Magnet:

A magnet is a material that attracts iron and objects containing iron. It can either be natural (such as lodestone) or artificially made.

Key Properties of a Magnet:

• Two Poles: Every magnet has two poles – the North Pole and the South Pole.
• Like Poles Repel: Poles of the same type repel each other.
• Unlike Poles Attract: Poles of opposite types attract each other.
• Alignment in North-South Direction: A freely suspended magnet always aligns itself in the North-South direction.
  
  

Magnetic Field:

• A magnet creates an invisible field around it where its influence can be felt. This is called the magnetic field.
• Identification of Magnetic Field:
• The direction of the magnetic field can be determined using a compass.
• SI Unit: The SI unit of the magnetic field is Tesla (T).

Properties of Magnetic Field Lines:

• Origin and Termination: Magnetic field lines originate from the North Pole and terminate at the South Pole.
• No Intersection: These lines never intersect each other.
  • One Direction at a Point: At any given point, only one direction of the magnetic field is possible.
  • If magnetic field lines were to intersect, it would imply two directions of the magnetic field at a single point, which is physically impossible.
• Density and Strength: The density of magnetic field lines indicates the strength of the field – higher density means a stronger field.
• Direction of Field Lines: The direction of the magnetic field is always from North to South.
  

Electric Current and Magnetic Effect:

• When an electric current flows through a wire, a magnetic field is created around it.
• Discovery: Hans Christian Oersted first observed that an electric current produces a magnetic field.
• Effect on Compass Needle: If a compass is placed near a wire carrying current, the needle of the compass deflects. This demonstrates that the flow of current creates a magnetic effect.
• Applications: The electromagnetic effect is the working principle behind devices like motors, transformers, and many other essential instruments.
  
  

Right-Hand Thumb Rule:

If you place the thumb of your right hand in the direction of the electric current flowing through a wire, the curled fingers of your right hand will indicate the direction of the magnetic field surrounding the wire.

 

Magnetic Effect of Electric Current:

When an electric current passes through a conductor, such as a wire, it creates a magnetic field around the conductor. This phenomenon is known as the magnetic effect of electric current.

 

Magnetic Field of a Current-Carrying Circular Loop:

When a circular loop of wire carries an electric current, a magnetic field is generated around the loop. This field has specific characteristics and properties that depend on certain factors.

Key Points:

1. Concentric Circles:

• Magnetic field lines are formed as concentric circles at every point along the circular loop.
• The direction of these magnetic field lines is determined using the Right-Hand Thumb Rule:
• If you hold your right hand such that your thumb points in the direction of the electric current flowing through the loop, the direction in which your fingers curl represents the direction of the magnetic field.

2. Size of the Circles:

 

• The magnetic field lines form concentric circles whose size increases as you move further away from the wire.
• This means that the magnetic field is strongest near the wire and gradually weakens as the distance from the wire increases.

3. Magnetic Field at the Center of the Loop:

 

• At the very center of the circular loop, the magnetic field lines are nearly straight.
• The magnetic field strength is at its maximum at the center of the loop due to the combined effect of the field lines from all parts of the loop converging there.

4. Magnetic Field Inside the Loop:

• Inside the circular loop, all the magnetic field lines point in the same direction.
• This uniform direction of the field lines indicates that the magnetic field inside the loop is uniform.
  
  

Factors That Affect the Magnetic Field of a Circular Loop:

1. Electric Current (Current):

• The strength of the magnetic field depends directly on the magnitude of the electric current flowing through the loop.
• Increasing the current increases the strength of the magnetic field.
  
  

2. Distance from the Wire:

• The strength of the magnetic field decreases as the distance from the wire increases.
• The magnetic field is strongest near the wire and weaker at greater distances.
  
  

3. Number of Turns in the Loop (Turns):

 

• If the circular loop has multiple turns, the strength of the magnetic field increases.
• Each turn of the loop contributes to the overall magnetic field, and the combined effect enhances the field strength.
• The total magnetic field is proportional to the number of turns in the loop.

4. Superposition of Magnetic Fields:

 

• The magnetic field created by each individual turn of the circular loop adds up to produce a stronger overall magnetic field.
• This superposition of the magnetic fields from all the turns amplifies the total magnetic field strength.
  
  

Solenoid:

Definition:

A solenoid is a cylindrical coil of wire made by winding the wire closely in the form of successive turns. When an electric current is passed through the solenoid, it generates a magnetic field similar to that of a bar magnet.

