physics exam 5

electrical current

when switch closes charge flows from source to receiver

When you touch your fingers (resistance 1000 Ω) to the terminals of a 6-V battery, how much current is moving through your fingers?

.006 A

Find the current if 5*10^20 electrons flow through a given point in 5 seconds. Hint: you might need to look up charge of an electron for this question.

16 A

A model electric train requires 6 V to operate. When it is connected to a 120-V household circuit, a transformer is needed. If the primary coil of the transformer has 360 turns, how many turns will the secondary coil have?

18 turns

Are the following statements true or false: 1) If I add one more light bulb to a series circuit consisting of four light bulbs, as a result, the rest of the light bulbs would be dimmer, because the current would decrease. 2) If two light bulbs are connected in series, current flowing through each of them can be different. 3) If light bulbs are connected in parallel, the total resistance of the circuit will be the sum of all resistances.

2 and 3 are false, 1 is true

Which of the following is TRUE: 1) Adding an extra light bulb in parallel Increases total resistance of the circuit. 2) If one out of three light bulbs, connected in parallel, is damaged, the current in the rest of the light bulbs would still flow.

2 only

Where in the diagram above, is a stronger magnetic field? (pic in phone)

where we have more field lines. (where blue ink is) more field lines=stronger field

The domains in the nail are randomly oriented. They can be induced into alignment, when a magnet is brought nearby. When you move the nail away from the magnet, ordinary thermal motion causes most or all of the domains in the nail to return to a random arrangement. If the field of the permanent magnet is very strong, the nail may retain some permanent magnetism of its own after the two are separated.

with non-ferromagnetic materials, although we could still have weak alignment in magnetic field, they never produce a strong magnetic field and they never retain magnetism after being removed from the magnetic field of a magnet

The solid wind-- protons and electrons emitted by the sun-- travels through the solar system. When the solar wind encounters Earth, the magnetosphere deflects most of the changed particles and shields our planet's surface. Solar winds that reach Earth collide with Earth's magnetic field rather than with the atmosphere.

High energy particles can cause radiation poisoning to humans and other mammals. They would be dangerous to unshielded astronauts, in outer space, or astronauts traveling to the moon. Large doses could be fatal. Still, solar storms-- and their effects-- are no problem for us on Earth's surface. Many of the charged particles pass near Earth and are trapped by its magnetic field. The trapped particles follow corkscrew paths around the magnetic field lines of Earth and bounce between Earth's magnetic poles high above the atmosphere

Bar magnets are moved into the wire coils at the same rate. Voltage induced in each coil causes a current, as indicated on the galvanometer. Ignore the electrical resistance in the loops in the coil, and rank from greatest to least the values of current readings.

I(B)>I(A) because there were more number of turns

How does the sum of the currents through the branches of a simple parallel circuit compare with the current in the voltage source?

I(current) at junction is shared with both lamps current splits into two currents. I=I(1)+I(2) voltage is constant

how much current is drawn by a 60 (omega-resistor) when a voltage of 12 V is impressed across it?

I=V/R= 12V/60(omega)=0.2A

Use ohm's law to determine the current in the lamp that has resistance of 80(omega) and is connected to 120V energy source

I=v/r= 120v/80(omega)= .75A

What does magnetic field look like inside the magnet

Inside a magnet, the field is always from right to left but outside the magnet it's always left to right -creating magnetic field inside the magnet: moving charge, always a charge inside anything, spinning/orbiting which creates magnetic field, spinning around the axis (motion of charge)

At 15 cents/kWh, what does it cost to operate a 1200-W hair dryer for 2 hours?

1200W=1.2 kW 1.2kW*2h*15 cents/kWh=36 cents

How much does it cost operating a 100-W lamp continuously for 1 day when the power utility rate is 15¢/kWh?

