2026 Exam Prep Questions for G22 MBBS - Electricity & Electrostatics

Practice flashcards covering key questions, formulas, and calculations for Electricity and Electrostatics.

What must be the distance $r$ between point charges $q_1 = 26.0\,\mu C$ and $q_2 = 47.0\,\mu C$ for the electrostatic force between them to have a magnitude of $5.70\,N$?

$r \approx 1.39\,m$. Calculated using Coulomb's law: $r = \sqrt{\frac{k q_1 q_2}{F}} = \sqrt{\frac{(9 \times 10^9)(26 \times 10^{-6})(47 \times 10^{-6})}{5.70}} \approx 1.39\,m$.

Two parallel horizontal metal plates are $25\,mm$ apart with a potential difference of $1000\,V$ applied across them. What is the electric field intensity between the plates?

$E = 4.0 \times 10^4\,V/m$ ($40,000\,N/C$). Calculated using $E = \frac{V}{d} = \frac{1000}{25 \times 10^{-3}} = 40,000\,V/m$.

If $30\,J$ of work is done transferring $5\,mC$ of charge from point B to point A in an electric field, what is the potential difference between B and A?

$V = 6000\,V$ ($6.0 \times 10^3\,V$). Potential difference is work done per unit charge: $V = \frac{W}{q} = \frac{30}{5 \times 10^{-3}} = 6000\,V$.

What is the net electric flux through a closed surface enclosing a net charge of $2.5\,\mu C$?

$\Phi \approx 2.82 \times 10^5\,N \cdot m^2/C$. Using Gauss's law: $\Phi = \frac{Q_{\text{enclosed}}}{\varepsilon_0} = \frac{2.5 \times 10^{-6}}{8.85 \times 10^{-12}} \approx 2.82 \times 10^5\,N \cdot m^2/C$.

If two charged small spheres a distance $R$ apart exert an electrostatic force $F$ on each other, what force is exerted if the distance is halved to $\frac{R}{2}$?

$4F$. Force is inversely proportional to the square of the distance ($F \propto \frac{1}{r^2}$), so $\text{F}' \propto \frac{1}{(\frac{R}{2})^2} = \frac{4}{R^2}$.

What maximum torque does an $HCl$ molecule with dipole moment $3.4 \times 10^{-30}\,C \cdot m$ experience in a $2.5 \times 10^5\,N/C$ electric field?

$\tau_{\text{max}} = 8.5 \times 10^{-25}\,N \cdot m$. Torque is maximum when $\theta = 90^\circ$ (where $\sin(90^\circ) = 1$): $\tau_{\text{max}} = pE = (3.4 \times 10^{-30})(2.5 \times 10^5) = 8.5 \times 10^{-25}\,N \cdot m$.

If the electric field magnitude is $E$ at a distance $d$ from a point charge, at what distance will its magnitude be $\frac{E}{4}$?

$2d$. For a point charge, $E \propto \frac{1}{r^2}$. For the field to drop to $\frac{E}{4}$, $r^2$ must increase by a factor of 4, meaning $r$ doubles to $2d$.

What is the electric field midway between two point charges, each equal to $30\,mC$, separated by $4\,cm$?

$E = 0\,N/C$. The charges are equal in sign and magnitude, so their electric fields at the midpoint are equal in magnitude ($6.75 \times 10^{11}\,N/C$) but point in opposite directions, canceling out completely.

Using Gauss's theorem, what is the electric flux of a field $E = 100\,N/C$ in the x-direction through a plane of area $A$ lying in the YZ plane?

$\Phi = EA = 100A\,N \cdot m^2/C$. The plane's normal points along the x-axis ($\theta = 0^\circ$), so $\Phi = EA \cos(0^\circ) = 100A\,N \cdot m^2/C$.

What is the surface charge density $\sigma$ on a sphere of radius $R = 1\,cm$ with a total charge $Q = 1\,\mu C$ uniformly distributed on its surface?

$\sigma \approx 7.96 \times 10^{-4}\,C/m^2$. Calculated as $\sigma = \frac{Q}{A} = \frac{Q}{4\pi R^2} = \frac{1 \times 10^{-6}}{4\pi (0.01)^2} \approx 7.96 \times 10^{-4}\,C/m^2$.

What plate area $A$ is required to construct a $1.0\,F$ parallel-plate capacitor with a plate separation of $1.0\,mm$?

$A \approx 1.13 \times 10^8\,m^2$. From $C = \frac{\varepsilon_0 A}{d}$, $A = \frac{Cd}{\varepsilon_0} = \frac{(1.0)(1.0 \times 10^{-3})}{8.85 \times 10^{-12}} \approx 1.13 \times 10^8\,m^2$ (approx. $113\,km^2$).

