When Can We Talk about Potential Energy

Q. A charge $q$ is projected into a uniform electric field $E$, work done when it suffers a displacement $Y$ along the field direction is

1. $qEY$
2. $\frac{qY}{E}$
3. $\frac{qE}{Y}$
4. $\frac{Y}{qE}$

Q.

Why Magnetic Force Does No Work?

Q. A cloud is at a potential of 8×10⁶ volt relative to the ground. A charge of 80 coulomb is transferred in lightening stroke between the cloud and the ground. Assuming the potential of the ground to remain constant the energy dissipated is
A)6.4×10⁸ Joule

Q. A molecule with dipole moment $p$ is placed in an electric field of strength $E$. Initially the dipole is aligned parallel to the field . If the dipole is to be rotated to be anti-parallel to the field , the work required to be done by an external agency is

1. $-2pE$
2. $-2pE$
3. $pE$
4. $2pE$

Q. A point charge $q$ is rotated along a circle in the electric field generated by another point charge $Q$. The work done by electric field on the rotating charge in one complete revolution is

1. Zero
2. Positive
3. Negative
4. Zero, if the charge $Q$ is at centre and nonzero otherwise

Q.

Answer the following:

(a) The top of the atmosphere is at about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface of the earth, the field is about 100 Vm⁻¹. Why then do we not get an electric shock as we step out of our house into the open? (Assume the house to be a steel cage so there is no field inside!)

(b) A man fixes outside his house one evening a two metre high insulating slab carrying on its top a large aluminium sheet of area 1m². Will he get an electric shock if he touches the metal sheet next morning?

(c) The discharging current in the atmosphere due to the small conductivity of air is known to be 1800 A on an average over the globe. Why then does the atmosphere not discharge itself completely in due course and become electrically neutral? In other words, what keeps the atmosphere charged?

(d) What are the forms of energy into which the electrical energy of the atmosphere is dissipated during a lightning? (Hint: The earth has an electric field of about 100 Vm⁻¹ at its surface in the downward direction, corresponding to a surface charge density = −10⁻⁹ C m⁻². Due to the slight conductivity of the atmosphere up to about 50 km (beyond which it is good conductor), about + 1800 C is pumped every second into the earth as a whole. The earth, however, does not get discharged since thunderstorms and lightning occurring continually all over the globe pump an equal amount of negative charge on the earth.)

Q.

electric potential of earth is taken to be zero.why?

Q. Assertion :Electric potential of earth is taken zero. Reason: No electric field exists on earth surface.

1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
3. Assertion is correct but Reason is incorrect
4. Both Assertion and Reason are incorrect

Q. A charge is fixed at one end of an insulating rod of length ‘L’ which rotates with uniform speed about an axis perpendicular to its length and passing through its other end as shown in the figure. This region of space contains a uniform electrostatic field $\overrightarrow{E}$ directed along positive X-axis. The work done by the field when the particle completes two full rotations is?

1. Zero
2. 
3. 
4. 

Q. A space shuttle is pulled closer towards the Earth by the Earth's gravity. In this case, the work done by gravity is .

1. Positive
2. Negative
3. Zero

Q. The work done to raise a mass m from the surface of the earth to a height h which is equal to the radius of the earth

Q.

The earth’s surface has a negative surface charge density of 10⁻⁹ C m⁻². The potential difference of 400 kV between the top of the atmosphere and the surface results (due to the low conductivity of the lower atmosphere) in a current of only 1800 A over the entire globe. If there were no mechanism of sustaining atmospheric electric field, how much time (roughly) would be required to neutralise the earth’s surface? (This never happens in practice because there is a mechanism to replenish electric charges, namely the continual thunderstorms and lightning in different parts of the globe). (Radius of earth = 6.37 × 10⁶ m.)

Q. Four bodies each of mass m are placed at different corners of a square of side a. Find work done on the system to take any one body to infinity.

Q.

A spaceship is stationed on Mars. How much energy must be expended on the spaceship to launch it out of the solar system? Mass of the space ship = 1000 kg; mass of the Sun = 2 × 10³⁰ kg; mass of mars = 6.4 × 10²³ kg; radius of mars = 3395 km; radius of the orbit of mars = 2.28 × 10⁸kg; G= 6.67 × 10^–11 m²kg^–2.

Q. Assertion :Work done by the electrostatic field on a charge moving around a closed circular or elliptical path will be zero. Reason: Electrostatic field is a conservative field.

1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
3. Assertion is correct but Reason is incorrect
4. Assertion is incorrect but Reason is correct

Q. A space shuttle is pulled closer towards the Earth by the Earth's gravity. In this case the work done by the internal forces of Earth-shuttle system will be?

