Physics 2 Exam 1

Is distilled(pure) water a good

conductor?

Not a good conductor — pure water

has almost no free ions; it's the

dissolved salts that conduct

Conduction. A metal instrument tray carrying −8 nC is touched to an identical

neutral tray, then separated. What is the charge on each?

−4 nC on each tray. Contact gives the SAME sign as the charging

object, and identical conductors share equally. Charge is conserved: −8 nC before, −8 nC

after.

Induction. A positively charged comb is held near — never touching — a steel

tray resting on a plastic cart. You briefly touch the far end of the tray with a

finger, lift the finger, and then remove the comb. Sign of the tray’s final charge?

Induction → negative. Your finger is the ground: electrons flow up through you toward

the comb. Lifting the finger traps them; removing the comb lets them spread out.

Induction gives the OPPOSITE sign to the charger. Order matters: finger off first, then

comb. The plastic cart keeps the tray from draining to Earth.

No transfer. The same comb is held near the tray and then removed — no one

ever touches the tray. What is the tray’s final charge?

No transfer → zero; still neutral. While the comb is near, charge separates within the

tray — negative pulled to the near side, positive left on the far side. But nothing enters or

leaves, so the total stays zero. Remove the comb and it redistributes.

Important

Important

Calculate the magnitude of the

electrostatic force between one Na⁺ ion

and its nearest-neighbor Cl⁻ ion. Express

your answer in nanonewtons.

State whether this force is attractive

or repulsive and explain how you know.

Attractive.

• Na+ carries +e and Cl− carries −e: the two charges have opposite signs.

• Coulomb’s sign rule: opposite charges attract, like charges repel.

• The force on each ion acts along the line joining the two ions and points

toward the other ion.

• This attraction, repeated throughout the lattice, is what holds the NaCl

crystal together.

p2

p3

p2

p3

p4

p5

p6

p2

p3

p4

p5

p2

p3

important

Important

1.What is the electric field midway between two negative charges?

a) equal charges

b) different charges (-1C, -2C)

(a) Zero. The field of a negative charge points toward it, so at the midpoint the two fields

are equal in magnitude and opposite in direction — they cancel.

(b) Not zero. Both fields still point toward their own charge, but the −2 C field is twice as

strong, so the net field points toward the −2 C charge

What is the direction of electric field midway between two

opposite sign charges?

Points from the + charge toward the − charge: both field vectors point that way, so they

add.

P2

Properties of electric field lines

1. Begin on positive charges; end on negative

charges (or come in from / go out to infinity).

2. Number of lines is proportional to charge

magnitude.

3. Field strength is proportional to line density.

4. Field direction is tangent to the line.

5. Lines never cross, and should be smooth curves.

6. Field lines meet a conductor’s

surface perpendicular

P2

p3