Grade 8 Ch 4 Electricity: Magnetic & Heating Effects

 

Ch 4 Electricity: Magnetic and Heating Effects

1. Introduction

• Electricity shows two important effects:
1. Magnetic effect
2. Heating effect

2. Magnetic Effect of Electric Current

2.1 Discovery

• Activity: A compass needle deflects when placed near a current-carrying wire.
• Current produces magnetic field around the wire.
• Stops when current is switched off.
• Hans Christian Oersted (1820): discovered this effect.

Key Observation

Compass Needle Behaviour:

• ✓ When current flows → Compass needle deflects from original direction
• ✓ When the current stops → Needle returns to the original direction

🔍 Why Does This Happen?

Understanding the Compass:

• Compass needle = tiny magnet
• Deflects when a magnet is brought near it
• Magnetic effect can act through non-magnetic materials

The Connection:

• Deflection indicates: Current-carrying wire has magnetic effect on compass needle
• When current stops → Magnetic effect disappears

🌐 Magnetic Field

Definition:

The region around a magnet or current-carrying wire where its magnetic effect can be felt (such as by compass needle deflection)

Key Points:

• A magnetic field exists when current flows
• The magnetic field disappears when the current stops

2.2 🧲 Electromagnets

🔍 What is an Electromagnet?

Observation:

• When current passes through a cylindrical coil → behaves like a magnet → deflects the compass needle
• When an iron nail is inserted in the core of the coil:
• The coil becomes a stronger magnet
• Deflection of the compass needle is much more
• Attracts iron clips
• When the current is stopped → coil loses its magnetic effect

Definition:

A current carrying coil that behaves as a magnet is called an electromagnet

Factors affecting strength:

• Amount of current.
• Number of turns in coil.
• Nature of core (iron makes it stronger).

Uses:

• Electric bell, motors, fans, speakers, telephones, cranes.

🧭 Polarity of Electromagnets

Basic Rule:

• When two magnets are brought close, unlike poles (North–South) attract each other

Finding Polarity:

• If the north pole of the magnetic compass is attracted towards end A of the electromagnet → end A is the south pole
• Repeat the procedure to find the polarity of end B
• The polarity of end B is opposite to the polarity of end A

Conclusion:

Feature	Detail
Poles in electromagnet	Two — North and South
Similar to	Poles in an electromagnet

2.3  🏗️ Lifting Electromagnets

 What are they?

• Strong electromagnets that may be hung on cranes

How they work:

Current State	Action
✅ Current ON	Electromagnet lifts iron/steel objects
❌ Current OFF	Magnetic field disappears → objects released

 

Applications

• Used in factories and scrap yards
• Purpose: Move, lift, and sort heavy metal items efficiently

2.4 Earth’s Magnetism

• Earth behaves like a giant magnet.
• Due to liquid iron movement in Earth’s outer core → produces currents → magnetic field.
• Protects Earth from harmful space radiation.
• Birds, animals, and navigators use it for direction.

3. Heating Effect of Electric Current 

Does a Current-Carrying Wire Get Hot?

3.1 Discovery: 

📌 Key Observation

Nichrome wire feels warm when current is passed through it

🔍 Why Does This Happen?

Resistance in Conductors:

• When electric current flows through any conductor → faces opposition or resistance to its flow
• Different conductors offer different levels of resistance

Conductor	Resistance Level
Nichrome wire	Higher resistance
Copper wire (same size & length)	Lower resistance

Energy Conversion:

• Resistance causes electrical energy → heat energy

Definition:

When current passes through a conductor → it gets heated. This warming is known as the heating effect of electric current

3.2 Factors Affecting Heat Produced

• Type of material.
• Thickness of wire.
• Length of wire.
• Time current flows.
• Amount of current.

3.3 Uses

• Light bulbs: filament glows due to heat.
• Appliances: electric iron, heater, immersion rod, kettle, hair dryer.
• Industry: used in furnaces to melt scrap steel.

3.4 Risks

• Excess heat → energy wastage, socket damage, fire.
• Need proper safety measures and wires.

