Download Chapter 7-2

Chapter 7-2
Electric Current
Electric Pressure
(Voltage)
• Charges flow from high-voltage areas to
low-voltage areas
• Think about it like water: water flows from
areas of high pressure to areas of low
pressure
• Voltage difference = the push that causes
electrons to flow
• Measured in volts (V)
Electric Current
• Flow of charges through a conductor
• Think of it as the number of electrons
flowing through a wire (or molecules of
water flowing through a pipe)
• Measured in amperes (A), commonly
called “amps.”
What makes a battery work?
1. A chemical reaction inside the
battery strips electrons off of atoms.
2. Electrons accumulate on the
negative terminal.
3. If a conductor is connected to the (-)
and (+) ends of the battery, electrons
will flow.
4. At the positive terminal, electrons
flow back into the carbon rod inside
the battery.
5. The battery lasts as long as the
chemical reaction inside can be
sustained.
Dry-cell batteries
Positive and negative terminals are separated by
a paste, not a liquid.
Wet-cell batteries
Positive and negative terminals are separated by a
liquid (in this case, sulfuric acid).
What makes a light bulb glow?
• Resistance = tendency of a material to oppose
the flow of electrons, measured in Ohms (Ω)
tungsten filament = high resistance, lots of
heat and light produced
copper wires = low resistance, no heat
or light produced
Is resistance good or bad?
Low resistance is good
for…
• wires that conduct
electricity
High resistance is good
for…
• light bulbs
• space heaters
• electric blankets
• electric stoves
• resistors – protect
delicate electronics
from too much electric
current passing through
them
What affects resistance?
• Type of material
• Insulator = material with high resistance
• Conductor = material with low resistance
• Thickness of wire
• Thinner wire = higher resistance
• Temperature of wire
• Warmer temperature = higher resistance
• Length of wire
• Longer wires = higher resistance
Ohm’s Law
• If you increase the water pressure on a pipe,
more water will flow through it.
• If you increase the voltage, more current will
flow through it.
• If you put obstructions in the pipe, less water
will flow through it.
• If you increase the resistance, less current will
flow.
Ohm’s Law
Voltage (V)
Current
(measured
in amps)
V
I
R
Resistance -measured in
Ohms (Ω)