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Physics 2 > Exam 1- Chapter 16-18 > Flashcards

Exam 1- Chapter 16-18 Flashcards

1
Q

What is the importance of electric force?

A

Electric forces between atoms and molecules hold them together to form liquids and solids. Electric forces are also involved in metabolic processes that occur within the body

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2
Q

What type of charges attract each other?

A

Opposite charges attract, like charges repel.

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3
Q

The law of conservation of electric charge

A

The net amount of charge produced in any process is zero (molecules can be charged, but the charges will add to zero). No net electric charge is created or destroyed. Just like energy, momentum, and mass, electric charge is conserved.

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4
Q

Charge

A

Symbolized by Q or q. Charge resides in atoms. Unit- coulomb (c)

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5
Q

Structure of an atom

A

Atoms have a large nucleus in the center which is positively charged. They are surrounded by a low density, negatively charged electron cloud. This is so electrons can easily move around. All atoms are neutral in charge because the electrons and protons balance out. Ions are charged.

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6
Q

Objects can be charged by

A

Rubbing. This causes a charge because electrons are moved from one object to the other. The object receiving the electrons will have a negative charge.

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7
Q

What does neutral mean?

A

It means that charges are balanced, not that there is no charge

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8
Q

Polarity

A

A polar molecule, such as water, is neutral overall, but the charges are not evenly distributed. In water, one end is a negatively charged oxygen, while the other end is 2 positively charged hydrogens.

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9
Q

Where does charge go if an object loses it?

A

An object that is charged by rubbing will hold the charge for a short time, The excess charge will leak off into water molecules in the air due to the polarity of water- electrons are attracted to the positive end of the water molecule.

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10
Q

Conductor

A

Charge flows freely in conductors, electrons have high kinetic energy. Generally these objects are made of metal. Everything is able to conduct to some extent, but it depends what object the conductor is compared to

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11
Q

Insulators

A

Almost no charge flow- flow takes effort. Made of mostly non metallic materials, includes wood and rubber

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12
Q

What happens when an object touches a conductor?

A

In a conductor, some electrons (free electrons) are bound very loosely to the nuclei. When a positively charged object is brought near a conductor, the electrons are attracted to the positive charge and move toward it. Electrons will move away from a negative charge.

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13
Q

If a neutral object is brought near a charged object, what happens?

A

It will move toward/be attracted to the charged object. For example, if the charged object has a negative charge, it will induce a positive charge in the neutral object and the charges will attract. The protons in the neutral object move toward the negative charge while the electrons are repelled.

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14
Q

If two neutral objects are placed next to each other, what is the nature of the forces between them?

A

Neither attractive nor repulsive, there will be no forces between the objects.

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15
Q

Charging by conduction

A

Charging by touch. The two objects that came into contact with each other end up with the same charge as electrons are transferred.

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16
Q

Charging by induction

A

When a charged object is brought near a neutral object, but the objects don’t touch. The charges in the neutral object are separated, so the objects will be attracted to each other.

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17
Q

Electrical grounding

A

The Earth can be considered “ground” because it is a reservoir of positive and negative charge. Ground can be used to induce a net charge. If a neutral object is grounded and a negatively charged object is brought near it, the repelled electrons will actually leave the neutral object and go to ground, causing a positive charge. The charge will go back to neutral when the negative object is taken away, but if connection to ground is severed, the positive charge will be permanent.

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18
Q

What occurs when a charged object is touched to the ground?

A

The opposite type of charge will be supplied, neutralizing the object’s charge.

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19
Q

Electroscope

A

A device that can be used for detecting charge. Two movable metal leaves, made of gold foil, are connected to a metal knob on the outside. If a positively charged object is brought near the knob of the electroscope, there is a separation of charge. Electrons go to the knob, and the leaves become more positive and repel each other. If charged by conduction, the whole electroscope has a net charge. Additional experimentation is required to determine the polarity of the charge.

