Electrostatics

Question 1

Electric charge

Answer 1

Describes a body related to an electric field. May be positive or negative. Bodies with like charges repel each other, while those with unlike charges attract.

Unit: Coulumb

Always linked to other bodies because it. cannot exist on it’s own.

Question 2

Most important charge carrier

Answer 2

The electron with a charge of -e

Question 3

Charge carriers

Answer 3

particles with electric charge

Question 4

Elementary charge

Answer 4

e=1,6*10^-19

Question 5

What does it mean that the electric charge is a discrete physical value?

Answer 5

It cannot assume any value. The electric charge of a body can only be the integer of the elementary charge.

Question 6

Proton

Answer 6

Has a charge of e+

Question 7

Cation

Answer 7

A positive ion formed from a neutral particle

Question 8

Anion

Answer 8

A negative ion formed from a neutral particle

Question 9

Coulomb’s law

Answer 9

The force law of electrostatic interaction, gives the force acting between two electrically charged point-like bodies. It’s an inverse square law, as is the gravitational force.

F= k* (q1*q2)/r^2

k=9*10^9 Nm/C^2

r^2 is distance

Question 10

What keeps the negative electrons around the positive atomic nucleus?

Answer 10

Couloumbs force

Question 11

Electric field

Answer 11

a model for understanding the electric interaction which helps to imagine how two bodies act on each other even if they are far away

Question 12

Field lines

Answer 12

Used to visualize the force field. The direction of a field line in a given point gives the direction of the force acting on the positive test charge

Question 13

The magnitude of a force field can be described by the…

Answer 13

density of the field lines

Question 14

Homogenous/uniform electric field

Answer 14

the positive test charge has the same strength(magnitude) and the same direction at each point.

Field lines are parallel and density is uniform.

E.G The electric field between the plates of a capacitor

Question 15

Inhomogenous electric field

Answer 15

Field lines may have different magnitude and different direction.

E.g: The electric field of a point like charge

Question 16

Electric dipole

Answer 16

two charges of equal magnitude but opposite sign (+q and -q) at a given distance. Electric field of this arrangement is inhomogeneous (dipole field).

Question 17

Electric dipole moment

Answer 17

Gives the strength of a dipole

P = q*d

Unit= coulomb*meter or debye

Question 18

Which weird molecule has an electric dipole moment?

Answer 18

H2O

electrons are not uniformly distributed due to high electronegativity of oxygen.

The electric field of the heart can be described with a dipole field.

Question 19

Electric field strength

Answer 19

E= F/q

Unit: N/C or V/m

A vector quantity

Is uniform (the same everywhere) in an homogenous electric field.

Question 20

Voltage

Answer 20

also called electric tension

The work needed to move a unit charge
Unit: Volt
U21= W/q

We assume we work against the electric field in this formula, and the work will be positive. U21 refers to two positions in electric field. Point #2 is at higher potential than #1, work is going from point 1 to point 2.

Question 21

How does voltage characterize field strength?

Answer 21

If the field strength is greater, more work must be done, and the voltage between the two points must be greater as well.

Question 22

Resting potential/voltage of a cell

Answer 22

-100mV

field points in the direction of the interior of the cell
charge moves from exterior to interior -> hence the negative sign.

Question 23

Electric potential

Answer 23

the voltage between two points is equal to the differences between their electric potentials.

Question 24

Equipotential surface

Answer 24

The set of all points which have the same electric potential value.

Electric field lines crosses equipotential surfaces at a right angle at every point of the electric field.

Question 25

Capacitor

Answer 25

An electrical component capable of storing electric charge and energy as well as creating homogenous electric field. Charging a capacitor gives one plate a positive charge and the other side a negative charge.

Question 26

Parallel plate capacitor

Answer 26

The simplest form of a capacitor. Two parallel metallic plates with vacuum or some other insulating material between them.

Question 27

Electric field strength of the capacitor

Answer 27

E=U/d Unit: V/m or N/C Electric charge and voltage is proportional to each other.

Question 28

Capacitance

Answer 28

The proportionality between the electric charge and the voltage. Unit: Farad (C/V) Measure of the charge-storing ability of a capacitor which gives the charge stored in the capacitor if its voltage is 1 V.

Question 29

What does the capacitance depend on?

Answer 29

The dimensions of the capacitor. - the greater the surface area of the plates, the more charge it can store at a given voltage -the lower the distance between the plates, the stronger the field and the more charge required to maintain a given voltage C= E0*Er*(A/d) ``` E0= absolute permittivity of vacuum, 8,85*10^-12 Er= the relative permittivity of the insulating medium between the plates ```

Question 30

Charging of capacitor

Answer 30

Both plates are neutral and one electron is moved from one plate to the other. The plates are now charged. Every electron moved the same way will require more work put in.

Question 31

The electric energy stored in the capacitor

Answer 31

The work needed to charge the capacitor which is stored as energy in the electric field of the capacitor. The more charge is transferred to the plates of the capacitor, the stronger the electric field and the greater the energy. W= 0,5*(q^2/C) = 0,5*C*U^2

Question 32

Connecting capacitors in series

Answer 32

C= C1+C2+C3+...

Question 33

Connecting capacitors in parallel

Answer 33

1/C=1/C1+1/C2+1/C3+...

Question 34

``` (PP) We double the distance between two electrons. How will the repulsive force acting between them change? A) It doubles B) It decreases by half C) It increases four times D) It decreases to its fourth ```

Answer 34

B?

Question 35

(PP) How can we obtain the resting potential of a cell? A) By measuring the electric current between two points B)By measuring the change in resistance between the two points C) By measuring the conductance between the two points D) By measuring the voltage between two points

Answer 35

D

Question 36

``` The voltage between the plates of a capacitor is increased by 100%. How does the capacitance change? A) It does not change B) Increases by 50% C) Decreases by 50% D) Increases by 100% ```

Answer 36

D

Question 37

A 20ohm resistor is connected to a 100 V voltage source. Which of the following statements is correct? A) The electric power is 2 MW B) The electric power is 0,5W C) The current flowing through it is 200 A D) The current flowing through is 5 A

Answer 37

A

Question 38

``` (PP) What is the Coulomb force acting between two electrons if the distance between them is 0,18 nm? A) 1,28 aN B) 7,11 nN C)7,11 fN D)44,4 GN ```

Answer 38

B

Question 39

(PP) The voltage between two points of the same equipotential surface... A) Is proportional to the distance between the two points B) Is inversely proportional to the square of the distance between the two points C) Is always zero D) Is a positive number

Answer 39

C

Question 40

(PP) In an electric field A) the direction of electric field lines shows the direction of electric force acting on any point charge B)...the density of electric field lines is proportional to the velocity change of the charged particle placed in the force field C)...a symmetric pattern of electric field lines indicates a homogenous field D)... the density of electric field line is uniform only if the field is inhomogeneous

Answer 40

C

Question 41

What is not true for electric charges? A) it cannot exist in itself, only connected to a particle with a mass B) it can create both attractive and repulsive forces C) it can be positive or negative D) electric charge is a quantity that does not have a least value, it can be divided indefinitely

Answer 41

D

Question 42

``` A 20 kOhm resistor is connected to a 100 V voltage source. What is its conductance? A) 0,2 S B) 5 mS C) 50 mS D) 200 S ```

Answer 42

D I = G*U