Physics: Principles and Problems
Physics: Principles and Problems
9th Edition
Elliott, Haase, Harper, Herzog, Margaret Zorn, Nelson, Schuler, Zitzewitz
ISBN: 9780078458132
Textbook solutions

All Solutions

Page 690: Assessment

Exercise 24
Step 1
1 of 2
Exercise scan
Result
2 of 2
The fields are filled as follows:

Top left-generator

Second on the right-back EMF

The bottom one-Lenz’s law.

Exercise 25
Step 1
1 of 2
The wire wound around an iron core is called the armature in an electric generator. This is where the mechanical energy is transformed into electrical energy.
Result
2 of 2
The wire wound around an iron core is called the armature.
Exercise 26
Step 1
1 of 2
Iron is used in the generator armature because of it’s magnetic permeability which allows for easy change of polarization thus creating a rapid change in magnetic flux.
Result
2 of 2
Because it is relatively easy to change magnetic polarization of iron.
Exercise 27
Step 1
1 of 2
The EMF induced by the change in magnetic flux is given as

$$
EMF=BLvsintheta
$$

and it will have it’s minimum when $sintheta=0$ and that is achieved when the wire moves parallel with the magnetic field, i.e. when it moves towards us (out of the page).

Result
2 of 2
The EMF is minimal when the wire moves in parallel with the field lines.
Exercise 28
Step 1
1 of 2
The conductor passing by the south magnetic field will have a reduction in magnetic flux (no more magnetic lines) so the induced EMF will try to compensate for it. That means that it would have to be positive by the right hand rule.
Result
2 of 2
The EMF will be positive.
Exercise 29
Step 1
1 of 2
The EMF induced by the change in magnetic flux is given as

$$
EMF=BLvsintheta
$$

and we see that it is proportional to the length of the conductor $L$. So if we increase the length of the wire we will increase the voltage (EMF), too.

Result
2 of 2
The EMF will increase.
Exercise 30
Step 1
1 of 2
The similarity between the two is the fact that both try to establish the connection between electric currents and magnetic field. However, in the case of Oersted we see that a steady current of electrons is enough the create a magnetic field. On the other hand, Faraday states that time dependent magnetic field is required to induce an electric current.
Result
2 of 2
They are similar because they both cope with the same problem essentially but the differ in the direction of research. Faraday goes from time-dependent magnetic field to electric currents and Oersted from steady state electric currents to magnetic fields.
Exercise 31
Step 1
1 of 2
In order to create an electric current with a bar magnet and a coil of wire one should either move the bar in and out of the coil. Or move the coil up and down the magnet.
Result
2 of 2
Moving the magnet in and out of the coil or coil up and down of the magnet.
Exercise 32
Step 1
1 of 2
The abbreviation EMF stands for electro-motive force and its name is inaccurate because it is not a force but rather a voltage.
Result
2 of 2
Electro-motive force but it is not a real force. Instead, it is a voltage(potential difference) .
Exercise 33
Step 1
1 of 2
In essence these two are almost exactly the same. The only difference is the purpose. In a generator, mechanical energy is converted into electrical while in a motor the electrical energy is converted into mechanical.
Result
2 of 2
Generators convert mechanical energy into electrical.

Motors convert electrical energy into mechanical.

Exercise 34
Step 1
1 of 2
The major parts of an AC generator are a permanent magnet, an armature, brushes and a slip ring.
Result
2 of 2
A permanent magnet, an armature, brushes and a slip ring
Exercise 35
Step 1
1 of 2
The reason for this lies in the fact that the power in the AC circuit goes from zero to its maximal value. The average being equal half of the maximum

$$
P_{max}=2P_{eff}
$$

This states that

$$
I^2_{max}R=2I^2_{eff}R
$$

Which finally gives that

$$
I_{eff}=0.707I_{max}
$$

Result
2 of 2
The reason is the oscillation in power output.
Exercise 36
Step 1
1 of 2
The water stored at the certain height behind the dam stands for the gravitational potential energy. This potential energy is transformed to kinetic energy while falling and that kinetic energy is transformed into rotational kinetic energy of turbines. The rotational kinetic energy of turbines is then used the create electrostatic potential energy via magnetic induction which is finally transformed into kinetic energy of electrons in terms of electrical current.
Result
2 of 2
Gravitational potential, kinetic energy of the free fall, rotational kinetic energy of turbines, voltage, electric current (kinetic energy of electrons).
Exercise 37
Step 1
1 of 2
Lenz’s law states that a current induced by a variable magnetic field will always flow in a direction so it’s magnetic filed opposes the change in magnetic flux.
Result
2 of 2
The magnetic field of the induced current always opposes the magnetic filed that has induced the current.
Exercise 38
Step 1
1 of 2
The reason for this is summarized by Lenz’s law. When the motor starts turning due to the electric current brought to it the magnetic field will occur to oppose the change in the magnetic flux and it will occur due to the induced current or so called back-EMF.
Result
2 of 2
Once turning, motor will induce some currents of its own.
Exercise 39
Step 1
1 of 2
The main reason for this is the rate at which the magnetic flux changes in two cases. In the case the switch is closed, the current will rise relatively slowly to reach it’s maximum value. Once we open the switch, the current will drop sharply to zero. This will create much larger back-EMF which in some cases can lead to a dielectric breakdown and hence the spark.
Result
2 of 2
Larger back-EMF is the reason for this effect.
Exercise 40
Step 1
1 of 2
As the name suggests alternating currents change with time by definition therefore the change of the magnetic field is permanent which brings the self-inductance to be permanent too. On the other hand, direct currents become steady after certain period and then EMF drops to zero, i.e. there is no more self-inductance.
Result
2 of 2
With AC magnetic field constantly changes leading to a persistent self-inductance whereas with DC that is not the case.
Exercise 41
Step 1
1 of 2
The reason for this is the fact that without a changing magnetic field there is no EMF.
Result
2 of 2
Without a changing magnetic field EMF is zero.
Exercise 42
Step 1
1 of 2
By definition, the ratio of the EMF in the primary and secondary circuit of a transformer depends on the ratio between the number of turns of the primary and secondary circuit.
Result
2 of 2
It depends on the ratio of the turns in the primary and secondary circuit.
Exercise 43
Step 1
1 of 2
In order to show this, we start from the formula

