In the figure shown below, a resistance of 150.4Omega is connected in series to an ammeter A of resistance 240Omega. A…

Year	2026	2025	2024	Total
Questions	18	13	19	50

Year

2026

2025

2024

Total

Questions

Solution & Explanation

Related Formula

Parallel combination of ammeter ( $RG = 240Ω$ $R_G = 240\Omega$) and shunt ( $S = 10Ω$ $S = 10\Omega$):

Rparallel = (RG \cdot S)/(RG + S)

Total equivalent resistance:

Req = Rₛₑᵣᵢₑₛ + Rparallel

Current Divider Rule:

Iammeter = Itotal \cdot ((S)/(RG + S))

Core Logic

Let's first calculate the equivalent resistance of the parallel branch (ammeter + shunt):

Rparallel = (240 \times 10)/(240 + 10) = (2400)/(250) = 9.6Ω

Now, add the series resistance ( $150.4Ω$ $150.4\Omega$) to find the total equivalent resistance of the circuit:

Req = 150.4 + 9.6 = 160Ω

Step 1: Finding Total Current

Use Ohm's Law to calculate the total current $Itotal$ $I_{\text{total}}$ drawn from the $20~V$ $20\mathrm{~V}$ battery:

Itotal = VReq = (20)/(160) = 0.125~A

Step 2: Calculating Ammeter Current

Using the Current Divider Rule, calculate the current $Iammeter$ $I_{\text{ammeter}}$ flowing through the $240Ω$ $240\Omega$ ammeter branch:

Iammeter = Itotal \times (10)/(240 + 10) = 0.125 \times (10)/(250)

Iammeter = 0.125 \times (1)/(25) = 0.005~A

Convert this into milliamperes ( $mA$ $\mathrm{mA}$):

Iammeter = 0.005 \times 1000 = 5~mA

Therefore, the reading of the ammeter is $5~mA$ $5\mathrm{~mA}$.

Simplified equivalent circuit showing ammeter shunt current split for Q25 — A circuit schematic containing a 20V battery connected in series with a 150.4 ohm resistor and a parallel combination of a 240 ohm ammeter and a 10 ohm shunt resistor.

Pattern Recognition

Notice how nicely the numbers are set up in JEE Mains questions! The parallel combination of $240Ω$ $240\Omega$ and $10Ω$ $10\Omega$ yields exactly $9.6Ω$ $9.6\Omega$, which beautifully combines with $150.4Ω$ $150.4\Omega$ to form a perfect integer sum of $160Ω$ $160\Omega$. This tells you that your intermediate steps are absolutely correct!

Chapter Mix

Class 12 Physics: Current Electricity: Measuring Devices

More Current Electricity Previous-Year Questions — Page 10

Q39 jee_main_2024_31_jan_evening Meter Bridge

The resistance per centimeter of a meter bridge wire is

r

, with

X Ω

resistance in left gap. Balancing length from left end is at

40 cm

with

25 Ω

resistance in right gap. Now the wire is replaced by another wire of

2r

resistance per centimeter. The new balancing length for same settings will be at

A. $20 cm$ $20 \text{ cm}$
B. $10 cm$ $10 \text{ cm}$
C. $80 cm$ $80 \text{ cm}$
D. $40 cm$ $40 \text{ cm}$

Solution

Related Formula

RleftRwire-left = RrightRwire-right

Core Logic

For a meter bridge, the balancing condition is independent of the absolute resistance of the bridge wire as long as it is uniform. The ratio of the resistances in the gaps balances with the ratio of lengths.

Meter Bridge diagram for Q39 - JEE Main 2024 Evening

Step 1: First Condition

(X)/(r ₁) = (25)/(r (100 - ₁))

Given $₁ = 40 cm$ $\ell_1 = 40 \text{ cm}$:

(X)/(r \times 40) = (25)/(r \times 60) (X)/(40) = (25)/(60)

Step 2: Second Condition

When replaced by a wire of $2r$ $2r$ per cm, the new lengths $₂$ $\ell_2$ will satisfy:

(X)/(2r ₂) = (25)/(2r (100 - ₂))

Notice that the $2r$ $2r$ terms cancel out entirely from both sides, leaving:

(X)/( ₂) = (25)/(100 - ₂)

Step 3: Conclusion

Since the ratio $X/25$ $X/25$ remains identical, the balancing length ratio $/ (100- )$ $\ell / (100-\ell)$ also remains identical. Therefore, $₂ = ₁ = 40 cm$ $\ell_2 = \ell_1 = 40 \text{ cm}$.

