Students can keep Class 8 Science Extra Questions and Class 8th Curiosity Chapter 4 Electricity: Magnetic and Heating Effects Important Extra Question Answer handy for quick reference during exams.
Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects Extra Questions
Class 8 Science Chapter 4 Extra Questions on Electricity: Magnetic and Heating Effects
Electricity: Magnetic and Heating Effects Class 8 Very Short Question Answer
Question 1.
What is an electrolyte?
Answer:
The liquid or paste form of chemicals inside an electric cell, which are used to give out electricity due to their reaction with the electrodes, is called an electrolyte.
Question 2.
What is an electrode?
Answer:
Metallic plates or carbon rods that are part of the electric cell and act as positive and negative terminals of the cell are called electrodes.
Question 3.
Define magnetic field.
Answer:
The region around a magnet where its magnetic effect can be felt is called the magnetic field of the magnet.
Question 4.
Name three types of portable sources of electricity.
Answer:
Voltaic cell, dry cell, and portable cell.
Question 5.
Name three variables on which the heating effect of electric current depends.
Answer:
- The length of the conducting wire.
- The nature of the metal used in conducting wire.
- The strength of the electric current.
Question 6.
Name three variables on which the magnetic effect of electric current depends.
Answer:
- The number of turns in the coil of conducting wire.
- The strength of the electric current.
- Presence of an iron nail/rod inside the coil of the conducting wire.
- The nature of the metal used in the conducting wire of which the coil is made.
Electricity: Magnetic and Heating Effects Class 8 Short Question Answer
Question 1.
What variables increase or decrease the magnetic effect of electric current?
Answer:
When we increase the current flowing through the coil (use a battery of two or more cells in place of one cell), the magnetic effect becomes stronger. When the number of turns in the coil is increased (the length of the conducting wire increases), the magnetic effect becomes stronger. When a conducting wire is coiled around a rolled sheet of paper and current is passed through the coil, it creates a magnetic field that becomes stronger when we insert an iron nail inside the sheet of paper roll.
Question 2.
What variables increase or decrease the heating effect of electric current?
Answer:
The heating effect of electric current depends on the nature of the metal used as a conductor. Nichrome produces more heat than copper. When the length of the wire in the circuit is increased, the heating effect increases. Also, the wires are kept thin to produce more heat. The amount of heat produced by the electric current flowing through the conductors also depends on the magnitude of the electric current and the duration of time for which the current is passed. A battery of two or more cells produces more heat when current is passed through a nichrome wire compared with a single cell. Also, the wire will be heated more if an electric current is passed for 1 minute compared to the passage of current for 30 seconds.
Question 3.
Explain how the discovery of the Voltaic cell was made?
Answer:
Two Italian scientists, Alessandro Volta and Luigi Galvani, are known to have discovered the Voltaic cell, also called the Galvanic cell. Galvani noticed a dead frog’s leg kicking when touched with two different metals, copper and iron. He believed the electricity that came from the frog itself stimulated muscular motion. But Volta thought that the electricity came from the metals. He tested this using saltwater-soaked paper in place of a frog’s leg and got the electric current. The two scientists thus agreed that the combination of metals and liquid generated an electric current. Thus, the first battery was invented.
Question 4.
How is the dry cell considered convenient over the Voltaic cell?
Answer:
A voltaic cell, also known as a Galvanic cell, uses a liquid electrolyte (a weak acid or a salt solution). This makes the battery inconvenient as a portable source of electricity. A dry cell uses the electrolyte in the form of a thick, moist paste. This makes the dry cell more convenient as a portable source of electricity.
Question 5.
In what way the rechargeable cells better than dry cells?
Answer:
The dry cells are considered single-use cells. Once the electric current given out by the dry cell weakens, it has to be discarded and disposed of. The rechargeable cells or batteries are used multiple times after recharging. The rechargeable cells thus run for longer periods and are preferred over dry cells. However, the rechargeable cells cannot last forever. These also become weak after being used many times, and need to be discarded. Mobile phone batteries need to be changed over time.
