers in the socket fixed on the walls of your room.
The tools used for repairing such as testers, screwdrivers, pliers etc. should have proper insulation.

12. Briefly explain the working of a hydro-electric power plant.

Ans.

B. Fill in the following blanks:

Rechargeable cells are also called secondary cells, storage cells or accumulators.
Electricity is the flow of electrons.
Primary cells are not rechargeable cells
Secondary cells are also called storage cells or accumulators.
Watt is the unit of electrical power.
In a dry cell chemical energy is converted into electrical energy.
Energy transfer is measured in joules (J)
Electrons flow from negative to positive terminal of the battery.
Uninterruptible power supply (UPS) is a device for giving a continuous supply of current.
The unit of electricity usage is kilowatt-hours.

C. Decide whether the following statements are TRUE/FALSE:

1. A dry cell is an improved form of the Leclanché cell. TRUE

2. Wood and rubber are insulators of electricity. TRUE

3. Light energy is converted into electrical energy in a primary cell. FALSE

4. An alloy having high a melting point is used to make fuse wire. FALSE

5. A thermal power plant is more costly than a hydro-electric power plant. TRUE

D. Choose the most appropriate answer:

1. Which of the following cells are recharged when run down?

a. Dry cell

b. Voltaic cell

c. Lead storage cell

d. Daniel cell

2. A safety fuse must be connected in series with:

a. Live wire

b. Earth wire

c. Neutral wire

d. All

3. The voltage of a single lead accumulator cell is:

a. 0.5 V

b. 1 V

c. 1.5 V

d. 2 V

4. The potential difference is measured by a:

a. Voltameter

b. Altimeter

c. Ammeter

d. None

5. In dry cells, the chemical energy is transformed into:

a. Potential energy

b. Kinetic energy

c. Light energy

d. Electrical energy