 

Key Features of a Solenoid:

1. Magnetic Field Inside the Solenoid:

 

• The magnetic field inside a solenoid is uniform.
• The magnetic field lines inside the solenoid are parallel to each other, indicating that the field is constant in strength and direction.

2. Magnetic Field Direction:

 

• Inside the Solenoid:
  • Inside the solenoid, the magnetic field lines move from the South Pole to the North Pole.
• Outside the Solenoid:
  • Outside the solenoid, the magnetic field lines move from the North Pole to the South Pole, similar to the field of a bar magnet.

3. Formation of Poles:

 

• The ends of the solenoid act as poles, just like a bar magnet.
• One end of the solenoid behaves as the North Pole, and the other behaves as the South Pole, depending on the direction of the current.

4. Applications:

 

• A solenoid is used to magnetize materials such as iron by placing them inside the solenoid.
• It is a crucial component in the creation of electromagnets, which are widely used in various devices and industrial applications.

 

Electromagnet vs Permanent Magnet

Aspect	Electromagnet	Permanent Magnet
Duration of Magnetism	Temporary – Magnetism ceases when the electric current is switched off.	Permanent – Magnetism remains constant over time.
Control over Strength	Can be varied by changing the current.	Fixed and cannot be changed.
Polarity	Polarity can be reversed by changing the direction of current.	Polarity cannot be changed.
Strength	Generally more powerful than a permanent magnet.	Comparatively weaker than an electromagnet.
Applications	Used in electric bells, motors, cranes, etc.	Used in compasses, door locks, etc.

 

Magnetic Force on a Current-Carrying Conductor

When a current-carrying conductor is placed in a magnetic field, it experiences a force.

Key Facts:

1. André-Marie Ampère’s Observation:

 

According to Ampère, a magnet exerts a force on a current-carrying conductor, equal in magnitude but opposite in direction to the force exerted by the conductor on the magnet.

2. Relationship Between Current and Force:

 

• If the electric current is perpendicular to the magnetic field, the force is maximum.
• If the direction of the electric current is parallel to the magnetic field, the force is zero.

3. Changing the direction of the current reverses the direction of the force.

Fleming’s Left-Hand Rule

Rule Description:

Stretch your left hand’s index finger, middle finger, and thumb such that they are mutually perpendicular to each other:

• Index Finger: Indicates the direction of the magnetic field.
• Middle Finger: Indicates the direction of the electric current.
• Thumb: Indicates the direction of the force acting on the conductor.

Additional Key Facts:

1. Magnetic Effects in the Human Body:

• The human heart and brain produce small but significant magnetic fields.

2. MRI (Magnetic Resonance Imaging):

• Magnetic fields are used to obtain detailed images of internal body organs.

3. Galvanometer:

• A device used to detect the presence and direction of electric current in a circuit.
  
  

Electromagnetic Induction

When a conductor or coil is placed in a changing magnetic field, an electric current is induced in it. This phenomenon is known as electromagnetic induction, and the current generated is called induced current.

 

Experiments and Observations of Electromagnetic Induction

1. Activity:

• Materials Required: A coil, a permanent magnet, and a galvanometer.
• Procedure:
  • Move the magnet towards the coil.
  • Hold the magnet stationary near the coil.
  • Move the magnet away from the coil.
• Observations:
  • When the magnet is brought closer to the coil, the galvanometer shows a momentary deflection, indicating the presence of induced current.
  • When the magnet is stationary, there is no deflection.
  • When the magnet is moved away from the coil, the galvanometer shows a momentary deflection in the opposite direction.

Explanation:

• Induced current is generated only when there is a change in the magnetic field (relative motion).
• If there is no relative motion between the magnet and the coil, no induced current is produced.

 

Use of Primary and Secondary Coils (Mutual Induction)

Materials Required:

• Two coils – one primary (Primary Coil) and one secondary (Secondary Coil).
• A switch, a battery, and a galvanometer.

Procedure:

• Electric current is passed through the primary coil.
• The ends of the secondary coil are connected to the galvanometer.
• The switch is turned ON and OFF.

Observations:

• When the switch is turned ON, the galvanometer shows a momentary deflection.
• When the current becomes steady, there is no deflection.
• When the switch is turned OFF, the galvanometer shows a momentary deflection in the opposite direction.

Explanation:

• This demonstrates that when the current in the primary coil flows or stops, an induced current is generated in the secondary coil.
• This phenomenon is called Mutual Induction.
  
