# electrostatics cw questions

#### k.ubhi2011

Joined May 5, 2011
3
hi, could anyone answer some of these questions thanks.

1. Two point-like charges of + 1 µ​
C and  2 µ C are placed at the co-ordinates (1, 0) m and (0, 1) m. What is the force between them? [4]

2. A solid sphere of radius 3 cm is made from a conductor and has a charge of
+ 4 µ​
C placed upon it. What is the electric field strength 1 mm from the
centre of the sphere? [4]

3. A parallel-plate capacitor has a surface area of 4 cm​
2, the plates are
separated by 2 mm and the space in between is filled with a dielectric
whose relative permittivity is 7. If the maximum voltage that can be applied
is 50 V, what is the maximum energy that can be stored? [4]

4. A piece of co-axial cable has an outer diameter of 6 cm and an unknown
inner diameter. It is known that the cable is filled with a dielectric whose
relative permittivity is 12. If the capacitance of 1 m of cable is 607 pF, what
is the inner diameter of the cable?​
[4]

5. A parallel plate capacitor has a surface area of 1 cm​
2 and its plates are
separated by 3 mm. In between the plates are three layers of dielectric,
each 1 mm thick, with relative permittivities of 3, 5 and 11 respectively. What
is the capacitance of the whole capacitor? [4]

6. Two particles, each of mass 20​
μg, are separated by 1 mm in air. How many electrons must be added to each particle so as to balance their force of
attraction due to gravity? [4]

7. Given that air becomes ionised at an electric field strength of 3 x 10​
6 Vm-1 ,
find the greatest charge per metre that can be placed on a straight wire of 1
cm diameter in air. [4]

8. Two dielectric materials of relative permittivity 3 and 7 respectively are
placed in contact with one another and subjected to an electric field at right
angles to the boundary between them. If the electric field strength in the
dielectric of relative permittivity 3 is 300 Vm​
-1, what is the electric field
strength in the other dielectric? [4]

9. A section of wire has a rectangular cross-section with a width of 0.3 mm and a height of 0.9 mm. If the current density in the wire is specified not to
exceed 30 kAm​
-2, what is the maximum current that can be allowed to flow
in the wire? [4]

10. A section of wire has a circular cross-section of radius 0.8cm. If it is made of material of resistivity 3 x 10​
-8 Ωm, and if the resistance is measured to be
450 Ω, how long is the piece of wire in question? [4]

11. An inductor whose inductance is 0.15 H is carrying a current of 50 mA. How
much energy is stored by the inductor?​
[4]

12. The magnetic field strength a distance of 1 cm away from a long straight
wire is measured to be 300 Am​
-1. How much current is flowing in the wire?​

[4]​

13. Calculate the magnetic flux density at the centre of circular loop of wire of
radius 5 cm in air carrying a current of 2 A.​
[4]

14. A length of conducting wire carrying a current of 25 mA is placed at an angle of 50​
o to a uniform magnetic field of flux density 3 T. If the force experienced
is 40 mN, what is the length of the piece of wire in question? [4]

15. A circular loop of wire has 100 turns, a radius of 1 cm and is carrying a
current of 5 mA. What is the magnetic flux density at the centre of the coil?​
[4]​

16. A solenoid with a circular cross-section has a length of 10 cm, a radius of 3cm and is wound with 2,000 turns per metre of wire. How much current must
flow in the wire to set up a magnetic flux density of 1 T at the centre of the​
solenoid, assuming that the inside of the solenoid is filled with air?

#### k.ubhi2011

Joined May 5, 2011
3
this is the orignal attachment given to make it clearer if required.

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#### russ_hensel

Joined Jan 11, 2009
825
Actually I have my own homework to do. How much can you learn if other people do the thinking work?