$0.36

The simple way to figure out the direction of the magnetic force on a current carrying wire is again using a different right hand rule, illustrated on the left

(lecture 31, slide 8)

Characteristics of parallel connections

-each device connects the same two points A and B of the circuit. The voltage is therefore the same across each device. -The current divides among the parallel branches. Ohm's law applies separately to each branch. The total current in the circuit equals the sum of the currents in its parallel branches. This sum equals the current in the battery or other voltage source. -As the number of parallel branches is increased, the overall resistance of the circuit is decreased. The overall resistance is lowered with each added path between any two points of the circuit. This means the overall resistance of the circuit is less than the resistance of any 1 of the branches. -easily overloaded. As more devices are plugged in and turned on, more pathways for current result in lowering of the combined resistance of each circuit. Therefore, a greater amount of current occurs in the circuits. The sum of these currents equals the line current, which may be greater than is safe. The circuit is then said to be overloaded. -diagram for circuit diagram for appliances connected to a household circuit (lecture 30 slide 10)

True or false: 1) The U shaped magnet only has a South Pole and does not have a North Pole. 2) If you break a bar magnet into two parts, you will end up with two smaller magnets instead of two pieces with one south pole and one north pole. 3) South and north poles of the magnets repel but south and south poles of two magnets attract each other.

1) false, 2) true, 3) false

if electrical energy costs 15 cents per KW*h, a 2000-W iron can operate for 4 hours at a cost of

120 cents or $1.20 a refrigerator, -first convert watts to kilowatts (2000W=2kW), take .15*4 to get .6 then take .6*2. -typically rated at around 500W, costs less for an hour but much more over the course of a month

Calculate the power of a hair dryer that operates on 120 V and draws a current of 10 A

1200 W

If we have a 4A fuse connected in series with an electrical device, which of the following would be the current that will melt the wire in the fuse and stop the current from flowing through this circuit.

10 A

Neon signs require about 12,000 V for their operation. Consider a neon-sign transformer that operates off 120-V lines. How many more turns should be on the secondary than on the primary?

100x more

A device in a 120-V circuit has a current rating of 30 A. What is the resistance of the device?

4 ohms

In a circuit of two lamps, connected in series, if the first lamp has a resistance of 10 ohms and total resistance of the circuit is 15 ohms, what is the resistance of the second lamp?

5 ohms

We have two fans in series, and voltage supplied is 120 V. If voltage drop in the first fan is 70 V, then the voltage drop in the second fan will be:

50 V

If 20 coulomb of charge flows through a circuit in 4 seconds, what is the current through this point?

5A

What is the voltage of the battery if 0.2 A current is drawn through the circuit when 30 ohm resistor light bulb is connected to it?

6 V

a simple design of the electric motor (lecture 31, slide 9)

A permanent magnet produces a magnetic field in a region in which a rectangular loop of wire is mounted to turn about the dashed axis shown. The current in the loop flips direction with each half-turn, and continuous rotation results .

voltage

electrical potential energy provided by the source to each charge. We also refer to voltage as potential difference of the source

What happens to the brightness of light from each lamp in a series circuit when more lamps are added to the circuit?

Adding more lamps in series produces greater circuit resistance, this decreases the current in the circuit, all voltages have to add up to the same total voltage, so the voltage drop across each lamp also decreases. Since power is the product of voltage and current, both these changes act to dim the lamps

Most Earth scientists think that moving charges looping around within the molten part of Earth create the magnetic field. Some Earth scientists speculate that the electric currents are the result of convection currents- heat rising from the central core and that such convection currents combined with the rotational effects of Earth produce Earth's magnetic field. Because of Earth's great size, the speed of moving charges need only be about a millimeter per second to account for the field.

As samples of rock are tested from different strata formed throughout geologic time, the magnetic field of Earth for different periods can be charted. This evidences shows that there have been times when the magnetic field of Earth has diminished to zero, followed by reversal of the poles. More than 20 reversals have taken place in the past 5 million years. The most recent occurred 700,000 years ago. The variation in the ion winds in Earth's atmosphere are respondible for nearly all of the fast fluctuation in Earth's magnetic field.

If we half the amount of current through the wire, how would the strength of magnetic field change around it?

produce half the amount (less)

We don't know exactly why Earth itself is a magnet. The configuration of earth's magnetic field is like that of a strong bar magnet placed near the center of Earth.

But Earth is not a magnetized chunk or iron like a bar magnet. It's simply too hot for individual atoms to hold to a proper orientation. So the explanation likely involves electric currents deep in the interior

A toy that draws a 0.3 A from 120 V outlet, operates for 1 hours. a second toy consumes 50W and is on for 1 hour. Which one costs more to operate, by how much more, if each kWh costs 10 cents?

