4.1 Charge & Current

Question 1

what is electric current defined as?

Answer 1

the rate of flow of charge

Question 2

what is the formula for electric current? and what is the unit?

Answer 2

I = ΔQ / Δt

amps/amperes, A

Question 3

what is the unit of charge?

Answer 3

coulombs, C

Question 4

what is one coulomb defined as?

Answer 4

one coulomb is defined as the amount of charge that passes in 1 second when the current is 1 ampere

Question 5

what electrical component can you use in a circuit to measure the current?

Answer 5

an ammeter, measures in amps

Question 6

what is the value for elementary charge?

Answer 6

e = 1.6 x 10^-19

Question 7

what is the charge of an electron?

Answer 7

-e or -1.6 x 10^-19

Question 8

what is the charge of a proton?

Answer 8

+e or 1.6 x 10^-19

Question 9

what is the smallest unit of charge?

Answer 9

e - because the net charge of a particle or object is quantised (will always be a multiple of e)

Question 10

in metals what are the charge carriers?

Answer 10

electrons, the movement of the electrons causing currents in the metal

Question 11

what are solutions that can conduct electricity called?

Answer 11

electrolytes

Question 12

what are the charge carries in electrolytes?

Answer 12

they are positive and negative ions

electrolytes are formed when ionic compounds, such as copper sulphate, are dissolved in water

Question 13

what is conventional current and why is it different to actual current?

Answer 13

conventional current flows from the positive + terminal to the negative - terminal of the cell, which is in fact the opposite way to the actual flow of electrons

Question 14

what is Kirchhoff’s first law?

Answer 14

the sum of the currents entering a junction must be equal to the sum of currents leaving a junction, because of conservation of charge

Question 15

what is the rule behind Kirchhoff’s first law?

Answer 15

charge is always conserved

Question 16

define mean drift velocity

Answer 16

mean drift velocity is the average velocity of the charge carriers (it is much less than the electrons’ actual speed but we take an average as they move randomly in all directions)

Question 17

what is the equation that links mean drift velocity to current? and what does each part represent?

Answer 17

I = nevA
where I = the electrical current flowing through the conductor in (amps)
n = the number of free electrons per unit volume, electron DENSITY (m^-3)
e = the charge on the electron, 1.6 x 10^-19 (C)
v = the mean drift velocity of the electrons
A = the cross sectional area of the wire or conductor (m^2)

Question 18

why are metals good conductors? and what does that mean for the mean drift velocity?

Answer 18

metals contain a large number of free electrons per unit volume, high value of n in I = nevA meaning the mean drift velocity is small even for a high current

Question 19

what is an insulator and what gives insulators their properties?

Answer 19

they are poor conductors (conduct very little electricity) and this is because they have very few free electrons, in fact many insulators have values close to zero for n in I = nevA

Question 20

what is a semiconductor?

Answer 20

semiconductors, such a silicon and germanium, have values of n between those of conductors and insulators, meaning that they have intermediate conduction properties - neither very good nor very poor in terms of their conducting properties

Question 21

what is a perfect insulator?

Answer 21

an insulator with no charge carriers and would therefore conduct nothing, but these do not exist in nature (real insulators have a very small n)