Electricity: Magnetic and Heating Effects Class 8 Long Question Answer
Question 1.
Explain the Structure of a Voltaic cell.
Answer:
A Voltaic cell is a plastic/glass container that contains two metal plates of two different materials and a liquid called an electrolyte. The metal plates are called electrodes. One of the plates acts as a positive terminal and the other as a negative terminal. This depends on the nature of the metal used. Copper is usually the positive terminal, and zinc is the negative terminal in a combination of copper and zinc plates.
The liquid is put in a glass or plastic container, and the metal plates are partly dipped in the electrolyte. The electrolyte is usually a weak acid or a salt solution. A chemical reaction between the plates and the electrolyte produces electricity. When the circuit is connected, the electric current starts flowing from the positive terminal through the circuit to the negative terminal. Over time, the chemicals get used up, and the cell stops working. It is called a “Dead” cell that cannot supply any more electricity.
Question 2.
Explain the structure of a dry cell.
Answer:
A dry cell consists of a zinc container, which acts as a negative terminal, and a carbon rod at the centre with a metal cap at the top end. The metal cap over the carbon rod acts as a positive terminal.
The zinc container is filled with a thick, moist paste electrolyte that surrounds the carbon rod. The chemical reaction between the electrolyte and the carbon rod produces electricity, and the current flows from the positive terminal of the cell through a circuit to the negative terminal. The dry cell is a single-use cell. It has to be disposed of once it is used up.
Question 3.
What is the magnetic effect of an electric current? Explain with examples.
Answer:
When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current. The magnetic field disappears when the current stops flowing. The magnets formed by passing an electric current through conducting wires are called electromagnets. Examples of electromagnets used in our daily life are electric doorbells, magnets used in loudspeakers, electric motors, fans, and many toys. Electromagnets of very high strength are used in industry to lift heavy loads of iron scrap to feed these into furnaces or for sorting heavy scrap items.
Question 4.
What is the heating effect of electric current? Explain with examples.
Answer:
When an electric current passes through a conductor, it gets heated. This warming is called the heating effect of electric current. When electric current flows through any conductor, it faces some opposition or resistance to its flow. This resistance converts some electric energy to heat energy. Different conductors have different resistances. Nichrome offers more resistance to the flow of current compared to copper. Thus, current flowing through a nichrome wire has a greater heating effect than copper wire. Nichrome is a material suitable for making heating elements. When electric current passes through wires of nichrome and other such materials, the wires become hot. The amount of heat produced in a wire depends on
- The nature of the material
- Thickness of wire (thin wires offer more resistance and thus more heat) and
- The length of the wire (Resistance offered and heat generated are directly proportional to the length of the wire).
While making elements of heating appliances from wires of these materials, the length of the wires is increased by making coils of the wires. Also, the wires are kept thin to produce more heat. The amount of heat produced by the electric current flowing through the conductors also depends on the magnitude of the electric current and the duration of time for which the current is passed.
A battery of two or more cells produces more heat when current is passed through a nichrome wire compared with a single cell. Also, the wire will be heated more if an electric current is passed for 1 minute compared to the passage of current for 30 seconds. Examples of daily use of heating effect in our houses are: room heaters, water heaters, electric stoves, electric kettles, electric irons, hair dryers, etc. In industry high high-temperature induction coils and furnaces are based on the heating effect of electric current.
Electricity: Magnetic and Heating Effects Class 8 Case Based Questions
I. An electric bell has an electromagnet fitted in its circuit. One or two electromagnets made of wires coiled on iron pieces are fixed on the bell case. When the bell switch is pressed, the current flows through these coils, and these become magnets. These magnets attract a metal strip, which has a hammer attached to its end near the gong of the bell. The metal strip moves fast towards the electromagnet, and the hammer strikes the gong. At the same time, with the movement of the metal strip from its position, the electric circuit breaks, the magnet stops working, and the strip moves back to its original position.