Energy consumed by first one= P*t=I*V*t= .3*120=36 Wh. second one: 50*1=50Wh second one costs more cost of first: 0.036kWh*10cents/kWh=.36 cents second one costs: 0.050kWh*10cents/kWh=.50 cents so the first one is about .14 cents cheaper

What happens to the current in other lamps if one of the lamps in a parallel circuit burns out

If one lamp burns out, the other lamps are unaffected. This is because current in each branch, according to Ohm's law, is equal to voltage/resistance, and since neither voltage nor resistance is affected in the other branches, the current in those branches is unaffected

When the north pole of one magnet is brought near the north pole of another magnet, they repel.

If opposite poles are brought together, they attract. Like poles: repel Opposite poles: attract

A charged particle moving through a magnetic field experiences a deflecting force, so a current of charged particles moving through a magnetic field also experiences a deflecting force.

If the particles are trapped inside a wire when they respond to the deflecting force, the wire will also be pushed. This happens because moving charge produces magnetic field that interacts with the magnets magnetic field. These are Newton's action-reaction forces

If the secondary coil has more turns than the primary, the alternating voltage produced in the secondary coil will be greater than that produced in the primary. In this case, the voltage is said to be stepped up.

If the secondary has twice as many turns as the primary, the voltage in the secondary will be double that of the primary. The ease with which voltages can be stepped up or down with a transformer is the principal reason that most electric power is AC rather than DC. We will only consider ideal transformers that gives output

Motors and generators look the same, but the roles of input and output are reversed.

In a motor, electrical energy is the input and mechanical energy is the output; in a generator, mechanical energy is the input and electric energy is the output. Both devices simply transform energy from one form into another. As the loop rotates, the induced voltage (and current) changes in magnitude and direction. One complete rotation of the loop produces on complete cycle in voltage (and in current)

Why is iron magnetic and wood is not?

Iron has magnetic domains (made of ferromagnetic material), wood does not.

If current was flowing from left to right in a horizontal wire, would it produce clockwise or counter-clockwise magnetic field around the wire, if viewed from the right.

Magnetic field lines would have counter-clockwise direction.

Almost all electric energy sold today is in the form of AC, traditionally because of the ease with which is can be transformed from one voltage to another. Large currents in wires produce heat and energy losses, so power is transmitted great distances at high voltages and correspondingly low currents.

Power is generated at 25,000 V or less and is stepped up near the power station to as much as 750,000 V for long-distance transmission, then stepped down in stages at substations and distribution points to voltages used in industrial applications (often 440 V or more) and for the home (240 V and 120 V). Energy then, is transferred from one system of conducting wires to another by electromagnetic induction

What is the resistance of an electric frying pan that draws 12A when connected to a 120V circuit?

R=V/I=120V/12A =10(omega)

Electric current can be produced in a wire simply by moving a magnet in the wire loops. This phenomenon of inducing voltage by changing the magnetic field in loops of wire is called electromagnetic induction. When a magnet is plugged into a coil that has twice as many loops as another coil, twice as much voltage is induced. The induced voltage makes a current, which makes an electromagnet, which repels the magnet in our hand. More loops means more voltage, which means we do more work to induce it so we trade mechanical energy for electrical energy.

The amount of voltage induced depends on how fast the magnetic field lines are entering or leaving the coil. Note, its not a complete loop, although steady current would not flow, we can still have free electrons in a metal moving briefly due to changing magnetic field. Also on the diagram (lecture 32 slide 4) when I have more turns in a coil I will have to do more mechanical work to move the magnet, because the coil resists magnet's motion but I will be inducing more voltage as well.

What happens to the brightness of the light from each lamp in a parallel circuit when more lamps are added in parallel?

The brightness of each lamp is unchanged as other lamps are introduced (or removed)

What happens to the current in other lamps if one lamp in a series circuit burns out?

The circuit is broken and all lamps will go out

A suspended magnet or compass points northward because Earth itself is a huge magnet.

The compass aligns with the magnetic field of Earth. The magnetic poles of Earth, however, do not coincide with the geographic poles

What happens to the current in the battery when more lamps are added in parallel?

