Punjab 9th Physics Ch 8 Magnetism Short Questions

**Temporary Magnets:** Materials that become magnetized only when placed in a magnetic field and lose their magnetism when the field is removed. Examples: Soft iron, paper clips.

**Permanent Magnets:** Materials that retain their magnetism even after the external magnetic field is removed. Examples: Steel, alnico, neodymium magnets.

The magnetic field of a magnet is the region around the magnet where its magnetic force can be detected. It is represented by magnetic field lines that emerge from the North pole and enter the South pole. The strength of the magnetic field is greatest near the poles and decreases with distance from the magnet.

Magnetic lines of force are imaginary lines used to represent the magnetic field around a magnet. They have the following properties:
• They emerge from the North pole and enter the South pole externally
• Inside the magnet, they run from South to North pole (forming closed loops)
• They never intersect each other
• They are closer together where the field is stronger (near poles)
• They repel each other laterally

**Uses of Permanent Magnets:**
• Loudspeakers and headphones
• Electric motors and generators
• Refrigerator door seals
• Compass needles
• Magnetic latches and clasps

**Uses of Electromagnets:**
• Electric bells and buzzers
• Magnetic relays and circuit breakers
• Electromagnetic cranes in scrapyards
• MRI machines in hospitals
• Maglev trains

Magnetic domains are small regions within a ferromagnetic material where groups of atoms (about 0.1 mm in size) are highly magnetized. Each domain behaves as a tiny magnet with its own North and South poles.

• In an unmagnetized material, domains are randomly oriented, canceling each other’s magnetic effects
• When magnetized, domains align in the same direction, producing a strong net magnetic field
• This domain theory explains why ferromagnetic materials can be magnetized and demagnetized

A current-carrying long coil (solenoid) forms a **solenoidal magnetic field**, which is similar to the magnetic field of a bar magnet.

Key characteristics:
• Magnetic field lines emerge from one end (North pole) and enter the other end (South pole)
• Inside the coil, field lines are nearly parallel and uniform
• The magnetic field is strongest at the center of the coil
• Field strength increases with: more turns, higher current, soft iron core

Applications: Electromagnets, transformers, inductors, relays

Paramagnetic Materials	Diamagnetic Materials
Weakly attracted to magnets	Weakly repelled by magnets
Have unpaired electrons	All electrons are paired
Magnetic susceptibility is small and positive	Magnetic susceptibility is small and negative
Examples: Aluminum, oxygen, platinum	Examples: Copper, silver, gold, water
Lose magnetism when external field is removed	Never retain magnetism
Domains align with external field	Induced magnetic field opposes external field

No, an isolated magnetic pole (magnetic monopole) is not possible. If we break a bar magnet into two equal pieces, we cannot get a North pole and South pole separately. Each piece will have both poles – a North pole and a South pole.

Even if a magnet is divided into thousands of pieces, each piece will be a complete magnet with its own North and South poles. This demonstrates that magnetic poles always exist in pairs.

**External Magnetic Field:**
• Direction: From North pole to South pole (outside the magnet)
• Field lines emerge from North pole and curve around to enter South pole

**Internal Magnetic Field:**
• Direction: From South pole to North pole (inside the magnet)
• Field lines complete the closed loop by running through the magnet’s interior

Together, external and internal field lines form continuous closed loops, which is a fundamental property of magnetic fields.

Maglev (Magnetic Levitation) trains use electromagnets for two main purposes:

**1. Levitation:**
• Electromagnets on the train and guideway create repulsive/attractive forces
• This lifts the train a few centimeters above the track, eliminating wheel friction
• Allows speeds up to 400 km/h (as in Japan’s bullet trains)

**2. Propulsion:**
• Propulsion electromagnets are installed along the guideway and train
• By alternating the polarity (push-pull effect), the train is propelled forward
• No engine or fuel needed for movement

Advantages: Silent operation, minimal maintenance, high speed, energy efficient

High field electromagnets use soft iron cores because:

• **High Permeability:** Soft iron can be easily magnetized; the induced magnetic field can be up to 1000 times stronger than the external field
• **Low Retentivity:** Soft iron loses its magnetism quickly when current stops, allowing precise control
• **Low Coercivity:** Requires minimal energy to magnetize and demagnetize
• **High Saturation:** Can support strong magnetic fields without losing effectiveness

These properties make soft iron ideal for applications requiring strong, controllable magnetic fields like transformers, electric bells, and industrial cranes.

