Class 12 Physics Notes Chapter 5 (Chapter 5) – Lab Manual (English) Book
Detailed Notes with MCQs of the key experiments and concepts from Chapter 5 of your Physics Lab Manual, which are crucial for understanding Earth's magnetism and related measurements. These topics frequently appear in various government examinations.
Chapter 5: Earth's Magnetism and Magnetic Elements
Core Concepts:
-
Earth's Magnetism:
- The Earth behaves as if a giant bar magnet is buried deep inside it.
- The Geographic North (NG) and Geographic South (SG) poles are the points defining the axis of Earth's rotation.
- The Magnetic North (NM) and Magnetic South (SM) poles are points where the Earth's magnetic field lines emerge or enter vertically. They do not coincide with the geographic poles.
- Geographic Meridian: A vertical plane passing through the geographic North and South poles at a given place.
- Magnetic Meridian: A vertical plane passing through the magnetic North and South poles at a given place. A freely suspended magnetic needle aligns itself in this plane.
-
Magnetic Elements of Earth: These quantities describe the Earth's magnetic field at a particular location.
- (a) Magnetic Declination (θ or α):
- Definition: The angle between the geographic meridian and the magnetic meridian at a place.
- Significance: It gives the direction of the magnetic North relative to the true North. Used for navigation.
- (b) Angle of Dip or Inclination (δ):
- Definition: The angle made by the direction of the Earth's total magnetic field (BE) with the horizontal direction in the magnetic meridian.
- Values: At the magnetic equator, δ = 0°. At the magnetic poles, δ = 90°.
- (c) Horizontal Component of Earth's Magnetic Field (BH):
- Definition: The component of the Earth's total magnetic field (BE) along the horizontal direction in the magnetic meridian.
- Significance: This component influences compass needles and is used in experiments like the tangent galvanometer and vibration magnetometer.
- Relationship with BE and δ: BH = BE cos δ
- Vertical Component (BV): BV = BE sin δ
- Resultant Field: BE = √(BH² + BV²)
- Relation between components and dip: tan δ = BV / BH
- (a) Magnetic Declination (θ or α):
Key Experiments and Instruments:
-
To determine the Angle of Dip (δ) using a Dip Circle:
- Apparatus: Dip Circle (a magnetic needle pivoted horizontally, free to rotate in a vertical plane, mounted on a vertical circular scale, which can rotate about a vertical axis over a horizontal scale).
- Principle: When the vertical circular scale of the dip circle is oriented exactly in the magnetic meridian, the magnetic needle aligns itself along the direction of the Earth's total magnetic field (BE), and the angle it makes with the horizontal is the angle of dip (δ).
- Procedure Outline:
- Level the base using levelling screws.
- Rotate the vertical scale until the needle reads 90° (becomes vertical). This sets the plane perpendicular to the magnetic meridian.
- Rotate the entire setup by 90° using the horizontal scale. Now the vertical scale is in the magnetic meridian.
- Read the angles shown by both ends of the needle (δ1, δ2).
- Reverse the needle in its bearings and read angles again (δ3, δ4).
- Rotate the instrument by 180° and repeat readings.
- Calculate the mean angle of dip.
- Precautions: Level base, clean pivots, avoid parallax error, keep away from other magnetic materials.
-
To determine the Horizontal Component of Earth's Magnetic Field (BH) using a Tangent Galvanometer:
- Apparatus: Tangent Galvanometer (TG), Battery, Rheostat, Ammeter, Commutator (reversing key), Connecting wires.
- Principle: Tangent Law. When a magnetic field (B) produced by a current-carrying coil is applied perpendicular to an existing uniform magnetic field (BH), a magnetic needle placed at the intersection point deflects such that it aligns itself along the resultant field, making an angle θ with the direction of BH. Mathematically, B = BH tan θ.
- Working: The coil of the TG is set in the magnetic meridian (so BH is parallel to the plane of the coil). The magnetic field (B) produced by the current (I) at the center of the coil is perpendicular to the plane of the coil (and hence perpendicular to BH).
- Field due to coil: B = (μ₀ n I) / (2r), where n = number of turns, r = radius of the coil, I = current.
- Formula Derivation: From Tangent Law, B = BH tan θ
(μ₀ n I) / (2r) = BH tan θ
Therefore, BH = (μ₀ n I) / (2r tan θ) or BH = K (I / tan θ), where K = (μ₀ n) / (2r) is the reduction factor of the TG. - Procedure Outline:
- Set up the circuit. Level the TG.