The current in the battery increases in the overall circuit, added paths means decreased resistance (there is resistance in a battery also, which we assume is negligible here)

I connected light bulb and electrical fan in series. If the filament of the incandescent light bulb breaks, will the fan stop or will it keep on operating?

The fan would stop working

A transformer for a laptop computer converts a 120-V input to a 24-V output. Compare the number of turns on primary coil and the secondary coil.

The primary coil has five times as many turns as the secondary coil.

The strength of magnetic field (number of magnetic field lines) depends on the amount of current through the wire.

The relation between the two is a direct one. If you triple the current the magnetic field will be three times bigger.

current

a flow of charge, pressured into motion by voltage and hampered by resistance

conductor

a material that allows easy movement of charge (plastic is NOT a conductor) (metals are good conductors) open circuits do not provide a path of complete conductors and switch provides that path

How does the magnetic field around the wire change if the current flowing through the wire quadruples? Would we have more or less of magnetic field lines?

We would have 4x stronger magnetic field and more field lines

Magnetic field produced by the coil of wire is very similar to a field created by a bar magnet.

With bar magnet, we could not "see" the field inside but from diagram on the right (lecture 31, slide 7), we see that magnetic field lines form complete loops

An ideal transformer has 50 turns in its primary and 250 turns in its secondary. 12-V ac is connected to the primary. Find (a) Voltage at the secondary; (b) The current in a 10-Ω device that is connected to the secondary; and (c) the power supplied to the primary.

a) 60 V b) 6A c) 360 W

A 4W night-light is plugged into a 120 V circuit and operates continuously for 1 year. Find the following (A) the current it draws (B) the resistance of its filament (C) the energy consumed in a year (d) then show that for a utility rate of 15 cents/kWh the cost for a year's operation is $5.25

a) P=4W and V=120V. From P=I*V we get I=P/V=4W/120V=.033A b)to find resistance we can use Ohm's law: V=I*R so R=V/I= 120V/0.033A=3600 ohms c) energy=power(inkWh)*time (hours)=0.004kW*8760h=35.04 kWh d) Total cost=cost per unit energy*amount of energy= 15 cents/kWh*35.04kWh= 525.6 cents= $5.46

True or false: a) Magnetized ferromagnetic material has most of its domains aligned which, as a result, strengthens the magnetic field. b) Iron can stay magnetized after being removed from magnetic field, if it was in contact with a very strong magnet. c) Only Iron can have magnetic domains because iron is the only ferromagnetic material that exists.

a) T b)T c)F

True or false: a) A step-up transformer in an electric circuit steps up voltage, but steps down current. b) The metal detectors people walk through at airports operate via Newton's law. c) A voltage will be induced in a wire loop even when the magnetic field within that loop does not change.

a) True, b) False, c) False.

If 100 V of ac is put across 100-turn transformer primary. a.) what will be the voltage output if the secondary has 200 turns? b) Assuming the answer to the preceding question is 200 V and the secondary is connected to a flood lamp with a resistance of 50 ohms, what will be the ac current in the secondary circuit? c.) what is the power in the secondary coil? d.) What is the power in the primary coil? e) What is the ac current drawn by the primary coil?

a) Two times greater, 200 V b) From Ohm's law, 200V/50 ohms=4A c) Power=voltage*current=200V*4A=800W d)By the law of conservation of energy, the power in the primary coil is the same, 800W e)8A, twice as much

Each of the transformers shown is powered with 100W, and all have 100 turns on the primary. The number of turns on each secondary varies as indicated. A) rank the voltage outputs of the secondaries from greatest to least b) rank the currents in the secondaries from greatest to least c) rank the power outputs in the secondaries from greatest to least

a.) V(B)>V(C)>V(A) **red markings on pic on phone b) **green markings on pic on phone** if it steps up voltage, it steps down current (if voltage halved, current is doubled) I(A)>I(C)>I(B) c) P(A)=P(B)=P(C) equal amount of power supplied

energy receivers

affect the circuits in which they are placed differently. can investigate these differences by measuring the current present (using Ammeter) when different devices are connected to the same energy source