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20
Q

How is the magnitude and direction of electric force determined?

A

Coulomb’s law gives the magnitude of electric force. The direction is always along the line joining two charges. If the charges have opposite signs, they attract, so the charges are directed towards each other.

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21
Q

Electric force is proportional to

A

The product of the two charges. It is inversely proportional to the distance between them.

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22
Q

Magnitude of charges produced by rubbing objects

A

Usually equals a microcoulomb (10^-6)

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23
Q

Elementary charge (e)

A

The fundamental charge, equal to the charge of an electron. e= 1.602*10^-19. This means that charges must occur in integer multiples of this number, and there can be no fractional amounts of e.

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24
Q

Electric flux

A

The electric field passing through a given area. It is proportional to the number of electric field lines passing through an area. It can be calculated as the sum of the fluxes through smaller pieces of a surface. In addition, electric flux is independent of the radius of a sphere

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25
Q

What is theta in the electric flux formula?

A

The angle between the electric field direction and a line drawn perpendicular to the area. Theta is zero when the field is perpendicular to the surface.

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26
Q

Gaussian surface

A

A closed surface through which an electric field passes. Gauss’ law only applies to these types of surfaces. The electric field tells us about the enclosed charge. A positive enclosed charge will result in an electric field that points out of an object. This is a mathematical (imaginary) surface, not a real surface

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27
Q

A Gaussian surface is most useful when

A

It matches the shape and symmetry of the electric field. A cylindrical Gaussian surface surrounds a cylindrical charge distribution, like a charged wire. Because the Gaussian surface matches the symmetry of the charge distribution, the electric field is perpendicular to the sides of the surface and no field will pass through the top or bottom surfaces (it’s parallel to these surfaces).

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28
Q

In a Gaussian surface, how does the electric field relate to an enclosed charge?

A

The electric field flows out of a closed surface surrounding a region of space with a positive charge and into a closed surface surrounding a negative charge. If a region of space has no net charge, the electric field can flow through this space, but the net flow is zero. Flow can also be called flux.

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29
Q

What is the purpose of using Gauss’s law? (2)

A
  1. Gauss’s law allows the electric field of some continuous distributions of charge to be found much more easily than does Coulomb’s law, especially when a Gaussian surface would have the same symmetry as an electric field.
  2. Gauss’ law is valid for moving charges, but Coulomb’s law is not.
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30
Q

Electric field

A

A charge Q will exert an electric field on nearby charge Q2. The electric field is the force that Q2 “feels” from Q,

31
Q

Electric field lines

A

Radiate outward from a positive charge, and inward toward a negative charge. The number of lines is proportional to the magnitude of the charge, and the closer together the lines are, the stronger the electric field is in that region.

32
Q

Electric dipole

A

The electric field lines due to two equal charges of opposite sign. The lines are curved and directed away from the positive charge toward the negative charge. The direction of the electric field at any point is tangent to the field line at that point.

33
Q

Electric fields in conductors

A

When charges are at rest, the electric field inside a conductor is zero (this is not true for nonconductors). This is because any net charge in a conductor distributes itself on the surface of the conductor. Also, the electric field is always perpendicular to the surface outside a conductor.

34
Q

Attractive forces

A

If Q2 is exerting an attractive force on Q1, the force is pulling Q1 towards Q2. Therefore, the force vector will be pointing from Q1 to Q2.

35
Q

How to changes to the magnitude of charge or mass affect the force of attraction between two objects?

A

Changing mass or change to the magnitude of charge will not change the force of attraction due to Newton’s third law (equal and opposite reaction).

36
Q

Do electric fields only exist at field lines?

A

No, electric fields exist at all points in space. Electric field lines are just used as a representation of the direction and density of the electric field.

37
Q

When a positive charge is exerting a field, what is the direction of force on a negative charge?

A

Opposite to the field. Positive charges create a field that points away from the charge, but a negative charge will experience an attractive force pulling it toward the charge.

