Chapter 4 Important Questions: Electricity: Magnetic and Heating Effects

✔ Correct Answer: (b) Gets deflected

The magnetic effect of electric current produces a magnetic field around the wire, which exerts a force on the compass needle and causes it to deflect. When the current stops, the needle returns to its original position.

✔ Correct Answer: (c) Nichrome

Nichrome wire is used because it offers high resistance to electric current, which means more electrical energy is converted to heat energy compared to copper or aluminium of the same size.

✔ Correct Answer: (b) Electrolyte

The electrolyte is a weak acid or salt solution in which the electrodes are dipped. A chemical reaction between the electrodes and the electrolyte produces electricity.

✔ Correct Answer: (b) A current carrying coil that behaves as a magnet

An electromagnet is formed when electric current flows through a coil, creating a magnetic field. When the current is switched off, the magnetic effect disappears — making it a temporary magnet.

✔ Correct Answer: (c) Increase the number of turns of the coil

Increasing the number of turns in the coil increases the strength of the magnetic field, making the electromagnet stronger. Using more cells (higher current) also increases strength.

✔ Correct Answer: (d) Zinc container

In a dry cell, the outer zinc container acts as the negative terminal ($-$), while the carbon rod covered with a metal cap at the centre acts as the positive terminal ($+$).

✔ Correct Answer: (b) Changing the direction of the electric current

The polarity of an electromagnet depends on the direction of current flow through the coil. Reversing the battery terminals reverses the current direction, which reverses the North and South poles.

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

A current-carrying nichrome wire shows both the heating effect (wire gets warm due to resistance) and the magnetic effect (compass needle deflects due to magnetic field around the wire).

✔ Correct Answer: (c) Hans Christian Oersted

Hans Christian Oersted (1777–1851), a Danish professor, discovered in 1820 that a current-carrying wire deflects a nearby compass needle, establishing the link between electricity and magnetism.

✔ Correct Answer: (b) They can be recharged and reused multiple times

Rechargeable batteries can be charged and used again many times, which prevents wastage, saves money over time, and reduces environmental pollution compared to single-use dry cells.

Magnetic field. The magnetic field is the region where the effect of a magnet or a current-carrying conductor can be detected, for example, by the deflection of a compass needle.

Iron. An iron core inside the coil greatly increases the strength of the electromagnet because iron becomes a strong temporary magnet when placed inside a current-carrying coil.

Electrolyte. The electrolyte is usually a weak acid or salt solution in which the electrodes are dipped. A chemical reaction between the electrodes and the electrolyte produces electricity.

Heating. The heating effect of electric current occurs because the conductor offers resistance to the flow of current, and some electrical energy is converted into heat energy.

Lemon juice (acid). The citric acid present in lemon juice acts as the electrolyte, allowing ions to move between the copper and iron electrodes and thus generating a small electric current.

False. Dry cells are more portable than Voltaic cells because they use a paste-like (moist) electrolyte instead of a liquid, so they do not spill and are compact and easy to carry.

True. The coil produces a magnetic field only when current flows through it. When the current is switched off, the magnetic effect disappears completely, making it a temporary magnet.

False. A battery of 2 cells provides a larger current, which creates a stronger magnetic field. Therefore, the electromagnet with 2 cells attracts more iron paper clips than one with a single cell.

True. The magnetic field around a current-carrying conductor exists only as long as current flows. Once the current is switched off, the magnetic field disappears immediately.

False. After being charged and used many times, rechargeable batteries slowly wear out and lose their ability to hold charge, eventually needing replacement.

The Voltaic cell uses chemical reactions to produce electricity; the electric iron uses heating effect; nichrome has high resistance suitable for heaters; electromagnet uses magnetic effect of current.

In a dry cell, the zinc container is the $-$ terminal, the carbon rod (with metal cap) is the $+$ terminal, and the paste is the solid electrolyte.

The labeled part is the Iron nail. In the electromagnet setup, the insulated copper wire is tightly wound around the iron nail, which acts as the core. When current flows through the coil, the iron nail becomes a temporary magnet (electromagnet) and can attract iron paper clips.