An electric motor is

an electrical machine that converts electrical energy into mechanical energy

electrical energy and power example

an incandescent bulb rated at 60 W draws a current of 0.5 A and a 100-W bulbs draws about .8 A Energy=power*time energy=current*voltage*time

the round prong connect the body of the appliance directly to ground (earth)

any charge that builds up on an appliance is therefore conducted to the ground, thereby preventing accidental shock

circuit

any path along which electrons can flow -for a continuous flow of electrons, there must be a complete path with no gaps

How does magnetic field strength relate to the closeness of magnetic field lines about a bar magnet?

bar magnet: density of magnetic field lines shows you how strong it is (closer to the magnet pole=more dense) more magnetic field lines=stronger field

if a flashlight uses 4 volt (4V) battery instead of 2V battery it is

brighter, more energy is delivered to the flashlight

If current is flowing from right to left, what would the direction of magnetic field lines be if viewed from the right?

clockwise circles vid on phone

If the resistance of a circuit remains constant while the voltage across the circuit decreases to a third its former value, what change occurs in the current?

current will be 3 times less

ohm's law

describes the relationship between current, voltage and resistance in an electrical circuit -voltage=circuit*resistance current=voltage/resistance resistance=voltage/current

Why will dropping an iron magnet on a concrete sidewalk make it a weaker magnet?

disaligning domains (shuffling them again when they hit the ground)

In the diagram below, which way would the current carrying wire be pulled due to the magnetic force exerted on it? (homework 31, question 5)

down

a complete path must be available from one side of the battery to the other in order for

electrical energy to move from battery to the light bulb (and make it glow)

A magnetic field is produced by the motion of electric charge. But if the motion of electric charges produces magnetism, what is moving in a common bar magnet? Turns out electrons in atoms are in constant motion. Two kinds of electron motion contribute to magnetism:

electron revolution: electron spin: in most common magnets, this is the main contributor to magnetism. every spinning electron is a tiny magnet. A pair of electrons spinning in the same direction make up a stronger magnet. A pair of electrons spinning in opposite directions, work against each other. The magnetic fields cancel. This is the reason why most substances are not magnets electrons behave as if they revolve about the atomic nucleus and spin about this own axes like tops

If you break a bar magnet in half, each half still behaves as a complete magnet. Break the pieces in half again, and you have four complete magnets. You can continue breaking the pieces in half and never isolate a single pole

even when your piece is one atom thick, there are two poles. Which suggests that atoms themselves are magnets

In a circuit of two lamps in parallel, if there is a voltage of 6V across one lamp, what is the voltage across the other lamp?

exact same amount of voltage drop on the second lamp as it is in the first lamp

Show that operating a 100W lamp continuously for 1 week when the power utility rate is 15 cents/kWh costs $2.52

first convert 100 W to kW=.1 kW. Now convert 1 week to hours, 7 days*24 hours=168 h. Next find total energy. Energy=power*time= 0.1 kW*168h=16.8 kWh. We were given the cost of each kWh is 15 cents so 16.8kWh would cost 16.8kWh*15cents/kWh=252 cents=$2.52

which way will the wire be pulled on this diagram?

force downward current going north (so thumb pointing north) go from north to south (in this case is left to right) palm is facing downward so force is downward (pic on phone)

parallel (circuit types)

form branches, each of which is a separate path for the flow of electrons. -Electrical devices directly connected to the same two points of an electric circuit are said to be connected in parallel. -The pathway for current from one terminal of the battery to the other is completed if only one lamp is lit. -A break in any one path does not interrupt the flow of charge in the other paths. Each device operates independently of the other devices lecture 30 slide 4&9 have pics to show these series

Which of the following would not increase safety of an electrician when working with wires that might carry the current?

having sweaty hands

just like thermal energy moves from hot object to colder object, electrical energy always moves from the

higher voltage to the lower voltage. the current will continue to move form one terminal to another until the battery runs down, for current to flow we need ends maintained at different voltages.

The main disadvantage of a series circuit:

if one device fails, current in the whole circuit ceases. Most circuits are wired so that it's possible to operate several electrical devices, each independently of the others. In your home, for example, a lamp can be turned on or off without affecting the operation of the other lamps or electrical devices. This is because these devices are connected not in series but in parallel with one another.

