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In this Class 12 Physics topic from Chapter 1, Electric Charges and Fields, students learn how electric flux is related to the net charge enclosed by a closed surface through Gauss’s law. The topic develops the idea of Gaussian surfaces, uses symmetry to simplify electric-field calculations, and applies the law to charged spherical shells, uniformly charged spheres, infinite line charges, and plane sheets. It also helps students understand the electric field inside conductors and choose suitable surfaces for solving electrostatic problems.
TOPIC PRACTICE
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25 questions
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Easy · Level 1View options
Net charge inside the surface
Colour of the surface
Only on material of the surface
Thickness of the surface
Easy · Level 1View options
Zero
Maximum
Always negative
Always positive
Easy · Level 1View options
Positive
Negative
Zero
Sign cannot be decided
Easy · Level 1View options
Negative
Positive
Zero
Always infinite
Easy · Level 1View options
As many lines enter as leave
It produces no field
Its charge becomes zero
It makes the surface disappear
Easy · Level 1View options
Zero
Positive
Negative
Infinite
Easy · Level 1View options
Positive
Zero
Negative
Always infinite
Easy · Level 1View options
Negative
Positive
Zero
Always maximum
Easy · Level 1View options
Gauss's law
Ohm's law
Hooke's law
Newton's law of cooling
Easy · Level 1View options
Net charge enclosed inside the surface
Colour of the surface
Thickness of the surface
Temperature of the surface
Easy · Level 1View options
Zero
Maximum
Always positive
Always negative
Easy · Level 1View options
Positive
Negative
Zero
Cannot be decided
Easy · Level 1View options
Negative
Positive
Zero
Always maximum
Easy · Level 1View options
Net flux will be zero
Net flux will be positive
Net flux will be negative
Net flux will be infinite
Easy · Level 1View options
Zero
Always positive
Always negative
Equal to area
Easy · Level 1View options
Positive
Negative
Zero
Not defined
Easy · Level 1View options
Negative
Positive
Zero
Always maximum
Easy · Level 1View options
It becomes double
It becomes half
It becomes zero
It remains unchanged
Easy · Level 1View options
Positive
Negative
Zero
Maximum negative
Easy · Level 1View options
Negative
Positive
Zero
Without sign
Easy · Level 1View options
Positive
Negative
Zero
Cannot be determined
Easy · Level 1View options
Negative
Positive
Zero
Maximum positive
Easy · Level 1View options
Positive
Negative
Zero
Depends on surface shape
Easy · Level 1View options
It becomes three times
It becomes nine times
It becomes one-third
It remains unchanged
Easy · Level 1View options
Total electric flux through a closed surface and net enclosed charge
Length of an open surface and electric current
Mass and velocity
Temperature and pressure
Question 1EasyLevel 1
On what does the net electric flux through a closed surface depend?
Correct answer: A
Gauss’s law states that the net electric flux through any closed surface is Φ = Q_enclosed/ε₀. Therefore, it depends on the algebraic sum of charges enclosed by the surface, not on its colour, material, thickness, or detailed shape. Charges outside may produce electric field on the surface, but their total contribution to closed-surface flux is zero. Hence option A is correct.
If the net charge inside a closed surface is zero, what is the net electric flux?
Correct answer: A
By Gauss’s law, the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. Substituting Q_enclosed = 0 gives Φ = 0. This conclusion concerns the net flux, not necessarily the electric field at every point; fields from internal opposite charges or external charges may still exist and cancel in total flux. Therefore option A is correct.
If a positive charge is placed inside a closed surface, what is the sign of net electric flux?
Correct answer: A
For a positive enclosed charge q, Gauss’s law gives Φ = q/ε₀. Since q and ε₀ are positive, the flux is positive. Equivalently, electric field lines emerge from a positive charge and leave the closed surface, producing outward flux. The surface’s shape does not change this sign. Thus option A is correct; negative or zero flux would require a negative or zero net enclosed charge.
If a negative charge is placed inside a closed surface, what is the sign of net electric flux?
Correct answer: A
For a negative enclosed charge q, Gauss’s law gives Φ = q/ε₀. Because q is negative while ε₀ is positive, the net flux is negative. In terms of field lines, they enter the closed surface toward the negative charge, so inward flux is counted as negative. The result is finite for a finite charge; it is not automatically zero or infinite. Hence option A is correct.
