Current & Circuits

Electric Current

The rate of flow of electric charge

Potential Difference (Voltage)

The work done per unit charge in moving a charge between two points in a circuit (the "push" driving current)

Resistance

The opposition to the flow of electric current

Material Effect on Resistance

Different materials have different resistivities

Length Effect on Resistance

Resistance is directly proportional to length

a longer wire has higher resistance

Cross-sectional Area Effect on Resistance

Resistance is inversely proportional to cross-sectional area

a thicker wire has lower resistance

Temperature Effect on Resistance (Metals)

In most metals

Ohm's Law

The current through a conductor is directly proportional to the voltage

V=IR

Series Circuit Current

The current is the same at all points

I_total = I_1 = I_2 = I_3...

Series Circuit Potential Difference

The total potential difference is shared between components

V_total = V_1 + V_2 + V_3...

the potential differenc across each component is dependant on it's resistance

Series Circuit Resistance

The total resistance is the sum of individual resistances

R_total = R_1 + R_2 + R_3...

Parallel Circuit Current

The total current splits between branches

I_total = I_1 + I_2 + I_3...

Parallel Circuit Potential Difference

The potential difference is the same across all components in parallel

V_total = V_1 = V_2 = V_3...

Parallel Circuit Resistance

The reciprocal of the total resistance is the sum of the reciprocals of individual resistances

1/R_total = 1/R_1 + 1/R_2 + 1/R_3...

and the total resistance is less than the smallest individual resistance

Resistor (Circuit Symbol)

A component that provides a fixed resistance

Filament Lamp (Circuit Symbol)

Provides resistance and emits light

its resistance increases with temperature (non-ohmic)

as current increase

Diode (Circuit Symbol)

Allows current to flow in practically only one direction (very high resistance in reverse)

LDR (Light Dependent Resistor) (Circuit Symbol)

A resistor whose resistance decreases as light intensity increases

Thermistor (Circuit Symbol)

A resistor whose resistance decreases as temperature increases

Energy Transferred Equation

E = IVt (Energy = Current x Voltage x Time)

Units for Energy Transferred

Joules (J) for energy

Power Definition

The rate at which energy is transferred or the rate at which work is done

Power Equation 1

P = E/t (Power = Energy / Time)

Power Equation 2

P = IV (Power = Current x Voltage)

Power Equation 3

P = I²R (Power = Current squared x Resistance)

Power Equation 4

P= V^2/R (Power=Voltage Squared / Resistance)

Power Unit

Watts (W)

AC (Alternating Current)

Current that constantly changes direction

Mains Electricity

AC supply (230V

DC (Direct Current)

Current that flows in only one direction

Battery/Cell Supply

DC

Live Wire (Color and Function)

Brown

carries the alternating potential difference (230V)

Neutral Wire (Color and Function)

Blue

completes the circuit

at 0V

Earth Wire (Color and Function)

Green and Yellow

safety wire

at 0V

Fuse Function

A safety device that melts and breaks the circuit if the current exceeds its rating

Fuse Placement

Always connected in the live wire

Circuit Breaker Function

An automatic switch that trips (turns off) when current is too high

Circuit Breaker Advantages

Faster than fuses

Double Insulation

Appliances with plastic casings and no exposed metal parts

I-V graph

Shows the relationship between the current and the potential difference

Linear component

straight line on a I-V graph

Non-linear Component

Curved line on a I-V Graph

Charge calculation

charge = current x time

Charge unit

coulombs

Standard Test Circuit

Used to investigate I-V graph for a range of componants

Ammeter placing

In series

Voltmeter placing

In parallel

Power rating

Shows the maximum safe power.

Household fuses

Protect the wires in the house.

Potential difference defintion 2

The energy transfered per coloumb of charge that passes between two points in an electric circuit.

Current definition 2

Current is the flow of electrical charge.

Conventional Current

Current flows from +ve to -ve.

Electron flow

electrons flow from -ve to +ve.