Download bulb is 4 X as bright

Brightness of a lightbulb
In USA, standard outlet voltage is 120 V. In
Europe, standard outlet voltage is 240 V.
How does the brightness of 100 W light
bulb compare when used in Europe vs in
US?
a)
b)
c)
d)
e)
bulb is ¼ X as bright
bulb is ½ X as bright
bulb has same brightness
bulb is 2 X as bright
bulb is 4 X as bright
how to approach this problem?
The physical properties of the bulb itself will
not change. The resistance will be the same.
What happens to the power of the bulb.
The power can change if the voltage is
different.
Power = V2/R
P1 = V12
R1
P2 = V22
R1
What is the correct response?
a)
bulb is ¼ X as bright
b)
bulb is ½ X as bright
c)
bulb has same brightness
d)
bulb is 2 X as bright
e)
bulb is 4 X as bright
Remember:
Matter is made of protons, neutrons, and
electrons
Protons are positively charged electrons
negatively charged neutrons have no charge
Like charges repel, opposite charges attract
An induced charge can be created in an
electrically neutral object by placing it in an
electric field.
Electric field lines are drawn from + charge
to – charge.
Where electric field is stronger, the field
lines are drawn closer together.
The electric force on an object depends on
both the object’s charge and the electric field
it is in.
Unless stated otherwise, the zero of electric
potential is at infinity
Equipotential lines show all points where a
charged object would feel the same electric
force.
Equipotential lines are always drawn
perpendicular to electric field lines.
Equipotential lines are parallel to the surface
The electric field between two charged
parallel plates is constant.
The electric field around a charged particle
depends on the distance from the particle.
Current is the flow of positive charges
Resistance is the property that impedes flow
of charge
Resistance in a circuit comes form the
internal resistance of the wires and from
resistors [and internal resistance of battery]
Resistance is related to current and voltage
by Ohm’s law
V = IR
Resistors in series: Req = R1 + R2 + . . .
One current flows through all resistors in
series
Every resistor experiences a voltage drop
related to its resistance.
Resistors in parallel:
1/Req = 1/R1 +1/R2 + . . .
All resistors in parallel have same
voltage drop.
Current divides in parallel circuit.
More current in path of least resistance
Use V I R table to organize circuit problem
Kirchoff’s current rule: sum of currents
entering junction equals sum of currents
exiting.
Kirhoff’s voltage rule: sum of voltages
around a closed loop is zero.
Write enough loop equations to include each
component at least once.
You choose a direction for the current and
label the sketch
If you guess wrong, then get negative value
for current
Ammeters are connected in series [ have
~zero resistance]
Voltmeters are connected in parallel to a
circuit. [~ infinite resistance]
Capacitors add in series like resistors in
parallel
Capacitors add in parallel like resistors in
series.
Capacitors are used to store charge. Charge
of capacitor Q = CV.
When first connected to a current, circuit
behaves as if capacitor was absent .
When connected to a circuit for a long time
they are fully charged and then capacitors
prevent flow of current.
Test tomorrow electrostatics and electric
circuits.