CBSE Class 8 Science Chapter 04 Electricity Magnetic And Heating Effects MCQs Set 03

Answer: (B) The magnetic field disappears completely

Explanation:
The chapter explicitly states that when current flows, the compass needle deflects due to the magnetic field produced by the current. When the current stops, the chapter notes that 'this magnetic effect disappears and the compass needle returns to its original direction.' The magnetic field is directly produced by the flowing current and ceases when current flow ceases.

Answer: (B) The iron nail amplifies and concentrates the magnetic field produced by the coil

Explanation:
When the iron nail was inserted in Activity 4.3, the chapter reports that 'the coil becomes a stronger magnet and the deflection of the magnetic compass needle is much more.' Iron is a ferromagnetic material that reinforces and concentrates the magnetic field created by the current-carrying coil, making the electromagnet significantly more powerful than the coil alone.

Answer: (C) It produces a larger current and creates a stronger magnetic field

Explanation:
• A battery with more cells provides greater current compared to a single cell
• The stronger current creates a more powerful magnetic field
• As a result, the compass needle shows greater deflection and the electromagnet attracts more iron paper clips

Answer: (B) Electromagnets produce a temporary magnetic field that exists only when current flows

Explanation:
The core discovery demonstrated in the introduction is that the nail acts like a magnet only when current flows through the wrapped wire, and the magnetic effect disappears when the circuit opens. This shows electromagnets are temporary magnets whose strength depends entirely on the presence of electric current.

Answer: (B) End A is a south pole

Explanation:
The chapter explains that unlike poles of magnets attract each other. If the north pole of the compass is drawn toward end A, then end A must be a south pole. The chapter uses this principle to help students determine electromagnet polarity: observing which compass pole is attracted reveals the electromagnet's pole at that location.

Answer: (B) Nichrome offers higher resistance to current flow compared to copper wire of the same dimensions

Explanation:
The chapter states that nichrome wire 'offers higher resistance compared to a copper wire of the same size and length' and that 'this resistance causes some of the electrical energy to be converted into heat energy.' The resistance of the nichrome material, not any special property, explains why it becomes hot when carrying current.

Answer: (B) A Voltaic cell has a liquid electrolyte while a dry cell has a paste-like electrolyte

Explanation:
The chapter explicitly defines the distinction: a Voltaic cell contains 'a liquid called an electrolyte, placed in a glass or plastic container,' whereas a dry cell's electrolyte is 'not a liquid but a thick moist paste.' This structural difference is the defining characteristic separating these two types of cells.

Answer: (B) It serves as the electrolyte that allows ions to move and enables current flow

Explanation:
The chapter states that 'the electrolyte is the lemon juice, which helps conduct electricity.' While the chemical reaction between the electrodes and electrolyte generates the electrical potential, lemon juice specifically enables current flow by acting as an ion-conducting medium between the copper and iron electrodes.

Answer: (C) The electromagnet becomes stronger

Explanation:
In the 'Think like a scientist' section, the chapter notes that 'the increase in number of turns of the coil also makes the coil a stronger magnet.' More coil turns mean the magnetic field contributions from each turn add up, creating a stronger overall magnetic field even if the current through each turn stays the same.

Answer: (B) The clips fell down because the magnetic effect disappeared

Explanation:
The chapter states that when students 'disconnect the wire from the cell to stop the flow of electric current in the wire,' they observe that the clips 'fall down.' This occurs because a current-carrying coil 'behaves as a magnet' only during current flow, and 'when the current is stopped, the cylindrical coil loses its magnetic effect.'

Answer: (B) High-resistance wire converts electrical energy into heat energy through the heating effect of current

Explanation:
The chapter explains that 'all these devices contain a rod or a coil of wire, called a heating element' and work 'on the same principle of the heating effect of electric current.' When current flows through a high-resistance material like nichrome, electrical energy is converted to heat, which is the intended function of such appliances.

Answer: (B) He used saltwater-soaked paper instead of a frog's leg

Explanation:
The chapter states that Volta 'used saltwater-soaked paper instead of the frog's leg and still got an electric current.' This test demonstrated that the combination of metals and liquid—not the frog tissue—generated electricity, supporting his hypothesis and leading to the first battery invention.

