Ch. 20 - Electricity
Section 20.1
Electric Charge
and Static
Electricity
p. 600
Electric Charge
 Causes subatomic particles to
attract/repel
 2 types
• + and –
 Everything affected by charge
 Neutral atom = protons
& e-’s
 Gain e-’s = - ion
 Lose e-’s = + ion
 Excess/shortage of e-’s
produce net charge
 Coulomb (C) - SI unit of
electric charge
6.24 x 1018 e-’s = 1 C
Electric Forces
 Opposite charges attract
 Electric force depends on charge &
distance
 2x distance = ¼ electric force
 Stronger than gravity
 Hold atoms together
Electric Fields
 The effect electric charge has on
other charges
 Strength depends on amt of
charge producing field & distance
from charge
 Exerts forces on charged object
placed in field.
 More net charge object has,
greater force on it
Static Electricity and Charging
 Static Electricity
 net accumulation of electric charges
on an object
 Charge can be transferred by
 Friction
 Contact
 Induction
Static Electricity and Charging
 During charge transfer, total charge
same before & after transfer
 Law of conservation of charge
Charging by friction
e-’s move from hair to balloon
 Atoms in rubber >attraction for e-’s
• Balloon net – charge
• Hair net + charge
--+++
Charging by contact
 Touch electrically charged object – you
become charged
 Sphere still has net charge, but reduced
Van de Graaff generator
Charging by Induction
 Transfer of charge w/o contact btwn
materials
 Walk across carpet
 Pick up extra e-’s; net – charge
 Repels e-’s in doorknob
 Leaves net + charge on doorknob closest
to hand
 Doorknob overall charge neutral, but
charges moved within it
Static Discharge
 Occurs when pathway forms for
charges to move
 Air becomes charged when hand near
doorknob
 Air provides path for e-’s
Static Discharge –
lightning
Charge built up from friction
btwn moving air masses in
clouds
 - charge in bottom of cloud
induces + charge in ground
 Charge in cloud increases
 Attraction increases
 Air charged – pathway for
e-’s
Lightning 5:06
Chapter 20.2
Electric Current and
Ohm’s Law
p. 604
Electric Current
 Continuous flow of
electric charge
 SI unit is ampere, or
amp (A)
 2 types:
• Direct current (DC)
• Alternating current
(AC)
Direct Current
 Charge flows in 1 direction
 Most battery opperated devices
• Flashlights
• Remotes
Alternating Current
 Current that regularly reverses its
direction
 Electric current in homes, businesses,
& schools often use AC
Electric
current
in a flashlight

e-’s flow from
- to + terminal
 Current in opposite direction!
 Scientists define current as direction +
charges flow
Conductors
 material that allows e-’s to
flow easily
• Metals like copper and
silver
 Made of ions in a lattice
• ions - atoms that gained
or lost e-’s
 Ions not free to move
 Each ion has e-’s loosely held
• Free e-’s conduct charge
Insulators
 material that doesn’t allow e-’s to move
through it easily
 e- are tightly held
 ex: plastic, wood, rubber, glass
Resistance
 e-’s in electric curent collide
 opposes flow of e-’s
 electrical energy converted
to thermal energy & light
 measured in ohms () Tungsten - high resistance
 Thickness, length, temp
affect resistance
Resistance depends on..
 wire thickness
• more resistance in
thinner wires
– milkshake in thin
v.s. thick straw
 wire length
• more resistance in longer wires
 Temp
• high resistance at high temps
• e-’s collide more often
Superconductors
 Can resistance ever be 0?
 Superconductors produce near zero
resistance when cooled to low temps.
Voltage
 Potential Difference (voltage)
 diff in electrical potential btwn 2 places
in electric field
• Similar to PE diff of water at top and
bottom of fountain
 large separation of charge creates high
voltage
• Large fountain – high PE
 “push” causing e- to move from - to +
 measured volts (V)
Voltage sources
 Fountains pump water to top, voltage
source increases PE of electric charges
 batteries
 solar cells
 generators
Ohm’s Law
•increasing voltage increases current
•Increasing resistance decreases
current
 A lightbulb with a resistance of 160  is
plugged into a 120-V outlet. What is the
current flowing through the bulb?
V
GIVEN:
R = 160 
V = 120 V
I=?
I
R
Electroscope
 Electroscope
 instrument that
detects presence of
electrical charges
 leaves separate
when they gain + or charge