10. Current, PD and Resistance

Question 1

what is the electric current?

Answer 1

Rate of flow of charge

Question 2

what is the equation to calculate current from time and charge?

give units

Answer 2

current = charge(c)/time(s)

I=q/T

Question 3

what is the definition of potential difference?

Answer 3

(voltage)
energy gained or lost by each electron (energy transferred per unit charge)

Question 4

how do you calculate potential difference from energy transferred and charge?

Answer 4

potential difference = energy transferred (J) /charge (C)
v= e/q

Question 5

what does an ammeter do?

Answer 5

measure current

Question 6

what does a voltmeter do and where should it be in a circuit?

Answer 6

• measures voltage
• should be paralell to component

Question 7

what is resistance?

Answer 7

what determines/reduces flow of current

Question 8

what happens to the current when the potential difference increases?

Answer 8

increases (electrons gain more energy, move faster)

Question 9

what happens to the current when the resistance of a circuit increases?

Answer 9

decreases

Question 10

what is the equation v=ir?

Answer 10

potential difference = current x resistance

Question 11

how do you convert from mA to A?

Answer 11

divide by 1000
there are 1000 milliamps in one amp

Question 12

what is the unit for resistance?

Answer 12

ohm

Question 13

what affects the resistance of a wire?

just list 4 things

Answer 13

• material
• length
• thickness
• temperature

Question 14

why does length affect resistance?

Answer 14

• length of wire is increased, electrons have to travel further
• chance of collisions increases
• causes resistance to increase

Question 15

why does thickness affect resistance?

Answer 15

• increasing thickness of wire increases area that electrons can flow through
• decreases chance of collisions w metal ions
• causes resistance to decrease

Question 16

why does temperature affect resistance?

Answer 16

1. ions vibrate faster
2. harder for electrons to flow
3. temp increases -> resistance increases

Question 17

what is ohms law?

Answer 17

• current is directly proportional to poterntial difference if temp is constant
• thus component that obeys ohms law has constant resistance (the gradient)

Question 18

why does the resistance of a wire increase when the current increases?

Answer 18

1. current increases -> more FREQUENT collisions of electrons w ions
2. rate of energy transer to ions increases
3. ions vibrate faster
4. resistance of wire increases

Question 19

what can a diode or LED be used to determine?

Answer 19

whether a current is flowing

Question 20

what are the similarities between resistors and bulbs?

Answer 20

1. reduce the current in a circuit
2. have some resistance
3. energy is transferred

Question 20

what are the similarities between resistors and bulbs?

Answer 20

1. reduce the current in a circuit
2. have some resistance
3. energy is transferred

Question 21

what are the differences between resistors and bulbs?

Answer 21

1. resistor has fixed resistance
2. bulb has a changing resistance
3. bulb emits light

Question 22

what are the similarities between bulbs and light emitting diodes?

Answer 22

1. emit light
2. resistance isn’t constant
3. energy transferred

Question 23

what are the differences between bulbs and light emitting diodes?

Answer 23

1. current can only flow in one direction
2. bulb -> current can flow in either direction

Question 24

what is the use of a thermistor?

Answer 24

sense diff temps and switch off an oven when it gets too hot

Question 25

why does the resistance of a thermistor drop with temperature?

Answer 25

as it gets hotter -> more electrons get excited + become available for conduction

resistance drops

Question 26

what are LDRS used for

Answer 26

sense light levels and switch on streetlamps when it gets dark

Question 27

why does the resistance decrease with the increase of light intensity?

Answer 27

as it gets brighter, more electrons get excited and become available for conduction -> R drops