SENAPREP
Static and current electricity Questions
Chapter: Electricity and Magnetism | Total Available: 35 Real Questions
Which of the following statements correctlyexplains/explain the existence of a positive forcebetween two electric charges?
1. Both the charges are positive.
2. Both the charges are negative.
3. Both the charges are oppositely charged.
Select the correct answer using the code givenbelow.
Correct Answer
📖 Governing Law / Core Concept
The existence of a force between electric charges is governed by Coulomb's Law, which states:
Where:
- F = Force between charges (N)
- k = Coulomb's constant (\(8.99 \times 10^9 \, \text{N m}^2/\text{C}^2\))
- q_1, q_2 = Magnitude of charges (C)
- r = Distance between centers of charges (m)
🧪 Step-by-Step Breakdown
The force between two like charges (both positive or both negative) is repulsive, resulting in:
- Positive force value (repulsion).
When one charge is positive and the other is negative:
- Negative force value indicating attraction.
Thus, only options 1 and 2 lead to a positive force.
🔍 Option Analysis
Option 1: Both positive charges result in repulsion.
Option 2: Both negative charges also lead to repulsion.
Option 3: Opposite charges attract, giving a negative force.
⚡ Mnemonic / Speed-Run
Remember: "Like Charges Repel, Unlike Charges Attract" to quickly identify charge interactions.
6. Visual Suggestion
6. Visual Suggestion
A schematic diagram showing two positive charges with arrows indicating repulsion, and two negative charges similarly represented, contrasting with arrows between a positive and a negative charge indicating attraction.
📖 Factual Verification & Reference
Verified against NCERT Class XI Physics, Chapter 2: Electrostatics.
If the current through an electrical machine running on direct current is 15 A and the machine runs for 10 minutes, the charge that passes through the machine during this time is:
Solution
Option (d) is correct.
Explanation:
- \( q \) = Charge (Coulombs)
- \( i \) = Current (Amperes) = 15 A
- \( t \) = Time (Seconds) \end{aligned}
- Charge, \( q = i \cdot t \)
- 1 Ampere = 1 Coulomb/second
A circuit diagram with a battery, switch, and resistor showing the direction of current and the flow of charge over time would help visualize the flow of electrons and resulting charge calculations.
Verified against NCERT Class XI Physics, Chapter 6 (Current Electricity).
Which one of the following is primarily responsible for conduction of current in a metal?
Correct Answer
📖 Governing Law / Core Concept
Electrical conductivity in metals is governed by the principle of free electron theory. In conductive materials, free electrons facilitate the transfer of electric charge.
🧪 Step-by-Step Breakdown
In a metal, the electrons are categorized as bound (not moving) and free (mobile).
1. Free electrons are weakly bound to the metal lattice due to the low ionization energy of metals.
2. The electric current \( I \) can be represented as:
Where:
- \( n \) = number density of free electrons (m−3)
- \( A \) = cross-sectional area of wire (m2)
- \( e \) = charge of an electron (Coulombs, C)
- \( v_d \) = drift velocity of electrons (m/s)
The drift velocity reflects how free electrons move in response to an electric field.
🔍 Option Analysis
Option A (Bound electrons) do not contribute effectively to conductivity as they are not free to move. Option C (Both bound and free electrons) is incorrect as bound electrons cannot conduct current. Option D (Ions) also contribute negligibly compared to free electrons.
⚡ Mnemonic / Speed-Run
Mneuonic: "Free to Flow" - only free electrons circulate to maintain electrical flow.
6. Visual Suggestion
6. Visual Suggestion
A schematic diagram showing a metal conductor with free electrons highlighted in motion, indicating their movement as current flows through the conductor.
📖 Factual Verification & Reference
Verified against NCERT Class XII Physics, Chapter 3 (Current Electricity).
