Chapter 11~ Resistance And Resistivity

Question 1

Definition of internal resistance :

Answer 1

It is defined as the resistance of the source that results in voltage ⚡ loss.

Question 2

2 formulae for E (e.m.f) that include internal resistance

Answer 2

1. E= V + Ir
2. E = IR + Ir
   Therefore, E = I (R + r)

R- resistance of external circuit
r- internal resistance

Question 3

Definition of electrical resistance :

Answer 3

The ratio of potential difference to the current

Question 4

Definition of ohm

Answer 4

’ 1 volt per ampere’

The ohm is the resistance of a component when a potential difference of 1 volt drives a current of 1 ampere through it.

Question 5

From the definition of resistance what is the formula related to it?

Answer 5

R = V/I

Question 6

Need to know the I - V characteristics of three components, namely:

Answer 6

1. Metallic conductor at constant temperature
2. A semiconductor diode
3. A filament lamp 💡

Question 7

State ohms law :

Answer 7

A conductor obeys Ohm’s law if the current is directly proportional to the potential difference across its ends.

( Think of I-V graph ~straight line through centre means proportional)

Question 8

Definition of an ohmic component :

Answer 8

Components that obey Ohm’s law. A component who’s I-V characteristic is that current is directly proportional to p.d. Across it

Question 9

If the current is directly proportional to the p.d across the component this means that the resistance is dependent or independent of both the current and the p.d? And why?

Answer 9

Independent,

Bec the ratio V/I is constant

Question 10

The three I- V characteristics for a metallic conductor :

Answer 10

1. Line passes through the origin
2. Current is directly proportional to voltage⚡
3. The resistance is constant and equal to
   R = 1/gradient
   ( BEC the graph is I - V.. If it were V- I than R would equal the gradient)

Question 11

A commercial resistor will give the same I-V characteristics as a

Answer 11

Metallic conductor

Question 12

The three I - V characteristics of a filament lamp:

Answer 12

1. Line passes through the origin
2. For very small currents and voltages ⚡ , the graph is roughly a straight line
3. At higher voltages ⚡, the line starts to curve. This suggests that the lamp’s resistance has increased ( the ratio of V/I is larger for higher voltages than for lower voltages)
   !!! Gradient decreases as voltage increases!!!

Question 13

The fact that the I-V graph is curved shows a that the resistance of the lamp 💡 depends on…?

Answer 13

The temperature of the its filament

Question 14

What are the 2 types of resistors:

Answer 14

1. NTC (negative temperature coefficient) - resistance decreases with increased temperature (we only learn this one in AS level)
2. PTC (positive temperature coefficient) - resistance increases with increased temperature

Question 15

4 uses of thermistors (why 🔼in resistance with temperature is useful)

Answer 15

1. Water sensors in cars and ice ❄ sensors on aircraft wings
2. Baby alarms
3. Fire sensors
4. overload protection in electric razor sockets (if the razor overheats, the thermistor’s resistance increases cutting the circuit)

Question 16

What is a diode? And an example that we discuss in AS level which is a semiconductor diode

Answer 16

Any component that allows electric current in only one direction. Example : light emitting diode (LED)

Question 17

2 I-V characteristics of a semiconductor diode (LED) :

Answer 17

1. The positive & negative values of current and voltage ⚡ are included.
   This is bec, when connected one way around (positively biased), the diode conducts & has a fairly low resistance.
   Connected the other way around (negative biased), it allows a tiny current and has almost infinite resistance.
2. For positive voltages less than its threshold voltage ⚡ , the current is almost zero hence the diode has almost infinite resistance. At its threshold voltage ⚡ it starts to conduct suddenly and the resistance decreases dramatically for voltages ⚡ greater than this threshold

Question 18

Which has a greater resistance, a pure metal or impure metal? And why?

Answer 18

Impure metal bec impurity atoms obstruct the free flow of electrons

Question 19

What 2 factors affect the resistance of a metal?

Answer 19

1. The temperature

2. The presence of impurities

Question 20

Formula for resistivity p :

Answer 20

p= RA/L

p-resistivity
R-resistance
A- cross sectional area
L-length

Question 21

Why does the resistance increase with temperature?

Answer 21

Because there are more frequent collisions 💥 between the conduction e-‘s and the vibrating ions of the metal obstructing their path to flow through.

Question 22

Threshold voltage ⚡

Answer 22

The minimum forward bias voltage ⚡ across a light emitting diode (LED) when it starts to conduct and emit light.