5. Electrostatics and Magnetism

Question 1

What is the charge of an electron and a proton?

Answer 1

e=- 1.6x10^-19 C

Proton=1.6x10^-19 C

They are the same but different changes.

Question 2

Why does static charge build up in drier air?

Answer 2

Static charge buildup or static electricity is more significant in drier air because lower humidity makes it easier for charge to become and remain separated.

Google: Drier conditions tend to result in a higher risk of static electricity buildup, which can lead to electrostatic discharges. This is due to the fact that the air moisture content is a natural conductor, earthing any potential static charge

Question 3

When placed one meter apart from each other, which will experience a greater acceleration: one coulomb of electrons or one coulomb of protons.

Answer 3

The electrons will experience the greater acceleration because they are subject to the same force as the protons but have a significantly smaller mass.

Question 4

Categorize the following materials as either conductors or insulators:

• blood
  -hair copper
• glass
• iron
• sulfuric acid
• distilled water

Answer 4

• blood- conductor
• hair- insulator
• copper- conductor
• glass- - insulator
• iron- conductor
• sulfuric acid- conductor
• distilled water- insulator

Question 5

What is the net charge of an object with one coulomb of electrons and 3 moles of neutrons?

Answer 5

The net charge will be -1C

Question 6

(Equation) What is the equation for Coulomb’s Law?

Or what is the equation for the electrostatic force?

Answer 6

F_e= the electrostatic force between two charges
k= Coulomb’s constant
q₁ & q₂ = the magnitude of the two charges
r= distance between charges.

** it’s important to note that force will change by a magnitude of distance squared when they forces are separated.

Very similar to the equation for gravitational force

Question 7

What is the difference between:

Test Charge
Source Charge

Answer 7

Test Charge: The charge placed inside the electric field

Source Charge: the charge that creates the electric field.

Key Concept: Electric fields are produced by source charges (Q). When a test charge (q) is placed in an electric field (E), it will experience an electrostatic force (Fe) equal to qE

This is because the equation of E=F_e/q is the same as F=q*E

Question 8

(Equation) What is the equation for the magnitude of an electric field?

Answer 8

E= the electric field magnitude in newtons per coulomb
Fe= magnitude of the force felt by the test charge q
q= test charge
k= electrostatic constant 8.99×10⁹ (N⋅m²)/C²)
Q=is the source charge magnitude
r= distance between the charges

Key Concept: by dividing coulomb’s law by the magnitude of the test charge, we arrive at two ways of determining the magnitude of the electric field at a point in space around the source charge.

Note: the second equation does not require a test charge.

Question 9

How are electric field vectors represented?

Answer 9

Electric field vectors can be represented as field lines that radiate outward from positive source charges and ratio inward to negative source charges.

Question 10

In what direction will positive and negative test charges move?

Answer 10

Positive test charges will move in the direction of the field lines

Negative test charges will move in the direction opposite of the field lines.

Key Concept: Field lines are used to represent the electric field vectors for a charge. They point away from a positive charge and point toward a negative charge. The denser the field lines, the stronger the electric field. Note that field lines of a single charge never cross each other.

Question 11

What is the electric field midway between to negative charges in isolation?

Answer 11

The electric field would be 0 because the two charges are the same. In this case, the fields exerted by each charge at the midpoint will cancel out and there will be no electric field.

In a negative charge, they are both exerting the same force towards them

Question 12

What are the field lines for an alpha particle (He ²⁺)

Answer 12

Because they are positive, they are pointing away from the hydrogen atom.

Question 13

How do distance and charge relate to electrostatic force and electric field?

Answer 13

This distance will change the force by a factor of the distance changed squared.
If the distances reduces, the charge will increase. If the distance increases, the charge will reduce.

Note the following equation and is divided by the distance between the two forces squared.

Book: Electrostatic force is directly related to each charge and related to the distance by an inverse square relationship. Electric field is unrelated to test charge but is still related to distance by an inverse square relationship. Note that it is the source charge that creates the electric field—not the test charge—so we cannot use the equation to determine a relationship.

Question 14

What is the definition of potential energy?

Answer 14

Potential energy- stored energy that can be used to do something or make something happen.

Question 15

(Equation) What is the equation for electric potential energy?

