Chapter 4: Electricity: Magnetic and Heating Effects

Yes! We can use a magnetic compass to detect current flow. When a current-carrying wire is placed near a compass, the needle deflects because the current produces a magnetic field around the wire. This is the magnetic effect of electric current — the compass acts as a current detector (galvanoscope). When the switch is turned OFF and no current flows, the needle returns to its original north-south direction.

Yes, temporary magnets can be made using electromagnets. When an insulated wire is wound around an iron nail and connected to a battery, the coil carrying current behaves like a magnet and the iron nail becomes magnetised. The iron nail attracts iron clips while the current flows, but loses its magnetism as soon as the circuit is broken. This is because the magnetic effect of electric current exists only as long as current flows.

Heat is generated in electrical appliances through the heating effect of electric current. When electric current flows through a conductor, the conductor offers resistance to the flow of current, and some electrical energy is converted into heat energy. Appliances like electric heaters, irons, and kettles contain a heating element (usually made of nichrome wire) that has high resistance, so it generates a large amount of heat. The greater the resistance and current, the more heat is produced.

A cell or battery is dead when the chemicals inside it are completely used up and it can no longer produce electric current — devices connected to it stop working. Not all cells and batteries can be recharged. Dry cells (single-use cells) cannot be recharged and must be disposed of once dead. However, rechargeable batteries (like lithium-ion batteries in phones and laptops) can be recharged multiple times by connecting them to an external power source.

The solution used in a Voltaic cell is called electrolyte.

It is usually a weak acid or salt solution that allows a chemical reaction with the electrodes to produce electricity.

A current carrying coil behaves like a magnet(electromagnet).

When current flows through the coil, it produces a magnetic field and has two poles — North and South — just like a bar magnet.

False.

Dry cells are more portable than Voltaic cells. Voltaic cells use a liquid electrolyte in a glass or plastic container, which can spill and is bulky. Dry cells use a thick moist paste as the electrolyte, making them compact, spill-proof, and easy to carry.

True.

An electromagnet works only because of the magnetic effect of electric current. When current flows through the coil, it produces a magnetic field and behaves like a magnet. As soon as the current is stopped, the magnetic field disappears and the coil is no longer a magnet.

False.

A battery with 2 cells provides more current than a single cell, which creates a stronger magnetic field. A stronger electromagnet can attract more iron paper clips. So the electromagnet with 2 cells attracts more clips, not fewer.

✔ Correct Answer: (c) Both (i) and (ii) are correct

Why (c) is correct: When electric current flows through a nichrome wire, it produces both the heating effect and the magnetic effect simultaneously. The high resistance of nichrome converts electrical energy to heat (heating effect), making the wire warm. At the same time, any current-carrying conductor produces a magnetic field around it (magnetic effect), which deflects a nearby compass needle.

Why other options are wrong:

• (a) Only (i): This is incomplete — the magnetic compass deflection (ii) also occurs, since all current-carrying wires produce a magnetic field.
• (b) Only (ii): This is incomplete — the wire does get warm due to the heating effect of electric current, especially nichrome which has high resistance.
• (d) Both wrong: Both effects (heating and magnetic) are real, well-established phenomena that occur simultaneously when current flows.

The correct matches are:

• (i) Voltaic cell → (d) Generates electricity by chemical reactions: A Voltaic cell produces electricity through a chemical reaction between the electrodes and the electrolyte.
• (ii) Electric iron → (c) Works on heating effect of electric current: An electric iron uses a heating element that heats up when current passes through it.
• (iii) Nichrome wire → (a) Best suited for electric heater: Nichrome has high resistance and generates a large amount of heat, making it ideal for heaters.
• (iv) Electromagnet → (b) Works on magnetic effect of electric current: An electromagnet is created by passing current through a coil, which produces a magnetic field.

✔ Correct Answer: (ii) generates more heat for a given current

Why (ii) is correct: Nichrome wire has high electrical resistance, which means it opposes the flow of current more than other metals. This opposition causes more electrical energy to be converted into heat energy, so it generates more heat for the same amount of current — making it ideal for heating devices.

Why other options are wrong:

• (i) Nichrome is not chosen because it is a good conductor — in fact, its high resistance (not good conductivity) is what makes it useful for heating.
• (iii) Nichrome is not necessarily cheaper than copper; it is chosen for its high resistance property, not cost.
• (iv) Nichrome is definitely not an insulator — it does conduct electricity, which is required for current to flow and generate heat.

Electric heating devices are more convenient and beneficial to society for several reasons:

1. Cleaner environment: Burning firewood or charcoal releases smoke, carbon dioxide, and harmful gases, causing air pollution. Electric devices produce no smoke or direct emissions at the point of use.
2. Safer to use: Electric devices have no open flames, reducing the risk of accidental fires and burns, especially indoors.
3. Better health: Traditional burning produces indoor air pollution that can cause serious respiratory diseases. Electric devices eliminate this health hazard.
4. Conservation of forests: Using firewood leads to deforestation. Switching to electric heating reduces dependence on wood, helping preserve forests and biodiversity.
5. Easier to control: Electric devices allow precise temperature control and can be switched ON/OFF instantly, making them more efficient and convenient to use.

(i) Direction of current:

(ii) Why the compass needle moves: When current flows through the coil, it produces a magnetic field around and inside the coil (magnetic effect of electric current). The compass needle, being a tiny magnet, responds to this magnetic field and gets deflected from its original north-south position. The needle points in the direction of the resultant magnetic field created by the electromagnet.

(iii) Effect of reversing battery terminals: If the battery terminals are reversed, the direction of current in the coil reverses. This reverses the polarity of the electromagnet (end A becomes the opposite pole and end B becomes the opposite pole). As a result, the compass needle would deflect in the opposite direction compared to the original deflection.

The electromagnet stopped lifting clips because the cell (battery) has become weak or dead. When the circuit stays closed for too long, the chemical energy inside the cell gets used up continuously. As the cell weakens, it can no longer supply sufficient current to create a strong enough magnetic field to lift the clips. However, the wire is still warm because even a small residual current flowing through the nichrome/copper wire produces the heating effect of electric current. Additionally, the wire retains some heat from the prolonged current flow.

The LED will glow in the case where lemon juice is used as the electrolyte (Fig. 4.12a). Lemon juice is a weak acid that can conduct electricity and act as an electrolyte, allowing a chemical reaction with the copper strip and iron nail to produce an electric current. Pure water is a very poor conductor of electricity and cannot act as an effective electrolyte, so no significant current flows and the LED will not glow in that case.

Yes, the coil will still deflect the compass, because a current-carrying coil produces a magnetic field regardless of whether there is an iron core or not. However, the deflection will be less than before. The iron nail (iron core) makes the electromagnet stronger by concentrating and amplifying the magnetic field. Without the iron nail, the magnetic field produced by the coil alone is weaker, so the compass needle deflects by a smaller angle.

The nichrome coil would be most suitable as a heating element. Among the four materials, nichrome has the highest electrical resistance, which means it opposes the flow of current the most and converts the maximum amount of electrical energy into heat energy. Iron, copper, and aluminium have lower resistance than nichrome, so they generate less heat for the same current. Additionally, nichrome can withstand high temperatures without melting or breaking down, making it ideal for heating applications.