chapter 27 .pdf
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Title: Fundamental of Physics
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1. THINK The circuit consists of two batteries and two resistors. We apply Kirchhoff’s
loop rule to solve for the current.
EXPRESS Let i be the current in the circuit and take it to be positive if it is to the left in
R1. Kirchhoff’s loop rule gives
1 – iR2 – iR1 – 2 = 0.
For parts (b) and (c), we note that if i is the current in a resistor R, then the power
dissipated by that resistor is given by P i 2 R .
ANALYZE (a) We solve for i:
12 V 6.0 V
A positive value is obtained, so the current is counterclockwise around the circuit.
(b) For R1, the dissipation rate is P1 = i 2 R1 (0.50 A)2(4.0 ) = 1.0 W.
(c) For R2, the rate is P2 = i 2 R2 (0.50 A)2 (8.0 ) = 2.0 W.
If i is the current in a battery with emf , then the battery supplies energy at the rate P =
i provided the current and emf are in the same direction. On the other hand, the battery
absorbs energy at the rate P = i if the current and emf are in opposite directions.
(d) For 1, P1 = i1 (0.50 A)(12 V) = 6.0 W.
(e) For 2, P2 = i 2 (0.50 A)(6.0 V) = 3.0 W.
(f) In battery 1 the current is in the same direction as the emf. Therefore, this battery
supplies energy to the circuit; the battery is discharging.
(g) The current in battery 2 is opposite the direction of the emf, so this battery absorbs
energy from the circuit. It is charging.
LEARN Multiplying the equation obtained from Kirchhoff’s loop rule by idt leads to
the “energy-method” equation discussed in Section 27-4:
i1dt i 2 R1dt i 2 R2 dt i 2 dt 0.
The first term represents the rate of work done by battery 1, the second and third terms
the thermal energies that appear in resistors R1 and R2, and the last term the work done on
2. The current in the circuit is
i = (150 V – 50 V)/(3.0 + 2.0 ) = 20 A.
So from VQ + 150 V – (2.0 )i = VP, we get
VQ = 100 V + (2.0 )(20 A) –150 V = –10 V.
3. (a) The potential difference is V = + ir = 12 V + (50 A)(0.040 ) = 14 V.
(b) P = i2r = (50 A)2(0.040 ) = 1.0×102 W.
(c) P' = iV = (50 A)(12 V) = 6.0×102 W.
(d) In this case V = – ir = 12 V – (50 A)(0.040 ) = 10 V.
(e) Pr = i2r =(50 A)2(0.040 ) = 1.002 W.
4. (a) The loop rule leads to a voltage-drop across resistor 3 equal to 5.0 V (since the total
drop along the upper branch must be 12 V). The current there is consequently
i = (5.0 V)/(200 ) = 25 mA. Then the resistance of resistor 1 must be (2.0 V)/i = 80 .
(b) Resistor 2 has the same voltage-drop as resistor 3; its resistance is 200 .
5. The chemical energy of the battery is reduced by E = q, where q is the charge that
passes through in time t = 6.0 min, and is the emf of the battery. If i is the current,
then q = i t and
E = i t = (5.0 A)(6.0 V) (6.0 min) (60 s/min) = 1.1 104 J.
We note the conversion of time from minutes to seconds.
6. (a) The cost is (100 W · 8.0 h/2.0 W · h) ($0.80) = $3.2 02.
(b) The cost is (100 W · 8.0 h/103 W · h) ($0.06) = $0.048 = 4.8 cents.
7. (a) The energy transferred is
(2.0 V) 2 (2.0 min) (60 s / min)
(b) The amount of thermal energy generated is
U i Rt G
H r RK
F 2.0 V IJ (5.0 ) (2.0 min) (60 s / min) 67 J.
H 1.0 5.0 K
(c) The difference between U and U', which is equal to 13 J, is the thermal energy that is
generated in the battery due to its internal resistance.
8. If P is the rate at which the battery delivers energy and t is the time, then E = P t is
the energy delivered in time t. If q is the charge that passes through the battery in time
t and is the emf of the battery, then E = q. Equating the two expressions for E and
solving for t, we obtain
q (120A h)(12.0V)
9. (a) The work done by the battery relates to the potential energy change:
qV eV e 12.0V 12.0 eV.
(b) P = iV = neV = (3.40 1018/s)(1.60 10–19 C)(12.0 V) = 6.53 W.
10. (a) We solve i = (2 – 1)/(r1 + r2 + R) for R:
3.0 V 2.0 V
3.0 3.0 9.9 102 .
1.0 10 A
(b) P = i2R = (1.0 10–3 A)2(9.9 102 ) = 9.9 10–4 W.
11. THINK As shown in Fig. 27-29, the circuit contains an emf device X. How it is
connected to the rest of the circuit can be deduced from the power dissipated and the
potential drop across it.