Answer 15

U= electric potential energy

k= electrostatic constant 8.99×10⁹ (N⋅m²)/C^²

Q= the source charge magnitude= electron x (e=1.6x10^-19 C)

q= the test charge magnitude = electron x (e=1.6x10^-19)

r= radius

Question 16

What is the definition of electric potential energy?

Answer 16

Electric potential energy is the amount of work required to bring the test charge from infinitely far away to a given position in the vicinity of a source charge.

Key Concept: Electric potential energy is the work necessary to move a test charge from infinity to a point in space in an electric field surrounding a source charge.

Key Concept: The electric potential energy of a system will increase when two like charges move toward each other or when two opposite charges move farther apart.

Key Concept: The electric potential energy of a system will decrease when two opposite charges move toward each other or when two like charges move farther apart.

Question 17

How does a change in electric potential energy from -4J to -7J reflect on the stability of a system?

Answer 17

In essence, it is more stable.

Book: A decrease in potential energy indicates that the system has become more stable. Keep in mind that negative numbers with larger absolute values are more negative, and represent a decrease in value from negative numbers with smaller absolute values (that is, –4 > −7 even though |−4| < |−7|).

Question 18

Compare the relationship between electric potential energy and coulomb’s law to the relationship between gravitational potential energy and the universal law of gravitation.

Answer 18

Electrical potential energy is Coulomb’s law multiplied by distance, whereas gravitational potential energy is the universal law of gravitation multiplied by distance.

Question 19

How does electric potential energy change between two particles as the distance between them increases?

Answer 19

If both particles have the same charge, the electrical potential energy decreases as distance increases.

If the two particles have opposite charges, then the electrical potential energy increases as distance increases.

The potential energy is the energy required to bring one particle closer to the other one. If they are the same charge, they repel each other, so will require more energy to get them closer together, increasing the potential energy.

Question 20

By what factor would electric potential energy change if the magnitude of both charges were doubled and the distance between them was halved?

Answer 20

As given by the equation U=(kQq)/r the electric potential energy would increase by a factor of eight if both charges are doubled and the radius is halved.

In other words U= (2 x 2)/ (.5)= 8

Question 21

(Equation) What is the equation for the electric potential?

Answer 21

V= electric potential measured in volts (V) (1 V = 1 J/C)

U= electric potential energy

q= the test charge magnitude = electron x (e=1.6x10^-19 C)

Q= the source charge magnitude= electron x (e=1.6x10^-19 C)

k= electrostatic constant 8.99×10⁹ (N⋅m²)/C²

²

Question 22

(Equation) What are the four equations that you should know for the MCAT based off of coulomb’s law?

Answer 22

From left to right, multiply by R
From top to bottom, divide by q

Question 23

What is the potential difference (voltage)?

Answer 23

Potential difference (voltage) is the change in electric potential that accompanies the movement of a test charge from one position to another.

Potential difference is path independent and depends only on the initial and final positions of the test charge.

Question 24

What is the difference between electric potential and voltage (potential difference)?

Answer 24

Electrical potential is the ratio of a charge’s electrical potential energy to the magnitude of the charge itself.

Voltage, or potential difference, is a measure of the change in electrical potential between two points, which provides an indication of the tendency toward movement in one direction or the other.

Question 25

How will a charge that is placed at a point of zero electric potential move relative to a source charge?

Answer 25

A charge will move in such a way to minimize its potential energy. Placing a charge at a point of zero electrical potential does not indicate that there is zero potential difference, so the charge may or may not move—and if it moves, it may move toward or away from the source charge depending on the sign of the source charge and test charge.

Question 26

True or False: The units of electric potential energy and electric potential are different.

Answer 26

True. Electrical potential energy is measured in joules (J), while electrical potential and potential difference (voltage) are measured in volts (V).

Question 27

What is an electric dipole?

Answer 27

Electric dipole: results from two equal and opposite charges being separated a small distance d from each other, can be transient (as in the case of the moment-to-moment changes in electron distribution that create London dispersion forces) or permanent (as in the case of the molecular dipole of water of the carbonyl function group)

In an external electric field, an electric dipole will experience a net torque until it is aligned with the electric field vector.

An electric field will not include any translational motion in the dipole regardless of its orientation with respect to the electric field vector.

Question 28

Define the following terms:
Equipotential lines

Answer 28

Equipotential lines are the sets of points within space at which the potential difference between any two points is zero. This is best visualized as concentric spheres surrounding a source charge. (like orbits on an atom)

Question 29

What is an electric dipole?

