Chapter 23 Physics- Electric Current

In metal wires

- Conduction electrons are charge carriers that are free to move throughout atomic lattice. - Protons are bound within the nuclei of atoms.

in fluids

- Positive ions and electrons constitute electric charge flow.

safety fuses

- are wires that melt when the given current is exceeded. - are connected in series along the supply line to prevent overloading in circuits. - are replaced by circuit breakers in modern buildings.

How much current is drawn by a 60- resistor when a voltage of 12 V is impressed across it?

0.2 A. From Ohm's law. current equals voltage/ resistance. 12/60 equals 0.2

A circuit consists of three lamps connected in series. If the current in one lamp is 1 A, what is the current in each of the other lamps? In the battery?

1 A for all. Current does not "pile up" anywhere in the circuit, even in the battery, where current is 1 A.

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

10 (symbol) rearrange Ohm's law: resistance= voltage/current 120/12

At 15 cents/ kWh what would it cost to operate a 1200-W hair dryer for 1 h?

18 cents. 1200 W = 1.2 Kw, so 1.2 kW x 1h x 15 cents /kWh= 18 cents

In a simple circuit consisting of a single lamp and a single battery, when the current in the lamp is 2 A, the current in the battery is 2 A. dependent on the internal battery resistance. 1 A. More information is needed

2A. The current is the same in both. So 2A in the battery means 2A in the connected circuit, choice (b). Although the amount of current may depend on the internal battery resistance, the 2 A of current in the battery is nevertheless the same as in the circuit.

What is the current in the battery of the parallel circuit if each of the three lamps carries 1 A?

3 A. Current in the battery is the total current in the circuit, the sum of currents in each branch.

What is the current in a bulb if the resistance of its filament is 2 ohms and it is connected across a 6-volt battery? 0.33 amps 3 amps 12 amps

3 amps

If the current in a lamp is 3 A, the current in the connecting wire is 3 A. zero. less than 3 A. more than 3 A.

3A. The same! Current doesn't bunch up. It flows from a wire into a lamp and flows out the other side. Current is more evident in the lamp resistance than in the connecting wires. Current in equals current out.

An electric current of 5 amperes in a wire carries _________. 5 joules per ohm 5 coulombs per second 5 coulombs per joule 5 volts per coulomb

5 coulombs per second

Another circuit consists of three lamps connected in parallel. If 6 V is across one lamp, what is the voltage across each of the other lamps?

6 V. The voltages across all branches of a parallel circuit are the same.

The amount of energy given to each coulomb of charge flowing from a 6-V battery is 6 joules. less than 6 joules. More information is needed. more than 6 joules.

6 joules. Voltage is defined in terms of the amount of energy per charge. Six volts is six joules per coulomb, not more than or less than.

LED

65% heat 35% light

What condition is necessary for the sustained flow of water in a pipe? What analogous condition is necessary for the sustained flow of charge in a wire?

A continuous pressure difference, often provided by a pump, is needed for water to flow. A continuous potential difference, often provided by a battery, is needed for charge to flow.

What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?

A difference in temperature is needed for heat to flow. A difference in electrical potential is needed for charge to flow.

Rank the voltage readings from greatest to least.

A,B,C

The actual speed of electrons drifting along in a common electric circuit is about the speed of A. a slow-moving snail. B. sound. C. a fast-moving automobile. D. light.

A. a slow-moving snail.

Which statement is correct? A. charge flows in a circuit B. voltage flows in a circuit C. resistance flows in a circuit D. all of the above

A. charge flows in a circuit

Compared to the resistance of two resistors connected in series, the same two resistors connected in parallel have A. less resistance. B. the same resistance. C. more resistance.

A. less resistance.

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 a 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.

What causes electric shock: current or voltage?

Although electric shock occurs when current is produced in the body, impressed voltage causes the current. So in this sense, voltage causes shock.

Electric current is a flow of electric charge that moves due to an electric pressure called A. All of these to some degree. B. voltage. C. amperage. D. None of these. E. wattage.