Answer: (B) Electromagnets can be turned on and off by controlling the electric current

Explanation:
The chapter notes that lifting electromagnets are 'hung to the cranes' and 'the crane operator can control the magnet by switching the current ON and OFF. When the current is turned ON, the electromagnet lifts the iron/steel objects; when the current is switched OFF, magnetic field disappears, and the objects are released.' This controllability is the key advantage for industrial use.

Answer: (B) Earth's magnetic field results from the movement of liquid iron in the core, which creates electric currents

Explanation:
In the 'A step further' section, the chapter explicitly states: 'Deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field.' This directly connects to the principle established earlier—that electric currents produce magnetic fields.

Answer: (B) Electromagnets have two distinct poles—North and South—just like permanent magnets

Explanation:
The chapter states: 'Just like a magnet, an electromagnet also has two poles—North and South.' The activity demonstrates this by showing that ends A and B have opposite polarities, paralleling the structure of permanent bar magnets with their two distinct poles.

Answer: (C) Rechargeable batteries can be charged and reused multiple times but eventually wear out

Explanation:
The chapter explicitly states that 'rechargeable batteries can be recharged and reused multiple times. This prevents wastage and saves money over time as well,' but it also notes that 'rechargeable batteries also do not last forever. After being charged and used many times, they slowly wear out.' This describes their advantage and ultimate limitation.

Answer: (B) The compass needle deflects from its original direction

Explanation:
The chapter explicitly describes that when current flows through the wire in Activity 4.1, the compass needle deflects from its original position because the current-carrying wire produces a magnetic field around it. This deflection is the key observation demonstrating the magnetic effect of electric current.

Answer: (B) The larger current from 2 cells generates greater heat in the conductor

Explanation:
The 'Think like a scientist' section states that heat generation depends on the magnitude of electric current. A battery with more cells provides larger current, which produces more heat in the same wire for the same duration.

Answer: (B) Electromagnets produce a temporary magnetic field that depends on current flow

Explanation:
The key distinction shown by Sumana's model is that electromagnets generate magnetic effects only when current flows. Unlike permanent magnets, their magnetism is temporary and can be switched on and off by controlling the electric current through the coil.

Answer: (B) An electrolyte that conducts electricity

Explanation:
The lemon juice serves as the electrolyte, which conducts electricity between the two different metal electrodes. The chapter explains that a Voltaic cell requires an electrolyte (usually a weak acid or salt solution) to enable chemical reactions that produce electric current—which is precisely why lemon juice works in this experiment.

Answer: (C) Iron significantly strengthens the magnetic field produced by the electromagnet

Explanation:
Activity 4.3 demonstrates that inserting an iron nail into the cylindrical coil produces much greater deflection of compass needles. The text explicitly states that inserting an iron core makes the coil 'a stronger magnet' and that 'for practical applications, most electromagnets have an iron core to make them stronger.'

Answer: (B) Nichrome has higher resistance and generates more heat for the same current

Explanation:
The chapter states that 'a nichrome wire offers higher resistance compared to a copper wire of the same size and length.' This higher resistance causes more of the electrical energy to convert into heat energy, making nichrome ideal for devices designed to generate heat.

Answer: (B) He obtained an electric current using saltwater-soaked paper instead of a frog's leg

Explanation:
• Galvani thought electricity came from the frog itself
• Volta believed it came from the metals
• To test his hypothesis, Volta used saltwater-soaked paper instead of frog tissue
• He still got electric current with just metals and liquid—proving the electricity came from their combination, not the biological tissue.

Answer: (B) Movement of liquid iron in Earth's core generates electric currents that produce a magnetic field

Explanation:
The chapter explicitly explains that 'deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field.' This demonstrates the same principle covered earlier—that moving electric current produces magnetism.

Answer: (B) The magnetic poles at each end reverse their positions

Explanation:
The 'Think like a scientist' section notes that students should 'repeat Activity 4.4 by changing the direction of the current,' and the text explains that 'its poles can be reversed by changing the direction of the current.' Reversing the battery terminals reverses the current direction, which reverses which end becomes the north pole and which becomes the south pole.