UEB - USF
ENVS120, 2023
Nguyen Quoc Hung
Homework 4: Electricity and Optics
1. How much current flows through a lamp with a resistance of 40 Ω when the
voltage across the lamp is 12 V?
I = U/R = 12V/40 Ω = 0,3A
2. Two pellets, each with a charge of 1 microcoulomb (10-6 C), are located 3 cm
(0.03 m) apart. What is the electric force between them?
Applying Coulomb's law ( taking Coulomb's constant
k = 9×109 N.m2/C2k = 9×109 N.m2/C2),
the electric force between the point charges is calculated as:
F = k×q1×q2/d2 = (9×109 N.m2/C2×10−6 C×10−6 C)/(0.03 m)2 = 10N
3. Find the current through a 800 W hair dryer connected to 120 V. What is the
resistance of the hair dryer?
P=IxU
I = P/V = 1200W/120V = 10A
P = I2 x R
R = P/I2 = 1200/102 = 12 Ω
4. Why is charge usually transferred by electrons rather than by protons?
Electrons are located in shells surrounding the nucleus of the atom,whereas
protons are present inside the nucleus. Electrons can move out of the shells on
providing energy, but movement of proton will require a very high amount of
energy (so as to break the nuclear force).
5. Suppose that the strength of the electric field about an isolated point charge has
a certain value at a distance of 1 m. How does the electric field strength compare
at a distance of 2 m from the point charge?
6. When you double the distance between a pair of charged particles, what
happens to the force between them? Does it depend on the sign of the charges?
7. Why is a good conductor of electricity also a good conductor of heat?
Both types of conduction are via free-moving electrons. 5000 billion freely moving
electrons in a penny repel one another. The electrons are attracted to the same
number of protons in the penny
8. Only a small percentage of the electric energy supplied to a common light bulb is
transformed into light. What happens to the remaining energy?
Most of the energy, more than 90%, of the electrical energy in an incandescent
lamp goes directly to heat. Thermal energy is the graveyard of electrical energy.
9. Consider a circuit with a main wire that branches into two other wires. If the
current is 8 A in the main wire and 4 A in one of the branches, how much current
is in the other branch?
Current passng through the second wire can be given as:
I = I1 + I2
I1 = 4A
I = 8A
I2 = I – I1 = 8A – 4A = 4A
Therefore, the amount of current leaving through the other branch is 4A
10.What is the net magnetic force on a compass needle? By what mechanism does
a compass needle align with a magnetic field?
- A compass points north because all magnets have two poles, a north pole and
a south pole, and the north pole magnet is attracted to the south pole of another
magnet
- The Earth is a magnet that can interact with other magnets in this way, so the
north end of a compass magnet is drawn to align with the Earth’s magnetic field.
Because the Earth’s magnetic North Pole attracts the “north” ends of other
magnets, it is technically the “South Pole” of our planet’s magnetic field.
11.Magnet A has twice the magnetic field strength of magnet B, and at a certain
distance it pulls on magnet B with a force of 50 N. With how much force does
magnet B then pull on magnet A?
Magnets A and B are pulling equally on each other (newtons 3rd) law. If A pulls on
B with 50N, then B also pulls on A with 50N.
12.Two charged particles are projected into a magnetic field that is perpendicular to
their velocities. If the charges are deflected in opposite directions, what does this
tell you about the particles?
If a charged particle having charge ‘g’ enters a magnetic field of strength ⃗B with a
velocity ⃗v then it experience a magnetic lorentz force. The forrce is given by:
⃗ ⃗
F =q ⃗v × B
Therefore if the particles deflect in opposite direction, the particle have the
opposite charges.
13.A common pickup for an electric guitar consists of a coil of wire around a small
permanent magnet. Why will this type of pickup fail with nylon strings?
The magnets magnetize the style strings, these vibrating strings induce voltage in
the coils producing sounds. If the strings were nylon no magnetization would
acquire between the pickups and strings, there would be nothing to amplify.
14.Do a pair of parallel current-carrying wires exert forces on each other?
- Yes
- The magnetic field generated by the current in 1 wire is perpendicular to the
current in the other wire and vise versa. When the current goes the same way in
the 2 wires, the force is attractive. When the currents go opposite ways, the force
is repulsive.
15.A nurse approaches his patient and counts 75 heartbeats in 1 min. Find the
frequency and period of the heartbeats?
T = 60/76 s
f = 76/60 1/s
16.A mosquito flaps its wings at the rate of 600 vibrations per second, which
produces the annoying 600-Hz buzz. Given that the speed of sound is 340 m/s,
how far does the sound travel between wing beats? In other words, find the
wavelength of the mosquito’s sound.
17.On a piano keyboard, you strike middle C, of frequency 262 Hz.
(a) Find the period of one vibration of this tone?
(b) If the sound leaves the instrument at a speed of 340 m/s, what is its wavelength
in air?
18.Microwave ovens typically cook food using microwaves with frequency of about 3
GHz. Find the wavelength of these microwaves traveling at the speed of light.
Wavelength = Speed/frequency
Speed of light: 3x108 m/s
Frequency = 3x109 Hz
Wavelength = 3x108/(3x109) = 0.1 m = 10cm
19.An oceanic depth-sounding vessel surveys the ocean bottom with ultrasonic
waves that travel 1530 m/s in seawater. When the time delay of an echo to the
ocean floor below is 4 s, find the depth of the water.
20.If the fundamental frequency of a guitar string is 220 Hz, what is the frequency of
the second harmonic? Of the third harmonic?
21.Why is the Moon described as a “silent planet”?
Because the moon has no atmosphere which means it has no wind or any
weather, so there is no sound to hear.There is also no medium for the sound to
travel through.Sound cannot travel through empty space (a vacuum)
22.Why is an echo weaker than the original sound? Discuss the role of distance.
23.Electrons on a radio broadcasting tower are forced to oscillate up and down an
antenna 535,000 times each second. Find the wavelength of the radio waves.
24.A spider hangs by a strand of silk at eye level 30 cm in front of a plane mirror.
You are 65 cm behind the spider. Show that the distance between your eye and
the image of the spider in the mirror is 1.25 m.
The spider's image is 30 cm in back of the mirror, so the distance from the image
to your eye is 30 cm + 30 cm + 65 cm = 125 cm = 1.25 m.
25.When light strikes glass perpendicularly, about 5% of the light is reflected at each
surface. Find the amount of light transmitted through a pane of window glass.
26.Are the wavelengths of radio and television signals longer or shorter than waves
detectable by the human eye?
The wavelengths of radio and TV signals are much longer than those of visible
light. Radio waves are on the order of 3 m-30,000 m. TV waves are on the order
of 0.3 m-3 m. Visible waves are on the order of 710m
27.If you fire a bullet through a board, it will slow down inside and emerge at a
speed that is less than the speed at which it entered. When light passes through
glass, does it similarly slow down and also emerge at a lesser speed?
28.Why does smoke from a campfire look bluish against trees near the ground, but
yellowish against the sky?
Particles in the smoke scatter mostly blue light, so against a dark background like
the trees near the ground, we see the smoke as blue