The force between electrically charged particles depends on the magnitude of each charge, their separation distance, and what else?

if the charge is same pole: repelling opposite: attraction direction matters, force could be towards each other or away from each other. Force is also direction as well (towards or away)

What is the source of magnetic force?

produced by magnetic poles (north and south), just like electric force is produced by electrical charges. never find isolated pole, always a pair.

if we have a current carrying wire, we can easily figure out the direction of magnetic field lines

if you place the thumb of your RIGHT hand in the direction of the current, your finger curl into the direction of magnetic field lines.

in a circuit of two lamps in series, if the current through one lamp is 1A, what is the current through the other lamp? Defend your answer

in series, the current is the same everywhere so whatever number she gives you it has to be the same everywhere. -No matter how many objects I collect, as long as they're in series, those two lightbulbs will have the same current going through them. Because the current is the same everywhere because there is only one path for it to take.

If a voltage of 6V is impressed across the circuit in the preceding question and the voltage across the first lamp is 2V, what is the voltage across the second lamp? defend your answer

in the series, each voltage drop (bulb?) adds up to the total voltage -4V of voltage dropped on the second one. 2+4=6

A current-carrying coil of wire is an electromagnet. The strength of an electromagnet is increased by simply increasing the current through the coil and the number of turns in the coil.

industrial magnets gain additional strength by having a piece of iron within the coil. Electromagnets powerful enough to lift automobiles are a common sight in junkyards. Magnetic domains in the iron core are induced into alignment, adding to the field.

Why does a piece of iron in a current-carrying loop increase the magnetic field strength?

iron can be magnetized, can behave like another magnet which has all of the domains arranged in the same direction and end up with two magnets making it a stronger magnet

what do you think will happen to the strength of electromagnetic if the iron core is replaced with aluminum or wood?

iron is ferromagnetic, it has domains and domains would align themselves within the magnetic fields. it will become magnetized, another magnet inside of electromagnet (even stronger) makes it strong enough to pick up very heavy objects. -Wood doesn't have ferromagnetic properties, cannot be magnetized. Doesn't contribute to electromagnetic fields. -aluminum wouldn't be as strong as iron.

magnetic field lines about a current-carrying wire bunch up when the wire is bent into a loop. If the wire is bent into another loop, overlapping the first, the concentration of magnetic field lines inside the loops is doubled.

it follows that the magnetic field intensity in this region is increased as the number of loops is increased. The magnetic field intensity is appreciable for a current-carrying coil of many loops

A common horseshoe magnet is simply a bar magnet that has been bent into a U shape

its poles are also at its two ends

The magnetic field of an individual iron atom is so strong that interactions among adjacent atoms cause large clusters of them to line up with one another. These clusters of aligned atoms are called magnetic domains.

like the alignment of iron atoms within domains, domains themselves can align with one another. Iron, nickel and cobalt are the few materials that have magnetic domains. Each domain is made up of billions of aligned atoms

we can qualitatively describe how much electricity is consumed

longer we use the microwave oven, the higher the electricity bill, the brightness of the light bulb and how hot it gets gives us an idea on how much electrical energy is converted to thermal energy and light

Is the rule for the interaction between magnetic poles similar to the rule between electrically charged particles?

magnetic poles (opposite, attract) same( repel), same for electrically charged particles

What produces a magnetic field?

magnets produced by moving charges (current), or it could be any moving charges. (electrons are even considered to be moving charges when they orbit around the nucleus)

insulators

materials that don't allow easy motion of charge (air is a good insulator)

Electrostatics

motionless/static charges and force of attraction or repulsion between them.

if we lift a box, we do work and the box gains gravitational potential energy (gravity will do work if i let go) the same is true if i (or battery)

move electrons farther from positive charges. I do work but electrons gain electric potential energy, if i let go, it will do work

Can you magnetize pure aluminum?

not something that has domains, because of that, when you bring magnets close to them (although all of those atoms are oriented the same way) if you remove the magnets then thermal energy would shuffle those up and they would be in random directions. doesn't retain magnetism

to prevent overloading in circuits, fuses may be connected in series along the supply line. in this way, the entire line current must pass through the fuse. the fuse is constructed with a wire ribbon that will heat and melt at a given current. if the fuse is rated at 20 A, it will pass 20 A but no more. A current greater than 20 A will melt the fuse, which "blows out" and breaks the circuit. Before a blown fuse is replaced, the cause of overloading should be determined and remedied.

