Power dissipated in heater at reduced voltage and RC circuit charge decay

[cseet]

HELP! PLSSS! - Physics

Hi All,
I badly need helpwith the below 2 questions... as for question 2, I worked out part a-c but I'm unable to complete part d... pls help and correct me if I am wrong.

really appreciate it!

cseet

1. A 125 W heater is designed to operate with an emf of 120 V.
Find the approximate power dissipated in the heater if the potential difference is decreased to 112 V. (Unit: W)

can somebody help me up with this?

2. A 5.2 F capacitor is charged to 1.4E2 V and is then connected across a 4.5E2 resistor. Answer the following questions:
All answers must have three significant digits unless otherwise specified.

(a) What is the initial charge on the capacitor? (Unit: microcoulombs, uC)
answer: 728 uC

(b) What is the initial current just after the capacitor is connected to the resistor? (Unit: milliamperes, mA)
answer: 311 mA

(c) What is the time constant of this circuit? (Unit: milliseconds, ms)
answer: 2.34 ms

but I'm Stuck on this question...
(d) How much charge is on the capacitor after 5.1 ms? (Unit: microcoulombs, uC)
can somebody help me up with this??

thanks again!
cseet

 

[Doc Al]

For #1: First find the resistance of the heater using the rated values for power and voltage. Now find the new power dissipated when the voltage is reduced.

For #2d: You found the time constant Τ, now use it. The charge at time t is: Q = Q₀e^(-t/Τ).

 

[cseet]

re

Hi Doc Al,

thanks so much! you're a gem!
cseet