Electromagnets have numerous practical applications:

1. **Magnetic Relay:** Used in circuits to control high-power devices with low-power signals
2. **Circuit Breaker:** Automatically breaks circuit during overload using electromagnetic trip mechanism
3. **Telephone Receiver:** Converts electrical signals to sound using vibrating electromagnet
4. **Electromagnetic Cranes:** Lift and move heavy iron/steel objects in scrapyards and factories
5. **Electric Bells:** Hammer strikes bell when electromagnet is energized
6. **MRI Machines:** Powerful electromagnets create detailed body images for medical diagnosis
7. **Particle Accelerators:** Guide and focus charged particles using controlled magnetic fields

**Right Hand Grip Rule for Solenoids:**

“Grip the solenoid with your right hand such that your fingers curl in the direction of conventional current flow (from positive to negative terminal) through the coil. Your extended thumb will then point toward the North pole of the electromagnet.”

**Applications:**
• Determining polarity of electromagnets
• Designing motors and generators
• Understanding magnetic field direction in coils

**Note:** For a straight current-carrying wire, the rule is modified: thumb points in current direction, fingers show circular magnetic field direction.

Soft iron is preferred for transformer cores due to these properties:

• **High Magnetic Permeability:** Concentrates magnetic flux, improving energy transfer efficiency between primary and secondary coils
• **Low Hysteresis Loss:** Minimal energy wasted as heat during repeated magnetization cycles
• **Low Eddy Current Loss:** When laminated, reduces unwanted circulating currents
• **Easy Magnetization/Demagnetization:** Responds quickly to alternating current changes
• **High Saturation Point:** Handles strong magnetic fields without losing effectiveness

These characteristics ensure transformers operate efficiently with minimal energy loss, making soft iron essential for power distribution systems.

**Answer:**

When two bar magnets are stored in a wooden box:
• The North pole (N) of one magnet should face the South pole (S) of the other magnet (N-S arrangement)
• This arrangement helps preserve magnetism by completing the magnetic circuit

**Identification:**
• **P (Magnetic Connector):** A soft iron keeper placed across the poles to maintain magnetic flux and prevent demagnetization during storage
• **Q (Non-magnetic Connector):** A spacer made of wood, plastic, or other non-magnetic material to prevent magnets from sticking together or damaging each other

Proper storage with keepers extends the life and strength of permanent magnets.

**Answer:**

To magnetize a steel bar using a solenoid:

**Setup:**
• Place the steel bar inside a long coil of insulated copper wire (solenoid)
• Connect the solenoid to a DC power source (battery) through a switch

**Current Direction:**
• Using the Right Hand Grip Rule: Current should flow such that when fingers curl in current direction, thumb points toward end A (desired N-pole)
• This means current flows counterclockwise when viewed from end A

**Process:**
• When DC current flows through the solenoid, a strong magnetic field is generated
• The steel bar becomes magnetized with end A as North pole and end B as South pole
• Steel retains magnetism after current is switched off (permanent magnet)

**Diagram Elements:**
– Solenoid coil with multiple turns
– Steel bar centered inside coil
– Battery with positive/negative terminals
– Switch to control current flow
– Labels: End A (N-pole), End B (S-pole), current direction arrows

**Answer:**

**Analysis:**
When a compass needle settles in the north-south direction at the midpoint between two magnets, it indicates that the magnetic fields from both magnets are aligned with Earth’s magnetic field at that point.

**Pole Identification:**
• If the compass North pole points toward one magnet, that magnet’s facing end is a South pole (opposite poles attract)
• The other magnet’s facing end must be a North pole

**Field Line Justification:**
• Magnetic field lines emerge from North poles and enter South poles
• At the midpoint, field lines from both magnets combine with Earth’s field
• The compass aligns with the resultant field direction

**Diagram:**
– Draw two bar magnets with facing ends labeled N and S
– Show field lines curving from N to S of each magnet
– Include Earth’s field lines running geographically North-South
– Compass needle aligned with resultant field at center point

**Answer:**

Yes, the reverse process is true. A changing magnetic field can induce an electric current. This phenomenon is called **Electromagnetic Induction**, discovered by Michael Faraday and described by Faraday’s Law of Induction.

**Faraday’s Law:**
“The induced electromotive force (EMF) in any closed circuit is equal to the negative rate of change of magnetic flux through the circuit.”

**Example: Electric Generator**
• A coil of wire is rotated within a magnetic field (or a magnet rotates inside a coil)
• As the coil cuts through magnetic field lines, the magnetic flux through the coil changes continuously
• This changing flux induces an EMF, which drives an electric current in the coil
• Mechanical energy (rotation) is converted to electrical energy

**Applications:**
• Power plants (hydroelectric, thermal, nuclear)
• Bicycle dynamos
• Wind turbines
• Alternators in vehicles

This principle is fundamental to modern electricity generation and distribution.