often, insulation that separates the wires in a circuit erodes and allows the wires to touch. this greatly reduces the resistance in the circuit, effectively shortening the circuit path, and is called a short circuit. -in buildings, we have circuit breakers, which use magnets or bimetallic strips to open a switch when the current is too great. utility companies use circuit breakers to protect their lines all the way back to the generators

Real life application of electromagnetic induction

on the road, it triggers traffic lights when a car drives over- and changes the magnetic field in-- coils of wire beneath the road surface. We send electromagnetic induction in the security systems of airports as we walk through upright coils and, if we are carrying any significant quantities of iron, change the magnetic field of the coils and trigger an alarm. we use it with an ATM card when its magnetic strip is swiped through a scanner

By convention direction of a current is

opposite of direction of electrons

what is magnetic domain?

part of a magnetic material that has atoms already aligned to other atoms nicely -atoms in a domain already aligned (like tiny magnets all facing one direction) elements that have a magnetic domain are known to be ferro-magnets

An electrical device draws 0.5 A from a 120V outlet. show that the toy consumes 60 W of power

power=I*V 0.5A*120V=60W

An electron has a charge of 1.6*10^-19C. If 200,000 electrons flow through a point in 0.0002 seconds, find current

q=1.6*10^-19C N=200,000 t=0.0002s I=q/t q(total)=N*q(electrons)=200,000*1.6*10^-19C/time first part is amount of charge moving past a point=3.2*10^-14/0.0002s=1.6*10^-10A

5 C of charge passes through a point in 20 seconds. What is the current in that point?

q=5C t=20s ----- I=? I=5C/20s=0.25A

magnetism

related to electricity history of magnetism on lecture 30 slide 13

What law of physics tells you that if a current-carrying wire produces a force on a magnet, then a magnet must produce a force on a current-carrying wire?

right hand rule: thumb going same direction of the current, fingers pointing to blue arrow (N->S) (vid on phone) means palm facing down=force down

uncontrolled charge transfer examples

sparks and lightning -the building up concentrations of electrical charge results in big electrical force that makes electrons flow toward a place of opposite charge

What two kinds of rotational motion do electrons in an atom appear to have?

spinning motion around it's own axis orbiting around the nucleus

A video game console requires 6 V to operate correctly. A transformer allows the device to be powered from a 120V outlet. If the primary has 500 turns, show that the secondary should have 25 turns.

step down transformer V(primary)=120 V Number of turns (primary)=500 (120/6=20 times less voltage) (500/20=25 turns) V(secondary)=6V Number of turns (secondary)=25 turns

A transformer has an input of 6V and an output of 36 V. If the input is changed to 12V, show that the output would be 72 V.

step up transformer Voltage (primary)=6V changed voltage of primary=12V Voltage(secondary)=36V Changed voltage of secondary= 72V 36V/6v= 6 times bigger 12V*6=72 V (6 times bigger)

At the micro level what is the difference between an unmagnetized iron nail and a magnetized iron nail?

still has domains but are not oriented the same way (unmagnetized iron nail) magnetized: same domain but twisted

from the definition power=energy per unit time, it follows

that energy=power*time. so an energy unit can be a power unit multiplied by a time unit such as kW*h (kilowatt*hours)

If you sprinkle some iron filings on a sheet of paper placed on a magnet, you'll see that the filings trace out an orderly pattern of lines that surround the magnet. The space around the magnet contains a magnetic field. The shape of the field is revealed by the filings, which align with the magnetic field lines that spread out from one pole and return to the other

the direction of the field outside a magnet is from the north pole to the south pole. Where the lines are closer together, the field is stronger. If we place another magnet or a small compass anywhere in the field, its poles line up with the magnetic field

electric circuits

the energy source, energy receiver and the wires collectively -a source can be battery or wall socket connected to power plant,it uses wires to deliver energy to receiver where one type of energy can be converted to another- useful energy. -in order to transfer energy from source to receiver, we need to have a closed circuit diagram on lecture 28, slide 5

Faradays law

the induced voltage in a coil is proportional to the product of its number of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within those loops

electric power

the rate at which electric energy is converted into another form, such as mechanical energy, heat, or light electric power in a circuit, is equal to the product of current and voltage

