What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Define drift velocity.

The average velocity acquired by free electrons in a conductor under an applied electric field, \(v_d=eE\tau/m\).

State Ohm's law and its limitation.

\(V=IR\) at constant temperature; it fails for non-ohmic devices like diodes and at high fields.

How does resistance depend on dimensions?

\(R=\rho l/A\): it increases with length and decreases with cross-sectional area.

How does resistivity of a metal vary with temperature?

It increases approximately linearly, \(\rho_T=\rho_0(1+\alpha\Delta T)\).

What is the relation between emf and terminal voltage?

\(V=\varepsilon-Ir\) while discharging; terminal voltage is less than emf by the drop \(Ir\).

State Kirchhoff's junction rule.

The algebraic sum of currents at a junction is zero, expressing conservation of charge.

State Kirchhoff's loop rule.

The algebraic sum of potential changes around any closed loop is zero, expressing energy conservation.

What is the balance condition of a Wheatstone bridge?

\(\dfrac{R_1}{R_2}=\dfrac{R_3}{R_4}\), when the galvanometer shows no deflection.

Why is a potentiometer preferred over a voltmeter for emf?

At balance it draws no current from the cell, so it measures true emf, not terminal voltage.

Give the expression for electric power dissipated in a resistor.

\(P=VI=I^2R=V^2/R\), the rate of heat production by Joule heating.

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Current Electricity — Class 12 Physics

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Chapter 3: Current Electricity

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Summary

Electric current is the rate of flow of charge, \(I=\dfrac{dq}{dt}\), arising from the drift of free electrons under an applied field. The average drift speed \(v_d=\dfrac{eE\tau}{m}\) leads to \(I=neAv_d\). Ohm's law states that for many conductors current is proportional to potential difference, \(V=IR\), where resistance \(R=\rho\dfrac{l}{A}\) depends on geometry and the material's resistivity \(\rho\). Resistivity rises with temperature for metals and falls for semiconductors. Resistors combine in series as \(R_s=R_1+R_2+\cdots\) and in parallel as \(\tfrac{1}{R_p}=\tfrac{1}{R_1}+\cdots\). A real cell has an emf \(\varepsilon\) and an internal resistance \(r\), so its terminal voltage is \(V=\varepsilon-Ir\). Kirchhoff's junction rule (charge conservation) and loop rule (energy conservation) solve complex networks. The Wheatstone bridge is balanced when \(\dfrac{R_1}{R_2}=\dfrac{R_3}{R_4}\), a principle exploited by the meter bridge and potentiometer for precise measurement. Electrical power dissipated is \(P=VI=I^2R=\dfrac{V^2}{R}\), and the heat produced obeys Joule's law.

Current and drift velocityOhm's law and resistivitySeries and parallel resistorsEmf and internal resistanceKirchhoff's rules and Wheatstone bridgeElectrical power and Joule heating

Key terms

Electric current

Rate of flow of charge, \(I=dq/dt\); SI unit ampere.

Drift velocity

Average velocity gained by free electrons under a field, \(v_d=eE\tau/m\).

Resistivity

Intrinsic material property \(\rho\) with \(R=\rho l/A\); unit \(\Omega\,m\).

Emf

Energy supplied per unit charge by a source, equal to terminal voltage on open circuit.

Internal resistance

Opposition to current within a cell, giving \(V=\varepsilon-Ir\).

Kirchhoff's rules

Junction rule (charge conservation) and loop rule (energy conservation) for circuits.

Important questions

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Practice quiz

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Class 12 Physics — Current Electricity (Practice Quiz)

10 Qs · ~10 min