**Answer:**

**Setup:**
Four identical solenoids arranged in a circle around center point O, labeled Top, Bottom, Right, and Left.

**Current Direction for Desired Field:**

**Solenoid 1 (Top):**
• Current: Counterclockwise (viewed from above)
• Result: Magnetic field at O points downward (toward solenoid when switched off)

**Solenoid 2 (Bottom):**
• Current: Counterclockwise (viewed from below)
• Result: Magnetic field at O points upward (toward solenoid when switched off)

**Solenoid 3 (Right):**
• Current: Clockwise (viewed from right)
• Result: Magnetic field at O points leftward (toward solenoid when switched off)

**Solenoid 4 (Left):**
• Current: Clockwise (viewed from left)
• Result: Magnetic field at O points rightward (toward solenoid when switched off)

**Explanation:**
Using the Right Hand Grip Rule for each solenoid:
• When current flows as specified, each solenoid produces a magnetic field at center O directed toward itself
• When one solenoid’s current is switched off, the net field from the remaining three solenoids points toward the inactive one
• This arrangement maintains symmetry and allows controlled magnetic field direction at the center

**Applications:** This principle is used in magnetic steering systems, particle beam focusing, and advanced electromagnetic devices.

**Methods to Identify Magnets vs. Magnetic Materials:**

**1. Repulsion Test (Most Reliable):**
• Bring the object near a known magnet
• If repulsion occurs, the object is a magnet (like poles repel)
• Magnetic materials are only attracted, never repelled

**2. Pole Identification:**
• Use a compass: A magnet will deflect the compass needle strongly at its poles
• Magnetic materials cause weak, uniform attraction without distinct poles

**3. Iron Filings Test:**
• Sprinkle iron filings around the object
• A magnet shows concentrated filings at poles with clear field line patterns
• Magnetic materials show uniform, weak attraction

**4. Suspension Test:**
• Suspend the object freely by a thread
• A magnet will align itself North-South due to Earth’s magnetic field
• Magnetic materials show no preferred orientation

**5. Breaking Test:**
• Break the object: A magnet produces two smaller magnets (each with N and S poles)
• Magnetic materials lose any induced magnetism when cut

**Key Difference:** All magnets are magnetic materials, but not all magnetic materials are magnets. Magnets produce their own persistent magnetic field; magnetic materials only respond to external fields.

**Magnetic Field Strength and Lines of Force:**

**Relationship:**
The strength of a magnetic field is represented by the density (closeness) of magnetic lines of force:
• **Stronger Field:** Lines are closer together (e.g., near magnet poles)
• **Weaker Field:** Lines are farther apart (e.g., far from magnet)

**Properties of Magnetic Lines of Force:**
1. Emerge from North pole, enter South pole externally
2. Form continuous closed loops (South to North internally)
3. Never intersect each other
4. Repel each other laterally
5. Tend to contract lengthwise (explains attraction between opposite poles)

**Diagram Examples:**

**1. Bar Magnet:**
“`
N ~~~~~~~~~~> S
|||||||||||||
(Lines dense at poles, sparse at sides)
“`

**2. Two Like Poles (Repulsion):**
“`
N <—-> N
Lines bend away, showing repulsive force
“`

**3. Two Unlike Poles (Attraction):**
“`
N ~~~~~~~~~~> S
Lines connect directly, showing attractive force
“`

**4. Earth’s Magnetic Field:**
“`
Geographic North ≈ Magnetic South
Field lines curve from magnetic South to North
“`

**Quantitative Measure:**
Magnetic field strength (B) is measured in Tesla (T) or Gauss (1 T = 10,000 Gauss). The number of field lines per unit area perpendicular to the field represents the magnetic flux density.

**Circuit Breaker:**

A circuit breaker is an automatic safety device that protects electrical circuits from damage caused by overload or short circuit by interrupting current flow.

**Working Principle (Electromagnetic Type):**

**Components:**
• Fixed contact and moving contact (carry current)
• Electromagnet coil (in series with circuit)
• Soft iron armature attached to moving contact
• Spring mechanism
• Trip lever and reset button

**Operation:**

**Normal Condition:**
• Current flows through coil and contacts
• Electromagnet produces weak magnetic field
• Spring holds contacts closed; circuit operates normally

**Overload/Short Circuit:**
• Excessive current flows through coil
• Electromagnet produces strong magnetic field
• Armature is pulled toward electromagnet
• Trip lever releases, spring opens contacts
• Circuit is broken; current stops flowing

**Reset Process:**
• After fault is corrected, press reset button
• Spring re-closes contacts; circuit resumes operation

**Advantages over Fuses:**
• Reusable (no replacement needed)
• Faster response time
• Can be manually switched off/on
• More precise current rating

