## 12th Class Physics Notes for Punjab Board

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### Q.4 Describe the force or forces on a positive point charge when placed between parallel plates

(a) with similar and equal charges (b) with opposite and equal charges

Since we know that the electric force on a point charge is given by the formula as
F = qE
a)  If the plates have equal and similar charges then the net force on positive point charge placed between them is zero because net electric field is zero as per the above formula.

b)  If the plates are oppositely charged then the net is not zero and the positive point charge moves from positive plate to negative plate

### Q.1) A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electrons by

i) increasing the potential difference
ii)   decreasing the length and the temperature of the wire

Drift velocity is the velocity which an electron attains in the presence of electric field.
i) The drift velocity increases with the increase in the potential difference because by increasing the potential the electric field increases. This increase in the electric field will cause an increase in the drift velocity.
ii)    Length and resistance are proportional to each other. Because if length is greater there would be larger number of atoms and chances of collision of electrons with atoms would be greater and resistance would be high and vice versa for smaller length. So by decreasing the length, the resistance will decrease due to low chances of collision of electrons with atoms. The decrease in the resistance will cause an increase in the drift velocity. By decreasing the temperature, resistance will also decrease due to which the current will increase. Due to this increase in the current the drift velocity will increase.

### Q.14 2) A current in a conductor produces a magnetic field, which can be calculated using Ampere's law. Since current is defined as the rate of flow of charge, what can you conclude about the magnetic field due to stationary charges? What about moving charges? 