Current Electricity and Magnetism | Class 8 | Science | Chapter 4 | Maharashtra State Board
Learn about the fundamentals of current electricity and magnetism in this lesson, exploring concepts such as electrical circuits, dry cells, electromagnets, and the workings of electric bells. Understand how potential differences cause electrons to flow and how electromagnetism is utilized in everyday devices .
Questions & Answers
1. Write proper words from the following group of words in the blanks and rewrite the completed sentences:
(magnetism, 4.5V, 3.0V, gravitational attraction, potential difference, potential, higher, lower, 0V)
a. Water in the waterfall flows from a higher level to the lower level because of …………… .
Ans.
Water in the waterfall flows from a higher level to the lower level because of gravitational attraction.
b. In an electric circuit, electrons flow from a point of potential to the point of ……….. potential.
Ans.
In an electric circuit, electrons flow from a point of lower potential to the point of higher potential.
c. The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the …………. of the cell.
Ans.
The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the potential difference of the cell.
d. Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be ……………… V.
Ans.
Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be 4.5 V.
e. An electric current flowing in a wire creates ………………… around the wire.
Ans.
An electric current flowing in a wire creates magnetism around the wire.
2. A battery is to be formed by joining 3 dry cells with connecting wires. Show how you will connect the wires by drawing a diagram.
Ans.
To connect three dry cells in series to form a battery, you need to connect the positive terminal of one cell to the negative terminal of the next cell. Here is a simple diagram to illustrate how to connect the wires:
This setup will increase the voltage output to the sum of the individual voltages of the cells while keeping the current capacity the same as a single cell
3. In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?
Ans.
If a bulb connected to a battery of two cells in series is not glowing, you can perform the following quick tests:
Check Connections: Ensure all connections are secure and correctly oriented, with the positive terminal of one cell connected to the negative terminal of the next.
Test Battery Voltage: Use a multimeter to check the total voltage of the battery. It should equal the sum of the two cells' voltages.
Inspect the Bulb: Ensure the bulb filament is intact and properly seated in its socket.
Check Wires: Look for loose, broken, or corroded wires that might disrupt the circuit.
Test with Known Good Battery: Connect the bulb to a different, working battery to see if it lights up, which can indicate if the issue is with the bulb or the original battery setup.
Examine the Switch: If there's a switch, ensure it is on and functioning correctly.
These steps will help identify and resolve the issue quickly.
4. Electric cells having potential difference 2V each have been connected in the form of a battery. What will be the total potential difference of the battery in both cases?
Ans.
(i)
When electric cells with a potential difference of 2V each are connected:
Series Connection: The total potential difference is the sum of all cell voltages. For example, three 2V cells in series give:
Total Voltage = 2V + 2V + 2V = 6V
(ii)
Parallel Connection: The total potential difference is the same as one cell. For example, three 2V cells in parallel give:
Total Voltage = 2V
In summary, series increases total voltage, while parallel keeps it the same.
5. Describe the construction, working and usefulness of a dry cell, with the help of a diagram.
Ans.
Dry Cell
A dry cell is a type of battery commonly used in portable electronic devices.
Diagram:
Construction:
Outer Casing: Zinc can (negative terminal).
Electrolyte Paste: Ammonium chloride and zinc chloride.
Carbon Rod: Center positive terminal.
Manganese Dioxide: Surrounds the carbon rod as a depolarizer.
Separator: Between zinc and manganese dioxide to prevent direct contact.
Working:
Chemical Reaction: When connected to a circuit, zinc reacts with the electrolyte, releasing electrons.
Electron Flow: Electrons move from the zinc (negative) to the carbon rod (positive) through the circuit, producing electrical energy.
Depolarization: Manganese dioxide prevents gas buildup by reacting with hydrogen.
Usefulness:
Portable: Compact and easy to transport.
Maintenance-Free: Does not leak or spill.
Variety: Available in different sizes (e.g., AA, AAA) for various devices.
6. Describe the construction and working of an electric bell with the help of a diagram.
Ans.
Construction:
Electromagnet: A coil of wire wound around an iron core. When current flows through the coil, it creates a magnetic field, turning the iron core into a magnet.
Iron Strip with Striker: Positioned near the electromagnet, this strip carries a hammer or striker that can hit the bell to produce sound.
Contact Screw: This screw touches the iron strip, allowing the electric circuit to be completed.
Battery and Switch: Provides the electric current to the circuit.
Working:
When the switch is pressed, current flows through the circuit, energizing the electromagnet.
The electromagnet attracts the iron strip, causing the striker to hit the bell and produce sound.
As the iron strip moves towards the electromagnet, it breaks contact with the screw, cutting off the current.
The electromagnet loses its magnetism, and the iron strip returns to its original position, restoring contact with the screw.
