Chapter 4: Current Electricity and Magnetism
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 4: Current Electricity and Magnetism
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 4: "Current Electricity and Magnetism." This chapter covers fundamental concepts of electric charge, current, potential difference, the working of various electric cells, and the fascinating relationship between electricity and magnetism, including applications like the electric bell.
Important Questions and Answers
Q1: Define 'Electric Charge' and state its SI unit.
Answer: Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types: positive and negative. The SI unit of electric charge is Coulomb (C).
Q2: What is 'Electric Current'? State its formula and SI unit.
Answer: Electric current is the rate of flow of electric charge.
Formula: Electric Current (I) = Charge (Q) / Time (t)
The SI unit of electric current is Ampere (A).
Q3: Define 'Potential Difference' and state its SI unit.
Answer: Potential difference (or voltage) is the work done to move a unit positive charge from one point to another in an electric field. It is the driving force for electric current.
Formula: Potential Difference (V) = Work Done (W) / Charge (Q)
The SI unit of potential difference is Volt (V).
Q4: Explain the analogy between water flow and electric current.
Answer: Just as water flows from a higher level to a lower level due to a difference in gravitational potential energy, electric current flows from a point of higher electric potential to a point of lower electric potential due to a potential difference. The potential difference acts like the 'pressure' that pushes the charge.
Q5: What is an 'Electric Cell'? What energy conversion takes place in it?
Answer: An electric cell is a device that converts chemical energy into electrical energy. It provides a potential difference that drives electric current in a circuit.
Q6: Describe the main components of a Dry Cell.
Answer: A dry cell typically consists of:
- A central carbon rod (positive terminal).
- A zinc casing (negative terminal).
- A paste of ammonium chloride (NH₄Cl) and zinc chloride (ZnCl₂) as the electrolyte.
- A mixture of manganese dioxide (MnO₂) and carbon powder packed around the carbon rod as a depolarizer.
Q7: What is a Lead-Acid Cell? Where is it commonly used?
Answer: A lead-acid cell is a type of rechargeable battery (storage battery). It is commonly used in automobiles (car batteries) and in Uninterruptible Power Supply (UPS) systems.
Q8: Name the positive and negative plates and the electrolyte in a Lead-Acid Cell.
Answer:
- Positive plate: Lead dioxide (PbO₂)
- Negative plate: Pure lead (Pb)
- Electrolyte: Dilute sulfuric acid (H₂SO₄)
Q9: What is the 'Magnetic Effect of Electric Current'?
Answer: The magnetic effect of electric current is the phenomenon where an electric current flowing through a conductor produces a magnetic field around it. This effect was discovered by Hans Christian Ørsted.
Q10: What is an 'Electromagnet'? How is it different from a permanent magnet?
Answer: An electromagnet is a temporary magnet created by passing electric current through a coil of wire wound around a soft iron core. Unlike a permanent magnet, an electromagnet's magnetism can be switched on or off, and its strength can be varied by changing the current or number of turns.
Q11: List three factors that affect the strength of an electromagnet.
Answer: The strength of an electromagnet depends on:
- The number of turns in the coil (more turns, stronger magnet).
- The amount of current flowing through the coil (more current, stronger magnet).
- The nature of the core material (a soft iron core significantly increases strength).
Q12: What is a 'Solenoid'? How does it behave when current passes through it?
Answer: A solenoid is a coil of insulated wire wound in the shape of a helix (like a spring). When electric current passes through a solenoid, it behaves like a bar magnet, producing a uniform magnetic field inside it.
Q13: Explain the working principle of an Electric Bell.
Answer: An electric bell works on the magnetic effect of electric current. When current flows, the electromagnet attracts an armature, causing a hammer to strike a gong. This movement breaks the circuit, demagnetizing the electromagnet, and the armature springs back, re-establishing contact. This cycle repeats, producing continuous ringing.
Q14: What is the role of the contact screw in an electric bell?
Answer: The contact screw in an electric bell makes and breaks the circuit. When the armature is attracted by the electromagnet, it moves away from the contact screw, breaking the circuit. When the electromagnet loses its magnetism, the armature springs back and touches the contact screw, completing the circuit again.
Q15: What is the function of manganese dioxide in a dry cell?
Answer: Manganese dioxide (MnO₂) acts as a depolarizer in a dry cell. It oxidizes the hydrogen gas produced at the carbon rod (positive terminal) during the cell's operation, preventing its accumulation, which would otherwise reduce the cell's efficiency and potential difference.
Q16: Why is a soft iron core used in an electromagnet?
Answer: A soft iron core is used in an electromagnet because it is easily magnetized when current flows and easily demagnetized when the current is switched off. This property makes it ideal for creating temporary magnets whose strength can be controlled.
Q17: What is the direction of conventional current flow?
Answer: By convention, the direction of electric current is taken as the direction of flow of positive charge, i.e., from the positive terminal to the negative terminal outside the cell.
Q18: What is the potential difference of a single lead-acid cell?
Answer: The potential difference of a single lead-acid cell is approximately 2 Volts (V).
Q19: Give an example of a device that uses an electromagnet.
Answer: An electric bell, electric crane, and electric motor are examples of devices that use electromagnets.
Q20: What is the role of the electrolyte in an electric cell?
Answer: The electrolyte in an electric cell is a substance that conducts electricity through the movement of ions. It facilitates the chemical reactions that produce electric charge and maintain the potential difference between the terminals.
Q21: How do like charges interact?
Answer: Like charges repel each other (e.g., positive repels positive, negative repels negative).