$$
EMF=BLv
$$

Where we can express the magnetic field as

$$
B=frac{F}{IL}
$$

and the force is given as $F=ma$ and the current is given as $I=frac{q}{t}$ so we obtain the following

$$
EMF=frac{mcdot acdot Lcdot vcdot t}{qcdot L}=frac{mcdot v^2}{q}
$$

Now, we can use the si units to obtain

$$
boxed{V=frac{textrm{kg}cdot frac{textrm{m}^2}{textrm{s}^2}}{C}=frac{J}{C}}
$$

Which is exactly a definition of volts.

Result
2 of 2
$$
V=frac{J}{C}
$$
Exercise 44
Step 1
1 of 2
The resistance will affect only the current since the EMF is by definition independent of the resistance and it is given as

$$
EMF=BLv
$$

since the resistance is a physical characteristic of the circuit it will affect only the current since

$$
I=frac{EMF}{R}
$$

Result
2 of 2
It will affect only the current.
Exercise 45
Step 1
1 of 2
The rotation of the armature is directly proportional to the rotation of the wheel. If the bike starts slowing down, the rotation of the armature will reduce and therefore it will cause lower EMF (light will be weaker).
Result
2 of 2
The rotation of the armature will slow down causing lower EMF.
Exercise 46
Step 1
1 of 2
The power received or delivered by a device is defined as

$$
P=IV
$$

Once we change the sign of the voltage, the current direction has to change too since

$$
V=IR
$$

and $R>0$ so that means that

$$
P_{inv}=(-V)times (-I)=VI=P
$$

This can be seen by another definition of the power

$$
P=I^2R
$$

Where we see that power is proportional to the square of the current, i.e. it is always positive.

Result
2 of 2
No, it doesn’t since the power is proportional to the square of the current.
Exercise 47
Step 1
1 of 2
There is no current since the there is no induced EMF in the wire. The reason for that is the definition of EMF which says

$$
EMF=BLvsintheta
$$

and since $theta=0$ we have that

$$
boxed{EMF=0}
$$

Result
2 of 2
There is no induced current since $EMF=0$.
Exercise 48
Step 1
1 of 2
Just after making the connection there is a back-voltage, therefore the current decreases. But after the current become steady the back-voltage is zero and the current is larger.
Result
2 of 2
Several tenths of a second after the connection is made
Exercise 49
Step 1
1 of 2
In order to solve this problem we are going apply the right-hand rule knowing that electrons are negatively charged. Then, we obtain that the current is out of the page.Exercise scan
Result
2 of 2
Out of the page.
Exercise 50
Step 1
1 of 2
The bulb will emit light in all the cases because the secondary circuit has the current flowing whenever there is a change in the magnetic field of the primary circuit i.e. whenever it’s current changes.
Result
2 of 2
The bulb will shine in all the cases.
Exercise 51
Step 1
1 of 2
This is a tricky question and can easily assume that the current is zero since the wire is moving from north to south> However, the wire is moving on the earth’s surface i.e. it is not colinear with the field (which is pointing down and and north)! Therefore, we will have a current, by the right hand rule that goes from east to west.
Result
2 of 2
The current will go from east to west.
Exercise 52
Step 1
1 of 2
a) In order to solve this problem we are going the use the first right hand rule which states that the current will move out of the page, i.e. to the left.

b) Using the third right-hand rule we determine that the force will point up i.e. it will work against the motion of the wire.Exercise scan

Result
2 of 2
a) To the left.

b) Up.

Exercise 53
Step 1
1 of 2
This is indeed, a very interesting problem and a powerful demonstration of every classical phenomenon of magnetism. So let’s solve it.

a) In the case when the south pole is down, the field lines are exiting the pipe on the north pole and entering the pipe on the south pole. The velocity of the bar is always pointing along the pipe so the current will run around the pipe (the right-hand rule) and not uniformly but close to the south pole the current will be clockwise and near the north pole it will be counterclockwise. In the middle of the bar, there is no current since the field lines are parallel with the pipe, i.e. with the velocity.

b) The induced current will produce a magnetic field which can be imagined as a field of a solenoid, it will be along the pipe. However, this field is not uniform, too. Close to the south pole, it will point down, near the north pole it will point up (the right-hand rule, again).

c) Finally, the main phenomenon is “slow-motion” falling of the bar. One can intuitively say that the forces should cancel out since the fields induced close to the poles point to different directions but the magnet has the poles that oppose to this directions, i.e. south pole will want field lines to go to it and the north pole would have them go from it. Therefore the force will be repelling in both cases.