This cycle repeats rapidly, causing the bell to ring continuously.
E X T R A
1. Rewrite the sentences after filling the blanks
1. The SI unit of electric charge is the ……………… .
Ans.
The SI unit of electric charge is the coulomb.
2. The SI unit of electric current is the ……………… .
Ans.
The SI unit of electric current is the ampere.
3. The SI unit of electric potential is the ………………. .
Ans.
The SI unit of electric potential is the volt.
4. 1 ampere = 1 coulomb per …………… .
Ans.
1 ampere = 1 coulomb per second.
5. When a glass rod is rubbed on a silk cloth, the glass rod acquires …………… charge.
Ans.
When a glass rod is rubbed on a silk cloth, the glass rod acquires positive charge.
2. Rewrite the following statements selecting the correct options:
1. The potential difference between the two electrodes of the lead-acid cell is nearly ………………. .
(a) 1 V (b) 1.2 V (c) 1.5 V (d) 2 V
Ans.
The potential difference between the two electrodes of the lead-acid cell is nearly 2 V.
2. The Ni-Cd cell delivers a potential difference of …………….. .
(a) 1 V (b) 1.2 V (c) 1.5 V (d) 2 V
Ans.
The Ni-Cd cell delivers a potential difference of 1.2 V
3. …………….. is a scalar quantity.
(a) Force (b) Acceleration (c) Velocity (d) Electric current
Ans.
Electric current is a scalar quantity.
4. The working of an electric bell is based on ………….. .
(a) the heating effect of electric current (b) the chemical effect of electric current (c) the magnetic effect of electric current (d) the optical effect of electric current
Ans.
The working of an electric bell is based on the magnetic effect of electric current.
5. The coulomb is the SI unit of electric ………….. .
(a) current (b) resistance (c) potential (d) charge
Ans.
The coulomb is the SI unit of electric charge
3. State whether the following statements are True or False:
1. The SI unit of electric potential is the ampere.
Ans.
False. (The SI unit of electric potential is the volt. OR The SI unit of electric current is the ampere.)
2. In the external circuit, the conventional current flows from the positive terminal of the cell to the negative terminal of the cell.
Ans.
True.
3. Very small current flows when lightning occurs.
Ans.
False. (A large current flows when lightning occurs.)
4. Sensation is felt by us due to a microscopically small current flowing to the brain.
Ans.
True.
5. In a car battery, a current is produced by the flow of both negatively and positively charged particles.
Ans.
True.
4. Identify the odd term:
1. Pressure exerted by a liquid, Electric current, Electric potential difference, Buoyant force.
Ans.
Buoyant force. It is a vector quantity. Others are scalar quantities.
2. Electric bulb, Electric heater, Electric bell, Electric iron.
Ans.
Electric bell. Its working is based on the magnetic effect of electric current. The working of other devices is based on the heating effect of electric current.
5. Answer the following questions in one sentence:
1. State the relation among the SI units of electric current, electric charge and time.
Ans.
1 ampere = 1 coulomb per second.
2. Name the positive terminal of the dry cell.
Ans.
The graphite rod at the centre is the positive terminal of the dry cell.
3. What constitutes an electric current in a metal?
Ans.
A continuous flow of electrons constitutes an electric current in a metal.
4. What is a battery?
Ans.
A group of two or more cells connected in series to obtain more potential difference is called a battery.
5. What is a solar cell?
Ans.
A solar cell is a device that converts solar energy into electrical energy.
6. State one characteristic of the Ni-Cd cell.
Ans.
The Ni-Cd cell is rechargeable.
7. Give one example in which the magnetic effect of electric current is used.
Ans.
The working of an electric bell is based on the magnetic effect of electric current.
6. Answer the following questions:
1. When do we get current electricity?
Ans.
We get current electricity when the electrons in an electrical conductor are made to flow. This flow occurs due to the potential difference between two points in a circuit. Electrons move from a point of lower potential to a point of higher potential, generating an electric current as they move through the conductor.
2. Explain the concept of electrostatic potential (electric potential).
Ans.
Electrostatic potential is the amount of electric potential energy per unit charge at a point in an electric field. It determines the direction in which a positive charge will move. A positive charge naturally moves from a point of higher electrostatic potential to a point of lower electrostatic potential, similar to how water flows from a higher level to a lower level.
3. What is the SI unit of electric potential?
Ans.
The SI unit of electric potential is the volt (V).
Note : This unit is named in honour of Alessandro Volta (1745-1827), Italian physicist, the inventor of the electric battery.
4. What is potential difference?
Ans.
Potential difference is the difference in electric potential between two points in a circuit. It determines the direction of electric current, causing it to flow from the point of higher potential to the point of lower potential. The potential difference is what drives the movement of electrons through a conductor.
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