Answer 29

Electric dipole- An electric dipole is the separation of charge within a molecule such that there is a permanent or temporary region of equal and opposite charges at a particular distance. (water molecule dipole)

Question 30

What is the voltage between two points on an equipotential line? Will this voltage cause a charge to move along the line?

Answer 30

There is no voltage between two points on an equipotential line, so there will be no acceleration along the line. However, there is a potential difference between different sets of equipotential lines, which can cause particles to move and accelerate.

Question 31

Why is the electrical potential at points along the perpendicular bisector of a dipole zero?

Dipole questions really are not on the MCAT

Answer 31

The perpendicular bisector of an electric dipole is an equipotential plane that is perpendicular to the axis of the dipole. As such, the equation is necessarily equal to 0 because cos 90° = 0.

Question 32

What is the behavior of an electric dipole when exposed to an external electric field?

Answer 32

A dipole will rotate within an external electric field such that its dipole moment aligns with the field.

Question 33

True or false: Any moving charge, whether a single electron traveling through space or a current through a conductive material, creates a magnetic field.

Answer 33

True.

The SI unit for magnetic field strength is the tesla (T)

Question 34

What is the difference between:

• diamagnetic
• paramagnetic
• ferromagnetic

Answer 34

Diamagnetic materials possess no unpaired electrons are slightly repelled by a magnet.

Paramagnetic materials possess some unpaired electrons and become weakly magnetic in an external magnetic field.

Ferromagnetic materials possess some unpaired electrons and become strongly magnetic in an external magnetic field.

Question 35

What is the Lorentz force?

Answer 35

The Lorentz force is the sum of the electrostatic and magnetic forces acting on a body.

Question 36

What are the requirements to have a:

• nonzero electric field?
• non-zero magnetic field?
• nonzero magnetic force?

Answer 36

To create an electric field, one needs a charge.

To create a magnetic field, one needs a charge that must also be moving.

To create a magnetic force, one needs an external electric field acting on a charge moving any direction except parallel or antiparallel to the external field.

Question 37

Which would experience a larger magnetic field:
an object placed five meters to the left of a current carrying wire, or an object placed at the center of a circle with a radius of five meters?

Answer 37

We need not determine the actual values of the magnetic fields in these two cases and can compare the two equations instead. The magnetic field created by the current-carrying wire is given by the magnetic field created by the loop of wire is given by and μ0, I, and r are the same in both equations. Therefore, the magnetic field at the center of the loop must be larger because the denominator in that equation does not include π.

Question 38

What is the palm rule for determining the magnetic force of a wire?

Answer 38

To answer this, you need the second palm rule: To determine the direction of the magnetic force on a moving charge, first, position your right thumb in the direction of the velocity vector. Then, put your fingers in the direction of the magnetic field lines. Your palm will point to the direction of the force vector for a positive charge, whereas the back of your hand will point in the direction of the force vector for a negative charge.

Essentiall, put your thumb in the direction of the current. Grab on to the wire with right hand. Your fingers grab onto the wire. The direction that your fingers grab (circular) is the direction of the positive current.

Question 39

(Equation) What is the equation of a magnetic field from a wire (straight wire)?

Answer 39

B= magnetic field
r= distance from the wire
µ₀= permeability of free space
I= current
π= 3.14

Notice that the straight wire contains π, but the circular wire does not.

Question 40

(Equation) What is the equation of a magnetic field for a loop wire?

Answer 40

B= magnetic field
r= distance from the wire
µ₀= permeability of free space
I= current

Note, it is the same equation, just without the pi.

This gives the magnitude of the magnetic field only at the center of the circular loop of current carrying wire with a radius r.

Question 41

(Equation) What is the equation for the magnetic force exerted on an object moving by?

Answer 41

F_b=qvBsinӨ

• F_b= the magnetic force
• q= the charge of the moving object
• v= the velocity of the moving charge
• B= magnitude of the magnetic field
• Ө= the smallest angle between the velocity vector and the magnetic field.

Question 42

(Equation) What is the equation for the magnetic force exerted on an object moving by a current-carrying wire?

Answer 42

F_b=ILBsinӨ

• F_b= the magnetic force
• I= The current
• L= length of the wire field
• B= magnitude of the magnetic field
• Ө= the smallest angle between the velocity vector and the magnetic field.