Thermal and Chemical Effects of Electric Current

Flashcards for topic Thermal and Chemical Effects of Electric Current

Intermediate58 cardsphysics

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Card 1

Front

What is the mathematical relationship between heat produced in a resistor, current, resistance, and time according to Joule's laws?

Back

H=i2RtH = i^2Rt, where:

  • HH = heat produced (Joules)
  • ii = current (Amperes)
  • RR = resistance (Ohms)
  • tt = time (seconds)

This formula encapsulates all three Joule's laws:

  • Hi2H \propto i^2 (proportional to square of current)
  • HRH \propto R (proportional to resistance)
  • HtH \propto t (proportional to time)

Example: Doubling the current quadruples the heat produced in the same time period.

Card 2

Front

How would you experimentally verify Joule's law stating that heat produced is proportional to resistance (H ∝ R)?

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Back

Experimental verification of H ∝ R requires:

  1. Setup:

    • Two identical calorimeters with equal amounts of fluid (e.g., K-oil)
    • Different resistors (R₁, R₂) immersed in each calorimeter
    • Both resistors connected in series (ensuring identical current)
    • Circuit includes: battery, rheostat, key, and measuring instruments
  2. Procedure:

    • Measure initial temperatures in both calorimeters
    • Pass current through the circuit for a fixed time
    • Continuously stir the fluid and measure final temperatures
    • Calculate temperature rise (ΔT₁, ΔT₂) in each calorimeter
  3. Analysis:

    • Calculate the ratio ΔT₁/ΔT₂
    • Compare with the ratio R₁/R₂
    • Verification is successful if ΔT₁/ΔT₂ ≈ R₁/R₂

Example: If resistors of 10Ω and 5Ω connected in series show temperature rises of 8°C and 4°C respectively, the law is verified since 8/4 = 10/5 = 2.

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Card 3

Front

What happens to the current direction in a copper-nickel thermocouple when one junction is heated?

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Back

In a copper-nickel thermocouple:

  • Current flows from copper to nickel at the cold junction
  • Current flows from nickel to copper at the hot junction
  • The galvanometer indicates the current direction
  • The magnitude of current increases with temperature difference until reaching the neutral temperature (approximately 390°C)
  • Beyond the neutral temperature, current decreases until reaching zero at the inversion temperature (approximately 780°C)
  • After exceeding the inversion temperature, the current direction reverses
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Card 4

Front

What is the mechanism of electric conduction in an electrolyte compared to metals?

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Back

Conduction in electrolytes:

  • Occurs through ion movement (both positive and negative ions)
  • Positive ions (cations) move toward cathode
  • Negative ions (anions) move toward anode
  • Involves chemical changes at electrodes
  • Rate of conduction depends on ion concentration and mobility

Conduction in metals:

  • Occurs through free electron movement only
  • No mass transfer or chemical changes
  • No directional preference for different charge carriers
  • Rate depends on electron density and lattice structure

This fundamental difference explains why electrolytes show chemical effects while metals only show thermal effects when current passes through them.

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Card 5

Front

What is a silver voltameter and how does it function as a precision measurement instrument?

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Back

A silver voltameter is a high-precision electrolytic cell used to measure electric current or charge with exceptional accuracy.

Key components and function:

  • Contains silver nitrate solution (1:5 to 1:6 ratio with water) as electrolyte
  • Uses a platinum cup as the cathode
  • Features a silver rod as the anode
  • Includes a porcelain cup surrounding the anode that is porous to the electrolyte

Working principle:

  • When current passes through the cell, silver from the anode dissolves into the solution
  • Silver ions deposit onto the platinum cathode
  • The mass of deposited silver is directly proportional to the charge passed (m = Zit)
  • Current can be calculated as i = m/(Zt) where Z is silver's electrochemical equivalent

Due to its high accuracy, silver voltameters are primarily used for standardization purposes in calibration laboratories.

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Card 6

Front

What is the structure and working principle of a Leclanché cell?