Pattern Recognition

Meter bridge balance point strictly depends on length ratio, NOT the specific resistivity or thickness of the wire (provided it is uniform). If external resistors don't change, the balance point never changes.

Chapter Mix

Class 12 Physics: Current Electricity

Q43 jee_main_2024_31_jan_evening Power Dissipation

By what percentage will the illumination of the lamp decrease if the current drops by

20%

A. $46%$ $46\%$
B. $26%$ $26\%$
C. $36%$ $36\%$
D. $56%$ $56\%$

Solution

Related Formula

Power (Illumination) is proportional to the square of the current for a constant resistance: $P = I² R$ $P = I^2 R$

Core Logic

Initial power: $P₁ = I₁² R$ $P_1 = I_1^2 R$

If current drops by 20%, the new current is:

I₂ = I₁ - 0.2 I₁ = 0.8 I₁

New power:

P₂ = (0.8 I₁)² R = 0.64 I₁² R = 0.64 P₁

Step 1: Calculate Percentage Change

Δ P % = (P₂ - P₁)/(P₁) \times 100%

Δ P % = (0.64 P₁ - P₁)/(P₁) \times 100%

Δ P % = (0.64 - 1) \times 100% = -36%

Step 2: Final Statement

The negative sign indicates a decrease. The illumination drops by $36%$ $36\%$.

Pattern Recognition

For squared relations $y = x²$ $y = x^2$, if $x$ $x$ changes by a factor $k$ $k$ ( $0.8$ $0.8$), $y$ $y$ changes by a factor $k²$ $k^2$ ( $0.64$ $0.64$). $1 - 0.64 = 36%$ $1 - 0.64 = 36\%$. This bypasses algebraic limits usually done for small changes (like $2Δ x / x$ $2\Delta x / x$) since 20% is too large for approximation.

Chapter Mix

Class 12 Physics: Current Electricity

Q51 jee_main_2024_31_jan_evening Power in DC Circuits

In the following circuit, the battery has an emf of

2 V

and an internal resistance of

(2)/(3) Ω

. The power consumption in the entire circuit is ______ W.

Power in DC Circuits diagram for Q51 - JEE Main 2024 Evening — The image shows a DC circuit with multiple resistors in parallel/series connected to a 2V battery with 2/3 ohm internal resistance.

Numerical Answer. Answer: 3 to 3

Solution

Related Formula

P = V²Req

Core Logic

To find total power, collapse the entire external circuit and battery internal resistance into a single equivalent resistance $Req$ $R_{eq}$ across the battery's ideal terminals.

Step 1: Equivalent Resistance Calculation

Analyzing the diagram: The circuit simplifies to an equivalent resistance $Req$ $R_{eq}$ combining the parallel/series elements along with the internal resistance $r = 2/3 Ω$ $r = 2/3 \, \Omega$. The final simplified equivalent resistance of the entire system calculates to:

Req = (4)/(3) Ω

Step 2: Calculate Power

P = V²Req

P = (2²)/(4/3)

P = (4)/(4/3) = 3 W

Pattern Recognition

Whenever "entire circuit" power is asked, include the battery's internal resistance inside $Req$ $R_{eq}$ so you can use $P = E² / Rtotal$ $P = E^2 / R_{total}$ directly.

Chapter Mix

Class 12 Physics: Current Electricity

Q jee_main_2024_31_jan_morning Resistor Circuits

Equivalent resistance of the following network is ________

Ω

Resistor Circuits diagram for Q53 - JEE Main 2024 Morning — A complex resistor bridge network connecting nodes A and B with multiple branches.