Q22: How do unlike charges interact?
Answer: Unlike charges attract each other (e.g., positive attracts negative).
Q23: What is the main difference between a dry cell and a lead-acid cell?
Answer: The main difference is that a dry cell is a primary cell (non-rechargeable), while a lead-acid cell is a secondary cell (rechargeable). Also, a dry cell uses a paste electrolyte, while a lead-acid cell uses a liquid electrolyte (dilute sulfuric acid).
Q24: What happens to the magnetic field of an electromagnet when the current is switched off?
Answer: When the current flowing through an electromagnet is switched off, the soft iron core immediately loses its magnetism, and the magnetic field disappears. This temporary nature is a key characteristic of electromagnets.
Q25: Why is it important to have a potential difference for current to flow?
Answer: A potential difference is essential because it provides the 'push' or 'force' that drives the free electrons in a conductor from a point of higher potential to a point of lower potential, thereby constituting an electric current. Without a potential difference, there would be no net flow of charge.
Exercise Solutions (From Screenshot Page 27)
Q1: Fill in the blanks.
- The SI unit of electric charge is Coulomb (C).
- The SI unit of electric current is Ampere (A).
- The SI unit of potential difference is Volt (V).
- The positive terminal of a dry cell is a carbon rod.
- The negative terminal of a dry cell is a zinc casing.
- A lead-acid cell uses dilute sulfuric acid as an electrolyte.
- An electromagnet is a temporary magnet.
- The strength of an electromagnet depends on the number of turns in the coil and the amount of current flowing through it.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Electric current - Ampere
- Electric charge - Coulomb
- Potential difference - Volt
- Dry cell - Non-rechargeable
- Lead-acid cell - Rechargeable
- Electromagnet - Temporary magnet
Q3: Give scientific reasons.
- A dry cell is called a dry cell.
Reason: A dry cell is called a dry cell because it does not use a liquid electrolyte. Instead, it contains a moist paste of chemicals (like ammonium chloride and zinc chloride) that acts as the electrolyte. This makes the cell more portable and less prone to leakage compared to cells with liquid electrolytes. - A lead-acid cell is called a storage battery.
Reason: A lead-acid cell is called a storage battery because it is a secondary cell, meaning it is rechargeable. The chemical reactions that produce electricity during discharge can be reversed by passing an external current through it, thereby storing electrical energy as chemical energy for later use. - An electromagnet is a temporary magnet.
Reason: An electromagnet is a temporary magnet because its magnetic properties are present only as long as electric current flows through its coil. When the current is switched off, the soft iron core quickly loses its magnetism. This temporary nature allows for controlled magnetic action, which is essential in many applications. - The strength of an electromagnet can be increased.
Reason: The strength of an electromagnet can be increased by:- Increasing the number of turns in the coil: More turns mean a stronger magnetic field.
- Increasing the amount of current flowing through the coil: A higher current produces a stronger magnetic field.
- Using a soft iron core: Soft iron is easily magnetized and enhances the magnetic field significantly.
Q4: Answer the following questions.
- Explain the structure and working of a dry cell.
Answer: Structure: A dry cell has a central carbon rod (positive terminal) surrounded by a paste of manganese dioxide and carbon powder. This is encased in a paste of ammonium chloride and zinc chloride, which acts as the electrolyte. The outer casing is made of zinc, which serves as the negative terminal. Working: Chemical reactions occur between the zinc casing, the carbon rod, and the electrolyte paste. These reactions cause a potential difference to develop between the carbon rod and the zinc casing. When the cell is connected to an external circuit, this potential difference drives electrons from the negative zinc terminal to the positive carbon terminal, producing an electric current. - Explain the structure and working of a lead-acid cell.
Answer: Structure: A lead-acid cell consists of a positive plate made of lead dioxide (PbO₂) and a negative plate made of pure lead (Pb). Both plates are immersed in a dilute solution of sulfuric acid (H₂SO₄), which acts as the electrolyte. Multiple such cells are connected in series to form a battery. Working: During discharge, chemical reactions convert lead and lead dioxide into lead sulfate, releasing electrons and producing electricity. During charging, an external electrical energy source reverses these chemical reactions, converting lead sulfate back into lead and lead dioxide, thereby storing energy. - Explain the magnetic effect of electric current.
Answer: The magnetic effect of electric current is the phenomenon where an electric current flowing through a conductor produces a magnetic field around it. This means that a current-carrying wire behaves like a magnet. The direction of the magnetic field depends on the direction of the current, and its strength depends on the magnitude of the current and the distance from the conductor. This effect is the basis for electromagnets, electric motors, and many other electrical devices. - Explain the working of an electric bell with a diagram.
Answer: (Diagram explanation - as I cannot draw, I will describe the components and their interaction.) Components: An electric bell consists of an electromagnet, an armature (a soft iron strip) with a hammer attached, a gong, a contact screw, and a battery/power source with a switch. Working:- When the switch is pressed, current flows from the battery through the electromagnet.
- The electromagnet gets magnetized and attracts the soft iron armature towards it.
- As the armature moves, the hammer attached to it strikes the gong, producing a ringing sound.
- Simultaneously, the movement of the armature breaks the contact with the contact screw.
- This breaks the circuit, causing the electromagnet to lose its magnetism.
- Without the magnetic force, the armature springs back to its original position, re-establishing contact with the contact screw.
- The circuit is completed again, and the process repeats. This rapid making and breaking of the circuit causes the hammer to continuously strike the gong as long as the switch is pressed.
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 4: Current Electricity and Magnetism.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.