**Definition**: When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This phenomenon is called the **magnetic effect of electric current**.
**Key Observation**: The magnetic field disappears when the current stops flowing.
**Historical Discovery**: Hans Christian Oersted (1777–1851), a Danish professor, discovered in 1820 that electricity and magnetism are linked. While demonstrating an electrical circuit, he noticed that whenever the circuit was closed or opened, a nearby magnetic compass needle deflected. This was a groundbreaking discovery showing the connection between electricity and magnetism.
**Apparatus Required**:
**Procedure**:
1. Make a switch using drawing pins, safety pin, and cardboard (as learnt in Grade 7)
2. Place the cell in the cell holder
3. Fix two nails to cardboard piece
4. Stretch the longer wire between the nails, keeping it slightly above the cardboard surface
5. Attach one end of the wire to the cell holder and another to the switch
6. Connect the second wire between the cell holder and switch
7. Place the magnetic compass beneath the wire between the two nails
8. Move switch to 'ON' position and observe compass needle
9. Move switch to 'OFF' position and observe
10. Repeat the switching several times
**Observations**:
**Explanation**: The compass needle is a tiny magnet that deflects when a magnetic field is present. The current-carrying wire creates a magnetic field around itself.
**Magnetic Field Definition**: The region around a magnet or a current-carrying wire where its magnetic effect can be felt (detected by compass needle deflection) is called a **magnetic field**.
The magnetic effect of electric current has important practical applications in:
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An **electromagnet** is a current-carrying coil of wire that behaves like a magnet. It produces a temporary magnetic field only when current flows through it.
**Key Difference from Bar Magnets**: Electromagnets can be switched ON and OFF by controlling the flow of electric current, whereas bar magnets have permanent magnetic properties.
**Apparatus Required**:
**Procedure**:
1. Wrap the insulated wire tightly around the iron nail in coil form
2. Secure the coil with adhesive tape
3. Connect the ends of the wire to the electric cell
4. Bring the nail close to iron paper clips and lift up
5. Note if clips hang from the nail
6. Disconnect the wire from the cell
7. Observe what happens to the clips
**Observations**:
**Important Note**: Keep the wire connected to the cell for only a few seconds to avoid weakening the cell quickly.
**Apparatus Required**:
**Procedure**:
1. Roll chart paper to make a cylinder with diameter roughly equal to pencil width
2. Secure with adhesive tape
3. Wind approximately 50 turns of insulated wire tightly on the cylinder to form a coil
4. Secure the wire with adhesive tape
5. Place compasses near the two ends of the cylindrical coil
6. Connect the two ends of the coil with cell terminals
7. Observe the compasses carefully
8. Disconnect the wire and observe compass needles
9. Insert an iron nail inside the paper cylinder
10. Repeat the observation steps
**Observations** (Without Iron Core):
**Observations** (With Iron Core):
**Conclusion**: An iron core significantly increases the strength of an electromagnet.
**Apparatus Required**:
**Procedure**:
1. Label the two ends of the coil as A and B
2. Place the magnetic compass near end A
3. Connect the coil to the cell
4. Observe which pole of the compass is attracted to end A
5. Note the polarity (remember: unlike poles attract)
6. Repeat procedure for end B
**Key Principle**: When the North pole of a compass needle is attracted to an end, that end is a **South pole** of the electromagnet (because unlike poles attract).
**Observations**:
**Through "Think Like a Scientist" Activities**, we learn:
#### 1. Effect of Electric Current (Number of Cells)
**Investigation**: Use 2 or 4 cells with the same coil
**Observations**:
**Conclusion**: Electromagnet strength **increases with increase in electric current**.
#### 2. Effect of Number of Turns
**Investigation**: Use 2 cells but vary the number of turns in the coil
**Observations**:
**Conclusion**: Electromagnet strength **increases with increase in number of turns**.
#### 3. Effect of Current Direction
**Investigation**: Change the direction of current in the electromagnet
**Observations**:
**Conclusion**: The electromagnet's poles can be **reversed by changing current direction**.
The strength of an electromagnet can be changed by:
1. **Changing the amount of electric current** flowing through the coil
2. **Changing the number of turns** of the coil
3. **Using both methods together** for maximum effect
---
**Why Does Earth Act Like a Magnet?**
Deep inside the Earth, the movement of liquid iron in the outer core creates **electric currents**, which generate a **magnetic field**.
