Physics Month 8

(a) Why do electronics workers may be asked to wear a wrist band that is clipped to a grounded surface that conducts electricity well? (b) Why did young boys have to go barefoot to carry gunpowder?

(a) So that any charge that builds by movement is discharged. (b) Because it was important that no static charge build up on their bodies as they ran back and forth.

When the rate of flow of charge past any cross section (of wire, say) is 1 coulomb (6.24 billion million electrons per second), the current is __ ampere

1

How does a negative ion differ from a positive ion?

A negative ion has extra electron(s); a positive ion has lost electron(s).

How is an electric field different from gravitational field?

An electric field interacts with charge (instead of mass). It can exert repulsive forces as well as attractive forces, and can therefore be shielded.

Why did the strings on top of the plastic "hat" on the Van de Graaff generator in class stand for apart from each other when the power was turned on?

Because the space that surrounds each of these things--the magnet, the black hole, and the electric charge --is altered. The space is said to contain a forcefield.

How are a gravitational field and an electric field similar?

Both exert forces.

Why is a good conductor of electricity also a good conductor of heat?

Both types of conduction are via free-moving electrons.

Gravitational forces depend on the property called mass. What comparable property underlies electrical forces?

Charge

If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24-volt battery, how much current would you draw?

Current = voltage/resistance = 24 V/1000 = 0.024 A

If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12-volt battery?

Current =voltage/resistance = 12 V/100,000 = 0.00012 A

Distinguish between DC and AC. Which is produced by a battery and which is usually produced by a generator?

DC--current flows in one direction and is produced by a battery; AC--direction alternates and is produced by a generator

Do an ampere and a volt measure the same thing, or different things? What are those things and which is flow and which is the cause of the flow?

Different quantities; the same potential difference (or voltage) causes a flow of charge, which is the current.

For the same two attracting objects (an electron and a proton), which force is stronger, the electric force between them or the gravitational force between them?

Electric force.

What is lightning?

Electrical discharge from cloud to cloud or to ground.

Which force--gravitational or electrical--repels as well as it attracts?

Electrical; gravitational force only attracts.

What is the difference between a good conductor and a good insulator?

Electrons are free to move in a good conductor.

Does voltage flow through a circuit, or is voltage established across a circuit?

Established across; only charge flows.

If you double the distance between an electron and another charged particle, what happens to the electric force between them?

FILL IN

How many amperes flow through 60-watt bulb when 120 volts are impressed across it?

From P = IV 60w = I x 120 V = 0.5 A

Is electric resistance greater in a short fat wire or a long thin wire?

Greater in a long, thin wire.

What is meant by the expression 'action at a distance' ?

Interaction between things that are physically apart.

Why is an electric field considered a vector quantity?

It has magnitude and direction.

What does it mean to say that charge is conserved?

It is neither created nor destroyed, only transferred.

What will happen to the electrical potential energy of a charged particle in an electric field when the particle is released and free to move?

It transforms to KE.

How does wetness affect the resistance of your body?

Lowers skin resistance.

How does the amount of charge on the inside surface of the sphere of a charged Van de Graaf generator compare with the amount on the outside?

None on the inside; all charges repel to the outside.

How do the electric field lines appear when the field has the same strength at all points in a region?

Parallel, equally spaced lines.

Which of these is a unit of power and which is a unit of electrical energy: a watt, a kilowatt, and a kilowatt-hour?

Power--watt, kilowatt; energy--kilowatt-hour

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?

Pressure difference; potential difference

Which has more mass--a proton or an electron?

Proton-more than 1800 times greater than the electron.

How do protons and electrons differ in their electric charge?

Same magnitude, but opposite charge

In a normal atom, how many electrons are there compared with protons?

Same number, no net charge.

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

Serves as a ground connection

Why does your hair stand out when you are charged by a device such as a Van de Graaff generator?

Strands if hair are charged with the same sign of charge and are mutually repelled.

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

Temperature difference; voltage difference

Imagine a "free" electron and "free" proton held midway between the plates of a charged parallel plate capacitor. When released, how do their accelerations and directions of travel compare? If we Ignore their attraction to each other, which reaches a capacitor plate first?

The acceleration of the electron would be greater (same f smaller m), and the directions of acceleration would be opposite (because directions of forces are opposite); electron first.

Why is it important that the resistance of an extension cord be small when it is used to power an electric heater?

The amount id energy converted to thermal energy depends on the resistance of the wire and the amount of current flowing in it. An electric heater draws a lot of current, so if the connecting are has a high resistance, much energy will be wasted in heating the wire.

Why are occupants safe inside a car struck by lightning?

The charges on the outside are mutually repelled and the electric fields inside cancel to zero.

How is the strength of an electric indicated with field lines?

The closer the lines, the stronger the field.

Five thousand billion billion freely moving electrons in a penny repel one another. Why don't they fly out of the penny?

The electrons are attracted to the same number of protons in the penny.

What is wrong with touching inside the components of a television set that has been turned off or even unplugged?

The energy transfer that occurs can be fatal where high voltages are present, even after the set has been turned off.

How does the concept of a field eliminate the idea of action at a distance?

There is actual contact between a field in an object.

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

Thick wires have less electrical resistance and will carry greater amounts of current without overheating.

Does charge flow through a circuit, or into a circuit?

Through

The SI unit for electrical potential energy is joule. What is the SI unit for electrical potential?

Volt

You are not harmed by contact with a charged balloon, even though its voltage is very high. Is the reason for this similar to why you are not harmed by the greater-than-1000 degrees C sparks from a Fourth of July-type sparkler?

Yes, both are amount of energy per some quantity.

a) What are the three main methods of charging objects? b) Which method involves no touching.

a) Contact, friction, induction b) induction

a) Why are metals good conductors? b) Why are materials such as rubber and glass good insulators?

a) Free electrons b) Bound electrons

a) what are electric field lines? b) How do their directions compare with the direction of the force that acts on a positive test charge in the same region?

a) Lines depicting an electric field b) They are the same.

a) If electrons are rubbed from cat's fur onto a rubber rod, does the rod become positively or negatively charged? b) How about the cat's fur?

a) Negatively b) positively

a) How do like charges behave? b) How do unlike charges behave?

a) Repel each other b) attract each other

The electrical field lines of force are shown as going from a _______ charge to a _______ charge.

positive; negative

Voltage is an electric _____ that can produce a flow of charge, or _______, within a conductor. The birds shown are safe because of the potential _______ across the bird's feet is negligible.

pressure; current; difference

Is an electron in a hydrogen atom the same as an electron in a uranium atom?

yes, all electrons are identical.