Currents and voltages with resistors

[runningirl]

Homework Statement

Two identical long bulbs are wired in series and connected to a 4.5-volt battery pack. The resistance of each bulb is 50 ohms.

a. Determine the voltage across each bulb.
b. Determine current through each bulb.
c. How does this compare to the current flowing out of the battery?
d. repeat this problem in parallel circuit

Homework Equations

The Attempt at a Solution

a. (4.5/50)(50)=4.5 V for both? but i don't think that's right since it's in a series...
b. 4.5/50 amps for both
c. shouldn't it be the same since it's in a series?
d. i had no idea??

 

[gneill]

Draw the circuit. Represent the bulbs as resistors. What's the total resistance that the battery pack "sees"?

 

[runningirl]

isn't it 100 ohms?

 

[gneill]

Yes. So what's the current supplied to the series circuit by the 4.5V battery pack to the 100 ohm net resistance?

 

[runningirl]

4.5/100

 

[gneill]

You should supply units for your numbers, so it's 4.5/100 amps, or 45 milliamps. In short form, 45 mA.

Now, with 45 mA flowing through a 50 ohm bulb, what's the voltage across it?

 

[runningirl]

oh, it's just 45(50) V.
would the current for each bulb still be 45...?
and how would this change for a parallel circuit>

 

[gneill]

All components connected in series share the same current, so both bulbs have 45 mA flowing through them.

Be sure to keep track of the orders of magnitudes for units. "45" here is in milliamps, or 1000ths of an amp.

Components in connected in parallel share the same voltage rather than the same current.

 

[runningirl]

is this how i would find current in the parallel circuit?
1/50+1/50=2/50=1/R
R=25 ohms
then to find circuit, i would need the voltage for each bulb...
but how would i find that? is it 2.75 for each?

 

[gneill]

is this how i would find current in the parallel circuit?
1/50+1/50=2/50=1/R
R=25 ohms
then to find circuit, i would need the voltage for each bulb...
but how would i find that? is it 2.75 for each?

Both bulbs are connected directly across the battery. A component connected directly across a voltage supply must have that supply voltage across it. The only possible voltage each bulb can have is that of the battery.

Since the voltage across each bulb is fixed at 4.5V, you can calculate the current in each bulb. Add them to find the total current.

Note:

In a parallel circuit
1. Parallel branches all share the same voltage
2. Total current is the sum of the currents in the branches

In a series circuit
1. All components share the same current
2. The total voltage is the sum of the individual voltages across all components.

 

[runningirl]

4.5/50=I for each bulb.
then .18 mA for the total current in the parallel circuit.

is that right?

 

[gneill]

4.5/50=I for each bulb.
then .18 mA for the total current in the parallel circuit.

is that right?

Correct.