Transcript Thought Question

Ohm’s Law

Much like how
gravitational potential
energy depends on a
masses location with in
the gravitational force
field of another mass,
electric potential
energy depends on a
charges location with
in the electric force
field of another
charged object.
Potential

Just like if you increase the mass of one of
the objects the PE increases, if you increase
the charge of one of the objects the PE also
increases.
 The ratio of electric PE to amount of charge is
known as electric potential, or voltage (V).
 V = PE/q
Potential

So, if the location of a charge(q) doesn’t
change, but the actual charge changes,
Voltage remains the same.

Voltage is independent of charge in the
field.
Electrons
move from a
point of high
electric
potential to a
point of low
electric
potential.

Comparing potential difference in a
battery to gravitational potential energy:
12 volts
q
0 volts
Note: this actually has nothing to do with Gravitational PE
Potential Difference
When there is a difference in potential
between two points in a circuit,
electrons will flow until there is no
longer a difference.
 Voltage, or potential difference is the
driving force for electrons in a circuit.

Unit
of potential difference or
“Electromotive Force” is the Volt (V)
Electromotive
The
Force (EMF)
maximum difference of potential
between the terminals of a battery or
generator
Current ( I )

What do we know about electrons in a
conductor?
– Conduction “free” electrons are free to move from
atom to atom.
– Current is simply the flow of charges or the net
movement of these electrons
Voltage does not go anywhere,
the charges move.
Current

When there is a Potential Difference between
two ends of a circuit, charges will move until
both ends reach a common potential.
 This is similar to how water flows from high
pressure to low pressure areas until there is
no longer a difference. (figure 34.1 in text)
Current
Current is measured in amperes (A)
(amps)
 I = q/t… so the units are 1A = 1C/s

The way we say current flows through a circuit is actually the
opposite direction that electrons move… I know… its weird
Resistance
Water
Pipe Re-visit
What do you notice about the flow of water through different
pipes?
Resistance ()
Resistance in a wire depends on four
different properties: Thickness, length,
and conductivity(how well electrons flow
in a material), and temperature.
 It is measured in Ohms ()

Resistance
 Thick
wire <resistance Thin wire
Resistance
 Short
wire <resistance Long wire
Resistance
 Cool wire <resistance Hot
most materials)
wire (for
Ohm’s Law

V = IR
Revisit the Do Now

What hurts you when you get shocked?

The faster the current passes through
you, the more damage it does (pg 537
in text)
Ohm’s Law and Electric Shock
Damaging effects of electric shock are
from current traveling through the body.
The initial cause of electric shock is the
voltage, however the current does the
damage.
 Your body typically offers 100,000 Ohms
of resistance. A current of 0.01 Amperes
could cause muscle spasms.

Power

What is a short circuit?

Battery example (note: don’t try this at
home!)
Power

Electrical Power (P) = current x voltage
(P = I x V)

1Watt = 1amp x 1volt
Power

Relate to our old concept of power:

1watt = 1amp x 1volt = (q/t) x (PE/q) =
PE/t = power