Download AP Physics Chp 18

AP Physics Chp 18
• Crooke’s Tube allowed Thomson to show that
atoms possessed smaller particles that also
had an electric charge
• Electrons
• Protons
• Neutrons were later
• Millikan determined the charge using oil drops
• e = 1.60 x 10-19 C
• Why do we call the +1 for protons and -1 for
electrons?
• Like charges repel and unlike attract
• Within an isolated system the net charge
remains constant ie. Rubbing a balloon on
your hair
• Conductors – allow charge to pass thru
• Insulator – do not allow charge to pass thru
• Charging by contact – balloon and hair
• Charging by induction (coming close)
– There has to be a way to allow the opposite
charge to leave the object to maintain the charge
• Coulomb’s Law
F k
q1 q2
r
2
• k is 8.99 x 109 Nm2/C2
• Can be either attractive or repulsive unlike
gravity
• How much force exists between two protons
in the nucleus of helium if they are 3.1 x 10-15
m apart?
• What’s the gravitational force?
• mp = 1.67 × 10-27 kg
• Electric Field
F
E
qo
• qo is a small, positive test charge
• E goes in the same direction as F if qo is +
• How large is the electric field that levitates a
proton in the air? m = 1.67 x 10-19 kg and
q = 1.6 x 10-19 C
• For a point charge
Ek
• IF q is + then E is away from qo
q
r
2
• Parallel plate capacitors
q
E
o A

E
o
σ is the q/A (charge density)
• Electric Field Lines ( lines of force)
• They go away from + towards –
• Don’t stop mid-way
• More charge = more lines