Physics OCR A-Level A Chapter 8 Charge and current

Electric Current

The rate of flow of charge

Equation for Current (when given time)

Current = Charge/Time
I = ΔQ/ Δt

1A is the same as...

one coulomb of charge passing a given point per second (1Cs^-1)

The coulomb

the electric charge flowing past a point in one second when there is an electric current of one ampere.

Charge Carrier

any particle which carries electric charge.

the elementary charge

1.60x10^-19

Quantised

Being limited to strict values, in this case multiples of e, not a continuous set of values.

Millikan's experiment

Determined the charge of an electron. Done by dropping charged oil droplets between two oppositely charged plates. The droplets experienced gravitational force, air resistance and upthrust. Some were held stationary some drifted slowly through the electric field. From this he calculated charge on electrons and that charge on the droplets was quantised.

Charge carriers in metals

electrons

Charge carriers in liquids

tend to be ions

structure of metal

a regular crystal structure or lattice of positive ions, surrounded by a number of free electrons. The metal ions are fixed in place and vibrate about a fixed point more vigorously at higher temps. Electrons are free to move and flow when the metal is given a positive and negative end.

A larger current may be due to...

A greater number of electrons moving past a given point each second
The same number of electrons moving faster through the metal.

Conventional Current

From the positive terminal to the negative.

Flow of electrons

From the negative terminal to the positive.

Ammeters

are used to measure the electric current at any part of the circuit, so is placed directly in series in the circuit at the point you want to measure the current.

An ideal ammeter will...

have the lowest possible resistance to reduce the effect they have on the current a high resistance would reduce the current it should be measuring. The ideal ammeter has zero resistance and so has no effect on the current.

Conservation of charge

electric charge can neither be created nor destroyed. The total amount of electrical charge in the universe is constant.

Kirchhoff's First Law

that for any point in an electric circuit the sum of currents into that point is equal to the sum of currents out of that point.
∑I(in) = ∑I(out)

Kirchhoff's First Law is based upon...

conservation of charge, where the charge (coulombs) is the product of the current (amperes)and the time (seconds). Charge cannot be created nor destroyed so the charge carriers entering s point in a given time must equal the total number of charge carries leaving the same point during that time.

Number density

the number of free electrons per cubic metre of material.

Semiconductors have...

a much lower number density than metals so in order to carry the same current the electrons in the semiconductors need to move faster.

Mean Drift Velocity

average distance travelled per unit time along the length of the wire

Equation for Current (given number density)

I = n q A v d

I = Anev
I - current
A- cross sectional area in M^2
n- number density
e- elementary charge
v- mean drift velocity

Assuming current remains constant, if the cross-sectional area of the wire halves, the drift velocity will.....

double

Assuming current remains constant, if the diameter of the wire doubles, the drift velocity will...

quarter

If the current halves and all other factors remain constant, the drift velocity will....

halve