EXPRESS The power absorbed by a circuit element is given by P = iV, where i is the
current and V is the potential difference across the element. The end-to-end potential
difference is given by
VA – VB = +iR + ,
where is the emf of device X and is taken to be positive if it is to the left in the diagram.
ANALYZE (a) The potential difference between A and B is
P 50 W
Since the energy of the charge decreases, point A is at a higher potential than point B; that
is, VA – VB = 50 V.
(b) From the equation above, we find the emf of device X to be
= VA – VB – iR = 50 V – (1.0 A)(2.0 ) = 48 V.
(c) A positive value was obtained for , so it is toward the left. The negative terminal is at
LEARN Writing the potential difference as VA iR VB , we see that our result is
consistent with the resistance and emf rules. Namely, starting at point A, the change in
potential is iR for a move through a resistance R in the direction of the current, and the
change in potential is for a move through an emf device in the opposite direction of
the emf arrow (which points from negative to positive terminals).
12. (a) For each wire, Rwire = L/A where A = r2. Consequently, we have
Rwire = (1.69 108 m )(0.200 m)/(0.00100 m)2 = 0.0011 .
The total resistive load on the battery is therefore
Rtot = 2Rwire + R =0.0011 6.00 .
Dividing this into the battery emf gives the current
1.9993 A .
The voltage across the R = 6.00 resistor is therefore
V iR (1.9993 A)(6.00 ) = 11.996 V 12.0 V.
(b) Similarly, we find the voltage-drop across each wire to be
Vwire iRwire (1.9993 A)(0.0011 ) = 2.15 mV.
(c) P = i2R = (1.9993 A)(6.00 )2 = 23.98 W 24.0 W.
(d) Similarly, we find the power dissipated in each wire to be 4.30 mW.
13. (a) We denote L = 10 km and = 13 /km. Measured from the east end we have
R1 = 100 = 2(L – x) + R,
and measured from the west end R2 = 200 = 2x + R. Thus,
R2 R1 L 200 100 10 km
4 13 km
(b) Also, we obtain
13 km 10 km 20 .
14. (a) Here we denote the battery emf’s as V1 and V2 . The loop rule gives
V2 – ir2 + V1 – ir1 – iR = 0 i
r1 r2 R
The terminal voltage of battery 1 is V1T and (see Fig. 27-4(a)) is easily seen to be equal to
V1 ir1 ; similarly for battery 2. Thus,
V1T = V1 –
r1 (V2 V1 )
r (V V )
, V2T = V2 – 1 2 1 .
r1 r2 R
r1 r2 R
The problem tells us that V1 and V2 each equal 1.20 V. From the graph in Fig. 27-32(b)
we see that V2T = 0 and V1T = 0.40 V for R = 0.10 . This supplies us (in view of the
above relations for terminal voltages) with simultaneous equations, which, when solved,
lead to r1 = 0.20 .
(b) The simultaneous solution also gives r2 = 0.30 .
15. Let the emf be V. Then V = iR = i'(R + R'), where i = 5.0 A, i' = 4.0 A, and R' = 2.0 .
We solve for R:
iR (4.0 A) (2.0 )
i i 5.0 A 4.0 A
16. (a) Let the emf of the solar cell be and the output voltage be V. Thus,
for both cases. Numerically, we get
FG V IJ r
We solve for and r.
0.10 V = – (0.10 V/500 )r
0.15 V = – (0.15 V/1000 )r.
(a) r = 1.003 .
(b) = 0.30 V.
(c) The efficiency is
Preceived 1000 5.0cm 2.0 10 W/cm
17. THINK A zero terminal-to-terminal potential difference implies that the emf of the
battery is equal to the voltage drop across its internal resistance, that is, ir.
EXPRESS To be as general as possible, we refer to the individual emf’s as 1 and 2 and
wait until the latter steps to equate them (1 = 2 = ). The batteries are placed in series in
such a way that their voltages add; that is, they do not “oppose” each other. The total
resistance in the circuit is therefore Rtotal = R + r1 + r2 (where the problem tells us r1 > r2),
and the “net emf” in the circuit is 1 + 2. Since battery 1 has the higher internal resistance,
it is the one capable of having a zero terminal voltage, as the computation in part (a)
ANALYZE (a) The current in the circuit is
r1 r2 R
and the requirement of zero terminal voltage leads to 1 ir1 , or
2 r1 1r2 (12.0 V)(0.016 ) (12.0 V)(0.012 )
Note that R = r1 – r2 when we set 1 = 2.
(b) As mentioned above, this occurs in battery 1.
LEARN If we assume the potential difference across battery 2 to be zero and repeat the
calculation above, we would find R = r2 – r1 < 0, which is physically impossible. Thus,
only the potential difference across the battery with the larger internal resistance can be
made zero with suitable choice of R.