Numerical Answer. Answer: 1 to 1

Solution

Related Formula

Rparallel = (1)/(Σ (1)/(Rᵢ))

Core Logic

By carefully identifying the nodes, we can see that a $6 Ω$ $6\,\Omega$ resistor in the middle branch is short-circuited by a direct zero-resistance wire path across it.

Removing the short-circuited $6 Ω$ $6\,\Omega$ resistor simplifies the circuit into three identical branches connected directly between the terminals $A$ $A$ and $B$ $B$.

Step 2: Equivalent Calculation

The simplified circuit consists of three identical $3 Ω$ $3\,\Omega$ resistors in parallel.

Req = 3 \times (1)/(3) = 1 Ω

Pattern Recognition

Always trace nodes directly connected by straight wires (zero resistance). Any resistor with both ends connecting to the exact same electrical node is shorted out and can be erased.

Chapter Mix

Class 12 Physics: Current Electricity

Q48 jee_main_2024_31_jan_morning Temperature Dependence Of Resistance

Two conductors have the same resistances at

0°C

but their temperature coefficients of resistance are

α₁

and

α₂

. The respective temperature coefficients for their series and parallel combinations are :

A. $α₁ + α₂, (α₁ + α₂)/(2)$
B. $(α₁ + α₂)/(2), (α₁ + α₂)/(2)$
C. $α₁ + α₂, (α₁α₂)/(α₁ + α₂)$
D. $(α₁ + α₂)/(2), α₁ + α₂$

Solution

Related Formula

RT = R₀(1 + α Δ T)

Step 1: Series Combination

Let base resistance at $0^° C$ $0^\circ \text{C}$ be $R$ $R$. For series:

Req = R₁ + R₂

(2R)[1 + αeq,s Δ T] = R(1 + α₁ Δ T) + R(1 + α₂ Δ T)

2 + 2αeq,s Δ T = 2 + (α₁ + α₂)Δ T

αeq,s = (α₁ + α₂)/(2)

Step 2: Parallel Combination

For parallel at $0^° C$ $0^\circ \text{C}$, $Req,0 = R/2$ $R_{\text{eq,0}} = R/2$.

Req,p = (R₁ R₂)/(R₁ + R₂)

(R)/(2) [1 + αeq,p Δ T] = (R² (1 + α₁ Δ T)(1 + α₂ Δ T))/(R(2 + (α₁ + α₂)Δ T))

(1)/(2) (1 + αeq,p Δ T) = (1 + (α₁ + α₂)Δ T)/(2(1 + (α₁ + α₂)/(2)Δ T))

Using binomial expansion for small $Δ T$ $\Delta T$:

1 + αeq,p Δ T \approx [1 + (α₁ + α₂)Δ T] [ 1 - (α₁ + α₂)/(2)Δ T ]

1 + αeq,p Δ T \approx 1 + (α₁ + α₂)Δ T - (α₁ + α₂)/(2)Δ T

αeq,p Δ T = (α₁ + α₂)/(2)Δ T

αeq,p = (α₁ + α₂)/(2)

Pattern Recognition

For two identical base resistances, the effective temperature coefficient is simply the arithmetic mean of their individual coefficients, regardless of whether they are in series or parallel.

Chapter Mix

Class 12 Physics: Current Electricity

Solution & Explanation

Related Formula

Core Logic

Step 1: Finding Total Current

Step 2: Calculating Ammeter Current

Pattern Recognition

Chapter Mix

Reference Study Guides

More Current Electricity Previous-Year Questions — Page 10

Solution

Related Formula

Core Logic

Step 1: First Condition

Step 2: Second Condition

Step 3: Conclusion

Pattern Recognition

Chapter Mix

Solution

Related Formula

Core Logic

Step 1: Calculate Percentage Change

Step 2: Final Statement

Pattern Recognition

Chapter Mix

Solution

Related Formula

Core Logic

Step 1: Equivalent Resistance Calculation

Step 2: Calculate Power

Pattern Recognition

Chapter Mix

Solution

Related Formula

Core Logic

Step 2: Equivalent Calculation

Pattern Recognition

Chapter Mix

Solution

Related Formula

Step 1: Series Combination

Step 2: Parallel Combination

Pattern Recognition

Chapter Mix

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