Electrical resistance

the ratio of the voltage of the energy source to the current moving through receiver. -describes how well a circuit component resits the passage of current -resistance of the wire is normally very small and negligible. -a property of materials and its shape -of some energy receivers in ohms is given here for fixed dimension lecture 28 slide 10

In what way are magnetic poles very different from electric charges?

we can have separately positive and negative electric charges but can never have just a north or south pole of a magnet isolated

the same current is resisted by the resistance of all the light bulbs so the total resistance to the current in the circuit is the sum of the individual resistances along the circuit path. The current in the circuit is numerically equal to the voltage supplied by the source divided by the total resistance of the circuit. The supply voltage is equal to the sum of the individual "voltage drops" across each device.

the voltage drop across each device is proportional to its resistance-ohm's law applies separately to each device. This follows from the fact that more energy is dissipated when a current passes through a large resistance than when the same current passes through a small resistance.

Whenever the primary switch is opened or closed, voltage is induced in the secondary circuit. We see that voltage is induced wherever a magnetic field is changing through the coil. Instead of opening and closing a switch to produce the change of magnetic field, suppose that alternating current is used to power the primary

then the frequency of periodic changes in the magnetic field is equal to the frequency of the alternating current

series (circuit types)

they form a single pathway for electron flow between the terminals of the battery, generator, or wall socket (which is simply an extension of these terminals) -same current exists almost immediately in all three lamps, and also in the battery, when the switch is closed. -electrons do not pile up in any lamp but flow through each lamp- simulataneously -some electrons move away from the negative terminal of the battery, some move all the way around the circuit (the same amount of current passes through the battery). -this is the only path of the electrons through the circuit. A break anywhere in the path results in an open circuit, and the flow of electrons stops. Burning out one of the lamp filaments or simply opening the switch could cause such a break pic on lecture 30 slide 5

Charged particles in space, produce cosmic rays and consist of protons, alpha particles, and other atomic nuclei stripped of electrons, as well as high-energy electrons. The protons may be remnants of the Big bang; the heavier nuclei probably boiled off from exploding stars. Cosmic rays are dangerous to astronauts and also a hazard to electronic instrumentation in space.

they're also deflected away by the magnetic field of Earth. Some of them are trapped in the outer reaches of Earth's magnetic field and make up the Van Allen radiation belts. The aurora borealis- "northern lights" is lighting of the sky caused by charged particles in the Van Allen belts striking atmospheric molecules. Disturbances in Earth's field often allow the ions to dip into the atmosphere, causing it to flow like a fluorescent lamp. Muons are example of many "secondary" cosmic rays reach Earth's surface and they are constantly bombarding us wherever you are on Earth.

It's more practical to induce voltage by moving a coil than by moving a magnet

this can be done by rotation the coil (or loop) in a stationary magnetic field. The arrangement is called a generator

According to Ohm's law

this current depends both on the voltage that is applied and on electrical resistance of the human body. The resistance of one's body, which depends on its condition, ranges from about 100 (omega) if it is soaked with salt water to about 500,000 omega if the skin is very dry. lecture 28, slide 14 has a chart on the current and its effects

A current of charges produces a magnetic field. When the current in the vertical wire on the diagram (lecture 31 slide 5) reverses direction, the compass needles turn around, showing that the direction of the magnetic field changes also

this is the effect that oersted first demonstrated in his classroom. Magnetic field lines (unlike electric field lines) are always complete loops.

when current passes through the coil, the magnet exerts a force on it and as a result the coil turns.

this way we'll have an electrical energy converted to the motion of the loop and mechanical work is being done

The forces that magnets exert on one another are similar to electrical forces because both can attract and repel without

touching, depending on which ends of the magnets are held near one another. Also like electrical forces, the strength of their interaction depends on the separation distance between the two magnets. Whereas electric charge is responsible for electrical forces, regions called magnetic poles give rise to magnetic forces. If you suspend a bar magnet at its center by a piece of string, you'll have a compass. One end, called the north-seeking pole, points northward, and the opposite end, called the south-seeking pole, points southward. North and South poles

if i connect different light bulbs to the circuit,

voltage will be the same for all of them since they are always connected to the same battery but current measured would be different.