Revision: Magnetic Effects of Current and Magnetism CUET (UG) Magnetic Effects of Current and Magnetism

The force experienced by a moving charge in the presence of a magnetic field, which depends on charge q, velocity v and magnetic field B, and which is opposite in direction on a negative charge compared to a positive charge, is called the magnetic force.

When both electric and magnetic fields act on a charge, the total force is called the Lorentz force.

Current passed through each of the two infinitely long parallel straight conductors kept at a distance of one meter apart in vacuum causes each conductor to experience a force of 2 × 10^-7 newton per meter length of the conductor.

A long solenoid is a coil whose length is much greater than its radius, producing a uniform magnetic field inside and nearly zero field outside.

OR

A solenoid is a long helical coil of insulated wire with many closely spaced turns, whose length l is much greater than its radius R (i.e., l ≫ R), such that it produces a strong, uniform magnetic field inside and a negligible field outside.

A device used to accelerate positively charged particles (α particles, deutrons, protons, etc.) to acquire enough energy to carry out nuclear disintegration is called a cyclotron.

The ampere is that constant current which, when maintained in each of two infinitely long, straight, parallel conductors of negligible circular cross-section, placed 1 metre apart in a vacuum, produces a force of 2 × 10⁻⁷ N per metre of length between them.

The turning effect experienced by a current-carrying loop placed in a uniform magnetic field, which forms the working principle of a moving coil galvanometer (MCG), is called torque on a current loop.

The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer when a unit current flows through it.  
Mathematically, it can be given by:

I_S = `(NBA)/k`

Where k is the couple per unit twist.

Current sensitivity is defined as the deflection e per unit current.

A Moving Coil Galvanometer (MCG) is a sensitive electromagnetic instrument used to detect and measure small electric currents (of the order of microamperes to milliamperes) by measuring the deflection of a current-carrying coil placed in a uniform magnetic field.

Deflection produced per unit current.

Deflection produced per unit voltage.

The current required to produce a unit deflection (1 division) on the scale.

The temperature above which a ferromagnetic substance becomes paramagnetic is called curie temperature. 

A line which joins all the places on earth, having zero angle of declination is called agonic line.

A line joining all the places on globe, having same angle of dip or inclination is called isoclinic line.

The path in a magnetic field in which a unit north pole tends to move when allowed to do so is known as magnetic field lines of force.

A line that joins all the places on earth, having the same angle of declination is called an isogonic line.

Substances which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field are called paramagnetic substances.

Substances which when placed in a magnetising field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.

Substances which when placed in a magnetic field are feebly magnetised in a direction opposite to that of the magnetising field are called diamagnetic substances.

Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.

\[\vec{E}=\frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\hat{r}\]

\[\vec F\] = q(\[\vec E\] + \[\vec v\] × \[\vec B\])

Vector Form: \[\vec F\] = q(\[\vec v\] × \[\vec B\])

Magnitude Form: F = qv B sin θ

Where:

• q = charge on the particle
• v = speed of the particle
• B = magnetic field strength
• θ = angle between \[\vec v\] and \[\vec B\]

Maximum magnetic force (when v ⊥ B): F_max = qv B

Combining the four dependencies: dB ∝ \[\frac {I dl sin θ}{r^2}\]​

Introducing the constant of proportionality \[\frac {μ_0}{4π}\]​​:

\[dB=\frac{\mu_0}{4\pi}\cdot\frac{Idl\sin\theta}{r^2}\]

\[{d\vec{B}=\frac{\mu_0}{4\pi}\cdot\frac{Id\vec{l}\times\hat{r}}{r^2}=\frac{\mu_0}{4\pi}\cdot\frac{Id\vec{l}\times\vec{r}}{r^3}}\]

For a finite conductor, integrate over the entire length:

\[{\vec{B}=\frac{\mu_0I}{4\pi}\int\frac{d\vec{l}\times\hat{r}}{r^2}}\]

\[\vec{B}=\frac{\mu_0IR^2}{2(x^2+R^2)^{3/2}}\hat{i}\]

Where:

• I = current
• R = radius of loop
• x = distance from centre along axis
• μ0 = permeability of free space

F = IL × B

B = μ₀​nI

Where:

• μ_0​ = permeability of free space
• n = number of turns per unit length
• I = current

Final energy in cyclotron: proportional to \[\ R_{exit}^2\]

f_a​ = f_c​

mv = p = q BR

Centripetal force provided by magnetic force: \[\frac{mv^2}{r}=qvB\]

Angular speed = \[\omega=\frac{qB}{m}\]

Period of circular motion = \[T=\frac{2\pi m}{qB}\]

Frequency = \[f=\frac{qB}{2\pi m}\]

\[F=BIL\sin\theta\]

Vector Form:

\[\vec{F}=I(\vec{L}\times\vec{B})\]

Special Cases:

• θ = 90^∘ → F = BILF 
• θ = 0^∘ → F = 0

\[F=\frac{\mu_0I_1I_2}{2\pi d}\times l\]

Per unit length:

\[\frac{F}{l}=\frac{\mu_0I_1I_2}{2\pi d}\]

Force acts along the line joining the wires

k = \[\frac{I}{\phi}=\frac{C}{NAB}\]

k is the reciprocal of current sensitivity. A galvanometer with a smaller figure of merit is more sensitive.