Result
2 of 2
a) Close to the south pole current is clockwise, close to the north pole it is counterclockwise.

b) Close to the south pole, the field is pointing down, close to the north pole the field is pointing up.

c) The force is always pointing up.

Exercise 54
Step 1
1 of 2
The reason for this behavior is the fact that the armature of the generator, when rotated, induces a current which in turn produces an opposing force and we need to put more work to rotate the generator.
Result
2 of 2
The reason for this is the force due to the induced current.
Exercise 55
Step 1
1 of 2
The reason for this is the absence of the back-EMF at the moment when the motor starts up. Soon after, back-EMF will rise thus reducing the current to the expected level. This is summarized by the Lenz’s law which says that a variable magnetic field (originating from the rotating armature in this case) will induce the EMF which will be in the opposite direction of the current used for running the motor and the total current will be lower than the initial one.
Result
2 of 2
The reason for this is the lack of the back-EMF during the start.
Exercise 56
Step 1
1 of 2
In essence, when we say a laminated core, we think of a core which metal parts are covered by some form of insulation. This insulation’s purpose is the reduction of Eddy currents that are generated within the core itself once the current is on.
Result
2 of 2
Because, lamination reduces Eddy currents within the core.
Exercise 57
Step 1
1 of 2
The fundamental low that states this is Lenz’s law which gives that there is always going to be a back-EMF that will try to diminish originally induced current.
Result
2 of 2
Lenz’s law.
Exercise 58
Step 1
1 of 2
The mutual induction process in a transformer goes as follows: The AC current in the primary coil changes it’s direction thus changing the magnetic flux in it’s vicinity. This change of the magnetic flux is detected by the second coil where now the current is induced in return. The voltage induced in the second coil linearly depends on the magnetic field of the primary coil.
Result
2 of 2
The oscillating AC current induced a change in magnetic field yielding with an induced voltage/current in the secondary coil.
Exercise 59
Step 1
1 of 2
This is a variation of problem 53, i.e. a part of it, so one should also check that one in order to see the detailed description of this phenomenon.

a) Close to the north pole the magnetic field lines move out of the pipe and with the velocity pointing down, the right-hand rule tells us that the direction of the induced current is $boxed{counterclockwise}$.

2) Now, the current induced creates a magnetic field which will point $boxed{up}$ the pipe, again obtained by the right-hand rule.

Result
2 of 2
a) Counterclockwise

b) Up

Exercise 60
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

From here, one can express the magnetic field as

$$
B=frac{EMF}{Lv}=frac{40}{20times 4}
$$

Finally, one obtains that

$$
boxed{B=0.5textrm{ T}}
$$

Result
2 of 2
$$
B=0.5textrm{ T}
$$
Exercise 61
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLvsinthetaapprox BLv
$$

Which after we plug in the values gives

$$
EMF=4.5times 10^{-5} times 75times 9.5times 10^2times frac{1000}{3600}
$$

Finally, we have that

$$
boxed{EMF=0.89textrm{ V}}
$$

Result
2 of 2
$$
EMF=0.89textrm{ V}
$$
Exercise 63
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

From here, one can express the speed of the wire as

$$
v=frac{EMF}{BL}=frac{10}{2.5times 0.2}
$$

Finally, one obtains that

$$
boxed{v=20frac{textrm{ m}}{textrm{ s}}}
$$

Result
2 of 2
$$
v=20 frac{textrm{ m}}{textrm{ s}}
$$
Exercise 64
Step 1
1 of 2
In order to solve this problem, we are going to use the definition of the effective voltage which says that

$$
V_{eff}=0.707V_{max}
$$

So once we plug in the values we get that

$$
V_{eff}=0.707times 565
$$

$$
boxed{V_{eff}=399textrm{ V}}
$$

Result
2 of 2
$$
V_{eff}=399textrm{ V}
$$
Exercise 65
Step 1
1 of 2
In order to solve this problem, we are going to use the definitions of the effective voltage, effective current and the effective power which say that

$$
V_{eff}=0.707V_{max}
$$

$$
I_{eff}=0.707I_{max}
$$

$$
P_{eff}=I_{eff}V_{eff}
$$

a) In the first case, once we plug in the values we get that

$$
V_{eff}=0.707times 150
$$

which gives that

$$
boxed{V_{eff}=106.1textrm{ V}}
$$

b) The same procedure holds for the effective current

$$
I_{eff}=0.707times 30
$$

which gives that

$$
boxed{I_{eff}=21.2textrm{ A}}
$$

c) Now, we combine part a and part b to find the answer to the last question, effective power

$$
P_{eff}=I_{eff}V_{eff}=106.1times 21.2
$$

This gives that

$$
boxed{P_{eff}=2.25times 10^3textrm{ W}}
$$

Result
2 of 2
$$
textrm{a) }V_{eff}=106.1textrm{ V}
$$

$$
textrm{b) }I_{eff}=21.2textrm{ A}
$$

$$
textrm{c) }P_{eff}=2.25times 10^3textrm{ W}
$$

Exercise 66
Step 1
1 of 2
In order to solve this problem, we are going to use the definitions of the effective voltage and Ohm’s law which say that