18. The currents i1, i2 and i3 are obtained from Eqs. 27-18 through 27-20:
1 ( R2 R3 ) 2 R3
R1 R2 R2 R3 R1 R3
1 R3 2 ( R1 R2 )
R1 R2 R2 R3 R1 R3
(4.0V)(10 5.0 ) (1.0 V)(5.0 )
0.275 A ,
(10 )(10 ) (10 )(5.0 ) (10 )(5.0 )
(4.0 V)(5.0 ) (1.0 V)(10 5.0 )
0.025 A ,
(10 )(10 ) (10 )(5.0 ) (10 )(5.0 )
i3 i2 i1 0.025A 0.275A 0.250A .
Vd – Vc can now be calculated by taking various paths. Two examples: from Vd – i2R2 =
Vc we get
Vd – Vc = i2R2 = (0.0250 A) (10 ) = +0.25 V;
from Vd + i3R3 + 2 = Vc we get
Vd – Vc = i3R3 – 2 = – (– 0.250 A) (5.0 ) – 1.0 V = +0.25 V.
19. (a) Since Req < R, the two resistors (R = 12.0 and Rx) must be connected in parallel:
R Rx 12.0 Rx
We solve for Rx: Rx = ReqR/(R – Req) = (3.00 )(12.0 )/(12.0 – 3.00 ) = 4.00 .
(b) As stated above, the resistors must be connected in parallel.
20. Let the resistances of the two resistors be R1 and R2, with R1 < R2. From the
statements of the problem, we have
R1R2/(R1 + R2) = 3.0 and R1 + R2 = 16 .
So R1 and R2 must be 4.0 and 12 , respectively.
(a) The smaller resistance is R1 = 4.0
(b) The larger resistance is R2 = 12
21. The potential difference across each resistor is V = 25.0 V. Since the resistors are
identical, the current in each one is
i = V/R = (25.0 V)/(18.0 ) = 1.39 A.
The total current through the battery is then itotal = 4(1.39 A) = 5.56 A. One might
alternatively use the idea of equivalent resistance; for four identical resistors in parallel
the equivalent resistance is given by
When a potential difference of 25.0 V is applied to the equivalent resistor, the current
through it is the same as the total current through the four resistors in parallel. Thus
itotal = V/Req = 4V/R = 4(25.0 V)/(18.0 ) = 5.56 A.
22. (a) Req (FH) = (10.0 )(10.0 )(5.00 )/[(10.0 )(10.0 ) + 2(10.0 )(5.00 )] =
(b) Req (FG) = (5.00 ) R/(R + 5.00 ), where
R = 5.00 + (5.00 )(10.0 )/(5.00 + 10.0 ) = 8.33 .
So Req (FG) = (5.00 )(8.33 )/(5.00 + 8.33 ) = 3.13 .
23. Let i1 be the current in R1 and take it to be positive if it is to the right. Let i2 be the
current in R2 and take it to be positive if it is upward.
(a) When the loop rule is applied to the lower loop, the result is
2 i1R1 0 .
The equation yields
(b) When it is applied to the upper loop, the result is
1 2 3 i2 R2 0 .
The equation gives
1 2 3
6.0 V 5.0 V 4.0 V
0.060 A ,
or | i2 | 0.060 A. The negative sign indicates that the current in R2 is actually downward.
(c) If Vb is the potential at point b, then the potential at point a is Va = Vb + 3 + 2, so
Va – Vb = 3 + 2 = 4.0 V + 5.0 V = 9.0 V.
24. We note that two resistors in parallel, R1 and R2, are equivalent to
R12 R1 R2
This situation consists of a parallel pair that are then in series with a single R3 = 2.50
resistor. Thus, the situation has an equivalent resistance of
Req R3 R12 2.50
(4.00 ) (4.00 )
25. THINK The resistance of a copper wire varies with its cross-sectional area, or its
EXPRESS Let r be the resistance of each of the narrow wires. Since they are in parallel
the equivalent resistance Req of the composite is given by
or Req = r/9. Now each thin wire has a resistance r 4 / d 2 , where is the resistivity
of copper, and A = d2/4 is the cross-sectional area of a single thin wire. On the other
hand, the resistance of the thick wire of diameter D is R 4 / D2 , where the crosssectional area is D2/4.
ANALYZE If the single thick wire is to have the same resistance as the composite of 9
thin wires, R Req , then
Solving for D, we obtain D = 3d.
LEARN The equivalent resistance Req is smaller than r by a factor of 9. Since
r 1/ A 1/ d 2 , increasing the diameter of the wire threefold will also reduce the
resistance by a factor of 9.
26. The part of R0 connected in parallel with R is given by R1 = R0x/L, where L = 10 cm.
The voltage difference across R is then VR = R'/Req, where R' = RR1/(R + R1) and
Req = R0(1 – x/L) + R'.
RR1 R R1
100 R x R0
R R R0 1 x L RR1 R R1 100 R R 10 x x 2 2
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