**Uses of Earth's Magnetic Field**:
**Scientific Significance**: This demonstrates that electromagnetic effects occur naturally on a planetary scale, not just in human-made devices.
---
**Lifting electromagnets** are **strong electromagnets hung to cranes** that use the principle of the magnetic effect of electric current to lift and move heavy metal objects.
1. **When current is turned ON**: The electromagnet creates a strong magnetic field that attracts iron/steel objects, lifting them off the ground
2. **When current is switched OFF**: The magnetic field disappears instantly, and the objects are released
**Industrial Uses**:
In Indian steel factories and recycling plants, lifting electromagnets are commonly used in scrap yards to separate magnetic materials from non-magnetic ones, making the recycling process efficient and cost-effective.
---
**Future Learning**: In higher grades, students will learn:
1. **Reciprocal Relationship**: Just as electricity produces magnetism (magnetic effect of current), **a moving magnet can also produce electric current**
2. **Scientific Significance**: This deep connection between electricity and magnetism is called **electromagnetism** and forms the theoretical foundation for modern technology
3. **Practical Implications**: This reciprocal relationship is vital to our daily lives and forms the basis of:
---
**Definition**: When an electric current passes through a conductor, the conductor gets heated. This warming is known as the **heating effect of electric current**.
**Scientific Explanation**: When electric current flows through a conductor, it encounters **resistance** (opposition to flow). Different conductors offer different levels of resistance.
**Resistance Definition**: The opposition offered by a conductor to the flow of electric current.
**Resistance Varies with Material**:
**Process**:
1. Electric current flows through the conductor
2. Conductor offers resistance to current flow
3. Electrical energy is converted into **heat energy**
4. The conductor becomes warm/hot
**Energy Conversion**:
**Electrical Energy → Heat Energy** (through resistance)
---
**Apparatus Required**:
**Why Nichrome Wire?** It has high resistance and heats up quickly, making the effect easily observable and safe for classroom use.
**Procedure**:
1. Mount two nails on cardboard piece approximately 5 cm apart
2. Tie the nichrome wire between the nails
3. Make connections as shown in Fig. 4.5 with switch in OFF position
4. Touch the nichrome wire with finger — record observation
5. Move switch to ON position for about 30 seconds
6. Move switch back to OFF position
7. Touch the nichrome wire momentarily (**DO NOT HOLD** to avoid injury)
8. Record observations about heat change
9. Repeat the last two steps to confirm observations
**Observations**:
**Safety Precaution**: Do not hold the nichrome wire for extended periods to avoid burns. Always work under teacher's supervision.
**Investigation**: Repeat Activity 4.5 with a battery of 2 cells instead of 1 cell
**Observations**:
**Conclusion**: **Heat generated increases with increase in electric current**.
The amount of heat generated in a wire depends on:
1. **Material of the wire** — Different materials have different resistances
2. **Thickness of the wire** — Thinner wires generate more heat than thicker wires (for same current)
3. **Length of the wire** — Longer wires generate more heat than shorter wires (for same current)
4. **Duration for which current flows** — More heat is generated if current flows for longer time
5. **Magnitude of electric current** — Higher current generates more heat
**Mathematical Relationship**: Heat generated is directly proportional to:
**Formula**: H = I²Rt (where H = heat, I = current, R = resistance, t = time)
---
**How They Work**: The filament is heated by electric current until it becomes **red hot** and glows, emitting light.
**Problem**: Much of the electrical energy is wasted as heat rather than light, making them inefficient.
All these appliances contain a **heating element** — a rod or coil of wire with high resistance:
#### 1. **Electric Room Heater**
#### 2. **Electric Stove / Hot Plate**
#### 3. **Electric Kettle**
#### 4. **Electric Iron**
#### 5. **Water Heating Immersion Rod (Geyser)**
#### 6. **Hair Dryer**
---
Problem: When electricity is transmitted through wires over long distances, a significant amount of electrical energy is **converted to heat** in the transmission wires, causing energy loss.
Impact: This is why electricity companies use **high voltage** for transmission (voltage loss is lower at high voltages) and then step it down for household use.