- Set the coil plane in the magnetic meridian (adjust until the needle reads 0-0 without current).
- Pass current (I), measure the deflection θ (average of both ends).
- Reverse the current using the commutator and measure deflection again.
- Take readings for different values of I, ensuring deflection is between 30° and 60° for better accuracy.
- Calculate BH using the formula.
- Precautions: Coil must be vertical and in the magnetic meridian, avoid parallax, keep other magnetic materials away, ensure deflection is within the optimal range.
-
(Related Concept) Vibration Magnetometer: (Sometimes used to determine BH or compare magnetic moments)
- Principle: A freely suspended bar magnet oscillates in a uniform horizontal magnetic field (like BH) with simple harmonic motion (for small amplitudes).
- Time Period (T): T = 2π √(I / (m BH)), where I = moment of inertia of the magnet, m = magnetic moment of the magnet.
- Determining BH: If 'I' and 'm' are known, BH can be calculated by measuring T: BH = (4π² I) / (m T²).
- Comparing Fields: If the same magnet oscillates in two fields B1 and B2, then (T1/T2)² = B2/B1.
- Comparing Moments: If two magnets (m1, I1) and (m2, I2) oscillate in the same field BH, then (T1/T2)² = (I1/m1) / (I2/m2).
Summary of Formulas:
- BH = BE cos δ
- BV = BE sin δ
- tan δ = BV / BH
- BE = √(BH² + BV²)
- Tangent Law: B = BH tan θ
- BH using TG: BH = (μ₀ n I) / (2r tan θ)
- Time Period (Vibration Magnetometer): T = 2π √(I / (m BH))
Units:
- Magnetic Field (B, BE, BH, BV): Tesla (T) or Gauss (G) [1 G = 10⁻⁴ T]
- Angle (θ, δ): Degrees or Radians
- Current (I): Ampere (A)
- Magnetic Moment (m): Ampere-meter² (Am²)
Multiple Choice Questions (MCQs):
-
At the magnetic poles of the Earth, the angle of dip is:
(a) 0°
(b) 45°
(c) 90°
(d) 180° -
The vertical plane passing through the geographic north and south poles at a place is called:
(a) Magnetic Meridian
(b) Geographic Meridian
(c) Magnetic Equator
(d) Geographic Equator -
A dip circle is initially in the magnetic meridian. If it is rotated by 90°, the dip needle will:
(a) Stay horizontal
(b) Stay vertical
(c) Show the true angle of dip
(d) Show an apparent dip of 90° -
The tangent galvanometer works on the principle of:
(a) Ampere's Law
(b) Biot-Savart Law
(c) Tangent Law
(d) Ohm's Law -
In the formula for BH using a tangent galvanometer, BH = (μ₀ n I) / (2r tan θ), the term (μ₀ n) / (2r) is known as:
(a) Resistance
(b) Reduction Factor
(c) Magnetic Permeability
(d) Deflection Constant -
If BE is the total intensity of Earth's magnetic field, BH is the horizontal component, and δ is the angle of dip, the correct relationship is:
(a) BH = BE tan δ
(b) BH = BE sin δ
(c) BH = BE / cos δ
(d) BH = BE cos δ -
The angle between the geographic meridian and the magnetic meridian at a place is called:
(a) Angle of dip
(b) Magnetic inclination
(c) Magnetic declination
(d) Magnetic latitude -
For maximum sensitivity and accuracy in a tangent galvanometer experiment, the deflection θ should ideally be around:
(a) 0°
(b) 30°
(c) 45°
(d) 90° -
The time period (T) of oscillation of a magnet in a vibration magnetometer is related to the horizontal component of Earth's field (BH) as:
(a) T ∝ BH
(b) T ∝ √BH
(c) T ∝ 1/√BH
(d) T ∝ 1/BH -
The SI unit of the horizontal component of Earth's magnetic field (BH) is:
(a) Ampere (A)
(b) Weber (Wb)
(c) Tesla (T)
(d) Henry (H)
Answer Key for MCQs:
- (c)
- (b)
- (b) - In a plane perpendicular to the magnetic meridian, only the vertical component acts on the needle, making it vertical.
- (c)
- (b)
- (d)
- (c)
- (c) - Sensitivity is maximum when tan θ is measured accurately, which occurs around 45°.
- (c)
- (c)
Study these concepts thoroughly, focusing on the definitions, principles of the instruments, and the formulas involved. Good luck with your preparation!