Ch 9: Electrostatics

Question 1

How do you define ‘current’?

Answer 1

the motion of charges through a material

Question 2

An insulator is a material that does not have ____ _______ and cannot carry a current.

Answer 2

free charges

Question 3

What is the magnitude of the elementary charge?

Answer 3

e = 1.6 x 10^-19 C

Question 4

The charge on an object can only be a whole number of + e’s. That is to say, that charge is _________. To remind us that charge is such, electric charge is usually denoted by the letter Q or q.

Answer 4

quantized

(p. 256)

Question 5

Total electric charge is always _________. However, this does not mean that electric charge cannot be created or destroyed, which happens all the time. For example, in the reaction e^- + e⁺ → γ + γ, an electron and its antiparticle (the ________, e⁺, which is essentially a positively charge electron) meet and annihilate each other, producing energy in the form of two gamma-ray photons (γ) which carry no charge. Charge has been destroyed, but the total charge (zero in this case) has been conserved.

Answer 5

conserved

positron

(p. 257)

Question 6

Coulomb’s constant is denoted by k and this fundamental constant in nature is equal in magnitude, to…

Answer 6

….10^-7 times the speed of light squared. Its value is 9 x 10⁹ N∗m²/C²

Question 7

If we had two objects each with a charge of 1 C, separated by a distance of 1 m, the electric force would be equivalent to 9 x 10⁹ N, which in tons is….

Answer 7

….one million tons.

(p. 260)

Question 8

State Coulomb’s law.

Answer 8

F_E = k |q₁ q₂ | / r²

Question 9

Remember that Coulomb’s law is an inverse square law, so if r increases by a factor of 2, then F_E will….

Answer 9

….decrease by a factor of 4.

Question 10

State Newton’s law of gravitation:

Answer 10

Question 11

What is the value of G, the gravitational constant?

Answer 11

6.7 x 10^-11 N∗m²/kg²

Question 12

The principle of superposition states that the net electric force on a charge (q) due to a collection of other charges (Q’s) is equal to…

Answer 12

…the sum of the individual force that each of the Q’s alone exerts on q.

(p. 262)

Question 13

Electric field lines (or vectors) always point AWAY from ________ source charges and toward ________ ones.

Answer 13

away from POSITIVE source charges and toward negative ones

p. 265

Question 14

If given the voltage (potential difference) between two electrodes, and the distance between them, how can you find the magnitude of the electric field?

Answer 14

Since an electric field can be given in volts per meter OR N/C. This means the electric field value is simply the voltage, divided by the distance in meters.

(see next card to explain the units V/m vs. N/C)

Question 15

Why is a volt/meter (V/m) the same as a newton/coulomb (N/C)?

Answer 15

1 V = 1 J/C and 1 J = N∗m

so 1 V = [1 N∗m]/C

[1 N/m]/C ÷ m = 1 N/C

Takeaway message: electric field strengths can be given in V/m or N/C.