**Problems Caused by Overheating**:
To minimize heating problems and prevent accidents:
**Important Safety Steps**:
**To Prevent Unnecessary Heating in Household Switchboards**:
---
**Application**: Electric arc furnaces use the heating effect of electric current to melt and recycle scrap steel.
**Process**:
1. Scrap steel is placed in a specially designed high-temperature furnace
2. Electric current produces intense heat (several thousand degrees)
3. High heat melts the scrap steel
4. Molten steel is converted into usable steel products
**Advantages**:
**Real-Life Context in India**: Many Indian steel factories and recycling units use electric arc furnaces for scrap steel recycling, particularly in regions like Maharashtra, Odisha, and Jharkhand.
---
Portable sources of electricity such as cells and batteries enable us to:
**Question**: How do these devices produce electrical energy?
---
A **Voltaic cell** (also called **Galvanic cell**) is a device that generates electric current through chemical reactions.
**Essential Components**:
1. **Two Metal Rods (Electrodes)**
2. **Electrolyte**
3. **Container**
**Step-by-Step Process**:
1. Two metal electrodes of different materials are placed in electrolyte
2. **Chemical reaction** occurs between the metals and electrolyte
3. This chemical reaction causes **charge separation** between the electrodes
4. One electrode becomes **positive terminal** (has deficiency of electrons)
5. Other electrode becomes **negative terminal** (has excess of electrons)
6. When circuit is connected externally, **electric current flows** from positive to negative terminal through the circuit
7. Inside the cell, ions flow through electrolyte to complete the circuit
**Initial State**: Cell works efficiently when new
**Over Time**:
**Dead Cell**: When all chemicals are exhausted, the cell can no longer supply electricity and is called a **dead cell**.
**Important Note**: In a simple Voltaic cell, the chemicals cannot be regenerated, so the cell cannot be recharged.
**Discovery and Invention**:
Two Italian scientists made crucial discoveries:
1. **Luigi Galvani (Late 1700s)**
2. **Alessandro Volta (1800)**
**Name Origins**:
**Historical Impact**: This invention was the first practical source of continuous electric current and laid the foundation for all modern battery technology.
---
To construct a practical Voltaic cell using common materials and demonstrate that a chemical reaction between two different metals and a liquid can generate electric current.
**Step 1: Prepare Individual Lemon Cells**
1. Take one lemon
2. Insert a copper wire/strip into the lemon at one point
3. Insert an iron nail into the lemon at another point
4. Keep the copper wire and iron nail separated by small distance
5. Do **NOT** let them touch each other
6. Repeat this process for all remaining lemons (5-6 total)
**Step 2: Connect Lemon Cells in Series**
1. Connect the copper wire of the first lemon to the iron nail of the second lemon using connecting wire
2. Connect the copper wire of the second lemon to the iron nail of the third lemon
3. Continue this pattern through all lemons
4. This creates a **series connection** where each lemon's positive terminal connects to next lemon's negative terminal
**Step 3: Connect LED to Detect Current**
1. Take the copper wire of the first lemon (this is the positive terminal of the battery)
2. Take the iron nail of the last lemon (this is the negative terminal of the battery)
3. Connect the LED between these two terminals using connecting wires
4. **Important**: The longer wire of the LED connects to the positive terminal (copper wire of first lemon)
5. The shorter wire of the LED connects to the negative terminal (iron nail of last lemon)
**Step 4: Observation**
**If LED Glows**:
**Understanding the Lemon Cell**:
**Important Concept**: A single lemon cell produces **very small voltage** (approximately 0.7-0.9 Volts).
An LED requires a **minimum voltage** to light up (approximately 2-3 Volts depending on LED type).
**Solution**: By connecting lemon cells in series:
**Analogy**: Just like connecting small water pipes end-to-end increases water pressure, connecting cells in series increases total voltage.
1. **Cost-effective** — Uses easily available materials
2. **Eco-friendly** — No hazardous chemicals
3. **Educational** — Demonstrates Voltaic cell principle
4. **Reusable** — Lemons can be used for extended period before deteriorating
1. **Low current output** — Cannot power high-load devices
2. **Short lifespan** — Lemon juice dries out or gets depleted
3. **Not practical** — Cannot be used for regular appliances
4. **Variable performance** — Depends on lemon freshness and juice acidity
The activity mentions we can also use other liquids as electrolytes:
All these work on the same principle as lemon juice.
