Electrostatics & Magnetism

Question 1

What unit measures charge, and what sign/s can it be given?

Answer 1

• coulomb (C)
• 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.

Question 2

What characteristics define charges that are attracted to each other?

Answer 2

Opposite charges

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.

Question 3

What characteristics define charges that repel each other?

Answer 3

Like charges

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.

Question 4

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

Answer 4

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

Question 5

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?

Answer 5

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.

Question 6

Define:

conductance

Answer 6

The ability of a material to transfer charge.

Conductance is generally higher in metals and lower in nonmetals.

Question 7

What property defines a good conductor?

Answer 7

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.

Question 8

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

Answer 8

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.

Question 9

Define:

insulation

Answer 9

It 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.

Question 10

What property defines a good insulator?

Answer 10

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.

Question 11

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

Answer 11

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.

Question 12

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

Answer 12

Coulomb’s law, 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)

Question 13

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

Answer 13

It indicates that the charges are the same sign, and will experience a repulsive force.

Question 14

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

Answer 14

It indicates that the charges are opposite in sign, and will experience an attractive force.

Question 15

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

Answer 15

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.

Question 16

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?

Answer 16

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.

Question 17

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

Answer 17

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.

Question 18

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

Answer 18

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.

Question 19

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

Answer 19

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.

Question 20

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

Answer 20

The charge will also move directly upwards.

Positive charges always follow the electric field.

Question 21

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

Answer 21

The charge will move directly downwards.

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

Question 22

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

Assume the neutron is initially stationary.

Answer 22

The neutron will not move at all.

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

Question 23

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

Answer 23

The proton will move toward the right.

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

Question 24

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

Answer 24

The electron will move toward the right.

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

Question 25

# Define: electric potential energy

Answer 25

It is the energy of **position for charges**, relative to each other. ## Footnote 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.

Question 26

What equation can be used to calculate electric potential energy?

Answer 26

U = Kq₁q₂ / r ## Footnote K = the Coulombic constant, measured in Nm²/C² q₁ and q₂ = charges on particles (C) r = distance between charges (m)

Question 27

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

Answer 27

Electric potential energy will be **halved**. ## Footnote U = Kq₁q₂ / r U is inversely proportional to r. Doubling r, then, must cut U in half.

Question 28

How does electric potential **differ** from electric potential energy?

Answer 28

**Electric potential** is equal to the electric potential energy of a charge divided by that charge. ## Footnote Electric potential is measured in units of volts (V).

Question 29

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

Answer 29

V = Kq / r ## Footnote 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)

Question 30

# Define: potential difference

Answer 30

Also called as **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. ## Footnote By convention, the moving charge is assumed to be a positive 1-coulomb test charge.

Question 31

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

Answer 31

Voltage must **double** as well. ## Footnote 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.

Question 32

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

Answer 32

**Equipotential lines**, as their name implies, give the positions where a test charge has the same potential with reference to a source. ## Footnote All charges produce equipotential lines that form closed, non-overlapping loops around them.

Question 33

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?

Answer 33

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

Question 34

# Define: electric dipole

Answer 34

It occurs when positive and negative charge are **separated** in an object or system. ## Footnote The simplest example of a dipole is a linear object with one positive and one negative end.

Question 35

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

Answer 35

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