Resistance

Question 1

what experiment did ohm carry out using a length of wire

Answer 1

• he placed a length of wire parallel to a voltmeter
• placed a rheostat in the circuit as a potential divider
• and gradually increased the pd from 0 to the highest value
• the pod across the wire and the current was recorded

Question 2

what did ohm discover

Answer 2

• that for metals, the current was proportional to the potential difference
• assuming the temperature remained constant

Question 3

what does ohms law then state

Answer 3

that any electrical component in which the pd across it and the current flowing through it is proportional is ohmic

Question 4

what is the name given to the components that are ohmic when an IV graph is drawn to calculate their resistance

Answer 4

an ohmic resistor / conductor

Question 5

what is the definition of resistance in a circuit

Answer 5

the opposition of an electric current flowing through a circuit

Question 6

what is then the definition of a resistor

Answer 6

any component in the circuit that slows down the flow of charge

Question 7

what is resistance caused by

Answer 7

• collisions between the flowing electrons and the lattice of positive ions present in the material
• as the electrons drift through the material

Question 8

what is the result of these collisions

Answer 8

• electrical energy (aka the kinetic energy of the electrons) is transferred to the kinetic and therefore thermal energy stores of the ions
• which is then dissipated as heat
• causing the component to heat up

Question 9

why does this inherently lead to some reduction in the current

Answer 9

because some electrical energy is lost as it is converted into other types of energy

Question 10

what equation is resistance defined by

Answer 10

• resistance = potential difference / current

- R = V / I

Question 11

what are the units for resistance

Answer 11

• ohms (greek letter for omega i think)

- and VA-1

Question 12

what is the main difference between ohms law and the definition of resistance

Answer 12

• resistance is DEFINED as R = V / I and this never changes
• but ohms law states that under certain conditions for a metal, the ratio of V / I is constant, meaning its resistance is constant

Question 13

how would you calculate the resistance from an IV characteristic graph and why

Answer 13

• change in x over change in y
• because they are plotted as I(y) against V(x)
• and R = V / I so you need to do the x divided by the y

Question 14

what does and IV graph for a filament lamp look like

Answer 14

• an elongated s shape

- passing through the origin

Question 15

what is simply happening that the two ends of the line for a filament lamp

Answer 15

• the unit of current is decreasing per unit change of pd

- or the pd is increasing per unit change of current

Question 16

why does this happen

Answer 16

• as the current increases and gets larger (either way it flows) the amount of electrons colliding with the lattice of positive ions increases
• this means that more electrical energy is being converted to the kinetic energy stores of the ions
• this causes them to vibrate more vigorously which therefore leads to the electrons having a harder time passing through them

Question 17

what does this increase in the kinetic energy of the ions and the electrons having a harder time passing through mean

Answer 17

• the flow of charge is slowed down
• meaning the current decreases
• this is what leads to the decrease in current per unit change of pd

Question 18

what correlation does this observation lead to and how

Answer 18

• an increase in the temperature of the filament leads to a decrease in the current flowing through it
• this is because the kinetic energy of the ions is mostly dissipated as thermal energy aka heat
• so this correlation can be drawn

Question 19

what does the IV graph look like for a semiconductor diode

Answer 19

• the line rests of the x(V) axis on one end of the graph
• then when it crosses to the other side (whether it be from the pos to the neg or vice versa)
• the line shoots up exponentially

Question 20

why does this occur

Answer 20

• a semiconductor only allows current to flow in one direction
• because of this, its resistance to the current flowing in the wrong / opposite direction is almost infinite
• which leads to the line resting on the x axis as the pd can change but the I is 0
• then when the direction of the current changes, it allows the current to flow through with little resistance
• leading to the current shooting up

Question 21

how can the infinite resistance of the diode in one direction be calculated using the graph

Answer 21

• when the line sits on the x axis, the change in V is whatever you want it to be
• but the change in y is pretty much 0 (horizontal line)
• if you use R = V / I, a value for V divided by basically 0 would be infinite

Question 22

what does the IV graph for a thermistor look like

Answer 22

• the inverted version of the filament lamp
• the lines get steeper as the current increases on both ends(neg and pos)
• passing through the origin

Question 23

why does a thermistor behave like this

Answer 23

• thermistors are built to have the resistance decrease as the temperature increases
• so as the temperature would normally increase as the current increases like in the filament lamp
• a decrease in its resistance means that the change in V over the change in I has to be reduced
• leading to the value of the current being increased almost exponentially