CS = \[\frac{\phi}{I}=\frac{NAB}{C}\]

Unit: div/A or div/μA

VS = \[\frac{\phi}{V}=\frac{NAB}{CG}\]

where G = resistance of the galvanometer coil.

Unit: div/V

\[B=\frac{\mu_0}{4\pi}\cdot\frac{m}{d^3}\]

\[B=\frac{\mu_0}{4\pi}\cdot\frac{2m}{d^3}\]

\[\tau=MB\sin\theta\]

Vector form: \[\vec{\tau}=\vec{M}\times\vec{B}\]

If we stretch the index finger, middle finger and thumb of the left hand mutually perpendicular to each other such that the index finger points along the direction of the magnetic field and the middle finger along the direction of current (moving charge), then the thumb represents the direction of the force F experienced by the moving charge.

The magnitude of magnetic induction (dB) at a point due to a small element of current carrying conductor is:
(i) directly proportional to current (dB ∝ I),
(ii) directly proportional to length of element (dB ∝ dl),
(iii) directly proportional to sine of angle between element and line joining its centre to the point (dB ∝ sin θ),
(iv) inversely proportional to square of distance (dB ∝ 1/r²).

Applications

• Magnetic field at centre of circular coil.
• Magnetic field on axis of the coil.
• Magnetic field at a distance from a straight current-carrying conductor.

Statement

The line integral \[\oint\vec{B}\cdot d\vec{l}\] taken around any closed loop equals μ₀ times the net steady current passing through the loop.

Proof (for a long straight wire)

• Consider an infinitely long straight wire carrying current I.

• By Biot–Savart law, field at distance r:
  B = \[\frac{\mu_0I}{2\pi r}\]

• Choose a circular Amperian loop of radius r, concentric with the wire.

• By symmetry, B is constant in magnitude and tangential (parallel to \[d\vec l\]) everywhere:
  \[\oint\vec{B}\cdot d\vec{l}=B\oint dl\] = B(2πr)

• Substituting B:
  \[\oint\vec{B}\cdot d\vec{l}=\frac{\mu_0I}{2\pi r}(2\pi r)\] = μ₀​I

Conclusion

\[\oint\vec{B}\cdot d\vec{l}=\mu_0I\]
The result is independent of the loop's radius, confirming the law's validity.

If the thumb, forefinger and middle finger of the left hand are stretched mutually perpendicular to each other, and the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb gives the direction of the force acting on the conductor.

Step 1: Torque due to current (Deflecting Couple):

• When current I flows through a coil of N turns, area A, in a field B: τ_deflecting = N I A B (Since radial field: sin⁡90° = 1)

Step 2: Restoring Torque (Spring):

• The phosphor-bronze strip/spring opposes the deflection. If ϕ is the angular deflection and C (or k) is the torsional constant of the spring, τ_restoring = Cϕ

Step 3: Equilibrium Condition:

• At equilibrium, deflecting torque = restoring torque: NIAB = Cϕ

Step 4: Current–Deflection Relationship:

Tangent law states that, if a magnetic field ‘B’ is applied at right angles to the horizontal component of the earth's field B_H, the needle comes to equilibrium at an angle ‘$\theta ’$ to the magnetic meridian such that, tan θ = `B/B_H`.

The magnetic susceptibility of a paramagnetic material varies inversely with its absolute temperature. Mathematically,

On cooling, paramagnetic substances get converted to ferromagnetic materials at the Curie temperature.

For ferromagnetic substances above the Curie temperature, the magnetic susceptibility is inversely proportional to (T − T_C), where T_C is the Curie temperature. Mathematically,

On heating beyond the Curie temperature (T_C(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.

• Electric field accelerates the particle; magnetic field keeps it in circular orbit of constant frequency.
• Resonance: polarity of Ds reverses as ion crosses gap after each semicircle.
• Cannot accelerate neutrons (uncharged) or electrons (small mass, high velocity).
• Ion speed is limited.

• A current-carrying conductor placed in a magnetic field experiences a force when the direction of current is not parallel to the magnetic field.
• The direction of force reverses when the direction of current or the direction of magnetic field is reversed, and no force acts when current flows parallel to the magnetic field.

• Torque depends on current, magnetic field strength and area of the loop.
• For a given perimeter, a circular loop experiences maximum torque (maximum area).
• Forms the working principle of the moving coil galvanometer (MCG).

• A current-carrying loop behaves like a magnetic dipole (bar magnet)
  
  
• Polarity Rule 
  Anticlockwise current → North pole (upper face)
  Clockwise current → South pole (lower face)

• A bar magnet behaves like a solenoid
• Both produce similar magnetic field patterns
• Solenoid Relation: M = NIA

• Diamagnetic substances are weakly repelled and have negative susceptibility.
• Paramagnetic substances are weakly attracted and obey Curie law.
• Ferromagnetic substances are strongly attracted and contain domains.
• Ferromagnets become paramagnetic above the Curie temperature.
• The comparison table is the most important revision tool for board preparation.