Physics 23 questions

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. The flow of charge persists as long as there is potential difference ; without a potential difference, or once both ends reach an equilibrium, flow ceases.

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. Once a common potential is reached, or difference ceases, flow ceases. Charge flow is actually particle* flow. Negative charge flow= electrons, negative ions. Positive flow = protons or positive ions are flowing.

Why are electrons, rather than protons, the principal charge carriers in metal wires?

Electrons are free to move through the metal, whereas protons are fixed in place. In metal wires, 1+ e- from each metal atom are free to move throughout the atomic lattice as conduction electrons white protons are bound inside nuclei. In conducting fluids like car batteries, positive ions are part of the charge flow.

What exactly is an ampere?

measurement unit for rate of electrical flow. One ampere = 1 coulomb of charge per second. *1 coulomb = 6.25 X 10^18 e-

Name two kinds of practical "electric pumps."

batteries and automobile alternators (electric generator). These do work to pull negative charges away from positive ones. By chemical disintegration or electromagnetic induction. The different values of energy per charge create potential difference. Van de Graaff is not practical! (single brief surge of voltage)

How much energy is supplied to each coulomb of charge that flows through a 12-V battery?

12 joules ex. common automobile battery provides 12V electrical pressure to circuit. 12 J energy are supplied to each coulomb of charge made to flow in the circuit.

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

Charge flows through and voltage is impressed across a circuit. *Water flows through and pressure changes across the the ends.

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?

Wide pipe and thick wire

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

Heated metal wires increase resistance because atoms at higher temperatures jostle into the way of moving electrons.

What is the unit of electrical resistance?

Ohm (Ω) , after Georg Simon Ohm (1826) discovered relationship in voltage, current, resistance

If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?

It is cut in half. current = voltage/resistance amperes = volts/ohms "the greater the voltage, the greater the current) (the greater the resistance, the smaller the current)

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

It is cut in half. "the greater the voltage, the greater the current" ∴ "the smaller the voltage, the smaller the current"

How does wetness affect the resistance of your body?

Wet bodies have lower resistance.

What is the function of the round third prong in a modern household electric plug?

It grounds the case to zero potential, by connecting the appliance body directly to ground. Any charge buildup is conducted to the ground.

Does a battery produce dc or ac? Does the generator at a commercial power station produce dc or ac?

A battery produces direct current, which refers to the flowing of charges in one direction. A commercial power station produces alternating current, where electrons in circuit are moved first in one direction then opposite.

What does it mean to say that a certain current is 60 Hz?

the current alternates back and forth at a frequency of 60 cycles per second.

What property of a diode enables it to convert ac to pulsed dc?

It is a one-way valve that allows electrons to in one direction only. Currents pass through a diode only half of each period, since alternating current changes its direction each half-cycle.

A diode converts ac to pulsed dc. What electrical device smoothes the pulsed dc to a smoother dc?

a capacitor. The ac input of the diode is output as pulsating dc. The capacitor provides continuous, smoother current by slow charging and discharging.

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 0.01 cm/s. The electric field propagates at nearly the speed of light. In DC circuits, electrons have drift velocity, and the large numbers make large currents possible.

Why does a wire that carries electric current become hot?

Moving electrons collide with atoms, which transfers their kinetic energy to atomic motion. "Bumping into" the anchored metallic ions in the path of e- makes the current-carrying wire hot.

What is meant by drift velocity?

The net flow of electrons along a wire. When an electric field is applied, interrupting e- motion, the electron drifts toward the right, very slow.

A tipped domino sends a pulse along a row of standing dominoes. Is this a good analogy for the way electric current, sound, or both travel?

This is a good model for sound travel but not how electric current travels. Electrons that are free to move in a conductor are accelerated by the electric field impressed upon them, not because they bump into one another. "Bumping" that does occur (drift) actually slows them down and offers resistance to their motion.

What is the error in saying the source of electrons in a circuit is the battery or generator?

Wires are full of mobile electrons. "unlike buying a water hose empty of water, you cannot buy an electric wire empty of electrons. The conducting circuit material is the source of the electrons. Electrons do not flow from power lines!"

When you make your household electric payment at the end of the month, are you billed for voltage, current, power, or energy?

Power utilities sell energy, while you supply the electrons. Energy flows from the outlet into the lamp, not electrons.

Where do the electrons come from that produce an electric shock when you touch a charged conductor?

The electrons originate in your body. Energy passes through your body, causing free electrons to vibrate in unison.

What is the relationship among electric power, current, and voltage?

Power is current times voltage. Watts = amperes X volts rate at which electric energy is converted into another form, ie mechanical energy, heat, light

Between a watt, a kilowatt, and a kilowatt-hour, which is a unit of power and which is a unit of energy?

A watt is power, a kilowatt is power, and a kilowatt-hour is energy. power = energy/unit time energy = power X time

How does the heat emitted by incandescent lamps affect their efficiency?

Heat emitted reduces the efficiency. CFLs emit less heat and more heat; a 25 W CFL = 100-W incandescent

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

1 amp. The same current flows through lamps in series. *the voltage will be split evenly among the lamps, though

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

4 volts. The sum of the voltages across each lamp must add up to the total voltage across both lamps.

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

6 volts in series connections, the voltage drop is proportional to its resistance. Ohm’s law applies separately to each device.

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

The sum of the currents in the branches equals the current through the source. 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.

What is the function of fuse or circuit breaker in a circuit?

To prevent over loading the circuit with current. Fuses may be connected in a series along a supply line. This way the entire line current must pass through the fuse. Cautions are made to "blow out" the circuit.

Does more current flow out of a battery than into it? Does more current flow into a lightbulb than out of it? Explain.

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Only a small percentage of the electric energy fed into a common lightbulb is transformed into light. What happens to the remaining energy?

the remaining is emitted as heat

Why are thick wires rather than thin wires usually used to carry large currents?

less resistance

Are automobile headlights wired in parallel or in series? What is your evidence?

parallel if both turn on at same time

Comment on the warning sign shown in the sketch. (Figure 1)

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If electrons flow very slowly through a circuit, why does it not take a noticeably long time for a lamp to glow when you turn on a distant switch?

that is circuit?