12 Electric current

Question 1

what are charge carriers?

Answer 1

charge particles which flow through a conductor

Question 2

what is current?

Answer 2

the rate of flow of charge in a wire or component

Question 3

what is the formula for current?

Answer 3

I = Q/t

Question 4

what is an insulator?

Answer 4

each electron is attached to an atom and cannot move away from the atom. no current can pass through

Question 5

what is a metallic conductor?

Answer 5

most electrons are attached to atoms but some are delocalised and are the charge carriers.

Question 6

what is a semiconductor?

Answer 6

the number of charge carriers increases with an increase of temperature so the resistance of a semiconductor decreases with temperature

Question 7

what is potential difference?

Answer 7

the work done (or energy transfer) per unit charge

Question 8

what is the formula for potential difference, work and Q?

Answer 8

V = W/Q

Question 9

what is the emf of a source?

Answer 9

the electrical energy produced per unit charge passing through the source

Question 10

what is the formula for electrical power, I and V?

Answer 10

P = IV

Question 11

what is resistance?

Answer 11

the pd across the component / the current through it

Question 12

what is the equation for resistance?

Answer 12

R = V/I

Question 13

what does ohm’s law state?

Answer 13

the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change

Question 14

what is an equivalent statement for ohm’s law?

Answer 14

the resistance of a metallic conductor under constant physical conditions is constant

Question 15

what is resistivity?

Answer 15

resistivity = RA / L

Question 16

how do you determine the resistivity of a wire?

Answer 16

• measure diameter using a micrometer at several points along the wire to give a mean. calculate A.
• measure resistance of different lengths of wire and plot graph of R against L
• resistivity is gradient x A

Question 17

what is a superconductor?

Answer 17

a material which has no resistivity at and below a critical temperature

Question 18

what are superconductors used for?

Answer 18

• make high power electromagnets that generate strong magnetic fields for devices like MRI scanners and particle accelerators
• high power computers

Question 19

what is the advantage of using a potential divider to measure the component characteristics?

Answer 19

the current and pd can be reduced to zero

Question 20

what are the IV characteristics for a wire?

Answer 20

straight line through origin
- resistance is constant

Question 21

what are the IV characteristics for a filament lamp?

Answer 21

curve with a decreasing gradient as the resistance increases as it becomes hotter

Question 22

what does a thermistor at constant temperature show on an IV graph?

Answer 22

straight line
the higher the temperature, the steeper the line

Question 23

at what pd does a diode conduct easily in the forward direction?

Answer 23

0.6 V and above

Question 24

why does the resistance of metal wire increase with temperature?

Answer 24

the positive ions vibrate more when the temperature is increased so its harder for the charge carriers to pass easily through the metal

Question 25

what does a positive temperature coefficient mean?

Answer 25

the resistance increases with temperature

Question 26

what does a negative temperature coefficient mean?

Answer 26

resistance decreases when temperature increases