Physics Electrics

Series:                                             Parallel:

Is = I = I                                            Is = I + I
Vs = V + V                                        Vs = V = V
Rs = R + R                                        1/Rs = 1/R + 1/R

Electrical current is a flow of charge  Current (A) = Charge (C) / time (s)  I=Q/t

Energy = charge x potential difference   E=QV

Electrical power (W) = potential difference (V) x current (A)   P=VI
   if   V=IR  so  P = I^2 R    OR     if   I=V/R  so   P = V^2 / R

E = I ( R + r )                    I = n x A x e x V  if    A (area) = pie x r^2

Resistance arises due to collisions       Electrical current flows easily              Heat rises with Resistance                     through copper but less easily          through nichrome due to more energy needed to pass electrons through

In conducting metals, when heated, the ions vibrate more, thus there are more collisions. In these metals the ions are positive and so the collisions cause electrons to loose energy. (positive temperature coefficient)

In a semi-conductor, the resistance decreases as temperature increases because as energgy increases in the material, more electrons are released and so more electrons are available. (negitive temperature coefficient)

Semi conductors have two stages: Valance (no conduction, electrons like to be here) and Conduction (conduction starts suddenly) you encourage electrons here by giving them more energy.

The resistance of a material of uniform cross-sectional area is found to be:

• Proportional to length (l)  
• Inversly proportional to it's cross-sectional area (A)

Resistance = ( resistivity x length ) / Area
Or resistivity (ohmic metres) = R x A / l

The EMF of a battery is the maximum energy per coloumb of charge

If E = I ( R + r ) with r being the internal resistance of the battery
E = IR + Ir  or  E = Ir + V  in the form of y = mx + c  the grdient of an energy against current graph gives the internal resistance

Resistors are ohmic conductors, due to the straight diagonal through the origin on a current against voltage graph resistance remains constant

non-ohmic conductors such as filimant lamps have increasing resistance as potential difference increases

Semiconductor diodes have very large resistance but as pd increases at a point, the resistance decreases rapidly

Thermistors and light-dependant resistors decrease in resistance as their dependant factor increases.

"Electric charge is conserved"             All the charge at a point equals zero so all that arrives at the point must also leave it

"Energy is conserved"                           Voltage of suply=sum of voltages across all components in a series circuit

Potential deviders are an application of Kirchhoff's second law and ohm's law
V across a resistor = total voltage in circuit x (R1 / R1 + R2)

Potenial deviders split a circuit's voltage across two sides of a total resistor. Increasing resistance of one side will decrease pd across the other side.
A thermistor connected to a resistor in series connected to a voltmetre works as a light metre. Increasing the resistance of the thermistor (i.e decreasing light intensity) will mean lower voltage reading on voltmetre.