The labeled part is the Carbon rod. The carbon rod is located at the centre of the dry cell and is covered with a metal cap. It acts as the positive terminal ($+$) of the dry cell, surrounded by the paste-like electrolyte.

Magnetic Effect of Electric Current: 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 as soon as the current stops flowing.

An electromagnet is a current-carrying coil that behaves as a magnet; it usually has an iron core to make it stronger. One practical use is in lifting electromagnets attached to cranes, which are used in factories and scrap yards to lift and sort heavy iron/steel objects.

Heating Effect of Electric Current: When electric current passes through a conductor, the conductor gets heated due to resistance offered to the flow of current — this is known as the heating effect of electric current. Two household appliances based on this effect are the electric iron and the electric kettle.

A Voltaic cell (also called a Galvanic cell) is an early type of electric cell in which a chemical reaction between two different metal electrodes and a liquid electrolyte produces electricity. Its two main components are the electrodes (two metal rods of different materials) and the electrolyte (a weak acid or salt solution).

Because dry cells use a solid paste electrolyte, they are more portable and convenient for everyday use compared to Voltaic cells, which can spill.

A current-carrying wire produces a magnetic field around it (magnetic effect of electric current). The compass needle is a tiny magnet, and the magnetic field of the wire exerts a force on it, causing the needle to deflect from its original north-south direction.

The strength of an electromagnet depends on:

1. The amount of electric current flowing through the coil — more current means a stronger magnet.

2. The number of turns in the coil — more turns produce a stronger magnetic field.

A rechargeable battery is a type of battery that can be charged and reused multiple times after it is discharged, because the chemical reactions inside it are reversible. Examples of devices that use rechargeable batteries include mobile phones (Li-ion battery) and electric vehicles (large rechargeable battery packs).

When the direction of electric current through the coil is reversed, the poles of the electromagnet are reversed — the end that was the North pole becomes the South pole and vice versa. This happens because the polarity of an electromagnet depends on the direction of current flow.

When electric current flows through a conductor, the conductor opposes (resists) the flow of current. This resistance causes some of the electrical energy to be converted into heat energy, which is why the wire gets heated — this is the heating effect of electric current.

Activity 4.1 — Observation:

A magnetic compass is placed below a wire connected in a circuit with a cell and switch. When the switch is turned ON, the compass needle deflects. When the switch is turned OFF, the needle returns to its original position.

Conclusion:

This shows that a current-carrying wire produces a magnetic field around it — the phenomenon known as the magnetic effect of electric current. The magnetic field exists only as long as current flows:

$\text{Current ON} \Rightarrow \text{Magnetic field present} \Rightarrow \text{Compass deflects}$

$\text{Current OFF} \Rightarrow \text{Magnetic field absent} \Rightarrow \text{Compass returns to normal}$

Nichrome vs Copper as Heating Element:

Nichrome offers higher resistance to the flow of electric current compared to copper of the same size. According to the heating effect of electric current, more resistance means more electrical energy is converted to heat energy. Therefore, nichrome generates more heat for the same current, making it ideal for heaters, irons, and stoves.

A lifting electromagnet is a strong electromagnet hung from a crane. When the crane operator switches the current ON, the electromagnet creates a powerful magnetic field and lifts heavy iron/steel objects (like scrap metal). When the current is switched OFF, the magnetic field disappears and the objects are released at the desired location.

Use in scrap yards:

In scrap yards, lifting electromagnets are used to efficiently sort, move, and lift large quantities of heavy metal items. They can quickly pick up and drop loads, making operations faster and safer compared to manual methods — separating magnetic metals like iron and steel from non-magnetic materials.

A Voltaic cell generates electricity through a chemical reaction between two different metal electrodes dipped in an electrolyte solution.

Lemon Cell (Activity 4.6):

• Electrodes: Copper wire ($+$) and iron nail ($-$) inserted into the lemon.

• Electrolyte: Lemon juice (citric acid), which conducts electricity.