B. voltage

Name two kinds of practical "electric pumps." Electric motors and a batteries Linear accelerators and Van de Graaf generators Batteries and Van de Graaf generators Batteries and automobile alternators

Batteries and automobile alternators

Why is there no proton current in a conductor with a potential difference applied across the circuit?

Because protons are bound inside the nuclei of atoms that are more or less in fixed positions

When current increases in any kind of lamp, what happens to its brightness?

Brightness increases when current (and therefore power) increases.

Rank the current readings in the ammeter, from greatest to least. (13)

C.B.A

Difference in potential between two points

Charges in conductor flow from higher potential to lower potential. • Flow of charge persists until both ends of conductor reach the same potential. • Maintained for continuous flow by pumping device

What is the relationship between current, resistance, and voltage difference?

Current = Voltage Difference / Resistance

Compared to the amount of electric current in the filament of a lamp, the amount of current in the connecting wire is A. definitely less. B. often less. C. actually more. D. the same. E. incredibly, all of these.

D. the same.

How much resistance must a 9-V device have to keep the current about 1 A? A. None of these. B. 1 ohm. C. 10 ohms. D. 4.5 ohms. E. 9 ohms.

E. 9 ohms

The current in a circuit is directly proportional to the impressed A. None of these. B. fear. C. resistance. D. All of these. E. voltage.

E. voltage

Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit?

Electric charge flows through a circuit; voltage is impressed across a circuit. Voltage is the electric pressure impressed across a circuit or across a circuit element that produces current. Voltage doesn't flow—charge does (flowing charge is current).

At what speed do electrons in a battery driven automotive circuit travel along a wire? At what speed does the electric field propagate along a wire? Electrons move at the speed of light. The electric field moves at 0.01 cm/s. Electrons travel at 0.01 cm/s. The electric field travels instantaneously. Electrons move at 0.01 cm/s. The electric field propagates at nearly the speed of light. Electrons travel at a few million km/hr. The electric field propagates at the speed of light.

Electrons move at 0.01 cm/s. The electric field propagates at nearly the speed of light.

True or False: Current is propagated through the conducting wires by electrons bumping into one another.

FALSE: Electrons that are free to move in a conductor are accelerated by the electric field impressed upon them.

When electrons flow in a thin lamp filament, they experience "friction." What is the practical outcome of this?

Heat and light!

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.

Consider members of a marching band standing at rest. You can set them into motion in two ways: (1) Give the last person in line a shove that cascades to the first person in line. (2) Issue the command "Forward, march." Which of these two is analogous to the way electrons move in a circuit when the switch is closed, and which is analogous to the way sound travels?

Issuing the command "Forward, march" is analogous to the way electrons move when they sense the electric field that energizes the circuit when the switch is closed. One marcher lurching against the other is analogous to the way sound travels.

What is the function of the round third prong in a modern household electric plug? It attaches the case to 120 volts potential. It provides safety against lightning strikes. It supplies a backup source of voltage. It grounds the case to zero potential.

It grounds the case to zero potential.

Does heating a metal wire increase or decrease its electrical resistance? Why?

It increases resistance because atoms at higher temperatures jostle into the way of moving electrons.

What does the power company provide to our homes? It provides electrons and the energy to move the electrons. It provides neither electrons nor the energy to move the electrons. It provides energy to move the electrons. It provides electrons that travel through the wire.

It provides energy to move the electrons.

More water flows through a thick hose than through a thin one connected to a city's water system (same water pressure).

Likewise for electric current in thick and thin wires connected across the same potential difference. Likewise shorter wire, less resistance. Also depends on temperature and material of the wire

Why are incandescent bulbs preferred over LEDs in a chicken coop to keep chicks warm on cold nights?

Most of the power dissipated by an incandescent bulb is in the form of heat.

If the two feet of a bird on the high-potential wire of a power line are widely spaced, won't it get a shock?

No, because there is no appreciable difference in electric potential between its feet.

If current in a wire moves in pulses, though always in the same direction, is this a mixture of dc and ac?

No, when the flow of charge is only in one direction, the current is dc. (ac alternates in direction.)

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.

As the number of parallel branches is increased, the overall resistance of the circuit is decreased.