Answer the following questions:
Question 1.
When does the electromagnet work?
Answer:
When electric current flows through a coiled thin conducting wire, it works as an electromagnet. The electromagnet works when the circuit is in the switch ON position.
Question 2.
When the flow of current stops in the coil, what happens to the magnetic field around the electromagnet?
Answer:
When the current stops flowing (the circuit is in the switch OFF position), the magnetic field around the electromagnet disappears, and then the electromagnet does not attract any magnetic substances.
Question 3.
What is the industrial use of an electromagnet?
Answer:
Applications of electromagnets are common in industrial activities that use magnetic substances. For example, heavy iron scrap is fed into furnaces at very high temperatures to melt it and make iron ingots and iron rods. Cranes fitted with very strong electromagnets are used to lift the iron scrap and to feed the same into the furnaces.
II. Voltaic cells were an important discovery, but they are not convenient for everyday use. Instead, dry cells are one of the most widely used electric cells today. They are called dry because the electrolyte is not a liquid but a thick, moist paste.
Answer the following questions:
Question 1.
Who and when discovered the Voltaic cell?
Answer:
Voltaic cell (named after the scientist Alessandro Volta), also known as a Galvanic cell (named after the scientist Luigi Galvani) is one of the earliest electric cells. In the late 18th century, Galvani noticed that a dead frog’s leg kicked when touched with two different metals. This could happen due to stimulation from electricity. Galvani believed that electricity came from the frog itself. But Volta experimented with saltwater-soaked paper and two different metals and still got an electric current. Thus, the Voltaic cell was discovered by finding the combination of metals and a liquid chemical that generated an electric current.
Question 2.
What is the difference between the electrolyte used in a dry cell and the one used in a Voltaic cell?
Answer:
In a dry cell, the outer shell made of zinc contains the chemicals in the form of a thick, moist paste, which serves as the electrolyte. The electrolyte in a Voltaic cell is a liquid.
Electricity: Magnetic and Heating Effects Extra Questions for Practice
Question 1.
When we use a zinc and a copper plate in a Voltaic cell, which plate is the positive electrode?
Question 2.
When we use 3 cells in place of a single cell to show the heating effect of electric current, what results we expect?
Question 3.
What device would you use to know if the electromagnet has two poles or not?
Question 4.
A Voltaic battery is made using iron nails, copper wires, and lemon cut pieces. When the LED glows, it shows that the battery is generating electricity. Will the LED glow if we interchange the terminals of the battery?
Question 5.
What liquids can be used in a voltaic battery?
Question 6.
What is the positive terminal of a dry cell?
Question 7.
What is the negative terminal of a dry cell?
Question 8.
Explain how the number of turns in the coil is related to the strength of the electromagnet.
Question 9.
What is the impact of using a battery of 2 or 3 cells in place of a single cell in an activity to explain electromagnets?
Question 10.
Are some metals more suitable for electromagnets than others?
Question 11.
The heating effect of electric current increases with the increase in the length of the conducting wire in an electric circuit. Explain.
Question 12.
Why is Nichrome more suited for making heating elements in electric appliances?
Question 13.
Name four combinations of two metals each that can be used in a Voltaic cell.
Question 14.
Explain the structure and functioning of a dry cell. What precautions should be taken so that the cell does not weaken quickly or suddenly be used up?
Question 15.
Why are Voltaic cells not convenient as a portable source of electricity? What improvements have made rechargeable cells more widely used compared to dry cells?
Question 16.
Explain the magnetic effect of electric current. On what factors does the strength of an electromagnet depend? What way the lifting electromagnets used in industry?
Question 17.
Write an activity to show that the electromagnets have two poles like a bar magnet.
Question 18.