The AC mains domestic supply current in India changes direction in every:
Correct Answer
📖 Governing Law / Core Concept
In alternating current (AC) systems, the frequency of the AC supply determines the time taken for the current to change direction. The formula for frequency (\( f \)) is given by:
Where \( T \) is the time period in seconds. In India, the frequency of AC is 50 Hz.
🧪 Step-by-Step Breakdown
To calculate the time period \( T \) of the AC supply:
The AC current inverts its direction once during each half-cycle. Therefore, the time for a complete direction change is:
🔍 Option Analysis
- Option (a): 50 s - Incorrect; far too long for practical AC usage.
- Option (b): 1/50 s - Incorrect; this would correspond to a higher frequency than typical AC.
- Option (c): 100 s - Incorrect; outside the range of domestic supply.
- Option (d): 1/100 s - Correct; matches the AC frequency in India.
⚡ Mnemonic / Speed-Run
Remember: For AC systems, frequency and time period are inversely related. 50 Hz translates to a direction change of 1/100 seconds.
6. Visual Suggestion
Include a cycle diagram illustrating the AC waveform, highlighting the direction change points every 1/100 seconds.
📖 Factual Verification & Reference
Verified against NCERT Class 12 Physics, Chapter on Alternating Current.
If the length of a copper wire is increased by twice, then its resistivity will be
1. Correct Answer
Option C is correct. The resistivity of a material remains unchanged with length changes.
📖 Core Concept
Resistivity is a fundamental property of materials, independent of geometric attributes such as length or cross-sectional area.
💡 3. Explanation
❌ 4. Option Analysis
- •Option A: Incorrect. Doubling the length influences resistance, not resistivity.
- •Option B: Incorrect. Resistivity does not diminish with an increase in length.
- •Option D: Incorrect. There is no correlation between the increase in length and resistivity.
NDA Speed-Run / Mnemonic Shortcut
Remember, resistivity (\( \rho \)) is solely a material characteristic and remains fixed regardless of length or shape variations.
5. Key Takeaways
Resistivity is unaffected by the wire's length or cross-sectional area.
Which one of the following statements is NOT correct?
1. Correct Answer
Option D is correct. A voltmeter has low resistance, and an ammeter has high resistance is incorrect.
📖 Core Concept
Accurate measurement of electrical parameters requires ammeters and voltmeters to connect the circuit correctly based on their resistance characteristics.
💡 3. Explanation
❌ 4. Option Analysis
- •Option A: Correctly states ammeters are connected in series.
- •Option B: Correctly states voltmeters are connected in parallel.
- •Option C: Correctly describes an ammeter's low resistance and a voltmeter's high resistance.
NDA Speed-Run / Mnemonic Shortcut
Remember: "Ammeters = Series + Low Resistance, Voltmeters = Parallel + High Resistance." This helps quickly recall their functions.
5. Key Takeaways
A voltmeter has high resistance and is used in parallel; an ammeter has low resistance and is used in series.
The magnetic field inside a long, straight solenoid carrying current
1. Correct Answer
Option D is correct. The magnetic field inside a long solenoid is uniform.
📖 Core Concept
A solenoid is a wire coil that generates a magnetic field when current flows through it, characterized by uniformity in the interior field lines.
💡 3. Explanation
❌ 4. Option Analysis
- •Option A: The magnetic field is not zero; it exists uniformly within the solenoid.
- •Option B: The magnetic field does not decrease toward the ends; it remains constant throughout the solenoid.
- •Option C: The magnetic field does not increase towards the ends; it is uniform along the entire length.
NDA Speed-Run / Mnemonic Shortcut
Recall: Inside a long solenoid, the magnetic field is uniform when current flows, represented by \( B = \mu_0 n I \).
5. Key Takeaways
The magnetic field inside a long solenoid carrying current is uniform and depends on current and the number of turns per unit length.
6. Visual Suggestion
Consider a diagram showing a solenoid with parallel magnetic field lines within, indicating uniformity, along with arrows showing current direction.
Which one of the following is NOT a basic property of electric charge?
1. Correct Answer
Option D is correct. Charges can be created and destroyed in an isolated system.