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 one of the branches

What is the relationship among electric power, current, and voltage? Power is current divided by voltage. Power is current times voltage. Power is voltage divided by current. Power is voltage.

Power is current times voltage.

Does a battery produce dc or ac? Does the generator at a commercial power station produce dc or ac? Both are dc. The battery is dc; the generator is ac. The battery is ac; the generator is dc. Both are ac.

The battery is dc; the generator is ac

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 added (or removed).

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

The circuit is broken, current ceases, and all lamps will go out.

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

The current in the battery increases by an amount that feeds the added branch(es). In the overall circuit, added paths means decreased resistance. (There is resistance in a battery also, which we assume is negligible here.)

What happens to a lamp when you take both ends of the wire connected to it and hold them to the same side of the 12-volt terminal of battery, and why? The lamp does not light up because there is no voltage difference applied across it. The lamp does not light up, because there is no voltage applied to it. The lamp lights up because a voltage difference is applied to it. The lamp lights up because voltage is applied to it.

The lamp does not light up because there is no voltage difference applied across it.

What happens when you have a voltage difference between the two ends of the lamp, and why? The lamp lights up because there is a voltage difference applied to it. The lamp lights up because voltage is applied to it. The lamp does not light up, because there is no voltage applied to it.

The lamp lights up because there is a voltage difference applied to it

How does the resistance of a thick piece of copper wire compare to the resistance of a thin piece of copper wire? The resistance of the thin piece is greater than that of the thick piece. Both the wire and narrow piece of copper provide the same resistance. The resistance of the thick piece is greater than that of the thin piece.

The resistance of the thin piece is greater than that of the thick piece.

A pipe is filled with water, and there is a piston at each end. If you apply unequal pressures at the two pistons, which way will the water flow in the pipe? Water will not flow in either direction. Water will flow from the piston with the lower pressure to the piston with the higher pressure. Water will flow from the piston with the higher pressure to the piston with the lower pressure.

Water will flow from the piston with the higher pressure to the piston with the lower pressure.

How does wetness affect the resistance of your body? Wet bodies have no resistance. Wet bodies have the same resistance as dry bodies. Wet bodies have lower resistance. Wet bodies have higher resistance.

Wet bodies have lower resistance

Is the voltage between two points in an electric circuit related to the flow of electrons between the points?

Yes, and we will soon see that it relates to the amount of energy given to the electrons.

Okay, so a potential difference across the ends of a wire produces current. Instead of saying potential difference, can we as well say voltage?

Yes. Recall from Chapter 22 that electrical potential and voltage are interchangeable terms. We now consider the difference in electric potential between two points in a conducting path, also measured in units of volts.

Rank the circuits illustrated according to the brightness of the identical bulbs, from brightest to dimmest.

all in the middle between brightest and dimmest

Rank the current readings in the ammeter from greatest to least.

all in the middle between greatest and least

What kind of current runs through the electric wiring in a home? direct current alternating current circular current

alternating current

When you flip a light switch what exactly travels from the switch to the lamps? Voltage An electric field All of the above The flow of electrons

an electric field. The signal carried in the electric field travels from the switch to the lamps. Not the electrons themselves, which travel at a snail's pace—the drift velocity. It is the signal that travels at nearly the speed of light, which is carried in an electric field. Voltage is impressed like pressure. Unlike current, voltage doesn't travel.

What factors does the resistance offered by a piece of conductor depend upon? neither the geometry nor the material of the conductor material of the conductor only the geometry of the conductor both the geometry and the material of the conductor

both the geometry and the material of the conductor

Which of these statements is correct? Voltage flows in a circuit. A battery is a source of electrons in a circuit. Charge flows in a circuit. None of these.

charge flows in a circuit

When the filament breaks in one lamp in a parallel circuit, lamps in other branches of the circuit normally _________. go out absorb energy from the damaged lamp continue glowing, but dimmer continue glowing, but slightly brighter

continue glowing, but slightly brighter

If your body resistance is how much current will you experience if you touch the terminals of a 12-V battery?

current= voltage/resistance. 12/100,000= 0.00012A

If your skin is very moist, so that your resistance is only and you again touch the battery terminals, how much current will you experience?

current= voltage/resistance. 12/1000= 0.012

The battery of your smartphone operates on dc. Both Neither ac.

dc. A smartphone is powered by a battery that supplies a steady flow of direct current, dc.