$$
V_{eff}=0.707V_{max}
$$

$$
I_{eff}=frac{V_{eff}}{R}
$$

a) In the first case, we can express the maximum voltage as

$$
V_{max}=frac{V_{eff}}{0.707}=frac{240}{0.707}
$$

which gives that

$$
boxed{V_{max}=339textrm{ V}}
$$

b) Ohm’s law will help us with the effective current

$$
I_{eff}=frac{V_{eff}}{R}=frac{240}{11}
$$

Finally, we have that

$$
boxed{I_{eff}=21.8textrm{ A}}
$$

Result
2 of 2
$$
textrm{a) }V_{max}=339textrm{ V}
$$

$$
textrm{b) }I_{eff}=21.8textrm{ A}
$$

Exercise 67
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLvsintheta
$$

Which has maximum, if other parameters are kept constant, when $theta=90^circ$.
From here, one can express the length of the wire as

$$
L=frac{EMF}{Bv}=frac{4.5}{0.05times 4}
$$

Finally, one obtains that

$$
boxed{L=22.5textrm{ m}}
$$

Result
2 of 2
$$
L=22.5textrm{ m };theta=90^circ
$$
Exercise 68
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

and Ohm’s law to find the current via

$$
I=frac{EMF}{R}
$$

After we plug in the given values in the first equation we have

$$
EMF=0.32 times 0.4times 1.3
$$

Finally, we have that

$$
EMF=0.166textrm{ V}
$$

Now, we can find the current in the circuit using Ohm’s law

$$
I=frac{EMF}{R}=frac{0.166}{10}
$$

which results in

$$
boxed{I=0.0166textrm{ A}}
$$

Result
2 of 2
$$
I=0.0166textrm{ A}
$$
Exercise 69
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

and Ohm’s law to find the current via

$$
I=frac{EMF}{R}
$$

Where $R=R_G+R_W$ since the wire and galvanometer are connected in series.
After we plug in the given values in the first equation we have

$$
EMF=0.02 times 0.1times 1
$$

Finally, we have that

$$
EMF=2times 10^{-3}textrm{ V}
$$

Now, we can find the current in the circuit using Ohm’s law

$$
I=frac{EMF}{R_G+R_W}=frac{0.002}{875+0.1}
$$

which results in

$$
boxed{I=2.3 times 10^{-6}textrm{ A}}
$$

Result
2 of 2
$$
I=2.3 times 10^{-6}textrm{ A}
$$
Exercise 70
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLvsintheta
$$

a) This equation explicitly involves the vertical component of the magnetic field which is given as

$$
B_v=Bsintheta=0.045times sin60^circ
$$

so we get that
$boxed{B_v=0.039textrm{ T}}$
b) The EMF is now found from the first equation

$$
EMF=BLvsintheta
$$

$$
EMF=0.039times 2.5times 2.4
$$

Finally, we have that

$$
boxed{EMF=0.234textrm{ V}}
$$

Result
2 of 2
$textrm{a) }B_v=0.039textrm{ T}$

$$
textrm{a) }EMF=0.234textrm{ V}
$$

Exercise 71
Step 1
1 of 2
a) In order to solve this problem we are going to use thermodynamic relations related to the efficiency coefficient so we have that

$$
P_{out}=eP_{in}
$$

So we can express the energy that is supplied to the turbine

$$
P_{in}=frac{P_{out}}{e}=frac{375times 10^6}{0.85}
$$

Finally, we have that

$$
boxed{P_{in}=441times 10^6textrm{ W}}
$$

b) Since the rate at which energy is supplied by the change in potential energy is given in a we have that

$$
E=PDelta t=441times 10^6times 1
$$

Finally

$$
boxed{E=441times 10^6textrm{ J}}
$$

c) To find the mass of the water that has to fall we have that

$$
E=mgh
$$

from which the mass is given as

$$
m=frac{E}{gh}=frac{441times 10^6}{9.81times 22}
$$

and finally, we have that

$$
boxed{m=2.04times 10^6textrm{ kg}}
$$

Result
2 of 2
$$
textrm{a) }P_{in}=441textrm{ W}
$$

$$
textrm{b) }E=441times 10^6textrm{ J}
$$

$$
textrm{c) }m=2.04times 10^6textrm{ kg}
$$

Exercise 72
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

Which after we plug in the values gives

$$
EMF=4times 0.2times 1
$$

Finally, we have that

$$
boxed{EMF=0.8textrm{ V}}
$$

Result
2 of 2
$$
EMF=0.8textrm{ V}
$$
Exercise 73
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

and Ohm’s law to find the current via

$$
I=frac{EMF}{R}
$$

a) After we plug in the given values in the first equation we have

$$
EMF=0.07 times 0.5times 3.6
$$

Finally, we have that

$$
boxed{EMF=0.126textrm{ V}}
$$

b) Now, we can find the current in the circuit using Ohm’s law

$$
I=frac{EMF}{R}=frac{0.126}{1}
$$

which results in

$$
boxed{I=0.126textrm{ A}}
$$

c) In order to minimize the change in the flux, the induced magnetic field will have the enter the page on the left side of the conductor i.e. it is clockwise when viewed from above.

d) Right hand rule tells us that the current flows clockwise in the circuit, i.e. from point A to B which means that point A is negative relative to B.