📖 Core Concept
Electric charge is a fundamental property of matter, obeying specific conservation laws and quantization principles.
💡 3. Explanation
- Additivity: Charges can be summed up.
- Conservation: Total charge remains constant in an isolated system.
- Quantization: The charge of any object is an integral multiple of the charge of an electron or proton.
❌ 4. Option Analysis
- •Option A: Charges can indeed be combined.
- •Option B: Charge conservation is a foundational principle in physics.
- •Option C: Charges only exist as integral multiples of electrons or protons.
NDA Speed-Run / Mnemonic Shortcut
Remember: Charge is conserved! Always look for statements regarding creation or destruction of charge to identify false claims.
5. Key Takeaways
Electric charge is quantized and conserved; it cannot be created or destroyed.
The magnetic field inside a current carrying very long solenoid is
Correct Answer
📖 Governing Law / Core Concept
The magnetic field \( \vec{B} \) inside an ideal long solenoid can be described by the formula:
where:
- \( \mu_0 \) = permeability of free space \((\text{T}\cdot\text{m/A})\)
- \( n \) = number of turns per unit length \((\text{m}^{-1})\)
- \( I \) = current through the solenoid \((\text{A})\)
🧪 Step-by-Step Breakdown
Inside a long solenoid, the magnetic field lines are densely packed, indicating a uniform field. The magnetic field is directed along the axis of the solenoid, and is constant in magnitude throughout its interior. Outside a long solenoid, the magnetic field is negligible.
The derivation shows that the contribution of the field from each loop adds up to yield a constant field inside:
🔍 Option Analysis
Option B (Non-uniform) is incorrect because the magnetic field has a constant value throughout the solenoid. Option C (Zero) is wrong as the field inside is significant. Option D (Highest at mid-point) also does not apply since the field is uniform throughout.
⚡ Mnemonic / Speed-Run
Use the mnemonic "Circuits Create Fields" to remember that current-carrying devices (like solenoids) create magnetic fields, which are uniform inside the coils.
6. Visual Suggestion
6. Visual Suggestion
Illustrate a long solenoid with arrows representing the uniform magnetic field lines inside it. Show denser lines indicating stronger field concentrations and sparse lines outside the solenoid, representing negligible magnetic field strength.
📖 Factual Verification & Reference
Verified against NCERT Class XII Physics, Chapter 5 (Magnetic Effects of Current).
For a material under static conditions, which one among the following statements is correct?
Correct Answer
📖 Governing Law / Core Concept
Charging methods and behavior of materials can be explained through the principle of electrostatics. Insulators do not conduct electricity but can be charged by friction (triboelectric effect) or induction.
🧪 Step-by-Step Breakdown
Charging materials can be categorized based on their ability to transfer charges:
- Insulators: Materials that do not allow free movement of electrons. They can hold a static charge.
- Conductors: Materials that allow free movement of electrons. In the case of a perfect conductor, charges redistribute instantly to maintain equilibrium.
In static conditions:
- Conductors cannot retain an external charge because any additional charge flows across their surface.
- Insulators can be charged by gaining or losing electrons, thus holding a static charge.
🔍 Option Analysis
- Option A: Correct, as insulators can accumulate charge.
- Option B: Incorrect, a perfect conductor cannot retain a charge; charges redistribute immediately.
- Option C: Incorrect, as insulators can hold charge but perfect conductors cannot.
- Option D: Incorrect, as insulators can be charged.
⚡ Mnemonic / Speed-Run
Remember: ICE for material properties: Insulators can hold charge, Conductors cannot hold charge (specifically perfect conductors), and Examples abound.
6. Visual Suggestion
6. Visual Suggestion
A flowchart depicting the charging methods: illustrate insulators being charged via friction and conductors demonstrating charge redistribution, showing electrons moving on an atomic level.
📖 Factual Verification & Reference
Verified against NCERT Class 12 Physics, Chapter 1 (Electric Charges and Fields).