4.1 Charge and Current

Question 1

What is electric current? State it’s units

Answer 1

The rate of flow of charge. It is measured in Amps

Question 2

Give the symbol equation relating charge, current and time (include units)::

Answer 2

I = (change in)Q / (change in)t

Q = charge (Coulombs)
I = current (Amps)
t = time (seconds)

Question 3

In electricity, what does “e” represent? What are the units of “e”?

Answer 3

e is the elementary charge - ie. a proton has charge +e, an electron has charge -e

units are coulombs

Question 4

Give two examples of possible charge carriers

Answer 4

Electrons (in metals)

Ions (in electrolytes)

Question 5

True or false: Current flows from negative to positive

Answer 5

False.

Conventional current is the “flow of positive charge” - it is in the opposite direction to the movement of the electrons in the circuit

Question 6

How can you measure the current in a circuit?

Answer 6

You can measure the current in a circuit with an ammeter connected in series with the component

Question 7

What is Kirchhoff’s first law?

Answer 7

All of the current going into a junction is equal to the current leaving the junction

ie. charge is conserved

Question 8

What is meant by “mean drift velocity”?

Answer 8

The average velocity of the charge carriers due to the applied electric field.

It has to be an average because they’re often moving randomly in all directions

Question 9

What equation uses the drift velocity to calculate current?

Answer 9

I = Anev

I = current
A = cross-sectional area of conductor
n = number density of charge carriers
e = the elementary charge
v = mean drift velocity

Question 10

What are the units of “number density of charge carriers” in the equation relating current and drift velocity?

Answer 10

m^-3

Question 11

Put conductors, semiconductors and insulators in order from highest “n” (number density of charge carriers) to lowest

Answer 11

Conductors
Semiconductors
Insulators