• Working: A chemical reaction between the copper, iron, and lemon juice generates a small electric current. When multiple lemons are connected in series, enough current is produced to light an LED:

$\text{Chemical energy} \rightarrow \text{Electrical energy}$

The LED glows when its positive terminal (longer wire) is connected to the copper (positive electrode) side.

Electric Heating Devices vs Burning Firewood:

1. No indoor air pollution: Electric heaters do not produce smoke or harmful gases like $\text{CO}_2$ and $\text{CO}$, unlike burning firewood, which causes respiratory diseases.

2. Cleaner and safer: There is no open flame, so the risk of accidental fires and burns is reduced significantly in households.

3. Easier to control: Electric devices can be turned on/off instantly and temperature can be precisely regulated, whereas controlling a wood fire requires constant attention.

These advantages contribute to better health, safety, and convenience for society, especially in urban areas.

Structure of a Dry Cell

Diagram — Draw and Label:

Draw a cylindrical cell. Label the following parts:

• Zinc container (outer casing) → Negative terminal ($-$)

• Carbon rod (at the centre) → covered with Metal cap → Positive terminal ($+$)

• Paste-like electrolyte → surrounds the carbon rod (moist solid, not liquid)

• Outer casing (cardboard/metal cover)

$\text{Dry Cell: } \underbrace{\text{Zinc (} - \text{)}}_{\text{container}} \longrightarrow \text{Electrolyte (paste)} \longrightarrow \underbrace{\text{Carbon rod (} + \text{)}}_{\text{with metal cap}}$

Dry Cell vs Voltaic Cell

What happens when a cell becomes 'dead'?

In both Voltaic and dry cells, electricity is produced by chemical reactions between the electrodes and the electrolyte:

$\text{Chemical Energy} \rightarrow \text{Electrical Energy}$

Over time, the chemicals get used up completely. When the chemicals are exhausted, no more chemical reaction can occur, and the cell stops producing electricity. At this stage the cell is called 'dead'. A dead dry cell cannot be recharged — it must be disposed of. Only rechargeable batteries can be restored by passing current through them in the reverse direction.

Experiment: Coil as an Electromagnet (Activity 4.3)

Materials Required:

Insulated wire (~100 cm), chart paper cylinder, iron nail, electric cell, two magnetic compasses, iron paper clips, adhesive tape.

Diagram — Draw and Label:

Draw a cylindrical coil (chart paper cylinder wound with insulated wire). Label:

• Cylindrical coil (insulated wire wound ~50 turns)

• Magnetic compass placed near each end (End A and End B)

• Electric cell connected to the two ends of the coil

• Iron nail inserted through the centre of the cylinder

• Iron paper clips placed near the ends of the nail

Procedure:

1. Wind ~50 turns of insulated wire tightly on a paper cylinder.

2. Place magnetic compasses near both ends of the coil.

3. Connect the coil to an electric cell and observe the compass needles.

4. Disconnect the cell and observe again.

5. Insert an iron nail into the cylinder, reconnect the cell and observe.

Observations:

Conclusion:

$\text{Current-carrying coil} \Rightarrow \text{Magnetic field} \Rightarrow \text{Acts like a magnet (Electromagnet)}$

• The coil with current acts as an electromagnet — it has two poles (North and South) like a bar magnet.

• Inserting an iron nail as core makes the electromagnet much stronger because iron becomes a strong temporary magnet inside the coil.

• The magnetic effect is temporary — it disappears when current stops, confirming the coil is an electromagnet, not a permanent magnet.

Heating Effect of Electric Current

What is it?

When electric current flows through a conductor, the conductor opposes the flow of current. This opposition is called resistance. The resistance causes electrical energy to be converted into heat energy. This warming of a conductor due to current flow is called the heating effect of electric current:

$\text{Electrical Energy} \xrightarrow{\text{Resistance}} \text{Heat Energy}$

Role of Resistance:

• Different conductors offer different levels of resistance.

• A conductor with high resistance (like nichrome) generates more heat for the same amount of current compared to a low-resistance conductor (like copper).