1. Zero
2. Insufficient data
3. Positive
4. Negative

Q.

A space shuttle is pulled closer towards the earth by earth's gravity. In this case the work done by the internal forces of earth - shuttle system is positive.but why don't we consider the concept of change in potential energy to find the workdone?

Q. Solve this:
Q.24. Three charges - q, Q and - q are placed respectively at equal distances on a straight line. If the potential energy of the system of three charges is zero, then what is the ratio of Q : q?
(1) 1 : 1
(2) 1 : 2
(3) 1 : 3
(4) 1 : 4

Q. A charge is fixed at one end of an insulating rod of length ‘L’ which rotates with uniform speed about an axis perpendicular to its length and passing through its other end as shown in the figure. This region of space contains a uniform electrostatic field $\overrightarrow{E}$ directed along positive X-axis. The work done by the field when the particle completes two full rotations is?

1. Zero
2. $4\pi LQ$
3. $2\pi LQ$
4. $\pi LQ$

Q. Figure shows the field lines of a point negative charge. In going from B to A, the kinetic energy of a small negative charge will:

1. increase
2. remains constant
3. decrease
4. data insufficient

Q.

Answer the following:

(a) The top of the atmosphere is at about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface of the earth, the field is about 100 Vm⁻¹. Why then do we not get an electric shock as we step out of our house into the open? (Assume the house to be a steel cage so there is no field inside!)

(b) A man fixes outside his house one evening a two metre high insulating slab carrying on its top a large aluminium sheet of area 1m². Will he get an electric shock if he touches the metal sheet next morning?

(c) The discharging current in the atmosphere due to the small conductivity of air is known to be 1800 A on an average over the globe. Why then does the atmosphere not discharge itself completely in due course and become electrically neutral? In other words, what keeps the atmosphere charged?

(d) What are the forms of energy into which the electrical energy of the atmosphere is dissipated during a lightning? (Hint: The earth has an electric field of about 100 Vm⁻¹ at its surface in the downward direction, corresponding to a surface charge density = −10⁻⁹ C m⁻². Due to the slight conductivity of the atmosphere up to about 50 km (beyond which it is good conductor), about + 1800 C is pumped every second into the earth as a whole. The earth, however, does not get discharged since thunderstorms and lightning occurring continually all over the globe pump an equal amount of negative charge on the earth.)

Q. An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. This is because

1. The magnetic forces do equal and opposite (but non-zero) work on each particle
2. The magnetic forces are necessarily negligible
3. The two magnetic forces are equal and opposite, so they produce no net effect
4. The magnetic forces do no work on each particle

Q. A charge is fixed at one end of an insulating rod of length ‘L’ which rotates with uniform speed about an axis perpendicular to its length and passing through its other end as shown in the figure. This region of space contains a uniform electrostatic field $\overrightarrow{E}$ directed along positive X-axis. The work done by the field when the particle completes two full rotations is?

1. Zero
2. 
3. 
4. 

Q. Why is it so that we have 2 points for Potential to be zero in line joining 2 charges while we have 1 point for Electric field to be zero??

Q. If there is a lone charge in the universe, and it moves a certain distance, what is the change in its electrical potential energy?

1. Infinite
2. Zero
3. Finite non-zero value
4. Impossible to say

Q.

A proton and an -particle enters a uniform magnetic field moving with the same speed. If the proton Takes 25s to make 5 revolutions, what is the periodic time for the -particle?

1. 5 s
2. 25 s
3. 50 s
4. 10 s

Q. Which of the following cannot be the shape of the path of a charged particle moving in a uniform magnetic field?

1. straight line
2. parabolic
3. circular
4. helical

Q. A space shuttle is pulled closer towards the Earth by the Earth's gravity. In this case the work done by the internal forces of Earth-shuttle system will be?

1. Positive
2. Negative
3. Zero
4. Insufficient data

Q. Two spheres of radii r and R carry charges q and Q respectively. When they are connected by a wire, there will be no loss of energy of the system if (a)qr=QR (b)qR=Qr (c)qr²=QR² (d) qR²=Qr²

Q. Two particles, A and B having equal charges, after being accelerated through the same potential difference enter into a region of uniform magnetic field and the particles describe circular paths of radii $R_1$ and $R_2$ respectively. The ratio of the masses of A and B is:

1. $\sqrt{R_1/R_2}$
2. $(R_1/R_2{)}^2$
3. $R_1/R_2$
4. $(R_2/R_1{)}^2$