Result
2 of 2
$$
textrm{a) }EMF=0.126textrm{ V}
$$

$$
textrm{b) }I=0.126textrm{ A}
$$

$$
textrm{c) Clockwise}
$$

$$
textrm{d) Point A is negative relative to point B. }
$$

Exercise 74
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

Now, we can express the number of turns in the secondary circuit as

$$
N_s=frac{V_s}{V_p}N_p
$$

a) In the case when $V_s=625$ V we have that

$$
N_s=frac{V_s}{V_p}N_p=frac{625}{120}times 150
$$

Finally,

$$
boxed{N_s=781}
$$

b) In the case when $V_s=35$ V we have that

$$
N_s=frac{V_s}{V_p}N_p=frac{35}{120}times 150
$$

Finally,

$$
boxed{N_s=44}
$$

c) In the case when $V_s=6$ V we have that

$$
N_s=frac{V_s}{V_p}N_p=frac{6}{120}times 150
$$

Finally,

$$
boxed{N_s=7.5}
$$

Result
2 of 2
$$
textrm{a) }N_s=781
$$

$$
textrm{b) }N_s=44
$$

$$
textrm{c) }N_s=7.5
$$

Exercise 75
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}=frac{I_p}{I_s}
$$

Now, we can express the number of turns in the secondary circuit as

$$
V_s=frac{N_s}{N_p}V_p
$$

a) In the case when $V_p=120$ V we have that

$$
V_s=frac{N_s}{N_p}V_p=frac{1200}{80}times 120
$$

Finally,

$$
boxed{V_s=1800textrm{ V}}
$$

b) Now, we can find the current in the primary circuit as

$$
I_p=frac{N_s}{N_p}I_s=frac{1200}{80}times 2
$$

Which gives that

$$
boxed{I_p=30textrm{ A}}
$$

c) Now we can find the power input and output as

$$
P_p=V_pI_p=120times 30
$$

$$
boxed{P_p=3600textrm{ W}}
$$

$$
P_s=V_sI_s=1800times 2
$$

$$
boxed{P_s=3600textrm{ W}}
$$

Result
2 of 2
$$
textrm{a) }V_s=1800textrm{ V}
$$

$$
textrm{b) }I_p=30textrm{ A}
$$

$$
textrm{c) }P_p=P_s=3600textrm{ W}
$$

Exercise 76
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

Now, we can express the number of turns in the secondary circuit as

$$
N_s=frac{V_s}{V_p}N_p
$$

a) In the case when $N_p=475$ and $V_s=9$ V and $V_p=120$ V we have that

$$
N_s=frac{V_s}{V_p}N_p=frac{9}{120}times 475
$$

Finally,

$$
boxed{N_s=36}
$$

b) The current is then found from

$$
I_p=frac{N_s}{N_p}I_s=frac{36}{475}times 0.125
$$

Which gives that

$$
boxed{I_p=9.4times 10^{-3}textrm{ A}}
$$

Result
2 of 2
$$
textrm{a) }N_s=36
$$

$$
textrm{b) }I_p=9.4times 10^{-3}textrm{ A}
$$

Exercise 77
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}=frac{I_p}{I_s}
$$

a) Now we can plug in the values in the above’s equation to have that

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

$$
frac{N_s}{N_p}=frac{120}{240}
$$

$$
boxed{frac{N_s}{N_p}=frac{1}{2}}
$$

b) Now, the same can be done for the primary current which gives

$$
frac{V_s}{V_p}=frac{I_p}{I_s}
$$

$$
frac{1}{2}=frac{I_p}{I_s}
$$

Finally, we have that

$$
I_p=frac{I_s}{2}=frac{10}{2}
$$

$$
boxed{I_p=5textrm{ A}}
$$

Result
2 of 2
$$
textrm{a) }frac{N_s}{N_p}=frac{1}{2}
$$

$$
textrm{b) }I_p=5textrm{ A}
$$

Exercise 78
Step 1
1 of 2
a) In order to solve this problem one has to find the secondary voltage and check whether or not $V_s>V_p$ which is a condition for a step-up transformer. We have that

$$
P_{out}=I_sV_s
$$

So the secondary voltage is given

$$
V_s=frac{P_{out}}{I_s}=frac{150}{5}
$$

$boxed{V_s=30textrm{ V}>V_p}$
so we have a step-up transformer.

b) The ratio of the output to input voltage is given as

$$
frac{V_s}{V_p}=frac{30}{9}
$$

$$
boxed{frac{V_s}{V_p}=3.33}
$$

Result
2 of 2
$$
textrm{a) Step-up transformer. }
$$

$$
textrm{b) }frac{V_s}{V_p}=3.33
$$

Exercise 79
Step 1
1 of 2
In order to solve this problem, we are going to use the relation of a transformer which gives that

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

So we get the ratio of the turns to be

$$
frac{N_s}{N_p}=frac{8}{24}=frac{1}{3}
$$

If we now reverse the circuits, we would have that

$$
frac{N_s}{N_p}=frac{3}{1}
$$

Turning back to the principal transformer equation one gets that

$$
V_s=frac{N_s}{N_p}V_p=3times 24
$$

Finally, we have that

$$
boxed{V_s=72textrm{ V}}
$$

Result
2 of 2
$$
V_s=72textrm{ V}
$$
Exercise 80
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}=frac{I_p}{I_s}
$$