• Heat generated depends on:

  • Material of the wire (resistance)

  • Thickness and length of the wire

  • Magnitude of current flowing

  • Duration for which current flows

$\text{More current} \Rightarrow \text{More heat generated (for same wire)}$

Heating Elements in Household Appliances:

All electric heating appliances contain a heating element — a rod or coil of high-resistance wire (usually nichrome) that gets red-hot when current passes through it.

Disadvantages of Heating Effect:

1. Energy wastage: Heat generated in transmission wires during electricity distribution is wasted energy.

2. Damage to appliances: Overheating can melt plastic parts of plugs and sockets.

3. Fire hazard: Excessive heating in wires can cause short circuits and fires if inappropriate wires are used.

Safety measure: Appropriate wires, plugs, and sockets rated for specified current must be used. Safety devices (like fuses) are placed in household circuits to prevent overheating incidents.

Hans Christian Oersted and His Discovery

Hans Christian Oersted (1777–1851) was a Danish professor who, in 1820, made the groundbreaking discovery that electricity and magnetism are linked. During a demonstration, he noticed that whenever an electrical circuit was closed or opened, the needle of a nearby compass deflected. He investigated this carefully and confirmed that:

$\text{Electric current in a wire} \Rightarrow \text{Magnetic field around the wire}$

This phenomenon is called the magnetic effect of electric current. His findings were published and verified by other scientists, leading to the development of electromagnets and many modern technologies.

Lifting Electromagnets — Practical Application

A lifting electromagnet is a powerful electromagnet hung from a crane. It works as follows:

Uses: In factories and scrap yards, lifting electromagnets efficiently lift, move, and sort heavy metal items. They separate magnetic metals (iron, steel) from non-magnetic materials.

Changing the Strength of an Electromagnet:

The strength of an electromagnet can be increased by:

1. Increasing the electric current (using more cells in the battery) → stronger magnetic field → more clips attracted.

2. Increasing the number of turns in the coil → stronger magnetic field.

3. Using an iron core inside the coil → iron becomes a strong temporary magnet, amplifying the effect.

$\text{Strength} \propto \text{Current} \times \text{Number of turns}$

Changing the Polarity of an Electromagnet:

The poles (North and South) of an electromagnet can be reversed by:

• Reversing the direction of electric current through the coil (i.e., reversing the battery terminals).

• The end that was North pole becomes South pole and vice versa.

This ability to control both strength and polarity makes electromagnets far more versatile than permanent magnets in industrial and technological applications.

Voltaic Cell — Diagram and Working

Diagram — Draw and Label:

Draw a glass/plastic container with liquid inside. Label:

• Glass container (holds the electrolyte)

• Electrolyte (weak acid or salt solution — liquid)

• Electrode 1 (one metal rod, e.g., zinc → negative terminal $-$)

• Electrode 2 (another metal rod, e.g., copper → positive terminal $+$)

• Connecting wire joining the two electrodes through an electric lamp

• Arrow showing electric current flowing from $+$ terminal through circuit to $-$ terminal

How it Generates Electricity:

$\text{Chemical reaction (electrodes + electrolyte)} \rightarrow \text{Electrical energy}$

A chemical reaction between the two different metal electrodes and the electrolyte causes electrons to flow from one electrode to the other through the external circuit, producing electric current. Over time, chemicals are used up and the cell becomes dead.

Lemon Cell — Construction and Working

Construction:

• Electrodes: Copper wire ($+$) and iron nail ($-$) inserted into each lemon.

• Electrolyte: Lemon juice (citric acid) inside the lemon.

• Multiple lemons connected in series (copper of one connected to iron nail of next).

• LED connected between the copper wire of the first lemon and iron nail of the last lemon.

Why does the LED glow?

The citric acid in lemon juice reacts chemically with the copper and iron electrodes, generating a small electric current in each lemon cell. When several lemon cells are connected in series, the voltages add up, providing enough current to light the LED:

$\text{Lemon juice (electrolyte)} + \text{Cu and Fe (electrodes)} \rightarrow \text{Electric current} \rightarrow \text{LED glows}$

Dry Cell vs Rechargeable Battery

Rechargeable batteries (especially Li-ion batteries) are increasingly important as the world transitions to cleaner, environmentally friendly sources of electrical power.