Everyday Applications Of The Heating Effect Of Electric Current

How an Incandescent Lamp Works:
Filament is heated by an electric current → lamp glows

Household Appliances
Using the Heating Effect:

• Electric room heaters
• Stoves
• Irons
• Immersion rods
• Water heaters
• Kettles
• Hair dryers

Common Feature:

• All these devices contain a rod or coil of wire called a heating element
• In some appliances where the element is visible → it can be seen glowing red hot

Useful vs. Problematic

Aspect	Details
✅ Useful	In many everyday appliances
❌ Energy loss	In wires during transmission
❌ Overheating	May damage plugs & sockets; plastic parts may melt
❌ Fire risk	Overheating may lead to fires
🛡️ Safety	Household circuits have safety devices to minimise such incidents

4. Electric Cells and Batteries How Does a Battery Generate Electricity?

4.1 Voltaic Cell (Galvanic Cell)

• Oldest type (also called Galvanic cell).
• Made of two electrodes in a liquid electrolyte.
• Produces current due to chemical reactions.
• Becomes “dead” when chemicals finish.
• Luigi Galvani & Alessandro Volta studied these cells.
• Activity: Lemon with copper and iron electrodes can light an LED.

Structure

• Two metal plates made of different materials (called electrodes)
• A liquid called an electrolyte (usually a weak acid or salt solution)
• Placed in a glass or plastic container
• Electrodes partly dipped in electrolyte

How it Works:

• Chemical reaction between plates and electrolyte → produces electricity
• When the circuit is connected, current flows from the positive terminal → through the circuit → to the negative terminal

Limitation:

 

Stage	What Happens
✅ Working	Chemicals produce electricity
❌ Dead	Chemicals get used up → cell stops working → cannot supply any more electricity

Lemon Cell Activity

Materials Required

• 5–6 juicy lemons
• Copper wires/strips (1–2 mm thick)
• Iron nails
• 1 LED
• Connecting wires

Procedure

• Insert a copper wire and an iron nail into one lemon, keeping them apart by a small distance
• Repeat for all remaining lemons
• Join copper wires and nails as shown in the diagram
• Connect the LED between:
• Copper wire of the first lemon
• Iron nail of the last lemon

Observation

• ✅ LED glows → Cell is working

Key Components:

Component	Material Used
Electrodes	Copper wires and iron nails
Electrolyte	Lemon juice (conducts electricity)
Alternative electrolyte	Salt solutions

4.2 🔋 Dry Cells

Why Dry Cells?

• Voltaic cells not convenient for everyday use
• Dry cells = most widely used electric cells today

Why ‘Dry’?

• Electrolyte is not liquid but a thick, moist paste
• Structure:
• Zinc container = negative terminal.
• Carbon rod in centre = positive terminal.
• Electrolyte = moist paste around carbon rod.
• Cannot be recharged (single-use).

Structure:

Part	Function
Zinc container	Acts as negative terminal
Carbon rod (centre) with metal cap	Surrounds the carbon rod
Paste-like electrolyte	Surrounds carbon rod

Type:

• Single-use cell → once used up → must be disposed of.

4.3 🔁 Rechargeable Batteries

• Can be used again and again after charging.
• Examples: mobile phones, laptops, inverters, vehicles.
• Lithium-ion batteries are widely used.
• New solid-state batteries being developed (safer, faster, longer life).

Advantages:

• Can be recharged and reused multiple times
• Prevents wastage
• Saves money over time

Applications by Size:

Size	Used In
Small	Watches, phones
Medium	Laptops, tablets
Large	Inverters, electric vehicles

Limitation:

• Do not last forever
• After being charged and used many times → slowly wear out

4.4 Battery Disposal

• Dead batteries contain harmful chemicals: acid, lead, cadmium, nickel, lithium.
• Must not be thrown in dustbin.
• Need to be recycled in e-waste centres.
• Recycling protects environment and saves resources.

5. Snapshots (Summary)

• Electric current produces magnetic effect (Oersted’s discovery).
• A coil with current behaves as an electromagnet (stronger with iron core).
• Electromagnets are widely used (bells, motors, cranes).
• Earth has a magnetic field due to liquid iron in its core.
• Current produces heat (heating effect).
• Used in bulbs, heaters, irons, furnaces.
• Cells and batteries produce current by chemical reactions.
• Dry cells are single-use, Rechargeable batteries can be reused.
• Recycling batteries is important for environment.