Now, we can express the number of turns in the secondary circuit as

$$
V_s=frac{N_s}{N_p}V_p
$$

a) In the case when $V_p=120$ V we have that

$$
V_s=frac{N_s}{N_p}V_p=frac{15times 10^3}{500}times 120
$$

Finally,

$$
boxed{V_s=3600textrm{ V}}
$$

b) Now, we can find the current in the primary circuit as

$$
I_p=frac{N_s}{N_p}I_s=frac{15times 10^3}{500}times 3
$$

Which gives that

$$
boxed{I_p=90textrm{ A}}
$$

c) Now we can find the power input andd output as

$$
P_p=V_pI_p=120times 90
$$

$$
boxed{P_p=10.8times 10^3textrm{ W}}
$$

$$
P_s=V_sI_s=3600times 3
$$

$$
boxed{P_s=10.8times 10^3textrm{ W}}
$$

Result
2 of 2
$$
textrm{a) }V_s=1800textrm{ V}
$$

$$
textrm{b) }I_p=30textrm{ A}
$$

$$
textrm{c) }P_p=P_s=3600textrm{ W}
$$

Exercise 81
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

From here, one can express the speed of the wire as

$$
v=frac{EMF}{BL}=frac{10}{2.5times 0.2}
$$

Finally, one obtains that

$$
boxed{v=20frac{textrm{ m}}{textrm{ s}}}
$$

Result
2 of 2
$$
v=20frac{textrm{ m}}{textrm{ s}}
$$
Exercise 82
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

From here, one can express the speed of the wire as

$$
v=frac{EMF}{BL}=frac{1}{0.2times 0.5}
$$

Finally, one obtains that

$$
boxed{v=10frac{textrm{ m}}{textrm{ s}}}
$$

Result
2 of 2
$$
v=10frac{textrm{ m}}{textrm{ s}}
$$
Exercise 83
Step 1
1 of 2
In order to solve this problem, we are going to use the definitions of the effective voltage

$$
V_{eff}=0.707V_{max}
$$

In our case, we can express the maximum voltage as

$$
V_{max}=frac{V_{eff}}{0.707}=frac{120}{0.707}
$$

which gives that

$$
boxed{V_{max}=170textrm{ V}}
$$

Result
2 of 2
$$
V_{max}=170textrm{ V}
$$
Exercise 84
Step 1
1 of 2
In order to solve this problem, we are going to use the definition of the effective current

$$
I_{eff}=0.707I_{max}
$$

In our case, we can express the maximum current as

$$
I_{max}=frac{I_{eff}}{0.707}=frac{2.5}{0.707}
$$

which gives that

$$
boxed{I_{max}=3.5textrm{ A}}
$$

Result
2 of 2
$$
I_{max}=3.5textrm{ A}
$$
Exercise 85
Step 1
1 of 2
In order to solve this problem, we are going to use the definition of the effective voltage

$$
V_{eff}=0.707V_{max}
$$

In our case, once we plug in the values we get that

$$
V_{eff}=0.707times 575
$$

which gives that

$$
boxed{V_{eff}=407textrm{ V}}
$$

Result
2 of 2
$$
V_{eff}=407textrm{ V}
$$
Exercise 86
Step 1
1 of 2
In order to solve this problem, we are going to use the definitions of the effective current which say that

$$
I_{eff}=0.707I_{max}
$$

So after we plug in the given values, we obtain that

$$
I_{eff}=0.707times 21.25
$$

which gives that

$$
boxed{I_{eff}=15.02textrm{ A}}
$$

Result
2 of 2
$$
I_{eff}=15.02textrm{ A}
$$
Exercise 87
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}=frac{I_p}{I_s}
$$

Now we can plug in the values in the above’s equation to have that

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

$$
frac{N_s}{N_p}=frac{440}{2.4 times 10^5}
$$

So the ratio between the secondary and primary turns is

$$
boxed{frac{N_s}{N_p}=frac{1}{545}}
$$

Result
2 of 2
$$
frac{N_s}{N_p}=frac{1}{545}
$$
Exercise 88
Step 1
1 of 2
In order to solve this problem we are first going the find the root means square current and then, from it, we are going to find the maximum current. So we have that

$$
I_{rms}=frac{P}{V_{rms}}
$$

And on the other hand

$$
I_{max}=sqrt{2}I_{rms}
$$

so we obtain that

$$
I_{max}=sqrt{2}times frac{P}{V_{rms}}=frac{1.41times 45times 10^3}{660}
$$

Finally,

$$
boxed{I_{max}=96textrm{ A}}
$$

Result
2 of 2
$$
I_{max}=96textrm{ A}
$$
Exercise 89
Step 1
1 of 2
In order to solve this problem we are going to use the relations for the maximum voltage in the circuit and the ratio of the currents and turns in a transformer. First, we find the effective current in the secondary circuit

$$
V_{effS}=0.707V_{peakS}=0.707times 60
$$

$$
V_{effS}=42.4textrm{ V}
$$

Now, the effective current is to be found from

$$
I_{effS}=frac{P}{V_{effS}}=frac{2times 10^3}{42}
$$

Which gives that

$$
I_{effS}=47.2textrm{ A}
$$

Now, we can use the fact that

$$
frac{I_{effP}}{I_{effS}}=frac{N_s}{N_p}
$$

from where we get the current in the primary circuit as

$$
I_{effP}={I_{effS}}frac{N_s}{N_p}=47.2timesfrac{10}{100}
$$

So we finally get that

$$
boxed{I_{effP}=4.72textrm{ A}}
$$

Result
2 of 2
$$
I_{effP}=4.72textrm{ A}
$$
Exercise 90
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the efficiency coefficient and Ohm’s law so we can start with the first one.

