CBSE physics
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A charge ‘q’ is moved from a point A above a dipole of dipole moment ‘p’ to a point B below the dipole in equatorial plane without acceleration. Find the work done in the process.
Work done, W = q x VAB = q X 0 = 0
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In what way is the behaviour of a diamagnetic material different from that of a paramagnetic, when kept in an external magnetic field?
Difference in the behaviour of a diamagnetic material and paramagnetic material:
| Diamagnetic | Paramagnetic |
| 1. Diamagnetic substance would move towards the weaker region of the magnetic field. | 1. Paramagnetic substance move towards the stronger region of the magnetic field. |
| 2. Diamagnetic substance is repelled by a magnet. | 2. Paramagnetic substance moves towards the magnet. |
| 3. Diamagnetic substance get aligned perpendicular to the field. | 3. Paramagnetic substance get aligned along the magnetic field. |
The plot of the variation of potential difference across a combination of three identical cells in series, versus current is shown below. What is the emf and internal resistance of each cell?
Potential difference, E = 6/3 = 2 V
Internal resistance, r = 
Given, three cells are connected in series,
r' = r/3 = 6/3 = 2 ohm
A battery of emf 12V and internal resistance 2 ohm is connected to a 4 ohm resistor as shown in the figure.
a) Show that a voltmeter when placed across the cell and across the resistor, in turn, gives the same reading.
b) To record the voltage and the current in the circuit, why is voltmeter placed in parallel and ammeter in series in the circuit?
a) Emf, E = 12 V
Internal resistance, r = 2V
Now, using the formula,
E = V + Ir
When the voltmeter is connected across the cell,
I = 
V1 = 12 - 2(2) = 8 V
When the voltmeter is connected across the resistor,
V2 = IR
= 2 x 4 = 8 V
That is, V1 = V2
Hence proved.
b) Voltmeter has very high resistance to ensure that it is connection does not alter the flow of current in the circuit. Current chooses the low resistance path. Therefore, voltmeter is connected in parallel to the load across which potential difference is to be measured.
Ammeter measures the value of current flowing through the circuit. Ammeter has a very low value of resistance to ensure that all the current flows through it. Hence, it should be connected in series.
Define an equipotential surface. Draw equipotential surfaces:
(i) in the case of a single point charge and in a constant electric field in Z-direction.
(ii) Why the equipotential surfaces about a single charge are not equidistant?
(iii) Can electric field exist tangential to an equipotential surface? Give reason.
i) Equipotential surface for a single point charge is:
ii) The equipotential surface about a single charge is not equidistant because V is inversely proportional to r.
Also, the equipotential surfaces about a single charge are not equidistant because electric field due to a single charge is not constant.
iii) Electric field cannot exist tangential to an equipotential surface because if the field lines are tangential, work will be done in moving a charge on the surface, which is against the theory of equipotential surface.
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Mock Test Series
Mock Test Series