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Flashcards in Electrostatics & Magnetism Deck (35)
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1
Q

What unit measures charge, and what signs or signs can it be given?

A

The coulomb (C) is the SI unit of charge. It can have either a positive sign (for current or protons) or a negative one (for electrons).

The MCAT is primarily concerned with the movement of current, which is conventionally positive.

2
Q

What characteristics define charges that are attracted to each other?

A

Opposite charges are attracted towards one another. In other words, a positive charge is attracted to a negative one and vice versa.

The attractive force varies proportionally with the magnitude of the charges.

3
Q

What characteristics define charges that repel each other?

A

Like charges are repulsed from one another other. In other words, a positive charge will be pushed away from another positive charge; the same will happen for two negative charges.

The repulsive force varies proportionally with the magnitude of the charges.

4
Q

What must be true of the total charge in a closed system?

A

Total charge in a closed system must remain constant. This is the law of conservation of charge.

5
Q

5 protons and 4 electrons collide with sufficient energy to create 4 neutrons and 1 proton. How do the total charges before and after the collision differ?

A

Total charge is the same before the collision as it is afterwards; in both cases, it is +1.

Initially, the total charge is (+5) + (-4) = +1. After the collision, the charge is 4(0) + (+1) = +1. This fits with the law of conservation of charge.

6
Q

Define:

conductance

A

Conductance is the ability of a material to transfer charge.

Conductance is generally higher in metals and lower in nonmetals.

7
Q

What property defines a good conductor?

A

A conductor must contain many electrical charges that are relatively moveable within the medium.

Common conductors include metals with high atomic masses, such as silver, copper, and gold.

8
Q

Calcium contains three times as many moveable electric charges than iron. Which metal is a better conductor?

A

Calcium is a better conductor; specifically, it has three times the conductance of iron.

The more charges contained within a material, and the more easily these charges can move, the higher the relative conductance.

9
Q

Define:

insulation

A

Insulation is the degree to which a material resists the transfer of charge. Good insulators are poor conductors, and vice versa.

Insulation is relatively high in polymers, amorphous crystals, and ionic solids. Insulation is low in most other substances.

10
Q

What property defines a good insulator?

A

An insulator has few free electrical charges within the medium, and those charges are difficult to move.

Common insulators include glass, quartz, rubber and teflon.

11
Q

Rubber contains 10,000 times as many moveable electric charges as paraffin. Which substance is a better insulator?

A

Paraffin is a better insulator; specifically, it has a resistance that is 10,000 times that of rubber. Resistance refers to a substance’s ability to hinder charge flow.

The fewer moveable charges a material has, the greater its relative resistance, and the better it is at insulating.

12
Q

What equation gives the relationship for the force between two charged particles separated by a given distance?

A

Coulomb’s Law gives this relationship. It is defined as:

F = Kq1q2 / r2

Where:

K = the Coulombic constant, measured in Nm2/C2
q1 and q2 = charges on particles (C)
r = distance between charges (m)

13
Q

In Coulomb’s Law, what is indicated by a positive value for F?

A

A positive value of F indicates that the charges are the same sign, and will experience a repulsive force.

14
Q

In Coulomb’s Law, what is indicated by a negative value for F?

A

A negative value of F indicates that the charges are opposite in sign, and will experience an attractive force.

15
Q

The distance between two positive charges is changed from r to 2r. How is the force between the charges affected?

A

The force between the charges will decrease to 1/4 of its original value.

According to F = Kq1q2 / r2, force is inversely proportional to the square of distance. Doubling the distance will reduce the magnitude of the force to one-quarter of its original value.

16
Q

A negative and a positive charge, separated by a distance of r, are replaced by equivalent charges that are both negative. How is the force between the charges affected?

A

Force will change from negative (attractive) to positive (repulsive).

The magnitude of force will be the same, since the strength of the charges is not changed.

17
Q

A positive test charge is placed next to a stationary positive charge. What direction will the electrostatic field lines point?

A

The field lines point away from the source charge.

By convention, all positive source charges have field lines with arrows pointing outwards, since a positive test charge will be repulsed.

18
Q

A positive test charge is placed next to a stationary negative charge. What direction will the electrostatic field lines point?

A

The field lines point toward the source charge.

