EEET 2249 Tutorial 2
Charge, Current, Voltage, Power and Energy Charge & Current
Q1
A wire carries a constant current of 10 mA. How many coulombs pass a cross
section of the wire in 30 s?
n(C) = i*T
= 10e-3 * 30
= 3e-1 C
(Ans: 0.3 C)
Q2
The charge entering the terminal of a device is given by q(t) = k1t + k2t2 C.
It is observed that i(0) = 5 A and i(4) = -3 A. Find the constants k1 and k2 .
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(Ans: k1 = 5 A and k2 = -1 A/s ).
Q3
The time varying current in an element is given by:
Calculate the total charge that has entered the element in (a) t = 6 s and (b) t = 10 s.
(Ans: q(6) = 8 C and q(10) = 5 C)
a)
q = (5-2)(3) + (6-5)(-1)
q = 8 C
b)
q = (5-2)(3) + (9-5)(-1)
= 5 C
Q4
In an electroplating bath silver ions transport charges from the positive silver
electrode to the negative electrode, which is an object being silver plated. When the
positive silver ions arrive at the negative electrode the charges on the silver ions are
neutralized and the silver is deposited on the surface of the negative electrode.
Given that 1.118 mg of silver transports 1 C of charge, determine the weight of silver
deposited when 5 A current is passed through the bath for 30 minute.
q = i*t
= 5 * 30 * 60
= 9000 C
m = 9000 * 1.118e-3
= 10.06 g
(Ans: 10.06 grams)
Voltage, Energy & Power
Q5
A and B are two points in a circuit with one source of energy. It is determined that to
move a +2 coulomb of charge from A to B, the source spends 10 joules of energy in 4
seconds.
(a) What is the voltage across the points A and B? (Mark polarities and give the
numerical value) (Ans: Polarities: +A and –B, voltage 5 V)
A+ B- 5V
(b) Assuming steady flow of charges over a 4 second period, calculate the power spent
and the current in the circuit? (Ans: power = 2.5 W, current = 0.5 A)
P = W/t
= 10/4
= 2.5 W
I = q/t
= 2/4
= 0.5 A
Q6
Four AA alkaline batteries are used in the 6 V power pack of a radio set. Each
battery has a useful energy of 50 watts seconds (joules). If the radio takes a steady current
of 2 mA from the power pack, how long can the batteries last? (Ans: 4.63 hours)
w = qV
q(total) = W/V
= 50 / 6
= 8.33 C (per battery)
= 33.32 C (total)
I = q/t
t(total) = q / I
= 33.32 / 2e-3 sec
= 4.63 hours
Q7
The terminals of a certain battery are labeled a and b. The battery voltage is Vab =
12V. To increase the chemical energy stored in the battery by 600J, how much charge
must be move through the battery? Should the electrons move from a to b or from b to a?
(Ans: 50C, b to a)
W = qV
q = W/V
= 50 C
Battery with Vab = 12V implies b is negative and a is positive. To charge, charge must go from b to a, so electrons must go from b to a.
Tags: Circuit Theory, Physics, solutions, tutorial