The entity that travels about the speed of light in an electric circuit is _________. voltage electric field current voltage and electric field, but not current

electric field

What type of field causes the electrons to do what they do in the wire? magnetic field electric and magnetic fields. electric field

electric field

Power utilities do not sell ______. They sell energy. You supply the electrons.

electrons

When your body undergoes electric shock, the source of moving electrons is _________. the object making electrical contact moisture between you and a voltage source mainly from the ground electrons already in your body

electrons already in your body

When you make your monthly household electric payment, which of the following are you billed for: voltage, current, power, energy? Energy Voltage Current Power

energy

series

forms a single pathway for electron flow between the terminals of the battery, generator, or wall outlet

parallel

forms branches, each of which is a separate path for the flow of electrons

When the filament breaks in one lamp in a series circuit, other lamps in the circuit normally _________. absorb energy from the damaged lamp go out continue glowing, but slightly brighter continue glowing, but dimmer

go out

In a circuit with two lamps in parallel, if the current in one lamp is 2 A, the current in the battery is greater than 2 A. None of these 1 A. 2 A.

greater than 2A. The current in the battery of a parallel circuit is equal to the sum of the currents in the branches. We know the current in one lamp is 2 A, which means the current in the battery must be at least 2 A. Whatever current is in the second lamp adds to the 2 A already in the battery.

circuit breaker

is an automatic switch that turns off when the current is excessive.

If both the current and the voltage in a circuit are doubled, the power is halved. is doubled. is quadrupled. remains unchanged if the resistance is constant.

is quadrupled. This tests knowledge of P = IV, pure and simple. Reading the equation, twice I and twice V means four times the power, choice (d).

Of what physical quantity is Hertz a unit of? speed at which the electrons travel down the wire number of times per second that the electrons move back and forth in the wire time taken by electrons as they travel down the wire

number of times per second that the electrons move back and forth in the wire

Consider a pair of identical lamps connected to a battery. They will glow brighter when connected in series. More information is needed. parallel. either in some cases.

parallel. Each identical bulb that is connected in parallel to the battery is impressed with the full battery voltage, which produces full brightness. But when connected in series, the battery voltage is divided amongst each bulb, and each glows dimmer. Adding bulbs in parallel doesn't change the brightness of each bulb. When bulbs are added in series, bulbs become dimmer

A flow of electric charge in a wire normally requires a _________. pump potential difference across the ends of the wire sustained energy difference across the ends of the wire voltage source

potential difference across the ends of the wire

A voltage source in a circuit provides electrical _________. grounding pressure resistance force

pressure

How does the current change if you increase the resistance, keeping the voltage difference the same? The current stays the same. The current decreases. The current increases

the current decreases

Ohm's law

the current in a circuit equals the voltage difference divided by the resistance. Consequences: ▪ For a fixed resistance, current doubles if the voltage is doubled. ▪ For twice the resistance and twice the voltage, the same current will flow. - Using Ohm's law, resistors are used to regulate current inside electrical devices.

When you double the voltage in a simple electric circuit, you double the resistance. the current. neither the current nor the resistance. both the current and the resistance.

the current. This tests knowledge of Ohm's law. A simple reading of I = V/R says the same change that occurs for voltage also occurs for current. If V is doubled, I is doubled. Doubling V does not double R. Current is directly proportional to the voltage, while the resistance is inversely proportional.

What is the flow of current proportional to? voltage at one end of the wire voltages at both ends of the wire voltage difference between the two ends of the wire

voltage difference between the two ends of the wire

Will water flow more easily through a wide pipe or a narrow pipe? Will current flow more easily through a thick wire or a thin wire? Narrow pipe and thick wire Wide pipe and thick wire Wide pipe and narrow wire Narrow pipe and thin wire

wide pipe and thick wire

In chemical batteries

• Work by chemical disintegration of zinc or lead in acid. • Energy stored in chemical bonds is converted to electric potential energy.