a) We have that by definition

$$
P_{out}=eP_{in}
$$

So the input power is given

$$
P_{in}=frac{P_{out}}{e}=frac{98times 10^3}{0.98}
$$

which gives that

$$
P_{in}=100times 10^3textrm{ W}
$$

b) Now, the maximum primary current can be found from the

$$
P=VI
$$

$$
I=frac{P}{V}=frac{100times 10^3}{600}
$$

So we obtain that

$$
I=167textrm{ A}
$$

Result
2 of 2
$$
textrm{a) }P_{in}=100times 10^3textrm{ W}
$$

$$
textrm{b) }I=167textrm{ A}
$$

Exercise 91
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

and Ohm’s law to find the current via

$$
I=frac{EMF}{R}
$$

a) After we plug in the given values in the first equation we have

$$
EMF=2 times 0.4times 8
$$

Finally, we have that

$$
boxed{EMF=6.4textrm{ V}}
$$

b) Now, we can find the current in the circuit using Ohm’s law

$$
I=frac{EMF}{R}=frac{6.4}{6.4}
$$

which results in

$$
boxed{I=1textrm{ A}}
$$

Result
2 of 2
$$
textrm{a) }EMF=6.4textrm{ V}
$$

$$
textrm{b) }I=1textrm{ A}
$$

Exercise 92
Step 1
1 of 2
In order to solve this problem, we are going to use the EMF equation which gives that

$$
EMF=BLv
$$

and Ohm’s law to find the current via

$$
I=frac{EMF}{R}
$$

After we plug in the given values in the first equation we have

$$
EMF=5times 10^{-5} times 7.5times 5.5
$$

Finally, we have that

$$
EMF=206times 10^{-5}textrm{ V}
$$

Now, we can find the current in the circuit using Ohm’s law

$$
I=frac{EMF}{R}=frac{206times 10^{-5}}{5times 10^{-5}}
$$

which results in

$$
boxed{I=41textrm{ A}}
$$

Result
2 of 2
$$
I=41textrm{ A}
$$
Exercise 93
Step 1
1 of 2
In order to solve this problem we are going to use Ohm’s law and power law equation which say that

$$
V=IR
$$

$$
P=VI
$$

From these two we have that

$$
P=frac{V^2}{R}=frac{100^2}{144}
$$

Finally the power that the resistor has to be able to handle is

$$
boxed{P=69.4textrm{ W}}
$$

Result
2 of 2
$$
P=69.4textrm{ W}
$$
Exercise 94
Step 1
1 of 2
In order to solve this problem we are going to use the definition of the transformer parameters which says

$$
frac{V_s}{V_p}=frac{N_s}{N_p}
$$

Now, we can express the number of turns in the primary circuit as

$$
N_p=frac{V_p}{V_s}N_s
$$

In the case when $N_s=20,000$ and $V_s=48,000$ V and $V_p=120$ V we have that

$$
N_p=frac{V_p}{V_s}N_s=frac{120}{48,000}times 20,000
$$

Finally,

$$
boxed{N_p=50}
$$

b) The current is then found from

$$
I_p=frac{N_s}{N_p}I_s=frac{20,000}{50}times 1times 10^{-3}
$$

Which gives that

$$
boxed{I_p=0.4textrm{ A}}
$$

Result
2 of 2
$$
I_p=0.4textrm{ A}
$$
Exercise 95
Step 1
1 of 2
This assumption directly violates the conservation of energy since it assumes that more energy would came out from the generator than it went into it. Generators do not create energy, but convert it from one form to another thus this is impossible.
Result
2 of 2
The conservation of energy.
Exercise 96
Step 1
1 of 2
By definition, the efficiency is the ratio of the output and input power, which in the case of a transformer becomes the ratio of the secondary and primary circuit. In percentages, it is given as

$$
boxed{e=frac{P_s}{P_p}times 100%}
$$

The secondary power can be found from the power law

$$
P_s=V_sI_s
$$

We can combine these two so the power in the primary circuit becomes

$$
P_p=frac{P_s}{e}times 100=frac{V_sI_s}{e}times 100
$$

but we know that $P_p=V_pI_P$ so we have that the primary current is

$$
I_p=frac{V_sI_s}{eV_p}times 100=frac{28times 25}{92.5times 125}times 100
$$

Finally, the current is given as

$$
boxed{I_p=6.05textrm{ A}}
$$

Result
2 of 2
$$
e=frac{P_s}{P_p}times 100%
$$

$$
I_p=6.05textrm{ A}
$$

Exercise 97
Step 1
1 of 2
In order to solve this problem, we have to understand that the secondary power and current belong to the ovens and that the primary power belongs to the transformer itself. We have that

$$
P_s=NV_SI_S
$$

where $N$ is equal to the number of users.

$$
P_s=NV_SI_S=8times 240times 35
$$

So we have that the power supplied to the users (ovens) is

$$
boxed{P_s=67.2times 10^3 textrm{ W}}
$$

Now, the primary power we find from the relation of the efficiency coefficient

$$
e=frac{P_{s}}{P_{p}}times 100
$$

$$
P_p=frac{P_s}{e}times 100=frac{67.2times 10^3 }{95}times 100
$$

Which gives that the primary power is

$$
P_p=70.7times 10^3 textrm{ W}
$$

Now, the dissipated power is the difference between the secondary and primary power

$$
P_{d}=P_p-P_s
$$

$$
boxed{P_{d}=3.5times 10^3textrm{ W}}
$$

Result
2 of 2
$$
P_s=67.2times 10^3 textrm{ W}
$$

$$
P_{d}=3.5times 10^3textrm{ W}
$$

Exercise 98
Step 1
1 of 2
When an electric motor can operate on both, AC and DC power we say that it is a universal motor. By its construction, universal motors do resemble DC series motors very much but there are a few significant modifications that make it possible for the universal motor to operate under AC power, too. In the universal motor, the stator’s field coils are attached in series to the rotor windings via a commutator.