Why does a charge placed outside a closed surface not contribute to net flux through it?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. Field lines produced by an external charge may enter the surface and must leave it again, so the inward and outward contributions cancel in the net total. Since the charge is not enclosed, Q_enclosed from it is zero and its net flux contribution is zero. Thus option A is correct.
If a closed surface contains one positive charge and an equal negative charge, what is the net electric flux through it?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. The positive and negative charges have equal magnitudes, so their algebraic sum is zero: Q_enclosed = +q − q = 0. Therefore Φ = 0, making option A correct. Electric fields may still exist at points inside the surface, but their total outward flux cancels.
If a closed surface encloses only the positive charge of a dipole while the negative charge remains outside, what is the net flux?
Correct answer: A
Gauss’s law gives Φ = Q_enclosed/ε₀ for any closed surface. Here, only the positive charge +q lies inside, while the negative charge is outside and is not included in Q_enclosed. Therefore Φ = +q/ε₀, which is positive. Option A is correct; the external charge can affect the field on the surface but cannot change the net flux determined by enclosed charge.
If a closed surface encloses only the negative charge of a dipole while the positive charge remains outside, what is the net flux?
Correct answer: A
For a closed surface, Gauss’s law states Φ = Q_enclosed/ε₀. In this case the enclosed charge is only −q, because the positive member of the dipole lies outside. Hence Φ = −q/ε₀, so the net flux is negative and option A is correct. The outside positive charge may contribute to the local electric field, but it contributes no net enclosed charge.
The concept of electric flux is most useful in understanding which law?
Correct answer: A
Electric flux measures the electric field passing through a surface, mathematically represented by the surface integral of the electric field. Gauss’s law directly relates the net electric flux through a closed surface to the charge enclosed: Φ = Qₑₙc/ε₀. Therefore, Gauss’s law is the correct answer. Ohm’s law concerns voltage and current, Hooke’s law concerns elasticity, and Newton’s law of cooling concerns heat transfer.
For a closed surface, total electric flux depends on what?
Correct answer: A
Gauss’s law states that the total electric flux through any closed surface is Φ_E = Q_enclosed/ε₀. Thus the determining quantity is the algebraic, or net, charge enclosed by the surface; external charges may affect the local field but contribute zero net flux through the closed surface. Therefore option A is correct, and colour, thickness, and temperature are irrelevant in this ideal electrostatic relation.
If the net charge inside a closed surface is zero, what is the total electric flux?
Correct answer: A
By Gauss’s law, the total electric flux through a closed surface is Φ_E = Q_net/ε₀. Substituting Q_net = 0 gives Φ_E = 0. Therefore option A is correct. This does not require the electric field to be zero at every point on the surface; fields from internal charges can cancel in net flux, and external charges can produce local fields without changing the total closed-surface flux.
If a positive charge is inside a closed surface, what is the sign of outward electric flux?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. A positive charge makes Q_enclosed positive, so the outward flux is positive. Field lines emerge from a positive charge, which also gives the same sign interpretation. Negative flux would correspond to net inward field lines, while zero flux would require zero net enclosed charge.
If a negative charge is inside a closed surface, what is the sign of outward electric flux?
Correct answer: A
By Gauss’s law, the net flux through a closed surface is Φ = Q_enclosed/ε₀. For a negative enclosed charge, Q_enclosed is negative, so the outward flux is negative. Physically, field lines point toward a negative charge and enter the surface, giving negative outward flux. Positive flux would require a net positive enclosed charge; zero would require zero net charge.
The same number of electric field lines leave and enter a closed surface. What can be said about the net flux?
Correct answer: A
For a closed surface, outward electric flux is taken as positive and inward flux as negative. If equal numbers of field lines enter and leave, their signed contributions cancel: Φnet = Φout − Φin = 0. Gauss’s law gives the same result when the net enclosed charge is zero. Therefore option A is correct; the sign cannot be positive or negative after exact cancellation.
If there is no net charge inside a closed surface, what is the total electric flux?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φnet = Qenclosed/ε0. If the algebraic net charge enclosed is zero, then Φnet = 0, regardless of the surface’s shape or area. External charges may produce electric field on the surface, but their entering and leaving contributions cancel. Thus option A is correct; the flux is not determined by area alone.
A positive charge is placed inside a closed surface. What is the sign of total electric flux through the surface?
