7- Current/Potential Difference Relationships

Question 1

Varying P.d and Current

Answer 1

• Rehostats vary current in circuits, or can be set up as potential dividers which vary P.D
• if the rheostat is set up in series it is a variable resistor
• if it is set up as the first branch of a parallel circuit, with the other branches leading directly into it, it is a potential divider

Question 2

Ohms law

Answer 2

• in metals, current and p.d are proportional as long as temperature remains constant
• Resistance =V/I, caused by collisions between electrons and conductor particles. Heat energy is often releases as electrons transfer kinetic energy
• resistance causes an increase in temperature
• Unit of resistance is Ohm
• if a conductor is ‘ohmic’ it has a constant resistance, however V/I graphs often show curves due to the increase of temperature in turn increasing resistance

Question 3

Semiconductors/diodes

Answer 3

• diodes only allow current through in one direction
• eg. LEDs only emit light when the current flows through them in one specific direction, so they’re graph only goes in the positive direction instead of crossing both positive and negative
• semi-conductors like thermistors release more electrons when they gain energy, increasing their current with heat rather than reducing it

Question 4

current/potential difference graphs

Answer 4

• for Ohmic conductors the I/V graph will be a straight line through the origin
• for filament bulbs the I/V graph will curve and start to even out (gradient decreases) as resistance increases with temperature, meaning there is less of an increase in current as p.d increases
• for a diode the line will be on the x-axis until a certain voltage, when it will suddenly allow a current to pass through and the gradient will increase
• for an NTC Thermistor the gradient will start to increase at higher p.d’s as the temperature increasing causes resistance to lower