current, potential difference and resistance (topic 2) Flashcards
what is meant by direct current
current that is always in the same direction
- potential difference = current × resistance
- calculate the potential difference across the battery in the circuit in the figure above
- power = current × potential difference
- calculate the power output of the battery in the figure above
- pd 12
15 JUST ADD THEM TOGETHER
calculate the reading on the voltmeter, resistance 15
4.5
replacing one of the resistors with a resistor of higher value will decrease, not change or increase the reading on the ammeter
decrease
(in a simple circuit, battery cell) and why (2)
- 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.
6/3 = 2
equal to
half
calculate the reading on voltmeter V2
3
describe how the student could use her circuit to investigate how the current
through a filament lamp affects its resistance (4)
- 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
estimate the resistance of the filament lamp when a current of 0.10A passes through the lamp
any value between 6.3 and 6.9
filament lamp
resistor at constant temperature
diode
(6)
- 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
explain why the student should open the switch after each reading (2)
- resistor would get hot if current left on
- so its resistance would increase
(2)
- 15
- 16 is nearer to that value than any other
(3)
- if current is above 5 A / value of fuse
- fuse melts
- breaks circuit
state what happens to the total resistance of the circuit and the current through the circuit when switch S is closed (2)
- total resistance decreases
- so current increases
name the component in the circuit used to change the potential difference across the resistor
variable resistor
the data is continuous
one of the points has been identified by the student as being anomalous, what is the most likely cause for this anomalous point
misreading the ammeter
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)
- yes, supports prediction
- or (straight) line passes through the origin
(6)
- 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
- not portable
- needs an electrical supply
to obtain a range of p.d. values
the resistance of the bulb increases as the potential difference across the bulb
increases, why (1)
temperature of the bulb increases
(6)
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)
- live
- react faster
- live and neutral
(4)
- 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
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)
- use smaller mass
- move mass closer to pivot
- reduce gap between coil and rocker
- more turns on coil
which lamp will be brighter at any potential difference, explain your answer, use the figure above to aid your explanation (2)
- 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
lamp B has the higher resistance at any potential difference, explain how the figure above shows this (2)
- lower current than lamp A for the same potential difference
- lower gradient than lamp A
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)
- 0 – 2 Volts
- for an ohmic conductor, current is directly proportional to potential difference
- so resistance is constant
circuit symbol for a thermistor
potential difference = current × resistance
- use the data given in figure 1 to calculate the resistance of the thermistor at
20 °C
9/0.025 = 360
in a thermistor, the resistance decreases as the temperature increases
gives a common understanding
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)
- replace Bunsen (and water) with a lamp
- replace thermometer with light sensor
potential difference = current × resistance
- divide 26 by 4 because current split into 4 = 6.5
- 230/6.5
- 35
(electric cooker has current of 26 A) (2)
- current exceeds maximum safe current for a 2.5 mm^2 wire
- a 2.5 mm^2 wire would melt
what is the difference between an alternating current and a direct current (2)
- a.c. is constantly changing direction
- d.c. flows in one direction only
(but would be 4 cells)
- 18 - 6
- 12
(2)
- need a.c.
- battery is d.c.
the transformer is 100% efficient, calculate the output current for the transformer shown in Figure 2
- 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)
(4)
- 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
- charge flow = current × time
- power = (current)^2 × resistance