**Applications:**
• Household electrical panels
• Industrial machinery protection
• Power distribution systems

**Correction and Explanation:**

The statement “A magnet attracts only a magnet” is **incorrect**. The accurate statement is: **”A magnet attracts magnetic materials, not only other magnets.”**

**What Magnets Attract:**

**1. Ferromagnetic Materials (Strongly Attracted):**
• Iron, nickel, cobalt, and their alloys
• These materials become temporarily magnetized when near a magnet
• Example: A magnet picks up iron nails (nails are not permanent magnets)

**2. Paramagnetic Materials (Weakly Attracted):**
• Aluminum, platinum, oxygen
• Attraction is very weak and usually not noticeable without sensitive equipment

**3. Other Magnets:**
• Opposite poles attract (N-S)
• Like poles repel (N-N or S-S)

**What Magnets Do NOT Attract:**

**1. Diamagnetic Materials (Weakly Repelled):**
• Copper, silver, gold, water, wood, plastic
• These materials create an opposing magnetic field

**2. Non-magnetic Materials:**
• Most organic materials, glass, rubber
• No significant magnetic interaction

**Why the Confusion?**
• People observe magnets attracting iron objects and assume those objects must be magnets
• In reality, the magnet induces temporary magnetism in ferromagnetic materials

**Key Concept:** Magnetism is a property of materials, not just permanent magnets. A magnet can attract any material that responds to magnetic fields, with ferromagnetic materials showing the strongest response.

**Comparison Based on Domain Theory:**

Property	Ferromagnetic	Paramagnetic	Diamagnetic
Domain Structure	Large domains with aligned atomic magnetic moments	No permanent domains; individual atoms have magnetic moments	No permanent domains; all electron spins paired
Response to External Field	Domains align strongly with field; material becomes strongly magnetized	Atomic moments weakly align with field	Induced moments oppose external field
Magnetic Susceptibility	Large and positive (10³ to 10⁵)	Small and positive (10⁻⁵ to 10⁻³)	Small and negative (-10⁻⁵ to -10⁻⁹)
Retention of Magnetism	Retains magnetism after field removal (permanent magnets possible)	Loses magnetism immediately when field removed	Never retains magnetism
Temperature Dependence	Loses ferromagnetism above Curie temperature	Susceptibility ∝ 1/T (Curie’s Law)	Nearly independent of temperature
Examples	Iron, nickel, cobalt, steel	Aluminum, platinum, oxygen	Copper, silver, gold, water, bismuth
Attraction/Repulsion	Strongly attracted to magnets	Weakly attracted to magnets	Very weakly repelled by magnets

**Domain Theory Explanation:**

**Ferromagnetic Materials:**
• Contain regions (domains) where atomic magnetic moments are spontaneously aligned
• External field causes domain walls to move, aligning more domains with the field
• After field removal, some alignment remains (hysteresis), creating permanent magnetism

**Paramagnetic Materials:**
• Individual atoms have magnetic moments, but thermal motion prevents spontaneous alignment
• External field causes slight preferential alignment of atomic moments
• Thermal agitation randomizes moments when field is removed

**Diamagnetic Materials:**
• All electrons are paired; no permanent atomic magnetic moments
• External field induces tiny opposing magnetic moments via Lenz’s law at atomic level
• Effect is universal but very weak; overshadowed by para/ferromagnetism when present

**Reasons Ferromagnetic Materials Are Ideal for Magnets:**

**1. Strong Magnetic Response:**
• Ferromagnetic materials have high magnetic permeability (μ >> μ₀)
• They concentrate magnetic flux, producing strong external fields
• Can be magnetized to high saturation levels

**2. Domain Alignment Capability:**
• Contain magnetic domains that can be aligned by external fields
• Once aligned, domains tend to stay aligned (high retentivity)
• This enables creation of permanent magnets

**3. High Coercivity (for hard ferromagnets):**
• Materials like steel, alnico, neodymium resist demagnetization
• Maintain magnetism despite external disturbances (temperature, vibration, stray fields)
• Essential for permanent magnet applications

**4. Customizable Properties:**
• Alloying and heat treatment can tailor magnetic properties
• Soft ferromagnets (iron): Low coercivity for electromagnets/transformers
• Hard ferromagnets (steel, rare-earth alloys): High coercivity for permanent magnets

**5. Practical Advantages:**
• Relatively abundant and cost-effective (iron, steel)
• Can be shaped, machined, and integrated into devices
• Compatible with various magnetization techniques (stroking, electric current, induction)

**Applications Enabled:**
• Permanent magnets: Speakers, motors, magnetic separators
• Electromagnet cores: Transformers, relays, MRI machines
• Magnetic storage: Hard drives, credit card strips

**Limitation Note:** Ferromagnets lose magnetism above their Curie temperature (e.g., iron: 770°C), which must be considered in high-temperature applications.