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Karnataka PUC II Electric Charges and Fields — practice questions

Explore 105 free Electric Charges and Fields questions in Karnataka PUC II Physics. Open each question or exercise to read its content, reference answers and available explanation.

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Electric Charges and Fields questions and exercises with answers

Showing questions 1–30 of 105.

A convex lens forms a real image at $60$ cm on one side of an object placed $20$ cm in front of it. Find the focal length of the lens.Read the answer and worked solution →When light passes from one medium to another, which property remains **unchanged**?Read the answer and worked solution →If 10⁹ electrons leave a body every second, the time required to accumulate a total charge of 1 C is approximately:Read the answer and worked solution →The electric field on the **equatorial line** of a dipole (perpendicular bisector, r >> a) has magnitude:Read the answer and worked solution →If a point charge is placed **outside** a closed Gaussian surface, the total flux through the surface is:Read the answer and worked solution →The principle that charge can be neither created nor destroyed is:Read the answer and worked solution →Which of the following is an insulator under normal conditions?Read the answer and worked solution →A body acquires a negative charge byRead the answer and worked solution →Two point charges of 2 μC each are placed 1 m apart in vacuum. The magnitude of the Coulomb force between them is closest toRead the answer and worked solution →Which scientist first experimentally demonstrated the quantisation of electric charge through his oil-drop experiment?Read the answer and worked solution →Approximately how many electrons are present in a charge of −1 C?Read the answer and worked solution →A cubic centimetre of copper contains roughly how many electrons in total?Read the answer and worked solution →If 10⁹ electrons leave a body every second, approximately how long is needed for the remaining body to develop a net positive charge of 1 C?Read the answer and worked solution →The reciprocal of resistivity ρ is calledRead the answer and worked solution →A Wheatstone bridge with arms P, Q, R and S is balanced whenRead the answer and worked solution →The force on a charge q moving with velocity v in a magnetic field B (Lorentz magnetic force) isRead the answer and worked solution →A current-carrying straight wire of length l in a magnetic field B experiences force of magnitudeRead the answer and worked solution →Electric potential is a:Read the answer and worked solution →An equipotential surface is perpendicular to:Read the answer and worked solution →Capacitance of a parallel-plate capacitor is:Read the answer and worked solution →Energy stored in a capacitor equals:Read the answer and worked solution →Capacitors in series share:Read the answer and worked solution →Time constant of an RC circuit is:Read the answer and worked solution →The smallest charge that can exist freely in nature isRead the answer and worked solution →An electric dipole of dipole moment $\vec{p}$ is placed in a UNIFORM electric field $\vec{E}$. The torque experienced isRead the answer and worked solution →The electric field at a distance $r$ from a POINT charge $q$ (in vacuum) has magnitudeRead the answer and worked solution →The SI unit of electric potential isRead the answer and worked solution →Electric potential at infinity in the standard syllabus convention is taken asRead the answer and worked solution →The SI unit of capacitance isRead the answer and worked solution →Electric field at any point inside a hollow charged conductor isRead the answer and worked solution →

Worked practice questions

A convex lens forms a real image at $60$ cm on one side of an object placed $20$ cm in front of it. Find the focal length of the lens.

  1. $10$ cm
  2. $12$ cm
  3. $15$ cm
  4. $20$ cm
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Correct answer: C. $15$ cm

Lens formula: $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$. Sign convention: $u = -20$ cm (object on the left), $v = +60$ cm (real image on the right). We have $\dfrac{1}{f} = \dfrac{1}{60} - \dfrac{1}{-20}$. That is $\dfrac{1}{f} = \dfrac{1}{60} + \dfrac{3}{60}$. That is $\dfrac{1}{f} = \dfrac{4}{60}$. So $\dfrac{1}{f} = \dfrac{1}{15}$. So $f = 15$ cm. **Trap.** Forgetting the sign on $u$ gives $\dfrac{1}{f} = \dfrac{1}{60} - \dfrac{1}{20} = -\dfrac{1}{30}$, leading to a negative $f$ — that's how option B (12) or A (10) creeps in.

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When light passes from one medium to another, which property remains **unchanged**?

  1. Wavelength
  2. Speed
  3. Frequency
  4. Amplitude
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Correct answer: C. Frequency

**Frequency** is set by the source and doesn't change at a boundary (otherwise oscillating dipoles at the interface would be discontinuous). Wavelength and speed both change in proportion: $\lambda_2 = \lambda_1 \, n_1/n_2$, $v = c/n$.

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If 10⁹ electrons leave a body every second, the time required to accumulate a total charge of 1 C is approximately:

  1. 200 days
  2. 200 hours
  3. 200 years (≈ 6.25 × 10⁹ seconds)
  4. 2 hours
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Correct answer: C. 200 years (≈ 6.25 × 10⁹ seconds)

**Charge per second:** 10⁹ × 1.6 × 10⁻¹⁹ = 1.6 × 10⁻¹⁰ C/s. **Time for 1 C:** 1 / (1.6 × 10⁻¹⁰) = 6.25 × 10⁹ s = 6.25 × 10⁹ / (365 × 24 × 3600) ≈ **198 years ≈ 200 years**. Shows 1 C is a very large unit for electrostatics.

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