CHPT 8 - Electric resistance

what is resistance

a measure of the hindrance of the flow of electrons along a conductor, It is caused by collisions between electrons and the (metal) ions that make up a conductor

what does increasing the length of the wire do to resistance

increases the resistance

resistance equation

voltage = current x resistance

V = I x R

measure of resistance

Ohms, Ω

what is ohmes law

it relates the current through a conductor to the potential difference across it

(can also just say the equation linking them)

why do ohmic conductors obey ohm’s law

• voltage is directly proportional to current

• the resistance of the component is constant due to linear gradient

• Line of best fit passes through the origin

an example of ohmic conductors

resistors

I-V characterisitic of a fixed resistor

directly proportional

I-V characteristics Bulb

• when the temp increases the atoms inside the wire vibrate more

• the resistance increases

• current decreases

I-V Characteristics for a diode

only allows current to flow in one direction

what is a fixed resistor

• a really long wire that is wound into a coil

• resistance remain constant

what is a thermistor

• Increasing temperature, decreases resistance

• These are used to turn on/ off circuits that are temperature dependant

what is a L.D.R

• As light intensity increases the resistance decreases

• used in street lamps to turn on/ off circuits

what is a variable resistor

• As the length of the wire increases the resistance increases

• used to vary the resistance so you can vary voltage of the circuit

• used in dimming lights

what happens if a thermistor is warm

its resistance is low , cold = high

what is the most well known semi conductor

silicon

advantages of a series circuit

• It requires fewer wires

• It will have lower resistance values

disadvantages of a series circuit

• there is no independent control for each component

• if one component breaks the whole circuit breaks

advantages of a parallel circuit

• each component can be controlled independently

• Even if one component breaks the others will continue to function

disadvantages of a parallel circuit

• requires many more wires, more complicated set up

• all branches have the same voltage as a supply making it more difficult to control the voltage for each individual component