---
1. **Magnetic Field**: The region around a magnet or current-carrying wire where magnetic effects can be detected (e.g., by compass needle deflection)
2. **Electromagnet**: A current-carrying coil that behaves like a magnet; can be switched ON and OFF
3. **Heating Effect of Electric Current**: The phenomenon of wire/conductor getting heated when current flows through it due to resistance
4. **Resistance**: Opposition offered by a conductor to the flow of electric current
5. **Voltaic Cell**: A device that generates electric current through chemical reactions between two different metals and an electrolyte
6. **Electrolyte**: A liquid that conducts electricity, usually a weak acid or salt solution
7. **Electrode**: Metal rod in a Voltaic cell; two different metals are used
8. **Dead Cell**: A cell that has exhausted its chemical components and cannot supply electricity
1. **Cause**: Electric current flows through conductor
**Effect**: Magnetic field is created around the conductor
2. **Cause**: Current-carrying coil wrapped around iron core
**Effect**: Strong electromagnet is formed
3. **Cause**: Number of turns in coil increases
**Effect**: Electromagnet becomes stronger
4. **Cause**: Current amount increases
**Effect**: Electromagnet becomes stronger
5. **Cause**: Current direction is reversed
**Effect**: Polarity of electromagnet reverses
6. **Cause**: Electric current flows through high-resistance wire
**Effect**: Wire gets heated
7. **Cause**: Current through conductor increases
**Effect**: Wire heats up more (heat ∝ I²)
8. **Cause**: Duration of current flow increases
**Effect**: More heat is generated (heat ∝ time)
9. **Cause**: Chemical reaction in Voltaic cell
**Effect**: Electric current is generated
10. **Cause**: Chemicals in Voltaic cell are exhausted
**Effect**: Cell becomes dead and cannot supply electricity
1. **Deflection of compass** proves magnetic effect of current
2. **Iron clips clinging to electromagnet** demonstrates electromagnet strength
3. **Nichrome wire heating** shows heating effect of current
4. **LED glowing in lemon cell** proves chemical generation of electricity
**Diagram 1: Simple Voltaic Cell**
```
Draw a glass container with:
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**Diagram 2: Electromagnet with Iron Core**
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Draw a cylinder wrapped with coiled wire:
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**Diagram 3: Heating Effect Setup**
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Draw two nails on cardboard with wire stretched between:
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**Diagram 4: Series Connection of Lemon Cells**
```
Draw multiple lemons in a row:
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**Heat Generated in Conductor**:
**H = I²Rt**
Where:
**Key Point**: Heat is proportional to the **square of current**, meaning doubling current produces **4 times** the heat.
1. **Lifting electromagnets in Indian steel factories** — Used to separate and move scrap metal in recycling plants in cities like Jamshedpur, Bhilai, and Kolkata
2. **Electric kettles in Indian households** — Common in all Indian homes for heating water for tea and cooking
3. **Immersion rods (geysers)** — Widely used in Indian homes, especially in northern regions for hot water supply
4. **Electric irons** — Used in Indian homes and laundries for pressing clothes
5. **Electric room heaters** — Essential in northern India during cold winters
6. **Earth's magnetic field** — Indian birds and animals (such as migratory birds, some fish species) use Earth's magnetic field to navigate during seasonal migration
---
---
This comprehensive set of notes covers every aspect of Chapter 4: Electricity: Magnetic and Heating Effects from the NCERT Curiosity Science textbook for Class 8. Students should be able to answer any examination question using these detailed notes.
Q1. When electric current flows through a wire, what magnetic effect is observed using a compass?
Answer: A — An electric current creates a magnetic field around the wire, which causes the compass needle to deflect because the compass needle is a tiny magnet sensitive to magnetic fields.
Q2. A coil of wire is wound tightly around an iron nail. When current flows through the coil, the nail can pick up iron filings. What is this device called?
Answer: B — A current-carrying coil that behaves as a magnet is called an electromagnet, especially when it has an iron core to make it stronger.
Q3. In the activity where a compass was placed near the ends of an electromagnet, it was observed that the north pole of the compass was attracted to end A. What does this tell us about end A?
Answer: B — Unlike magnetic poles attract each other; since the north pole of the compass is attracted to end A, end A must be the South pole of the electromagnet.
Q4. What happens to an electromagnet made from a coil of wire wrapped around an iron nail when the electric current is switched off?
Answer: B — Unlike a permanent magnet, an electromagnet only attracts objects while current flows through it; switching off the current stops the magnetic field and the electromagnet loses its attractive force.
