8 - Charge and current

Question 1

Define Current

Answer 1

Rate of flow of charge.

Question 2

Define charge

Answer 2

A physical property, either positive or negative, measured in coulombs (C) or relative charge

Question 3

define relative charge

Answer 3

simplified measurement of electric charge of a particle or object, measured as multiples of elementary charge

Question 4

Define elementary charge

Answer 4

The charge equivalent to the charge of a proton, symbol e

1.6 x 10^-19 C

Electron has charge -e and proton has charge +e

Question 5

How many electrons in a Coulomb?

So what’s the charge of an electron?

Answer 5

1 Coulomb needs 6.25x10^18 electrons

charge of electron = 1 / 6.25x10^18 = 1.6 x 10^-19 C (absolute charge) = -1.6 x 10^-19 C

Question 6

current formula

Answer 6

I = Q / t

Question 7

What is net charge?

Answer 7

Size of charge on particular object is quantised and expressed as multiple of e

Question 8

Define quantisation

Answer 8

The availability of some quantities (eg, energy or charge) only in discrete values. 
// Charge on an object is quantised, only in integer multiples of e.

Question 9

Define coulomb

Answer 9

Unit of charge

Question 10

Formula of net charge

Answer 10

Q = +- n e

=> Q = charge
n = number of electrons
e = elementary charge

Question 11

Electric current in metals vs electrolytes

Answer 11

Current in metals is the movement of electrons

Current in electrolyte is the movement of ions

Question 12

Conventional current vs electron flow

Answer 12

Electron flow: from negative terminal to the positive terminal (attracted to positive)

Conventional current: from positive terminal towards negative terminal

Question 13

How does a larger current arise?

Answer 13

1) greater number of electrons moving past a given point each second (eg wire with greater cross sectional area)
2) same number of electrons moving faster through the metal

Question 14

How to measure current

Answer 14

Use ammeter

Placed in series -> lowest possible resistance in order to reduce the effect they have on the current. Ideal ammeter has 0 resistance

Question 15

Define Kirchhoff’s first law

Answer 15

At any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point, electrical charge is conserved

Question 16

Define conservation of charge

Answer 16

A conservation law which states that electrical charge can neither be created nor destroyed – the total charge in any interaction must be the same before and after the interaction

Question 17

Define number density

Answer 17

The number of free electrons/ charge carriers per cubic metre of a material
symbol n
unit m ^-3

Question 18

Define semiconductors

Answer 18

A material with a lower number density than a typical conductor, for example silicon

Question 19

How fast do you charge carriers move?

Answer 19

Most charge carriers, like electrons, move slowly. Free electrons repeatedly collide with the positive metal ions as they drift through the wire towards the positive terminal.

Question 20

New equation for electric current

Answer 20

I = Anev

I = Electric current in the conductor, Amperes (A)
A = Cross-sectional area of the conductor (m^2)
n = Number density
v = mean drift velocity of the charge carriers (ms ^-1)

Question 21

Effect of changing cross-sectional area

Answer 21

The narrower the wire, the greater the drift velocity must be in order for the current to be the same

Question 22

Relative number densities of conductors, semiconductors, and insulators

Answer 22

Conductors have a very high number density of the order of 10^ 28 m-³
insulators have a much lower value
semiconductors in between the two, with number density is around 10^ 17 m-³

Question 23

Define mean drift velocity

Answer 23

The average displacement or distance travelled of the electrons along the wire per second

Symbol v

Unit ms^-1