Transcript “wet” or “dry”. Solar cells convert light energy into electrical energy

the flow of
charged particles;
can be positive or negative,
but usually negative (electrons)
through a conducting metal
electric cell - a device that
converts one form of energy
to electrical energy
Chemical cells convert chemical
energy into electrical energy.
Chemical cells can be
“wet” or “dry”.
Solar cells convert light energy
into electrical energy.
A generator converts mechanical
energy into electrical energy.
battery - two or more cells
connected in series or in parallel
Electric current is measured
in Amperes, in honor of
Andre Marie Ampere.
One Ampere is the flow of
one Coulomb of charge per second.
1 Amp = 1 Coulomb per second
= 1 C/s
I = Q/t
Ammeter
a device that measures current
Voltmeter
a device that measures
electric potential difference
power = work/time
.
= (work/charge) (charge/time)
.
= electric potential difference current
P (Watts) = V (Volts).I (Amps)
Analogies of simple circuits are these links:
Water circuit analogy link
Air flow link
Resistance
determines the amount of current flow
= the ratio of potential difference to current
R=
V
The SI unit of
resistance is the
I Ohm, W, named
in honor of Georg Simon Ohm.
One Ohm of resistance is the resistance
such that one Volt of potential difference
is needed to obtain a current of one Amp.
The resistance of a circuit element depends on:
1. the length of the conductor
as length increases, resistance increases proportionally
2. the cross-sectional area of the conductor
as area increases, resistance decreases proportionally
3. the resistivity of the conductor
as resistivity increases, resistance increases proportionally
Ohm’s Law
The ratio of potential difference to current
is constant.
If R = V/I is a constant value
for a given resistor, then that
resistor is said to obey Ohm’s Law.
Click here and here to link to pages describing resistor
color codes.
Resistivity
The resistivity, r, of a conductor is equal to the
resistance of a wire 1 cm long and having
a cross-sectional area of 1 cm2.
l
R =r
A
R = resistance, measured in Ohms
r = resistivity, usually in units of W.cm
l = length, measured in cm
A = cross-sectional area, measured in cm2
Many circuit elements do not
obey Ohm’s Law. Resistors
that get hot, like light bulbs
and heating elements, do not
keep a constant resistance.
Resistance generally increases as
objects become hotter.
Click here and here to run
simulations of Ohm’s Law.
Series
Resistor Circuits
1. total resistance is the sum of the
separate resistors
RT = R1 + R2 + R3 + ...
2. current is the same through each resistor
IT = I1 = I2 = I3 = ...
3. total potential difference is the sum of each
VT = V1 + V2 + V3 + ...
In other words, in a series circuit,
resistance and voltage add,
but current stays the same.
R,
W
E = 12 V
R1
R3
R2
R1
8.0
R2
2.0
R3
5.0
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
R3
R2
V,
V
I,
A
P,
W
R1
8.0 6.4 0.80 5.1
R2
2.0 1.6 0.80 1.3
R3
5.0 4.0 0.80 3.2
RT = 15 Ω
VT = 12 V
IT = 0.80 A
PT = 9.6 W
Parallel
1. reciprocal of the total resistance is the
sum of the reciprocals of the separate
resistors
1/RT = 1/R1 + 1/R2 +1/R3 + ...
2. total current is the sum of the current
through each resistor
IT = I1 + I2 + I3 + ...
3. potential difference is the same across
each resistor
VT = V1 = V2 = V3 = ...
In other words, in a parallel circuit,
resistance adds as reciprocals,
voltage stays the same, and current splits.
R,
W
E = 12 V
R1
R1
12
R2
8.0
R3
12
R2
R3
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
V,
V
I,
A
R1
12 12
R2
8.0 12 1.5 18
R3
12 12
1.0 12
1.0 12
R2
R3
P,
W
RT = 3.42 Ω
VT = 12 V
IT = 3.50 A
PT = 42 W
Kirchhoff’s Rules
Loop Rule: The sum of the potential differences
around any closed circuit loop is zero.
Junction Rule: The sum of the currents
into any circuit junction is zero.
Go to link1, link2, link3, link4, link5, and link6 to view
pages and simulations examining
Kirchhoff’s Loop and Junction Rules.
The site linked
here allows you
to build and test
your own series,
parallel, and/or
combination circuits.
For a complete interactive tutorial on
electricity and magnetism, go here.