Resistance

Question 1

Resistance

Answer 1

The resistance (R) of a conductor is the ratio of the p.d. across it to the current flowing through it

• is a scalar quantity
• unit of resistance is the ohm

Question 2

The ohm

Answer 2

A conductor has a resistance of 1 ohm if the current through it is 1 ampere when the p.d. across it is 1 volt

Question 3

Measuring resistance

Answer 3

• using an ammeter and voltmeter

- using an ohmmeter

Question 4

Ohmic conductors

Answer 4

Conductors (mainly metals and some liquids) that obey ohms law

Question 5

Ohm’s law

Answer 5

For certain conductors (mainly metals), at constant temperature the current flowing through them is directly proportional to the p.d. across them

The constant of proportionality is the resistance (R) of the conductor

Question 6

Practical resistors

Answer 6

Some conductors are specially made to have a resistance of a specific value

Question 7

Variable resistors

Answer 7

One type of variable resistor is called a rheostat

Question 8

Resistors in series

Answer 8

If two or more resistors are connected in series the total resistance of the combination is equal to the sum of the resistances of each resistor.

R = R1 + R2 + R3

Question 9

Resistors in parallel

Answer 9

If resistors of resistances R1, R2 and R3 ohms are connected in parallel with each other, the resistance of the combination can be calculated from the Formula:

1/R = 1/R1 + 1/R2 + 1/R3

Question 10

Factors affecting the resistance of a conductor

Answer 10

• its temperature
• its length
• its cross-sectional area
• the material from which it is made

Question 11

Effect of increasing temperature: metallic conductors

Answer 11

The resistance of a metallic conductor increases as the temperature increases

The resistance changes linearly with temperature

Question 12

Effect of increasing temperature: insulators and semiconductors

Answer 12

The resistance of an insulator or a semiconductor decreases as the temperature increases

Question 13

The thermistor

Answer 13

‘Thermistor’ is an abbreviation for the word ‘thermal resistor’.

A thermistor is a semiconductor whose resistance decreases rapidly with increasing temperature

Question 14

The resistance of a uniform conductor: length

Answer 14

The resistance of a uniform conductor is directly proportional to its length

R is proportional to l
(For a fixed cross-sectional area A)

Question 15

The resistance of a uniform conductor: cross-sectional area

Answer 15

The resistance of a uniform conductor is inversely proportional to its cross-sectional area

R is proportional to 1/A
(For a fixed length l)

Question 16

Resistivity

Answer 16

If a conductor of length l and cross-sectional area A has a resistance R, the constant p (rho) given by:

Rho = RA/l

Is called the resistivity of the material in the conductor

• rho is called the resistivity of the material in the conductor
• the unit of resistivity is the ohm meter

Question 17

Practical uses of a Wheatstone bridge

Answer 17

• temperature control

- fail-safe device