4.1 Charge and Current

module 4

Define electric current.

Rate of flow of charge.

I = Q/t

Unit: ampere, A

Define the coulomb.

Charge passing a point when current of 1 A flows for 1 s.

1 C = 1 A s

What is elementary charge?

Smallest unit of charge.

e = 1.60 x 10^-19 C

electron = -e

proton = +e

What does quantised charge mean?

Charge only exists in whole-number multiples of e.

Q = ne

n = number of electrons/protons added or removed

How do you find number of electrons from current and time?

Use:

Q = It

Q = Ne

So:

N = It/e

What carries current in metals?

Free electrons.

Electron flow:

negative to positive

Conventional current:

positive to negative

What carries current in electrolytes?

Ions.

Positive ions move to cathode.

Negative ions move to anode.

Define conventional current.

Direction of flow of positive charge.

It goes from positive terminal to negative terminal.

Opposite to electron flow in metals.

State Kirchhoff's first law.

Current entering a junction equals current leaving.

Physics link:

conservation of charge

When do you use Kirchhoff's first law?

Use at junctions.

current in = current out

Useful for parallel branches.

Define mean drift velocity.

Average velocity of charge carriers along a conductor.

Electrons still move randomly, but have a net drift.

State the drift velocity equation.

I = Anev

I = current

A = cross-sectional area

n = number density

e = elementary charge

v = mean drift velocity

When do you use I = Anev?

Use when a question includes:

current

number density

wire area/diameter

drift velocity

How do you find drift velocity?

Rearrange:

I = Anev

v = I / Ane

How do you find cross-sectional area of a wire?

If radius is given:

A = pi r^2

If diameter is given:

r = d/2

then:

A = pi r^2

Why is drift velocity small in metals?

Metals have very large number density n.

From:

I = Anev

large n means small v for same current.

How does number density affect drift velocity?

From:

I = Anev

For same I and A:

v is inversely proportional to n

Lower n means higher v.

Compare conductors, semiconductors and insulators using n.

Conductors:

large n

Semiconductors:

medium n, changes with temperature/light

Insulators:

very small

Semiconductors

A material that has the ability to change its number of charge carriers, and so its ability to conduct electricity. Light dependent resistors and thermistors are both examples.