current, potential difference and resistance (topic 2)

Question 1

what is meant by direct current

Answer 1

current that is always in the same direction

Question 2

• potential difference = current × resistance
• calculate the potential difference across the battery in the circuit in the figure above

Answer 2

Question 3

• power = current × potential difference
• calculate the power output of the battery in the figure above
• pd 12

Answer 3

Question 4

Answer 4

15 JUST ADD THEM TOGETHER

Question 5

calculate the reading on the voltmeter, resistance 15

Answer 5

4.5

Question 6

replacing one of the resistors with a resistor of higher value will decrease, not change or increase the reading on the ammeter

Answer 6

decrease

Question 7

(in a simple circuit, battery cell) and why (2)

Answer 7

• y
• shows a direct current / p.d.
  or
• a battery / cell gives a direct current
  or
• a battery/cell gives a constant current/p.d.

Question 8

Answer 8

6/3 = 2

Question 9

Answer 9

equal to

Question 10

Answer 10

Question 11

Answer 11

half

Question 12

calculate the reading on voltmeter V2

Answer 12

3

Question 13

Answer 13

Question 14

describe how the student could use her circuit to investigate how the current
through a filament lamp affects its resistance (4)

Answer 14

• ammeter used to measure current
• voltmeter used to measure potential difference
• resistance of variable resistor altered to change current in circuit or change potential difference (across filament lamp)
• resistance (of filament lamp) calculated, resistance = pd/current
• resistance calculated for a large enough range of different currents that would allow a valid conclusion about the relationship to be made

Question 15

estimate the resistance of the filament lamp when a current of 0.10A passes through the lamp

Answer 15

any value between 6.3 and 6.9

Question 16

Answer 16

filament lamp
resistor at constant temperature
diode

Question 17

(6)

Answer 17

• switch on
• read both ammeter and voltmeter
• adjust variable resistor to change the current
• take further readings
• draw graph
• of V (pd) against I (current)
• take mean
• R = V / I
• allow take the gradient of the graph

Question 18

explain why the student should open the switch after each reading (2)

Answer 18

• resistor would get hot if current left on
• so its resistance would increase

Question 19

(2)

Answer 19

• 15
• 16 is nearer to that value than any other

Question 20

(3)

Answer 20

• if current is above 5 A / value of fuse
• fuse melts
• breaks circuit

Question 21

state what happens to the total resistance of the circuit and the current through the circuit when switch S is closed (2)

Answer 21

• total resistance decreases
• so current increases

Question 22

name the component in the circuit used to change the potential difference across the resistor

Answer 22

variable resistor

Question 23

Answer 23

the data is continuous

Question 24

one of the points has been identified by the student as being anomalous, what is the most likely cause for this anomalous point

Answer 24

misreading the ammeter

Question 25

does the data the student obtained support the hypothesis: 'the current through a resistor is directly proportional to the potential difference across the resistor', give a reason for your answer (1)

Answer 25

- yes, supports prediction - or (straight) line passes through the origin

Question 26

(6)

Answer 26

- read V (pd) and I (current) - use thermometer to read temperature - apply heat - read V and I at least 3 other temperatures - use thermometer to read temperature at regular intervals of temperature - determine R (resistance) from V / I - range of temperatures above 50 °C - graph of R against T

Question 27

Answer 27

- not portable - needs an electrical supply

Question 28

Answer 28

to obtain a range of p.d. values

Question 29

the resistance of the bulb increases as the potential difference across the bulb increases, why (1)

Answer 29

temperature of the bulb increases

Question 30

(6)

Answer 30

cost: - halogen are cheaper to buy - 6 halogen lamps cost the same as one LED - however LEDs last longer - need to buy more halogen lamps to last the same time as one LED - also costs more to run a halogen than LED - LED has lower maintenance cost (where many used, eg large departmental store lighting) energy: - LED works using a smaller current - LED wastes less energy - LEDs are more efficient (22% more) - LED produces less heat - LED requires smaller input (power) for same output (power)

Question 31

Answer 31

- live - react faster - live and neutral

Question 32

(4)

Answer 32

- ammeter - to measure current - variable resistor - to vary current or - switch - to stop apparatus getting hot or - fuse - to break circuit if current is too big

Question 33

a teacher said that the values of current were too high to be safe, suggest two changes that would allow lower values of current to be used in this investigation (2)

Answer 33

- use smaller mass - move mass closer to pivot - reduce gap between coil and rocker - more turns on coil

Question 34

Answer 34

Question 35

which lamp will be brighter at any potential difference, explain your answer, use the figure above to aid your explanation (2)

Answer 35

- A is brighter because it has a higher current than lamp B at any p.d. - therefore A has a higher power output than bulb B

Question 36

lamp B has the higher resistance at any potential difference, explain how the figure above shows this (2)

Answer 36

- lower current than lamp A for the same potential difference - lower gradient than lamp A

Question 37

both lamps behave like ohmic conductors through a range of values of potential difference, use the figure above to determine the range for these lamps, explain your answer (3)

Answer 37

- 0 – 2 Volts - for an ohmic conductor, current is directly proportional to potential difference - so resistance is constant

Question 38

circuit symbol for a thermistor

Answer 38

Question 39

potential difference = current × resistance - use the data given in figure 1 to calculate the resistance of the thermistor at 20 °C

Answer 39

9/0.025 = 360

Question 40

in a thermistor, the resistance decreases as the temperature increases

Answer 40

Question 41

Answer 41

gives a common understanding

Question 42

a student plans to investigate how the resistance of a light-dependent resistor (LDR) changes with light intensity, the student starts with the apparatus shown in Figure 2 but makes three changes to the apparatus, one of the changes the student makes is to replace the thermistor with an LDR, describe what other changes the student should make to the apparatus (2)

Answer 42

- replace Bunsen (and water) with a lamp - replace thermometer with light sensor

Question 43

potential difference = current × resistance

Answer 43

- divide 26 by 4 because current split into 4 = 6.5 - 230/6.5 - 35

Question 44

(electric cooker has current of 26 A) (2)

Answer 44

- current exceeds maximum safe current for a 2.5 mm^2 wire - a 2.5 mm^2 wire would melt

Question 45

what is the difference between an alternating current and a direct current (2)

Answer 45

- a.c. is constantly changing direction - d.c. flows in one direction only

Question 46

Answer 46

(but would be 4 cells)

Question 47

Answer 47

- 18 - 6 - 12

Question 48

(2)

Answer 48

- need a.c. - battery is d.c.

Question 49

the transformer is 100% efficient, calculate the output current for the transformer shown in Figure 2

Answer 49

- 18/12 = 1.5 - 1.5 x 2 = **3** (the output current is equal to the input potential difference divided by the output potential difference, and we multiply this fraction by the input current)

Question 50

(4)

Answer 50

- because the potential of the live wire is 230V - and the potential of the electrician is 0 V - so there is a large potential difference between live wire and electrician - current passes through his body

Question 51

- charge flow = current × time - power = (current)^2 × resistance

Answer 51