Download Today: Electricity and Magnetism

Today: Electricity and Magnetism
Basic Ideas
Electrical Circuits
ÆA circuit is always a circle.
ÆA battery provides work to move
charges (Q) around the circuit. The
units here are volts (V) which equals
the work per unit charge. To confuse
college students, the common name
for the physical quantity comes from
the name of the units – voltage (V). To
the right, we assume the battery
provides V = 6 V.
ÆThe black lines are wires. We
assume the voltage never changes
along a continuous wire.
6V
V
0V
ÆThe charge passing a given point
per unit time is the current I. The units
are amps (A).
ÆThe jagged symbol represents a
resistor. The voltage does change
across a resistor.
ÆFor convenience, lets define the
green ground as the zero volts.
0V
R
Electrical Circuits – Ohm’s Law
Ohm’s Law
The voltage change across a
resistor:
V=IR
6V
V
0V
0V
R
Electrical Circuits – Power
Power
ÆCharge Q gains potential energy
QV while being pushed across the
battery.The rate at which energy is
gained is QV/t = (Q/t)V = IV.
Remember, energy per time equals
power. So, P=IV.
ÆIn turn, the charge loses energy
while crossing the resistor. The rate
at which energy is lost is also P=IV.
6V
R
0V
ÆWe can also write the relationship
as P = IV = I(IR) = I2R.
ÆThe energy is changed to heat for
a typical resistor. For a light bulb, it
is heat + light.
Heats up
0V
Resistors in Series
Resistors in Parallel
Magnetism
Magnetism