1. A metallic object holds a charge of -6.7 · 10 -5 C. What total number of elec

Question

1. A metallic object holds a charge of -6.7 · 10 -5 C. What total number of electrons does this represent? (e = 1.6 · 10 -19C is the magnitude of the electronic charge.)

2. Two point charges, separated by 1.6 cm, have charge values of +2.7 and -2.5 µC, respectively. What is the value of the mutual force between them? [Answer in Newtons]

3. Charge A and charge B are 1.60 m apart, and charge A is +2.80 C and charge B is +2.30 C. Charge C is located between them at a certain point and the force on charge C is zero. How far from charge A is charge C?

4. Two point charges are separated by 10.0 cm and have charges of +2.00 µC and -2.00 µC, respectively. What is the electric field at a point midway between the two charges? (ke = 8.99 ´ 109 N·m2/C2)

Explanation / Answer

1) charge is quantized so

N*e = 6.7*10-5 where N = no of electrons and e = charge on one electron

N* 1.6 *10-19 = 6.7 *10-5

N = 4.1875* 1014

2) F = k Qq/ r2

= 9*109 *2.7*10-6 *(-2.5*10-6) / (1.6 *10-2)2

= - 237.3 N where -ve sign shows force of attraction

3) lets say its is X m apart from charge A

now balance forces

k (2.80)q/ x2 = k (2.30)q/ (1.6-x)2

X = 0.839 m

4) electric field will add up because of opposite sign

= 2( kq / r2 )

= 2*8.99*109 *2 *10-6 / ( 5*10-2)2   we take r = 5 cm due to mid point

= 1.4384 *107 N/c

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