By convention, all negative source charges have field lines with arrows pointing inwards, since a positive test charge will be attracted.

19
Q

Describe the motion of a positive test charge that is placed exactly at the midpoint between two equal, positive stationary charges.

A

The test charge will remain immobile.

The two positive charges repel the test positive charge equally; in other words, each charge cancels the other’s force vector. There will be no net force on the test charge.

20
Q

What direction will a positive charge move when introduced into a uniform electric field pointing directly upwards?

A

The charge will also move directly upwards.

Positive charges always follow the electric field.

21
Q

What direction will a negative charge move when introduced into a uniform electric field pointing directly upwards?

A

The charge will move directly downwards.

Negative charges always move exactly opposite compared to the electric field.

22
Q

What direction will a neutron move when introduced into a uniform electric field pointing directly upwards?

Assume the neutron is initially stationary.

A

The neutron will not move at all.

An uncharged, or neutral, particle will not be accelerated in any direction by an electric field.

23
Q

What direction will a proton move when introduced into a uniform electric field pointing towards the right?

A

The proton will move toward the right.

Protons are positive, and positive charges always move in the same direction as the field lines.

24
Q

What direction will an electron move when introduced into a uniform electric field pointing towards the left?

A

The electron will move toward the right.

Electrons are negative, and negative charges move in the opposite direction compared to the field lines.

25
Q

Define:

electric potential energy

A

Electric potential energy is the energy of position for charges, relative to each other.

This is the same as the electrostatic force between charges times the distance between them. Electric potential energy may be positive or negative, depending on the sign of the charges.

26
Q

What equation can be used to calculate electric potential energy?

A

U = Kq1q2 / r

K = the Coulombic constant, measured in Nm2/C2
q1 and q2 = charges on particles (C)
r = distance between charges (m)

27
Q

If the distance between two positive charges doubles, what will happen to the electric potential energy between them?

A

Electric potential energy will be halved.

U = Kq1q2 / r
U is inversely proportional to r. Doubling r, then, must cut U in half.

28
Q

How does electric potential differ from electric potential energy?

A

Electric potential is equal to the electric potential energy of a charge divided by that charge.

Electric potential is measured in units of volts (V).

29
Q

What equation can be used to calculate electric potential, measured in volts (V)?

A

V = Kq / r

Where:

K = the Coulombic constant, measured in Nm2/C2
q = stationary source charge (C)
r = distance between the source charge and the positive test charge (m)

30
Q

Define:

potential difference

A

Potential difference, also called voltage, is the potential energy per charge that is lost or gained when a test charge is moved between two positions. Alternatively, potential difference is simply the difference in electric potential between two points.

By convention, the moving charge is assumed to be a positive 1-coulomb test charge.

31
Q

What change in voltage is necessary to hold a unit charge in position, if the source charge is suddenly doubled?

A

Voltage must double as well.

Remember, voltage (or potential difference) is simply the difference in electric potential between two points. Electric potential is calculated using the equation V = Kq / r. According to this equation, voltage and charge (of the source) are directly proportional. Doubling one thus requires the other to double as well.

32
Q

What concept describes the set of equal-energy positions where a positive test charge can be placed, when adjacent to a stationary charge?

A

Equipotential lines, as their name implies, give the positions where a test charge has the same potential with reference to a source.

All charges produce equipotential lines that form closed, non-overlapping loops around them.

33
Q

A single, stationary particle has a charge of 5 C. One test charge is placed 1 cm away, while another is placed 2cm away. Can these two positions be on the same equipotential line?

A

No. At these positions, the electrostatic potential energy must be different.

According to U = Kq1q2 / r , the potential energy is inversely proportional to the distance between the fixed charge and the test charge. Doubling the distance will halve the energy, as long as neither charge is changed. These positions , then, cannot have equal energy and cannot be equipotential.

34
Q

Define:

electric dipole

A

An electric dipole occurs when positive and negative charge are separated in an object or system.

The simplest example of a dipole is a linear object with one positive and one negative end.

35
Q

Describe the motion of a dipole placed into an electric field.

A

The dipole will rotate so that the positive end is aligned with the electric field and the negative end is exactly opposite to the field.

Assuming that the dipole is neutral overall, it will not undergo any translational motion in the field.