The mechanism that allows for the universal motor to operate well on AC power is the following: the currents in the armature and the field coils are synchronized i.e. they change the direction at the same time, thus making the polarity of the fields synchronized, too. The resulting mechanical force will therefore never change the direction and the rotation will remain constant.

Result
2 of 2
The currents in the coils and the armature are changing direction at the same time.
Exercise 99
Step 1
1 of 2
In order to solve this equation, we are going to use the Doppler effect formula which says that

$$
f_{E}=f(1pmfrac{v}{c})
$$

Since the galaxy is moving towards the Earth we will have that

$$
f_{E}=f(1+frac{v}{c})=4.56times 10^{14}times(1+frac{2.75times 10^6}{3times 10^8})
$$

FInally, one obtains that

$$
boxed{f_{E}=4.6times 10^{14}textrm{ Hz}}
$$

Result
2 of 2
$$
f_{E}=4.6times 10^{14}textrm{ Hz}
$$
Exercise 100
Step 1
1 of 2
In order to solve this problem, we are going to use the Doppler effect formula which says that

$$
f_{E}=f(1pmfrac{v}{c})
$$

Since the frequency measured on Earth is lower than the original frequency, the galaxy is moving away from the Earth. So we will have that

$$
f_{E}=f(1-frac{v}{c})
$$

From here, the speed can be expressed in a couple of steps

$$
frac{f_E}{f}=1-frac{v}{c}
$$

$$
1-frac{f_E}{f}=frac{v}{c}
$$

FInally, one obtains that

$$
v=c(1-frac{f_E}{f})
$$

Now, we can insert the given values to get that

$$
v=3times 10^8times (1-frac{6.14times 10^{14}}{7.29times 10^{14}})
$$

$$
boxed{v=4.73times 10^{7}frac{textrm{ m}}{textrm{ s}}}
$$

Result
2 of 2
$$
v=4.73times 10^{7}frac{textrm{ m}}{textrm{ s}}
$$
Exercise 101
Step 1
1 of 2
In order to solve this problem, we are going to use the formula that connects electric charge and voltage with the capacitance

$$
C=frac{q}{Delta V}
$$

Now, we can express the charge as

$$
q=CDelta V=22times 10^{-6}times 48
$$

Finally, we have that

$$
boxed{q=1.06times 10^{-3}textrm{ C}}
$$

Result
2 of 2
$$
q=1.06times 10^{-3}textrm{ C}
$$
Exercise 102
Step 1
1 of 2
To solve this problem, one should use the power law equation which tells us that

$$
P=IV
$$

and Ohm’s law which says

$$
I=frac{V}{R}
$$

Now, we get that

$$
P=frac{V^2}{R}
$$

So, we can solve this for the voltage to get that

$$
V=sqrt{PR}=sqrt{frac{P_{max}R}{2}}=sqrt{0.5times 5times 22}
$$

Finally, we have that

$$
boxed{V=7.4textrm{ V}}
$$

Result
2 of 2
$$
V=7.4textrm{ V}
$$
Exercise 103
Step 1
1 of 2
In order to solve this problem we are going to divide it into two sub-systems, the parallel with three resistors one and the parallel one with two resistors that are connected to each other in series. We have then that

$$
frac{1}{R_{p1}}=frac{1}{R}+frac{1}{R}+frac{1}{R}=frac{3}{R}
$$

$$
R_{p1}=frac{R}{3}
$$

and for the second part connected in paraller

$$
frac{1}{R_{p2}}=frac{1}{R}+frac{1}{R}=frac{2}{R}
$$

$$
R_{p2}=frac{R}{2}
$$

Now, the equivalent resistance of the circuit is

$$
R_{eq}=R_{p1}+R_{p2}=frac{R}{3}+frac{R}{2}=frac{5R}{6}
$$

Now, we can plug in the given values

$$
R_{eq}=frac{5times 85}{6}
$$

Finally,

$$
boxed{R_{eq}=71Omega}
$$

Result
2 of 2
$$
R_{eq}=71Omega
$$
Exercise 104
Step 1
1 of 2
In order to solve this problem, we are going to use the formula that defines the force acting on a charged particle by a magnetic field

$$
vec F=qvec vtimes vec B=qvB
$$

If we now plug in the values we obtain the following

$$
F=1.6times 10^{-19}times 2.1 times 10^6 times 0.81
$$

Finally

$$
boxed{F=2.72 times 10^{-13}textrm{ N}}
$$

The acceleration of the muon can be found via Newton’s second law

$$
vec F=mvec a
$$

$$
a=frac{F}{m}=frac{2.72times 10^{-13}}{9.11times 10^{-31}}
$$

which gives that

$$
boxed{a=3times 10^{17}frac{textrm{ m}}{textrm{ s}^2}}
$$

Result
2 of 2
$$
F=2.72 times 10^{-13}textrm{ N}
$$

$$
a=3times 10^{17}frac{textrm{ m}}{textrm{ s}^2}
$$

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