 

1. Fill in the blanks:

(i) The solution used in a Voltaic cell is called electrolyte.

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

 

2. Choose the correct option:

(i) Dry cells are less portable compared to Voltaic cells.(True/False)

Answer: False

(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)

Answer: True

(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)

Answer: False

 

3. An electric current flows through a nichrome wire for a short time.

(i) The wire becomes warm.

(ii) A magnetic compass placed below the wire is deflected.

Choose the correct option:

(a) Only (i) is correct

(b) Only (ii) is correct

(c) Both (i) and (ii) are correct

(d) Both (i) and (ii) are not correct

Answer:- (c) Both (i) and (ii) are correct

 

4. Match the items in Column A with those in Column B.

Answer:-

Column A	Column B
(i) Voltaic cell	(d) Generates electricity by chemical reactions
(ii) Electric iron	(c) Works on heating effect of electric current
(iii) Nichrome wire	(a) Best suited for electric heater
(iv) Electromagnet	(b) Works on magnetic effect of electric current

 

5. Nichrome wire is commonly used in electrical heating devices because it

(i) is a good conductor of electricity.

(ii) generates more heat for a given current.

(iii) is cheaper than copper.

(iv) is an insulator of electricity.

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

 

6. Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.

Answer:- Electric heating devices are more convenient and safer for society because:

1. They do not produce smoke, so they help keep the air clean and reduce indoor air pollution.

2.They are easier to use-just switch them on and off; no need to gather or burn wood or charcoal.

3.They save time and effort, especially for women and children in rural areas who often collect firewood.

4.They reduce deforestation, as less wood is needed for burning.

5.They are safer, as there is a lower risk of fire accidents or breathing problems.

 

7. Look at the Fig. 4.4a. If the compass placed near the coil deflects:

(i) Draw an arrow on the diagram to show the path of the electric current.

Answer:-

(ii) Explain why the compass needle moves when current flows.

Answer:- The compass needle moves when current flows because the electric current through the wire produces a magnetic field around it. This magnetic field interacts with the magnetic field of the compass needle, causing it to deflect from its original north-south direction.

(iii) Predict what would happen to the deflection if you reverse the battery terminals.

Answer:- If you reverse the battery terminals, the direction of the electric current will reverse, flowing from end B to end A. This will cause the magnetic field to reverse, and the compass needle will deflect in the opposite direction compared to its original deflection.

 

8. Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in introduction story). After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons

Answer:- The lifting electromagnet stopped lifting the iron paper clips because Sumana forgot to move the switch to the OFF position, which likely caused the battery to drain or the cell to become “dead.” When a cell is used for a long time without a break, the chemicals inside it get used up, and it can no longer supply enough electric current to produce a strong magnetic field. This weak magnetic field is not strong enough to attract the iron paper clips. The wire wrapped around the iron nail is still warm because, even with a weak current, the resistance in the wire converts some electrical energy into heat energy, known as the heating effect of electric current.

 

9. In Fig. 4.11, in which case the LED will glow when the switch is closed?

Answer:-  In Fig. 4.11, the LED will glow when the switch is closed in case (a), where the electrodes are placed in lemon juice. This is because lemon juice acts as an electrolyte, enabling a chemical reaction between the copper strip and iron nail to generate electric current, which can power the LED. In case(b), with pure water, the LED will not glow as pure water does not conduct electricity effectively due to the absence of ions.

 

10. Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?

Answer:- Yes, the coil will still deflect the compass even after Neha removes the iron nail. This is because a current-carrying coil of wire produces a magnetic field, and the compass needle responds to that magnetic field.

However, the deflection will be less than before. This is because when the iron nail is present inside the coil, it acts as a core and makes the electromagnet stronger. Without the iron nail, the magnetic effect is weaker, so the compass needle shows less deflection.

11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.12.

When current is passed through the coils, compass needles placed near the coils will show deflection.

(i) Only in circuit (a)

(ii) Only in circuits (a) and (b)

(iii) Only in circuits (a), (b), and (c)

(iv) In all four circuits

Answer:- (iv) In all four circuits