38
Q

How does acquiring a positive charge change the mass of an object?

A

The mass of the object decreases. This is because electrons are removed from the object to create the positive charge, and electrons have mass.

39
Q

As charged plates are brought closer together, how does the electric field change?

A

The electric field does not change, in contrast to point charges. The electric field between plates is parallel with uniform density.

40
Q

If a positive test charge is brought near a positively charged conductor, how will the electric field change?

A

The electric field will decrease because the charges in the conductor will be repelled and will move along the conductor’s surface to get away from the test charge. They will then be at a greater distance from the test charge and will create a smaller field.

41
Q

Why are we unaware of the electric force acting between two everyday objects?

A

Because most objects are electrically neutral, so the positive and negative charges will balance each other out.

42
Q

What is the best location to be safe during a lightning storm?

A

The inside of a metal car. This is because metal is a conductor, so the electric charge will pass along the exterior of the car. Lightning travels from clouds to the tallest conductors in the area. If a person is near a tall object, or is the tallest object in an area (like a grassy field), they are more likely to be struck by lightning. Being inside a building will likely shield a person from lightning even if it isn’t a conductor.

43
Q

How to add electric field vectors

A

Keep in mind that a charge further away from a test point will generate a weaker electric field that will have a smaller influence on a test point. For example, in a square, the two closest corners would only be counted for the resultant. The vectors go from tip to tail, and the resultant vector is drawn from the tail of the first drawn vector to the tip of the final drawn vector

44
Q

A metal ball is attracted to a positively charged rod. What is the charge of the ball?

A

Negative (opposites attract) or neutral. Charging by induction can cause the charges to separate in conductors, so the negative charges within the object will still be attracted to the positively charged object as they are closer than the other positive charges (Coulomb’s law). The process occurs differently in insulators because electrons don’t move freely, but they can reorient themselves within atoms.

45
Q

How are electric potential and electric potential energy affected by changes in charge?

A

Electric potential is determined by the electric field and is independent of the charge placed in the field. It is not affected by a change in charge. The electric potential energy is proportional to charge and will therefore be affected when the charge changes.

46
Q

Why is the electric field zero halfway between two identical point charges?

A

The electric field is a vector, so the field halfway between charges is the sum of the field for each charge. The fields are the same magnitude, but point in opposite directions, so the net electric field is zero. The electric potential is scalar, so the potentials of each charge will sum at the midpoint and will be a nonzero value.

47
Q

How are electric field and electric potential related?

A

Electric field is proportional to the change in electric potential. If the electric potential is constant, then the electric field must be zero.

48
Q

How much work does it take to bring 3 charges together?

A

3W. It takes W to bring two charges together. It takes 2W to bring in a third charge, since the third charge is repelled by both of the original charges. The total work is 3W when the work values are added together.

49
Q

A proton and an electron are in a constant electric field created by oppositely charged plates. The proton is released by the positive plate and the electron is released by the negative plate. Which feels the larger electric force?

A

The magnitude of the force is the same for both but in opposite directions. Force is the product of the electric field and charge. The electric field is the same and the magnitude of charge is the same between the particles.

50
Q

A proton and an electron are in a constant electric field created by oppositely charged plates. The proton is released by the positive plate and the electron is released by the negative plate. When the proton and the electron strike the opposite plate, which one has more kinetic energy?

A

Both acquire the same kinetic energy. In this situation, the increase in kinetic energy is equal to the decrease in electric potential energy, which is the product of the object’s charge and the change in electric potential
through which it passes. The change in potential for each object has the same magnitude, but they have opposite signs since they move in opposite directions in the field. The magnitude of the charge of each object is the same, but they have opposite signs. Therefore, they experience the same change in electric potential energy and experience the same final kinetic energy.

51
Q

Which factors determine capacitance?

A

The capacitance is determined by the shape of the capacitor (area of plates and separation distance) and the material between the plates (dielectric).

52
Q

Capacitance

A

The constant ratio of the charge on the plates to the potential difference between them.

