Electrostatics I Questions and Answers - Physics Form 1 Topical Revision

Questions

1. Two isolated and insulated spheres A and B carry the same positive  charge. Sketch the electric lines of force of their field when placed close to each other but not touching some.
2. State the observation on the leaves of a positively charged electroscope  when a negative charge is brought near it.
3. The fig shows sketches of two types of houses built in a lightning prone area. State with reason which house is safer to stay in during lightning and thunderstorms?
   
4. The diagram below shows a circuit with a capacitor C and a lamp L. When the sketch is closed at Y, the lamp L lights. When the switch is closed at X, L does not light. Explain the observation.
   
5. In the clothing and textile industries the machines experiences electrostatics forces at certain points. Suggests one method of reducing these forces.
6. State two other factors to be considered in constructing a capacitor other than surface area of the plates.
7. State the precaution that is taken when charging a metal object.
8.  
   1.  
      1. State coulombs law of electrostatic force
      2. Define capacitance
   2. Describe how the type of charge on a charged metal rod can be determined
   3. The fig. Shows hollow negatively charged sphere with a metal disk attached to an insulator placed inside. State what would happen to the leaf of an uncharged electroscope if the metal disk were brought near the cap of the electroscope. Give a reason for your answer.
      
   4. State two ways of charging the magnitude of the deflection of the leaf of an electroscope.
   5. The fig below shows an arrangement of capacitors connected to a 10V d.c supply.
      
      Determine:
      1. The charge stored in the 2μF capacitor.
      2. The total capacitance of the arrangement.
9. Explain why the leaf of an uncharged electroscope diverges when a charged object is brought near the cap.
10. A glass rod can be charged positively by rubbing it with silk. Explain what happens when the glass rod is being charged.

Answers

1.  
   
2. The leaf in the electroscope falls
3. Metal roofed house. Because there is less resistance of the flow of charges to the ground so if struck by lighting it would conduct it to the ground. The other one would burn or have the people inside struck by the lightning.
4. At x the capacitor is charged only once and the keeps charging and discharging in opposite directions hence current keeps alternating at the a.c frequency. This lights the bulb continuously.
5. Earthing the machines/using spikes.
6. Material used between the two plates of the capacitor.
7. Well insulated / avoid touching 
8.  
   1. Ability to store charge given by the quantity of charge it can store per unit p.d
   2. Bring it near a charged electroscope (say +vely). If not, charge the electroscope – vely and bring the rod near.  If divergence is observed then they have the same charge.  Note that if decrease in divergence is observed in both cases then the rod is simply a conductor and it’s not charged.
   3. Nothing would happen to the leaf of the electroscope. This is because in a hollow charged conductor, the charged conductor and not inside 
   4. - Earthing or using another
      - Charged body
   5.  
      1. Q= CV
         = 2x10
         = 20μC
      2. Series = ¹/_c = ¹/₃ + ¹/₃
         ¹/_c = ²/₃
         C= ³/₂ = 1.5μF
         Parallel Ct = 1.5 + 2
         =3.5 μF.
9. If the charged object has a positive charge. Its proximity to the cap draws electrons into the cap, leaving a shortage of electrons in the leaf. Both sides of the leaf are therefore positively charged, and repel each other.
   
   If the charged object has a negative charge, then its proximity to the cap pushes the electrons from the cap to the farthest regions of the conductor. The leaf is the farthest region from the cap, so it gains an excess of electrons. Both sides of the leaf are therefore negatively charged, and repel each other.
10. Glass happens to lose electrons easily, and silk grabs them away from the glass atoms, so after rubbing the glass becomes positively charged and the silk becomes negatively charged.