Question

Two light bulbs, one with a resistance of 30 ohms and the other 40 ohms, are arranged in two different circuits with a 120 volt source.

A] The two bulbs are first connected in parallel to the source.
i. Draw a schematic of the circuit –clearly labeled.
ii. Determine the current, voltage and power through each bulb.

B] The two bulbs are now connected in series to the source.
i. Draw a schematic of the circuit –clearly labeled.
ii. Determine the current, voltage and power through each bulb.

C] Rank the bulbs in each situation described, from greatest to least of their brightness.

a. 30ohminparallelcircuit

b. 40 ohm in parallel circuit

c. 30 ohm in series circuit

d. 40 ohm in series circuit

D] Explain how a voltmeter would be connected in each type of connection to verify the voltage calculated for each bulb in part A and B.

E] Explain how an ammeter would be connected to each type of connetion to verify the current calculated for each bulb in part A and B.

Answer 1

Vnet = I * RNet

RNet in parallel = 1/Rnet = 1/30 +1/40

Rnet = 17.14 ohms

Inet = 120/17.14 = 7 Amps

Current Across R1 = V/R1 = 120/30 = 4 AMps

V1 = V2 = 120 Volts

Current across R2 = 120/40 = 3 Amps

Power across R1 (30 ohms in parallel) = I1^2 R1 = 4^2* 30 = 480 Watts

Power across R2 (40 ohms in parallel) = I2^2 R2 = 3^2 * 40 = 360 Watts

----------------------------------------------

B: when in series

Rnet =R1 + R2

Rnet = 30 + 40 = 70 ohms

in series, current remains same

Inet = I1 = I2 = V/Rnet = 120/70 = 1.714 Amps

V1 = I1R1 = 1.714* 30 = 51.42 VOlts

V2 = I2R2 = 1.714 * 40 = 68.56 Volts

POwer across R1 (30 ohms in series) = P1 = V^1^2/R1 = 51.42^2/30 = 88.1333 Watts

Power across R2 = P2 (40 ohms in series) = V2^2/R2 = 68.56^2/30 = 156.68 Watts

---------------------

C.

from above Power values,

we get

a. 30ohminparallelcircuit > b. 40 ohm in parallel circuit >d. 40 ohm in series circuit > c. 30 ohm in series circuit

------------------------------------------------

D.

Voltmeter mus t be connected in parallel

and

E. AMmeter must be connected in SERIES