A Voltaic cell, also known as a Galvanic cell, contains two metal plates made of different materials and a liquid called an electrolyte, placed in a glass or plastic container. The plates, called electrodes, are partly dipped in the electrolyte, which is usually a weak acid or a salt solution. A chemical reaction between the plates and the electrolyte produces electricity.
(a) In what way is the Voltaic cell different from the dry cell?
(b) Who discovered the Voltaic or the Galvanic cell? What was the difference of opinion between the two scientists about the electricity generation by the cell, and how was it resolved?
MCQs
Question 1.
Which of the following appliances is NOT based on the heating effect of electric current?
(a) Electric bell
(b) Electric iron
(c) Water heater
(d) Electric stove
Question 2.
Dry cell uses _______________ electrolyte that is why it is considered as a convenient portable source of electricity.
(a) coloured
(b) solid
(c) liquid
(d) Thick, moist paste
Question 3.
In a dry cell, the negative terminal is the bottom of the zinc container. Which is the positive terminal of the cell?
(a) Bottom of the carbon rod
(b) Bottom of the zinc container
(c) The paste of chemicals inside the cell
(d) Metallic tip over the carbon rod.
Question 4.
The strength of the heating effect of electric current depends on:
(a) The nature of the metal used in the coil
(b) Number of turns in the coil
(c) The strength of the current flowing through the coil
(d) All of these
Question 5.
The strength of the electromagnet does NOT change with a change in
(a) The nature and thickness of the conductor
(b) Time duration for which the current is passed
(c) Strength of the electric current
(d) The number of turns in the coil
Fill in the blanks
Question 1.
When we increase the current flowing through the coil by using battery of two or more cells in place of one cell, the magnet becomes _______________
Question 2.
When we increase the current flowing through the coil by using battery of two or more cells in place of one cell, the heating effect of electric current _______________
Question 3.
Example of the use of heating effect of electric current in industry are high _______________ furnaces.
Question 4.
While making elements of heating appliances from wires of these materials, length of the wires is _______________ by making coils of the wire.
Question 5.
Batteries used in laptops, backup devices like inverters and in motor vehicles are bigger batteries that are _______________
True or False
Question 1.
Any of the two plates in a Voltaic cell can be a positive terminal, irrespective of the metal used.
Question 2.
The magnetic effect produced by a coil when the current through it remains unchanged when we add more batteries to the circuit.
Question 3.
The heating effect of electric current is caused due to resistance in the conductor to the flow of electric current.
Question 4.
Dry cells use thick, moist paste electrolytes.
Question 5.
The dry cells are single-use cells. Once these become weak, they have to be disposed of.
Question 6.
A doorbell is an example of the heating effect of electric current.
Match the following
Question 2.
| Column A | Column B |
| 1. Examples of the heating effect of electric current | (a) Dry cell electrolyte |
| 2. Examples of the magnetic effect of electric current | (b) Metallic tip over the carbon rod |
| 3. A weak acid or salt solution in liquid form | (c) Room heater, electric stove, hair dryer, electric iron |
| 4. An electrolyte in the form of a thick, moist paste | (d) Magnets in door bells, loudspeakers, electric motors, and fans |
| 5. Positive terminal of the dry cell | (e) Electrolyte in a Voltaic cell |
Assertion and Reason Questions
Two statements are given- Assertion (A) and Reason (R). Please read them carefully and choose the correct option.
Question 1.
Assertion (A): When an electric current passes through a conductor, it gets heated. This warming is called the heating effect of electric current.
Reason (R): When electric current flows through any conductor, it faces some opposition or resistance to its flow. This resistance converts some electric energy to heat energy.
Codes:
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Question 2.
Assertion (A): While making elements of heating appliances from wires of materials like nichrome, the length of the wires is increased by making coils of the wire.
Reason (R): The increase in the length of the conducting wire does not change the heating effect of the electric current.
Codes:
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.