Correct answer: A
Gauss’s law gives Φnet = Qenclosed/ε0. A positive charge means Qenclosed > 0, and ε0 is positive, so the total outward flux is positive. The exact value depends on the charge magnitude, but its sign is fixed. Option A is therefore correct. Negative flux would correspond to net negative enclosed charge, while zero would require zero net enclosed charge.
A negative charge is placed inside a closed surface. What will be the sign of total electric flux?
Correct answer: A
By Gauss’s law, the total flux through a closed surface is Φnet = Qenclosed/ε0. For an enclosed negative charge, Qenclosed is negative while ε0 is positive, so the net outward flux is negative. Therefore option A is correct. A positive sign would require a net positive enclosed charge, zero would require zero net charge, and “always maximum” is not a valid consequence of charge sign.
The charge enclosed by a closed surface is doubled. What happens to the total electric flux through the surface?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. Thus, for the same medium and unchanged ε₀, flux is directly proportional to the net enclosed charge. If Q_enclosed changes to 2Q, then Φ changes to 2Φ. Therefore option A is correct. The surface’s shape or the charge outside it does not alter this total-flux relation.
If a closed surface encloses only the positive charge of a dipole, what will be the net flux?
Correct answer: A
Gauss’s law states that the net electric flux through a closed surface is Φ = Q_enclosed/ε₀. If the surface encloses only the positive charge +q of the dipole, then Q_enclosed = +q, so Φ = +q/ε₀ and its sign is positive. The negative charge outside the surface contributes no net enclosed charge, although its field may pass through parts of the surface. Therefore, option A is correct; zero applies only when the total enclosed charge is zero.
If a closed surface encloses only the negative charge of a dipole, what will be the net flux?
Correct answer: A
By Gauss’s law, the net flux through a closed surface is Φ = Q_enclosed/ε₀. When the surface contains only the negative dipole charge −q, the enclosed charge is negative, giving Φ = −q/ε₀. Thus the net flux has a negative sign. The positive charge outside the surface can affect the local electric field and the flux through individual portions, but it does not change the net enclosed charge. Hence option A is correct, while zero would require equal positive and negative charge inside.
The net charge inside a closed surface is positive. What is the total outward electric flux?
Correct answer: A
Gauss’s law gives the total outward flux through a closed surface as Phi = Q_enclosed/epsilon-naught. Since the enclosed net charge is positive, the quotient is positive. Equivalently, more electric field emerges outward from positive charge than enters the surface. Thus option A is correct. The magnitude would require the numerical charge, but its sign is determined immediately; zero would require zero net enclosed charge.
The net charge inside a closed surface is negative. What is the total outward electric flux?
Correct answer: A
For a closed surface, Gauss’s law states Phi_out = Q_enclosed/epsilon-naught. A negative enclosed charge therefore produces a negative total outward flux. Physically, field lines tend to enter the surface toward the negative charge, opposite to the outward area direction. Hence option A is correct. A positive or zero flux would require positive or zero net enclosed charge, respectively; the magnitude cannot be called maximum without numerical information.
A closed surface contains two positive charges and one negative charge. If the net enclosed charge is positive, what will be the total outward electric flux?
Correct answer: A
Gauss’s law gives the total electric flux through a closed surface as Φ = Q_enclosed/ε₀. Since the algebraic sum of the two positive charges and the negative charge is stated to be positive, Q_enclosed > 0 and the net outward flux is positive. The detailed shape of the surface can change the field distribution, but not the total flux. Thus A is correct; B and C contradict the sign of enclosed charge, and D confuses local distribution with total flux.
If the charge enclosed by a closed surface is made three times, how will the total flux change?
Correct answer: A
Gauss’s law gives Φ = Q_enclosed/ε₀ for a closed surface. With the same physical constant ε₀, flux is directly proportional to the net enclosed charge. If Q_enclosed changes to 3Q_enclosed, then Φ changes to 3Φ. Therefore option A is correct. Nine times would require both charge-related factors to be doubled, and the other choices contradict direct proportionality.
Gauss's law states that the net electric flux through a closed surface equals the net charge enclosed divided by the permittivity of free space: Φ = Q_enclosed/ε₀. Hence option A correctly names the two related quantities. The law is not a relation between an open-surface length and current, mechanical quantities, or thermal variables. The word “closed” is essential because the theorem concerns a complete closed boundary.
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