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Back

Structure:

  • Carbon electrode (positive terminal)
  • Zinc electrode (negative terminal)
  • Both electrodes immersed in NH₄Cl (ammonium chloride) solution
  • Carbon electrode packed in a porous cup containing MnO₂

Working principle:

  1. When current flows, Cl⁻ ions combine with zinc
  2. NH₄⁺ ions move toward carbon electrode, separating into NH₃ and H⁺
  3. H⁺ ions enter the porous cup, giving positive charge to carbon electrode
  4. MnO₂ acts as a depolarizer by reacting with hydrogen to form Mn₂O₃ and water
  5. This prevents hydrogen accumulation on the carbon electrode (polarization)

The cell produces approximately 1.5 V but has slow depolarizing action, making it suitable for intermittent use rather than continuous current draw.

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Card 7

Front

Compare and contrast a dry cell with a standard Leclanché cell, including their applications and limitations.

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Back

Similarities:

  • Both are primary cells based on the same electrochemical principles
  • Both use zinc as negative electrode and carbon as positive electrode
  • Both employ MnO₂ as a depolarizer

Differences:

  • Structure:
    • Standard Leclanché: Liquid NH₄Cl electrolyte with porous cup
    • Dry cell: NH₄Cl and MnO₂ in paste form contained in a zinc container (which serves as the negative electrode)
  • Internal resistance:
    • Standard Leclanché: Higher (around 1Ω)
    • Dry cell: Very low (around 0.1Ω)
  • Portability:
    • Standard Leclanché: Less portable due to liquid electrolyte
    • Dry cell: Highly portable, sealed to prevent drying

Applications:

  • Both suitable for intermittent use (flashlights, toys, remote controls)
  • Dry cells preferred for portable applications due to their sealed nature

Limitations:

  • Neither suitable for continuous high-current applications
  • Both subject to polarization if used continuously
  • Limited shelf life due to chemical deterioration even when not in use
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Card 8

Front

What chemical reactions occur at each electrode in a Daniell cell, and how do they prevent polarization?

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Back

At Zinc (Negative) Electrode:

  • SO₄²⁻ ions combine with zinc to form ZnSO₄
  • Negative charge transfers to zinc electrode
  • Reaction: Zn + SO₄²⁻ → ZnSO₄ + 2e⁻

At Copper (Positive) Electrode:

  • Cu²⁺ ions from CuSO₄ solution receive electrons
  • Copper deposits on copper electrode
  • Reaction: Cu²⁺ + 2e⁻ → Cu

Polarization Prevention:

  • The H⁺ ions move through porous barrier from H₂SO₄ solution
  • They combine with SO₄²⁻ in CuSO₄ solution to form H₂SO₄
  • This arrangement prevents hydrogen gas accumulation at the anode
  • Without this mechanism, hydrogen buildup would create an opposing potential, reducing cell effectiveness (polarization)
  • The continuous chemical cycle maintains stable electrode potentials
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Card 9

Front

What are the primary chemical reactions occurring during the discharging process of a lead accumulator?

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Back

During discharge of a lead accumulator:

At the negative electrode (Pb):

  • SO₄²⁻ ions move toward the Pb electrode
  • SO₄²⁻ gives up negative charge and forms PbSO₄
  • Reaction: Pb + SO₄²⁻ → PbSO₄ + 2e⁻

At the positive electrode (PbO₂):

  • H⁺ ions move toward the PbO₂ electrode
  • H⁺ gives up positive charge and reduces PbO₂ to PbO
  • PbO reacts with H₂SO₄ to form PbSO₄ and water
  • Overall reaction: PbO₂ + 2H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + H₂O

Net result: PbSO₄ forms at both electrodes, and the specific gravity of the acid decreases as H₂SO₄ is consumed.

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Card 10

Front

How does the direction of current flow differ between a discharging and charging secondary cell, and what energy conversions occur in each case?

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Back

Discharging:

  • Current direction: Exits positive terminal → flows through external circuit → enters negative terminal
  • Energy conversion: Chemical energy → Electrical energy
  • Process occurs spontaneously due to the potential difference between electrodes

Charging:

  • Current direction: External source forces current into positive terminal → through cell → out negative terminal
  • Energy conversion: Electrical energy → Chemical energy
  • Process requires an external power source with emf greater than the cell's emf
  • The reversal of current direction drives the reversal of chemical reactions
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