Current & Resistance

rate at which charges move through cross section of wire or given medium; movement of electric charge

1 ampere A = 1 coulomb/sec c/s

positive or negative charges in motion

• current consisting of positive charges that goes in direction of electric field, from positive to negative
• represents the effect of the actual motion of the charge carriers (equivalent conventional current of negative is in opposite direction; same direction for positive)

net velocity of the charge carrier moving in an electric field, taking all of their collisions and zig zags into account

• potential difference is applied across conductor
• charge carriers have to be "pumped" through the circuit; work has to be done on them to maintain v
• electric field force sets the electrons in movement
• charges collide with atoms in the device and convert electrical potential energy to kinetic energy to move

they convert energy into electrical energy, this energy is supplied to charge carriers

• batteries convert chemical energy to electrical energy
• generators convert mechanical energy to electrical energy

• charges move only in one direction, potential difference is fixed
• generated by batteries and generators

• terminals of the source of the electrical potential are always changing signs, no net motion
• only generated by generators

surface perpendicular to the motion of the charges

charge flows through a closed path and returns to its starting point

rate at which work is done; rate of energy transfer

amount of heat produced when current flows through (joule heating: heating of the resistor)

max potential difference of a device

• actual potential difference between the terminals 
• less than the emf because of the internal resistance of the battery (caused by collisions between charges and atoms within battery)

opposition of the motion of charge through a conductor, quantified by the geometric characteristics of the material through which the charge is moving

1 ohm = 1v/amp

• usually resistivity and temperature increase together
• for semi-conductors, there is an inverse relationship

• ratio of a potential difference applied across a conductor to the current through the conductor is constant
• graph of current vs. potential difference is linear with a constant slope equal to the inverse of the resistance

materials that follow Ohm's Law; constant resistance over a wide range of potential differences

• higher potential difference means greater current
• directly related

shorter objects have less resistance; longer objects have greater resistance

greater area has less resistance; smaller area has greater resistance

lower temperatures have less resistance; higher temperatures have more resistance

inverse relationship; current decreases when resistance increases

• has very little resistance so more energy gets dissipated, lacks a resistor 
• usually the circuit is cut off from a capacitator so recharging does not occur; burns out quickly
• low resistance increases current, can cause circuit to overheat and catch fire

provide a single conducting path without junction; only one path for the charge to follow

connected with junctions providing separate conducting paths; some charged go one way, others go the other

graphic representation of an electric circuit with symbols representing circuit components

path through which charges can be conducted; set of electrical components connected so that they provide complete paths for charge movements

element that dissipates energy, increases the resistance and is a type of resistor (e.g. light bulb)

has a source of potential difference and electrical energy (e.g. battery) and a load (e.g. light bulb)

closed-loop path for electrons to follow, path from one battery terminal to the other is complete, switch is closed to allow steady current flow

incomplete path and no charge flow and no current, switch is open, circuit does not work

• potential difference across a load equals the terminal voltage
• conservation of energy says that the energy gained must equal the energy dissipated in a charge's trip in a circuit

1 w= 1 vxa

---| |--- longer side is positive, shorter side is negative

inversely proportional

• a single resistor can replace a group of series or parallel resistors; represents total current found by using individual resistance values
• for series circuits, add the sum of the individual resistances
• for parallel circuits, do the same with inverses

• constant throughout series circuits (conservation of charge, current is the same throughout all resistors)
• proportioned across each resistor in parallel circuits

• proportioned across each resistor in series circuit 
• constant throughout each resistor in parallel circuits (sides of bulbs are connected to common points, so voltage across is the same)

• series require all elements to conduct
• parallel do not require all elements to conduct; provides separate alternate pathways

inversely related; low equivalent resistance can be created with a group of high resistances in parallel

consist of series and parallel circuits