What is the equivalent capacitance $C_{\text{eq}}$ of three capacitors ($1\,\mu F$, $2\,\mu F$, and $3\,\mu F$) connected in series?

$C_{\text{eq}} = \frac{6}{11}\,\mu F \approx 0.545\,\mu F$. In series: $\frac{1}{C_{\text{eq}}} = \frac{1}{1} + \frac{1}{2} + \frac{1}{3} = \frac{11}{6}$, so $C_{\text{eq}} = \frac{6}{11}\,\mu F$.

How many time constants $t$ must elapse before a capacitor in an RC circuit is charged to within $1.0\%$ of its equilibrium charge?

$t \approx 4.6\text{ time constants}$ ($\ln(100) \approx 4.605$). During charging $Q = Q_0(1 - e^{-t/\tau})$. At $99\%$ charge, $e^{-t/\tau} = 0.01 \implies \frac{t}{\tau} = \ln(100) \approx 4.6$.

An electric motor takes a current of $1.5\,A$ at $250\,V$. If its efficiency is $80\%$, what is its power output?

$P_{\text{out}} = 300\,W$. Power input $P_{\text{in}} = IV = (1.5)(250) = 375\,W$. Power output $= 0.80 \times 375 = 300\,W$.

An electric oven element is rated at ($240\,V$, $2000\,W$). What is its electrical resistance?

$R = 28.8\,\Omega$. Using $P = \frac{V^2}{R}$, $R = \frac{V^2}{P} = \frac{(240)^2}{2000} = 28.8\,\Omega$.

A circuit operates on a current of $2.5\,mA$. How many electrons pass through this circuit in $1.0\,s$?

$N \approx 1.56 \times 10^{16}\text{ electrons}$. Total charge $Q = It = (2.5 \times 10^{-3})(1.0) = 2.5 \times 10^{-3}\,C$. Number of electrons $N = \frac{Q}{e} = \frac{2.5 \times 10^{-3}}{1.60 \times 10^{-19}} \approx 1.56 \times 10^{16}$.

How can a galvanometer of coil resistance $200\,\Omega$ that gives full-scale deflection at $10\,mA$ be converted into a voltmeter reading up to $100\,V$?

Connect a $9800\,\Omega$ ($9.8\,k\Omega$) resistor in series with the galvanometer. Total resistance needed $R_{\text{total}} = \frac{V}{I_g} = \frac{100}{10 \times 10^{-3}} = 10,000\,\Omega$. Series resistance $R = 10,000 - 200 = 9800\,\Omega$.

A potentiometer wire of length $100\,cm$ balances a standard cell of emf $1.5\,V$ at $60.0\,cm$. What is the emf of a cell that balances at $80.0\,cm$?

$E_2 = 2.0\,V$. On a potentiometer, emf is directly proportional to balance length: $\frac{E_1}{l_1} = \frac{E_2}{l_2} \implies E_2 = 1.5 \times \frac{80.0}{60.0} = 2.0\,V$.

What is the electric field inside a spherical shell of uniform surface charge density?

Zero. By Gauss's law, any Gaussian surface inside the shell encloses zero net charge, so the electric field is zero everywhere inside.

How is the electric potential difference between two points in an electric field defined?

The work done per unit charge in moving a positive test charge from one point to the other ($V = \frac{W}{q}$).

An electron is shot horizontally into a vacuum chamber containing a uniform downward electric field. What is its acceleration?

Upward (opposite to the field direction), constant in magnitude: $a = \frac{eE}{m}$.

A parallel plate air capacitor has a capacitance of $2\,\mu F$. What is its capacitance when filled with a dielectric of dielectric constant $6.0$?

$12\,\mu F$. Adding a dielectric multiplies the original capacitance by $K$: $C = K \times C_0 = 6.0 \times 2\,\mu F = 12\,\mu F$.

Why is a potentiometer used to accurately measure the internal resistance of a cell with a tiny internal resistance ($0.01\,\Omega$)?

A potentiometer draws no current from the cell at balance, allowing it to compare potential differences accurately without potential drop errors.

What is the electric field intensity $E$ just outside a charged plane conductor in air?

$E = \frac{\sigma}{\varepsilon_0}$, where $\sigma$ is the surface charge density and $\varepsilon_0$ is the permittivity of free space.

By what method can both electrical insulators and conductors be charged?

Contact (touching or rubbing with a charged object).

How does moving the plates of a parallel-plate capacitor closer together affect its capacitance?

It increases the capacitance, because capacitance is inversely proportional to plate separation ($C = \frac{\varepsilon_0 A}{d}$).

How is Kirchhoff's first rule (junction rule) mathematically expressed and what principle does it represent?

$\sum I = 0$. It represents the law of conservation of electric charge at any circuit junction.