Circuits in Action

Question 1

define emf (electromotive force)

Answer 1

the force per unit charge that’s transferred into the circuit at the power supply, measured in volts

Question 2

define potential difference

Answer 2

the energy per unit charge transferred out of the circuit due to the resistance of components in the circuit

Question 3

what is the difference between emf and pd?

Answer 3

emf is a measure of the energy delivered to the charge carriers in the circuit, pd is a measure of energy delivered from the charge carriers out of the circuit

Question 4

define terminal pd

Answer 4

the pd across all components in a circuit external to the power supply (what’s measured when you put a voltmeter across the terminals of a power supply) - always less than emf

Question 5

define internal resistance (r)

Answer 5

the resistance of the power supply

Question 6

define load resistance (R)

Answer 6

the total resistance of all components external to the power supply

Question 7

what are the 2 equations for emf?

Answer 7

E = I(R + r)
E = V + Ir

Question 8

what are ‘lost volts’?

Answer 8

the difference between the emf and the terminal pd

Question 9

what happens to the no. of lost volts when the load resistance decreases?

Answer 9

terminal pd decreases, current in a circuit increases, the energy dissipated due to the internal resistance increases and the no. of lost volts increases

Question 10

3 step method for finding the emf of a cell experimentally?

Answer 10

• measure terminal pd with voltmeter
• measure corresponding current w/ ammeter
• vary I & V by changing load resistance with variable resistor

Question 11

how do you find emf and internal resistance from an I/V graph?

Answer 11

emf - y-intercept of the graph
r - gradient of graph

Question 12

what is the internal resistance when connecting cells/batteries in series?

Answer 12

combined internal resistance will always add to give a greater (total) internal resistance

Question 13

what is the emf when connecting cells/batteries in series?

Answer 13

the combined emf will either add or subtract depending on the direction the cells are placed in the circuit

Question 14

what happens to the total internal resistance when 2 sources of equal emf are combined in parallel?

Answer 14

the total internal resistance decreases and can be found using the rule for resistors in parallel (1/Rtot = 1/R1 + 1/R2, etc)

Question 15

which cell/battery is the effective emf when 2 are connected in parallel?

Answer 15

the effective emf = the emf of the cell with the greatest emf

Question 16

what are potential divider circuits?

Answer 16

circuits that consist o 1 or more resistor placed in series which act to split or divide the total pd across the circuit

Question 17

what are 3 uses for potential divider circuits?

Answer 17

• to alter the output voltage
• to respond to changes in temp
• to respond to changes in light intensity

Question 18

what’s a potentiometer?

Answer 18

a potential divider with a sliding contact along a single wire or resistor which allows the user to smoothly alter the output resistance

Question 19

what’s a semiconductor?

Answer 19

a material that conducts only under certain conditions
a material whose electrical properties vary depending on external conditions or by adding impurities

Question 20

what is reverse bias? (when yapping abt diodes)

Answer 20

the diodes resistance is very high therefore the current that flows is very small

Question 21

what is forward bias? (diodes)

Answer 21

the direction current is allowed to flow in a diode

Question 22

what is a thermistor?

Answer 22

semiconductor components whose resistance varies according to the ambient temperature

Question 23

how does the resistance of an NTC thermistor change as temperature increases?

Answer 23

as temp increases, resistance decreases

Question 24

how does the resistance of an PTC thermistor change as temperature increases?

Answer 24

as temp increases, so does resistance

Question 25

explain the shape of the IV characteristic graph of an NTC thermistor

Answer 25

- as more current flows, the temperature of the NTC thermistor increases - greater temperature increases the energy of the ions and releases extra delocalised electrons - therefore the resistance of the thermistor decreases due to the increase in electron number density - at higher current, the V/I ratio will be smaller, leading to an increase of the gradient - therefore resistance decreases because I/V = 1/R

Question 26

why does an increase in temperature increase the resistance of a PTC thermistor?

Answer 26

an increase in temperature causes greater vibration of lattice ions in the semi-conductor material, this makes it more difficult for electrons to pass through the structure, increasing the resistance of the thermistor

Question 27

why does an increase in temperature decrease the resistance of a NTC thermistor?

Answer 27

an increase in temperature also releases extra conduction electrons, this allows a greater current to flow, therefore decreasing the resistance of the thermistor

Question 28

what are LDRs & how do they work?

Answer 28

components whose resistance changes when exposed to different intensities of light, they do this as the absorption of photons of light allows electrons to be released from their bounding ions and become available for conduction

Question 29

what is a photon?

Answer 29

a particle of light energy

Question 30

what happens to the resistance of an LDR when light intensity increases & why does this happen?

Answer 30

- when light intensity increases, the resistance of the LDR decreases - this happens because greater light intensity delivers more energy to the LDR which releases more conduction electrons, higher light intensity and therefore increases the electron number density

Question 31

define calibrated

Answer 31

to correlate the readings of (an instrument) with known readings in order to check the instrument's accuracy

Question 32

define response time

Answer 32

the time taken for a sensor to respond to the change in outside conditions

Question 33

define sensitivity

Answer 33

the change in reading on your instrument, per unit change in outside conditions

Question 34

define resolution

Answer 34

the smallest change that an instrument can detect