Electric charge and current

Question 1

What does ‘quantised’ mean?

Answer 1

Fundamental particles have charges of fixed value

Question 2

What is the elementary charge?

Answer 2

1.6 x 10^-19

Question 3

Unit for charge

Answer 3

Coulomb

Question 4

Unit of current

Answer 4

Ampere

Question 5

How does current in metals behave?

Answer 5

Temperature increases = frequency of vibrations increase = electrical resistance increases

Question 6

What direction does conventional current move in?

Answer 6

From + to -

Question 7

What direction do electrons flow in?

Answer 7

From - to +

Question 8

How does current behave in electrolytes?
use the example of sodium chloride

Answer 8

• Electrolytes are molten ionic compounds in which ions are free to move
• In sodium chloride, common salt dissolved in water separates into Na+ and Cl- ions.
• Na+ cations attracted to -cathode, Cl- anions are attracted to +anode
• On reaching anode, Cl- releases an electron that flows through the metal part of the circuit
• In the cathode, electrons combine with the Na+ ions

Question 9

How do you measure current?
Where should the component be placed?

Answer 9

• An ammeter
• In series, because it has low resistance so it would create a short circuit

Question 10

What is conservation of charge?

Answer 10

Electric charge can neither be created nor destroyed. In any closed system, the total amount of charge is constant

Question 11

What is Kirchhoff’s 1st Law?

Answer 11

the total of the currents entering a point must equal the total of the currents leaving the point

Question 12

How do you increase current?

Answer 12

• Increase the number of charge carriers passing the point (e.g. using a wire with a bigger cross-sectional area)
• Increase the speed of the charge carriers so more pass every second

Question 13

What is number density?

Answer 13

The number of free electrons per unit volume

Question 14

what is the equation for number of electrons?

Answer 14

n = q/e

n - number of electrons
q = charge
e = elementary charge

Question 15

how does current behave in metals?

Answer 15

• electrons in the outer shell of the metal atom become free of the atom and move (delocalised electrons)
• the remaining positive ions are fixed in the metal lattice
  -> they vibrate around fixed
  points; as temp. increases,
  vibrations increase
• introducing a positive and negative charge at either end of the wire causes the conduction electrons to move towards the positive charge (electric current)

Question 16

what is electrical resistance caused by?

Answer 16

the free electrons colliding with the metal ions, which makes it harder for the electrons to move
- resistance increases with temperature in metals

Question 17

what are the similarities between electric current in a metal wire and in an ionic solution?

Answer 17

similarities:
both are examples of flows of charge

differences:
in metals, charge carriers are negative
in ionic solutions, charge carriers can be both positive or negative

in metals, charge carriers are electrons
in ionic solutions, charge carriers are ions

Question 18

why the net charge on a particle or an object is quantised and a multiple of e?

Answer 18

The net charge of a particle is due to the gain or loss of electrons

Question 19

what is mean drift velocity?

Answer 19

average velocity attained by charged particles

Question 20

what is number density?

Answer 20

number of free electrons per unit volume (m-3)

Question 21

how does number density impact conductivity?

Answer 21

higher number density = better conductor

Question 22

what is the number density for a conductor?

Answer 22

approx. 10^28m-3

Question 23

what is the number density for a semiconductor?

Answer 23

approx. 10^16 - 10^18m-3

Question 24

what is the number density for an insulator?

Answer 24

near zero

Question 25

why do semiconductors get hot?

Answer 25

the electrons are moving very fast and they bump into lattice ions (faster than in a conductor because semiconductors and conductors can carry the same current, which suggests SC's electrons move faster)

Question 26

what is the relationship between temperature and conductivity in semiconductors? why?

Answer 26

increasing temperature = increasing conductivity -> because as the SC heats up, more energy is given to the valence electrons which allows them to jump the conduction band and contribute to electric current

Question 27

show that the equation I = Anev is homogenous with base units

Answer 27

I = Anev A = [m^2] x [m^-3] x [C] x [ms^-1] A = [m^3] x [m^-3] x [C] x [s^-1] A = [C] x [s^-1] A = A yes, it is homogenous

Question 28

why do the lights come on quickly?

Answer 28

all free electrons respond when a potential difference is applied between the ends of a wire due to the electric field of the wire - they move en masse due to the electric field

Question 29

what is the relationship between cross-sectional area and mean drift velocity?

Answer 29

as cross-sectional area decreases, mean drift velocity increases