Electric Circuits

Electric current

Rate of flow of electric charge

Potential difference

Energy transferred per unit charge between two points in a circuit

Resistance

Measure of how difficult it is for charge carriers to pass through a component

Ohm's law

For an ohmic conductor, current is directly proportional to the potential difference across it, given temperature is constant

Principle of charge conservation

The total electric charge in a closed system does not change

Kirchoff's first law

Total current flowing into a junction is equal to the total current flowing out of that junction

Distribution of current in a series circuit

Current is the same everywhere

Distribution of current in a parallel circuit

The sum of currents in each parallel set of branches is equal to the total current

Principle of conservation of energy

Energy cannot be created or destroyed, only transferred from one form to another

Kirchoff's second law

The sum of all the voltages in a series circuit is equal to the battery voltage

Distribution of potential differences in a series circuit

The total sum of the voltages across all elements is equal to the supply p.d

Distribution of potential differences in a parallel circuit

The p.d across each branch is the same

Distribution of resistance in a series circuit

Rₜ = R₁ + R₂ + R₃ + ...

Derivation of resistance in series

V = V₁ + V₂ + V₃

V = IR₁ + IR₂ + IR₃

V = I(R₁ + R₂ + R₃)

R = R₁ + R₂ + R₃

Distribution of resistance in a parallel circuit

1/R = 1/R₁ + 1/R₂ = 1/R₃ + ...

Derivation of resistance in parallel

I = I₁ + I₂ + I₃

I = V/R₁ + V/R₂ + V/R₃

I = V(1/R₁ + 1/R₂ + 1/R₃)

1/R = 1/R₁ + 1/R₂ + 1/R₃

Power

Rate of transfer of energy

Current-voltage graph of an ohmic conductor

Current-voltage graph of a diode

Current-voltage graph of a filament bulb

Current-voltage graph of a (Negative Temperature Coefficient) thermistor

Resistivity

A measure of how easily a material conducts electricity

What are the variables in the equation ρ = RA/l?

ρ: Resistivity

R: Resistance

A: Cross-sectional area

l: Length

What are the variables in the equation I = nAve?

I: Current

n: Charge carrier density (number of electrons per unit volume)

A: Cross-sectional area

v: mean drift velocity

e: electron charge

How does length of a wire affect p.d?

Because R = ρl/A, as length increases, resistance increases. Using Ohm's law (V = IR), as resistance increases, potential difference also increases

Potential divider circuits

A circuit with several resistors in series connected across a voltage source used to produce a fraction of the source p.d

Variable potential divider circuits

A potential divider circuits where one resistor is a variable resistor, meaning you can vary the potential difference output

Electromotive force

The energy transferred by a cell per coulomb of charge that passes through it

Internal resistance

Energy lost due to electrons colliding with atoms inside the battery

Terminal potential difference

The p.d across the resistance R

Lost volts

The p.d across the resistance r

Lattice structure

Provides a medium for vibration of the atoms about their equilibrium position. As temperature of the solid increases, intensity of the vibration of atoms also increases

How does lattice vibrations affect resistance?

The more intense the vibrations, electrons are more likely to collide with the atoms, causing them to slow down. This in turn increases the resistance of the material

Temperature increase in a semiconductor

As temperature increases, its atoms gain energy and once they gain enough energy they begin to release electrons. This increases the number of charge carriers available which decreases resistance

Negative temperature coefficient thermistors

As temperature increases, resistance decreases

Metallic conductors

As temperature increases, resistance increases

Light-dependant resistors

Made from photoconductive materials. Because of the photoelectric effect, as light intensity increases, electrons are released and resistance decreases