  

Fleming’s Right-Hand Rule

Rule Description:

Stretch the thumb, index finger, and middle finger of your right hand such that they are mutually perpendicular:

• Index Finger: Represents the direction of the magnetic field.
• Thumb: Represents the direction of motion of the conductor.
• Middle Finger: Represents the direction of the induced electric current.

Importance:

• This rule is used to understand the working of generators.
• It helps determine the direction of induced current.

 

Alternating Current (AC)

Definition:

The electric current that reverses its direction periodically after a fixed interval of time is called Alternating Current (AC).

Key Facts:

• In India, the frequency of alternating current is 50 Hz.
• This means the current changes direction 50 times per second.
• Time for 1 Cycle (T) = 1/50 seconds = 20 milliseconds.
• 50 Hz Frequency:
• In 1 second, the current changes direction 50 times, meaning 100 alternations (50 in one direction and 50 in the opposite direction).

Advantages and Disadvantages of Alternating Current

 

Advantages	Disadvantages
Can be transmitted over long distances with minimal energy loss.	Cannot be stored easily.
Its voltage can be increased or decreased using transformers.	Difficult to use directly in electronic devices.
Suitable for large-scale energy consumption.	Can cause more damage to devices.

 

Direct Current (DC)

Definition:

The electric current that does not change its direction over time is called Direct Current (DC).

Sources of Direct Current:

• Batteries
• Cells
• Accumulator Cells

Advantages and Disadvantages of Direct Current

Advantages	Disadvantages
Direct current can be stored.	Energy loss is high during long-distance transmission.
Suitable for electronic devices.	Difficult to increase or decrease voltage.
Can be stored in batteries and used to power devices.	Requires large transformers and equipment.

 

Feature Alternating Current (AC) Direct Current (DC)

Direction Change Changes direction periodically. Does not change direction.

Energy Loss Minimal energy loss over long distances. High energy loss over long distances.

Storage Cannot be stored. Can be stored.

Usage Used in homes and industries. Used in batteries and electronic devices.

Voltage Variation Can be increased or decreased using transformers. Voltage is difficult to vary.

Household Electric Circuit

For electricity supply in homes, a well-organized circuit is required to ensure safety and efficiency. A household electric circuit primarily uses three types of wires.

 

 

 

 

 

Wires Used in Household Electric Circuit

Live Wire (Phase Wire):

 

Color: Red

Function: Supplies electric current.

Voltage: 220 V

Neutral Wire:

 

Color: Black

Function: Returns the electric current back to the supply board.

Voltage: 0 V

Earth Wire (Ground Wire):

 

Color: Green

Function: Prevents electric shocks by channeling leakage current to the ground.

Voltage: 0 V (typically).

Special Fact:

In India, the potential difference between the live wire and neutral wire is 220 V.

 

Components of Household Electric Supply System

Pole:

Electricity provided by the electricity board is delivered to houses through poles.

 

Main Supply Fuse:

Installed on the main supply line to protect against overload or short circuits.

 

Electric Meter:

Measures the amount of electrical energy consumed in the house.

 

Distribution Box:

Contains the main switch and separate fuses for each circuit.

 

Separate Circuits:

Different parts of the house have separate circuits, enhancing safety.

 

Importance of Earth Wire

If an electric appliance's metal casing has leakage current, the earth wire carries it to the ground.

Prevents electric shocks.

Provides a low-resistance path for leakage current, ensuring the safety of appliances.

Short Circuit

Definition:

When the live wire and neutral wire accidentally come into contact, it is called a short circuit.

 

Consequences:

The circuit's resistance becomes very low.

Excessive electric current starts flowing, causing overload.

Increased risk of fire or damage to appliances.

Overloading

Definition:

When more electric current flows through a circuit than its capacity, it is called overloading.

 

Causes:

Sudden increase in supply voltage.

Connecting multiple electrical appliances to a single socket.

Use of faulty wires or appliances.

Safety Measures in Household Electric Circuit

Electric Fuse:

 

A thin wire that melts when excess current flows, breaking the circuit.

Protects against overloading and short circuits.

Earth Wire:

 

Directly channels leakage current to the ground, preventing electric shocks.

Miniature Circuit Breaker (MCB):

 

An automatic switch that turns off when excess current flows in the circuit.

To reset, the switch needs to be turned on manually.

More effective than traditional fuses.

Important Questions

How many types of wires are used in a household electric circuit? Explain their functions.

What is the importance of the earth wire?

What is a short circuit? Write its causes and consequences.

What are the main causes of overloading?

What is the difference between an electric fuse and an MCB?

What safety devices are used in household electric circuits?