Resistance

Question 1

define resistance

Answer 1

the opposition to flow of charge

Question 2

what is the equation for resistance?

Answer 2

R = V/I
- it is the ratio of PD and current I over a component

Question 3

how do measure resistance?

Answer 3

a variable power supply can be used to set PD and current to required values
OR
use a variable resistor to adjust current and PD

Question 4

what is Ohm’s Law?

Answer 4

for metals kept at a constant temperature, the current through the wire is directly proportional to the PD across it
i.e resistance is constant
-> the graph is a straight line through origin
(resistance = 1/gradient

Question 5

what is the problem with Ohm’s Law?

Answer 5

most materials don’t obey it

Question 6

why does resistance occur?

Answer 6

• when a current passes through a metal, the temperature will increase, which causes resistance R to increase
  (for a constant pd V, the current will fall with time)
• this is because the moving electrons collide frequently with the metal lattice ions, which transfers energy to them -> temperature increases
• the ions vibrate more, which makes it harder for electrons to pass through the metal lattice -> more work must be done by electrons
• this increases resistance

Question 7

how does resistance change in a semiconductor?

Answer 7

• the resistance falls with increasing temperature

Question 8

why is copper metal a good conductor?

Answer 8

it has lost an electron to form positive ions and ‘free’ electrons

Question 9

a metal wire is maintained at a constant temperature. which graph represents the relationship between dissipated power and current in wire?

Answer 9

a quadratic graph