MCAT Electric Charge and Force

Question 1

Elementary charge

Answer 1

Protons and electrons which are smallest units of charge

Proton: +1q = +1C/ ( 6.24x 10^18) = 1.6 x 10^19

Electron: -q= -1C/ (6.24x10^18) = -1.6x10^19

Question 2

How many electrons and protons are in neutral atom?

Answer 2

6.24 x10^18 for both electron and protons.

Keep in mind you’re talking about how many protons and electrons so it won’t be negative.

Question 3

How can we find the charge of an object?

Answer 3

Q = n * qe
Where Q is the charge of the object.

n is some integer of the elementary charge

qe is the charge

Question 4

Law of conservation of charge

Answer 4

Law that states that charge can only be transferred not created or destroyed. Electrons are the unit of charge transferred as protons are trapped in the nucleus.

Potential energy of electric charge = qEd
and KE = 1/2mv^2

qEd = 1/2mv^2

Question 5

What happens when we rub two objects together

Answer 5

There is a flow of charge with one object gaining an excess of electrons.

We can find the number of electrons transferred by calculating n via the following equation: Q= n* qe

Question 6

Electrostatic force

Answer 6

The force exerted by charged objects that acts on other charged objects.

It’s a vector having both magnitude and direction.

States that the electrostatic force is proportional to the magnitude of charge but inversely proportional to the square of the radius.

When Fe is positive it’s a repulsive force.
When Fe is negative it’s an attractive force.

Question 7

Electric field

Answer 7

A sphere of influence created by a charged object that exerts a force on another charged object that enters it. Either an attractive force or a repelling force.

E = Fe( electric force)/ d distance

Si unit us Newtons/ Coulomb

Electric field and electric force are vector quantities. When in the direction it’s an attractive force when they’re in the opposite direction it’s an repelling force.

Question 8

Describe the field lines for charged objects

Answer 8

Positive charges field lines point away and towards negative charge.

We can determine the strength of the electric field via the spacing between the vector lines. If there is less spacing then the electric field is strong if there is more spacing then the electric field is weak.

Question 9

What happens when we have oppositely charged plates?

Answer 9

It creates a uniform electric field. We can determine The acceleration that happens to charged objects that’s placed in this field by calculating a from F=ma.

F is the electric force Fe.

Question 10

Voltage

Answer 10

Electric potential difference. Electric potential energy per unit charge.

V = electric potential energy/ q

SI unit: volts

Question 11

Current, describe the movement of current

Answer 11

The movement of charge

SI unit: Ampere which is equivalent to C/s.

Current goes in the direction that a positive charge will take in the electric field but since electrons only move electrons move against current.

Current (positive charge) moves from high voltage to low voltage whereas negative charges move from low voltage to high voltage.

Question 12

Conductivity v. Resistivity

Answer 12

Conductivity is the ease by which electrons flow through a material.

Resistivity is the resistance that a material has against the flow of electrons.

Conductivity and resistivity are inversely proportional to each other.

Question 13

Conductors v. Insulators

Answer 13

Conductors are materials that allow flow of charge because their electrons are spread out over the material.

Insulators is material that impede the flow of charge because their electrons don’t spread out.

Question 14

What determines the resistance of a material?

Answer 14

Proportional to the length and resistivity constant but inversely proportional to the area.

SI unit is in ohm

Question 15

Describe how current is created in a circuit?

Answer 15

A battery applies a voltage by attaching the positive end ( high voltage) to the negative end ( low voltage).

This allows current to flow from high voltage to low voltage.

Question 16

Describe the circuit elements
- Battery
- Resistor

Answer 16

The battery is represented as a long line which represents the positive charge (high voltage end) and a shorter line which represents the negative charge (low voltage).

Resistor is represented as jagged lines and serves to impede the flow of charge.

Question 17

How is the circulatory system analogous to a circuit?

Answer 17

• The heart acts like a battery creating a pressure differential that acts like voltage.
• Blood acts like current as it flows in response to the pressure differential created.
• Vascular resistance acts like resistance to the flow of blood.

Question 18

Ohm’s law

Answer 18

States that the current a circuit is equal to the voltage applied to the resistance.

V = IR

Question 19

How to find the electric potential energy of a circuit?

Answer 19

E = Q* V
Where Q is charge and V is voltage.

Question 20

How to find the power of a circuit

Answer 20

Power is the change in energy per time.

P = IV= V^2/R=I^2R

Question 21

What happens to the total voltage supplied by the battery?

Answer 21

Because of the internal resistance of the circuit the some of the voltage supplied by the battery is dissipated.
We can calculate the actual potential difference via:

Vterm = emf-IR

Question 22

Series circuit v. Parallel circuit

Answer 22

• Series circuit is a circuit in which circuit element is parallel so there is a linear flow of charge.
• Parallel circuit is a circuit in which the circuit elements branch off creating split flow of charge.

Question 23

Kirchoff junction rule

Answer 23

The current that enters a junction equals the current that leaves a junction.

Question 24

Kirchoff loop rule

Answer 24

The total voltage in a circuit equals zero. Follows the law of conservation of energy.

Question 25

How do we find the current, voltage, and resistance for a series circuit?

Answer 25

Current is constant across all resistors while the sum of the voltages equals the total voltage of the circuit.

To find the resistance you add the resistances of each resistor so that the total resistance will be greater than the individual resistances.

Question 26

How to find the current, voltage, and resistance for a parallel circuit?

Answer 26

The current is additive across resistors while the voltage equals the total voltage.

To find the resistance you add the inverse of each resistance. Because of this the total resistance is always less than each individual resistance.

Question 27

• Ammeters
• Ohmeters
• Voltmeters

Answer 27

• Ammeters - measures current. Must be added to circuit in series with low resistance.
• Ohmeters- measure resistance.
• Voltmeters- measure voltage. Must be added in parallel and have a high voltage.

Question 28

Capacitors

Answer 28

Devices that store electrical energy by separating charge.
C= Q/V, where Q is charge and V is voltage.

SI unit for capacitance is the farad.

Question 29

Parallel plate capacitors

Answer 29

A battery supplies charge to the plates and they flow between the plates, being stored by them

The flow of charge happens until the capacitor voltage is equal to the battery voltage.

Question 30

Describe how electric potential energy for capacitors

Answer 30

U = 1/2CV^2
or
U= 1/2QV
or
U=1/2Q^2/C

Question 31

How do we calculate the capacitance of parallel capacitors?

Answer 31

We add the capacitances so that the equivalence capacitance is equal to the individual capacitances.

Question 32

How do we calculate the capacitance of series capacitors?

Answer 32

We add the inverse of each of the capacitors resulting in a equivalent capacitance that’s less then each capacitance.