4.2.1.4 Resistors

Question 1

How does resistance work?

Answer 1

• electric current is the flow of charge (which is made up of electrons)
• as the electrons move through a conductor they collide with metal ions in the metal
• electrical energy carried by the electrons is transferred into other forms e.g. thermal

Question 2

What does resistance tell us?

Answer 2

• resistance tells us the potential difference required to drive a current through a component
• the higher the resistance the more energy that is transferred as the current moves through the component

Question 3

Ohm’s law:

Answer 3

• the current flowing through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across it
• this means the resistance remains constant as the current and potential difference change
• at a constant temperature resistance is a constant

Question 4

Example of an ohmic conductor:

Answer 4

resistor

Question 5

Graph for current and potential difference of ohmic conductors:

Answer 5

Question 6

For what components is resistance not constant?

Answer 6

• the resistance of components e.g. lamps, diodes, thermistors and LDRs is not constant
• changes with the current through the component

Question 7

What is the resistance of a filament lamp like?

Answer 7

• the resistance of a filament lamp increases as temperature increases
  
  • metal ions in wire move faster at a higher temperature making it harder for electrons to flow through wire - more E of electrons transferred to thermal E
• if resistance increases graph becomes non-ohmic

Question 8

Graph for current and potential difference of a filament lamp:

Answer 8

Question 9

Circuit which measures current and potential difference of diode:

Answer 9

Question 10

What is the resistance of a diode like?

Answer 10

• the current through a diode flows in only one direction
• the diode has a very high resistance in the reverse direction

Question 11

Graph of current and potential difference in a diode:

Answer 11

Question 12

What is the resistance through a thermistor like?

Answer 12

the resistance of a thermistor decreases as temperature increases

Question 13

Applications of thermistors in circuits:

Answer 13

• thermistors are sensitive to temperature change
• measure the temperature of a device
• thermostat - switches the heating system in a house on or off as necessary

Question 14

Graph to show relationship between resistance and temperature for a thermistor:

Answer 14

Question 15

Graph of current and potential difference across a thermistor:

Answer 15

temperature is constant

Question 16

What is the resistance across an LDR like?

Answer 16

resistance of an LDR decreases as light intensity increases

Question 17

Applications of LDR’s in circuits:

Answer 17

• switching lights on when it gets dark
• alarm clocks, street lights, light intensity meters, and burglar alarm circuits

Question 18

Graph to show relationship between resistance and light intensity for an LDR:

Answer 18

Question 19

Graph of current and potential difference across an LDR:

Answer 19

Question 20

How may a thermostat be helpful in a household?

Answer 20

• look for trends in energy consumption
• reduce energy consumption
• reduce bills