Electric Current and Power

lecture 6

current

flow of charges

flow of charges requires

electrical potential difference

in electrostatics, the electric field inside the metal is

zero, the electric potential is constant, and the charges are at rest

if a metal carries a current, the electric field is 

not zero and the potential is not constant 

a voltage source is a 

two terminal device that maintains a fixed potential difference between its terminals

battery

chemical reaction at the terminals

generator

a conducting coil moving in a magnetic field

an ideal voltage source produces

a potential difference that is independent of whatever is connected to it (independent of load resistance and output current)

the SI unit for current

ampere, amp , A

in order for a current to flow,

there must be a continuous conducting path from one terminal of a voltage source to the other terminal 

circuit

continuous conducting path from one terminal of a voltage source to the other terminal 

if the path is broken, say with a switch, the circuit is said to

be open and no current will flow

for a given voltage source, the amount of current

produced is dependent on the devices in the circuit

in general, devices can have

• capcitance

• resistance

• inductance

for a single path, any steady current must be the 

same at all location along the path 

batteries do not create

charge or current 

light bulbs do not consume

charge or current

when current flows in a metal conductor, it is

free electrons that are moving

electron (negative charge) move

from lower potential to higher potential

the direction of a current is defined to

be the direction of a POSITIVE charge would move move

current flows from

high potential to low potential (positive terminal to the negative terminal)

resistance is a measure of

how hard it is for a current to flow given a potential difference

resistance is measured in

ohms

ohm’s law states

the resistance of a device is a constant, independent of the applied potential difference and the current

ohmic devices

devices that obey ohm’s law

non-ohmic devices

devices that do not obey ohm’s law

resistance of a device depends on

its geometry and on the material it is constructed of

rho (p) is the

resistivity of the material

conductivity

a material’s intrinsic ability to conduct electricity

the inverse of conductivity is

resistivity (p)

materials with large resistivity make 

poor conductors

materials with small resistivity make

good conductors

the resistivity of most material is

temperature dependent and varies with temperature

a is the

temperature coefficient of resistivity

if (a) is positive,

resistivity increases with temperature

is (a) is negative,

resisitivity decreases with temperature

series 

one connected after the other in a chain 

parallel

side-by-side connection

a voltmeter is connected “externally”,

in parallel with the circuit element across which the voltage is to be measured 

voltmeter measures

the potential difference between two points

ammeter is used to measure

the current flowing in the circuit

ammeter must be in

series with other elements

most electrical devices transform electrical energy into

some other form of energy

the voltage source provides the 

the electrical energy by transforming some other form of energy 

in a circuit, a battery or power supply provides

the potential difference to drive a current through the circuit

the rate at which energy is moved, used, or transformed, is known as

power

heating due to currents is an energy loss mechanism and is sometimes referred to as 

ohmic losses 

household circuits are designed so that the

potential difference between the two sides of a plug is 120V

wires are heated due to

ohmic losses

wires are rated based on

how much current they can handle without getting too hot

large diameter wires have a smaller resistance and

can handle larger currents

small diameter wires have a larger resistance and

can’t handle larger currents

circuit breaker or fuses are sensitive to

the current and will break the circuit if it gets too high

junctions are

branch points were the current has a choice of paths

between auctions, along a single path, the current

is the same at all points

if there are not junctions in the circuit, the current is the

same everywhere

at junctions, the total current flowing in the junction must equal

the total current flowing out of the juction

as we move around a circuit, the voltage can go

up or down due to various circuit elements (voltage sources, resistors, capacitors)

but if we make a full loop, returning to our starting point, we must return to the same

voltage 

there are two ways of connecting 2 resistors to a voltage source

series and parallel

series circuit

one connected after the other in a chain

series: since there are no junctions between them, the current

is the same that flows through all

series: the potential difference across each of them

will be different, but will add up to the voltage source

parallel circuit 

side-by-side connection

parallel: the potential difference across each resistor

is the same as voltage source

series - current

the same for all

some circuits are combinations of resistors in series with resistors in parallel

• these can be handled step-wise

• there is an equivalent resistance for these circuits