Q5. A student made two electromagnets: Electromagnet X with 20 turns of wire and Electromagnet Y with 50 turns of wire. Both use the same iron nail and are connected to the same battery. Which electromagnet will be stronger?
Answer: B — The strength of an electromagnet increases with the number of turns of the coil; therefore, Electromagnet Y with 50 turns will produce a stronger magnetic field than Electromagnet X with 20 turns.
Q6. In a scrap yard in Delhi, a large crane uses a lifting electromagnet to move steel plates. Why is the electromagnet more useful than a permanent magnet for this job?
Answer: B — The electromagnet can be controlled by switching the current ON to lift the steel plates and OFF to release them, making it practical for sorting and moving heavy metal items repeatedly.
Q7. A school science exhibition shows an electromagnet that can pick up 10 iron paper clips when connected to a single cell. If the same coil is connected to a battery with 3 cells instead, what change would you expect?
Answer: B — A battery with more cells provides a larger electric current; a larger current creates a stronger magnetic field, allowing the electromagnet to attract more iron paper clips.
Q8. Student A says that an electromagnet and a bar magnet are the same because both have North and South poles. What is the key difference that Student A has overlooked?
Answer: B — The critical difference is that an electromagnet is temporary and depends on electric current to function, whereas a bar magnet is permanent and retains its magnetic properties indefinitely.
Q9. In Activity 4.3, when an iron nail was inserted into the cylindrical coil of wire, the deflection of the magnetic compass needles increased significantly. Which of the following best explains why?
Answer: B — The iron core acts as a material that gets magnetized by the coil's magnetic field and amplifies it, creating a much stronger electromagnet than the coil alone.
Q10. A scientist wants to create an electromagnet that can reverse its poles without changing the coil or the battery. Based on your understanding of electromagnets, what must the scientist change?
Answer: C — Reversing the direction of electric current through the coil reverses the direction of the magnetic field, which causes the poles of the electromagnet to reverse.
What happens to a magnetic compass needle when electric current flows through a nearby wire?
The compass needle deflects from its original direction because the current-carrying wire produces a magnetic field.
Define magnetic field.
The region around a magnet or current-carrying wire where its magnetic effect can be felt, such as by deflection of a compass needle.
What is the name of the phenomenon where electric current produces a magnetic effect?
The phenomenon is called the magnetic effect of electric current.
What is an electromagnet?
A current-carrying coil that behaves as a magnet and can attract iron and steel objects while current flows.
How many poles does an electromagnet have?
An electromagnet has two poles — North and South — just like a bar magnet.
What happens to an electromagnet when the electric current is switched off?
The electromagnet loses its magnetic effect and no longer attracts iron or steel objects.
Name two factors that affect the strength of an electromagnet.
The strength depends on the amount of electric current flowing through the coil and the number of turns of the coil.
What is a lifting electromagnet used for?
Lifting electromagnets are used in factories and scrap yards to lift, move, and sort heavy metal items by switching current ON and OFF.
Who discovered the magnetic effect of electric current and in what year?
Hans Christian Oersted, a Danish scientist, discovered it in 1820.
How can the polarity of an electromagnet be reversed?
By changing the direction of the electric current flowing through the coil.
What is the magnetic effect of electric current? [1 mark]
Define: when current flows through conductor, magnetic field is produced around it. This phenomenon is called magnetic effect of electric current.
Describe in brief what happens when the electric current is switched OFF in an electromagnet. How is this property useful in lifting electromagnets used in scrap yards? [2 marks]
When current stops, magnetic field disappears and electromagnet loses its magnetic effect. This allows the crane operator to release heavy metal objects at will by switching current off.
An electromagnet is made by wrapping insulated wire around an iron nail and connecting it to an electric cell. List three ways in which the strength of this electromagnet can be increased. Explain one of them briefly. [3 marks]
Three ways: (1) increase number of turns in coil, (2) increase electric current (use more cells), (3) use better iron core material. Pick one and explain how it increases magnetic field strength.
Draw and label a diagram of an electromagnet setup using a coil of wire, an iron nail, and a magnetic compass. Describe an activity (Activity 4.4) that proves an electromagnet has two poles like a bar magnet. How can you reverse the polarity of an electromagnet without changing anything else? [5 marks]
Diagram must show: coil wound on iron nail, battery connections, compass placement. Activity 4.4: bring compass to each end and observe which pole is attracted (opposite poles at each end). Polarity reversal: change direction of current flow through coil.
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