Chapter 8: Charge and current

Physics AS Level - OCR Gateway A completed ♡

The current and voltage in a series circuit

Current: Same at any point

Voltage: splits up at each component

The current and voltage in a parallel circuit

Current: Splits up at each branch

Voltage: same across each branch

Definition of electric current

Rate of flow of positive charge carriers (not just electrons)
e.g. electrons are common in metals

ions are common in liquids

Unit of current

Amperes (Amps)

Symbol of current

I (capital i)

Symbol of charge

Q

Unit of charge

Coulombs (C)

Quantisation of charge

Charge comes in definite finite quantities

Positive and negative charge has a definite minimum magnitude, known as the elementary charge

Elementary charge

The definite minimum magnitude of positive and negative charge

elementary charge (e) = 1.6 × 10^-19 C

Net charge

Gain / loss of electrons by an object
Q = ± ne

Net charge on the object in coulombs

Number of electrons (either added or removed)

Elementary charge (1.6 × 10^-19 C)

Electrons in metals

Closely packed and arranged in a crystal lattice structure

• Positive metal ion - fixed

• free electrons (attracted to +ve) that carry charge and move through as electric current

Electric current in metals

The greater the rate of charge flow, the greater the electric current in the wire. A larger current can be due to:

• greater number of electrons passing a given point / second

• same number of electrons moving through faster

Conventional current vs. electron flow

Flow of positive charge from the positive to negative terminal.

Flow of negative charge from the negative to positive terminal.

Kirchhoff’s Laws

1. current flowing into a junction = current flowing out (conservation of charge)

2. the potential difference across the battery is the same as that across the rest of the circuit

Resistance rule - Kirchhoff’s second law (series)

V_T = V₁ + V₂

[USING OHMS LAW => V = IR]

I_TR_T = I₁R₁ + I₂R₂

[IN SERIES I (current) IS THE SAME]

R_T = R₁ + R₂

Resistance rule - Kirchhoff’s second law (parallel)

I_T = I₁ + I₂

[USING OHMS LAW => V = IR re-arrange to I = V]

V_T/R_T = V₁ /R₁ + V₂/R₂

[IN PARALLEL V (voltage) IS THE SAME]

1/R_T = 1/R₁ + 1/R₂

Mean drift velocity

Average velocity of charged particles as it travels through a conductor.

Number density

Number of free electrons per cubic meter of material:

• larger number = better conductor

Semi-conductors

Less free electrons - electrons need to move faster for the same current, so they heat up more.

I = Anev [on the formula sheet]

I = current (Amps)

A = cross sectional area of the conductor (m² )

n = number density (m^-3)

e = elementary charge (C)

v = drift velocity (ms^-1)

If the area is decreased, what happens to the mean drift velocity?

The electrons have to move faster to maintain the same current.