53
Q

A battery establishes a voltage on a parallel plate capacitor. After the battery is disconnected, the distance between the plates is doubled without loss of charge. How does this affect the capacitance and voltage?

A

Capacitance decreases and voltage increases. When the plates were connected to the battery, a charge was established on the plates. As the battery is disconnected, this charge remains constant on the plates. The capacitance decreases as the plates are pulled apart, since the capacitance is inversely proportional to the separation distance. For the charge to remain constant with smaller capacitance, the voltage between the plates increases.

54
Q

Is electric field a vector or a scalar?

A

Vector

55
Q

How will changing the sign of a charge affect electric potential and electrical potential energy?

A

Electric potential will not be affected, but electric potential energy will change its sign

56
Q

Electric potential

A

Electric potential energy per unit charge

57
Q

Electric current

A

Any flow of charge. Current is not a vector

58
Q

Complete circuit

A

A continuous conducting path. This is the only way that a current can flow through

59
Q

Ohm’s law

A

V=IR, when R is a proportionality constant that’s independent of V. This means that the law requires the ratio of voltage to current to be constant. This is usually only true for metal conductors, so Ohm’s law is not a fundamental law of physics.

60
Q

Resistivity

A

A constant of proportionality that depends on the material used. Silver has the lowest conductivity and is the best conductor. Resistivity depends somewhat on temperature, with the resistance of a material increasing as temperature increases.

61
Q

Direct current (DC)

A

Current moves in one direction

62
Q

Alternating current (AC)

A

The current reverses direction many times per second, and is usually sinusoidal.

63
Q

Superconducting

A

At very low temperatures, the resistivity of certain metals can become zero.

64
Q

Peak voltage

A

The maximum values the voltage oscillates between in an AC current

65
Q

Peak current

A

The average current is zero because with a sinusoidal curve, the current is positive as often as it is negative.

66
Q

When connected to a battery, a lightbulb glows brightly. If the battery is reversed and reconnected to the bulb, how will the brightness be affected?

A

The brightness will be the same, regardless of which direction the current is flowing

67
Q

When a battery is connected to a lightbulb properly, current flows through the lightbulb and makes it glow. How much current flows through the battery compared to the lightbulb?

A

The same amount of current flows through both. In a loop, the current will be the same at every point in the loop.

68
Q

Electrons carry energy from a battery to a lightbulb. What happens to the electrons when they reach the lightbulb?

A

The electrons are not used up. In the lightbulb, electrons at higher potential energy enter the lightbulb and give off energy as they pass through the bulb. The number of electrons entering and exiting the bulb is the same.

69
Q

Is current or charge used by as they pass through a resistor?

A

Neither are used up. The potential energy is removed from electrons, but the overall number of electrons doesn’t change.

70
Q

The unit kilowatt-hour is a measure of

A

Energy, because it includes a unit of time it can’t represent power

71
Q

Why might a circuit breaker open if you plug too many electrical devices into a single circuit?

A

The current becomes too high. As devices are added to a circuit, the voltage doesn’t change. However, each device creates a new path for current to flow through. The circuit breaker will open if the amount of current is too much.

72
Q

Why isn’t it dangerous for a bird to land on a power line, but it is dangerous for a ladder to touch a power line?

A

Birds are good conductors, but they create a parallel loop with the wire. There is a very small voltage drop, so there is little current flow. A ladder provides a path from a high voltage area to a zero voltage area (ground), so there would be a large amount of current flow.

73
Q

Why does the light come on immediately when the light switch is turned on?

A

Electrons are located throughout the circuit, not just at the switch. When the switch is turned on, the electric potential across the circuit creates an electric field within the wire and all the electrons in the wire move. Therefore, the electrons in the wire are instantly pushed by the voltage difference.

74
Q

When is Gauss’ law used?

A

When we don’t have a point charge, like if we have a shape of a charge

Physics 2 (4 decks)

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