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Electricity index:




1) The electric charge is quantized. This means that:

a) it cannot be destroyed

b) it can be positive or negative

c) it can only be a multiple of a defined quantity

d) none of the above

2) A metallic sphere has a negative electric charge of 3.2 * 10-4 C. How many electrons does it have?

(electron charge: 1.6 * 10-19 C)

a) 2*1015

b) 200

c) more than 2*1015

d) 2*1010

3) Three neutral spheres, electrically insulated from the environment, are touching each other as shown in the picture below. A positively charged glass rod approaches the spheres, as shown, but never touching any sphere. While the rod is there, the spheres are separated from each other.

After the rod is put away, the spheres stay charged on the following manner:

a) 2 positively charged and one negatively charged

b) 2 neutral and 1 positive

c) 1 neutral, 1 positive, 1 negative

d) 2 neutral and 1 negative

4) A metallic object, positively charged,with charge +Q, approaches a gold leaf electroscope previously charged negatively, with charge -Q.


I. As the object approaches the electroscope, the leaves open up even more.

II. As the object approaches the electroscope, the leaves remains at the same positions.

III. If the object touches the electroscope terminal, the leaves must definitely move down and close.

We can say that:

a) Only I is correct

b) II and III are correct

c) Only III is correct

d) None are correct

5) The charges Q1 and Q2 keep charge q in equilibrium at the position shown below.

For this to happen the ratio Q1/Q2, in absolute values, must be:

a) 2/3

b) 3/2

c) 2

d) 9

6) A tiny oil drop , with mass m=9.6*10-15 kg and charge q = -3.2*10-19 C, falls vertically in a vacuum. In a given moment, an electric field is turned on in this region, pointing downwards. The intensity of the field is tuned until the drop falls at constant speed. What is the value of the electric field in this situation?

a) 3*105 N/C

b) 2*107

c) 5*103

d) 8*10-3

7) At a point P, close to an electric charge, the absolute value of the electric field is 500 N/C and the electric potential is -3*103 V. The distance from P to the charge, and the charge value are respectively:

(K = 9*109 )

a) 6 m and 2*10-6 C

b) 3 m and -2 *10-6 C

c) 3m and 2*10-6 C

d) 6 m and -2*10-6 C

8) A proton has mass m and charge e. An alpha particle has mass 4m and charge 2e. If these 2 particles are placed in an electric field, they will be subjected to forces Fp and Falpha, whose ratio Fp / Falpha equals:

a) 1/4

b) 1/2

c) 2

d) 4