4.3 Electrical Circuits

Question 1

what is Kirchoff’s Second Law?

Answer 1

Kirchoff’s Second Law states that in any closed loop. the sum of the e.m.f is equal to the sum of the products of the current and the resistance
(in other words the total potential difference is equal to the sum of all the potential differences of the components)
∑Ɛ = ∑IR

Question 2

what is Kirchoff’s First law?

Answer 2

Kirchoff’s First law states the sum of currents entering a junction in a circuit is equal to the sum of the currents leaving the junction

Question 3

what is Kirchoff’s Second Law a consequence of?

Answer 3

the conservation of energy

Question 4

what is Kirchoff’s First Law a consequence of?

Answer 4

the conservation of charge

Question 5

what is the equation for total resistance in a series circuit?

Answer 5

Rtotal = R1 + R2 + R3

opposite to spring constant and capacitors

Question 6

what is the equation for total resistance in a parallel circuit?

Answer 6

1 / Rtotal = 1 / R1 + 1 / R2 + 1 / R3 opposite to capacitors and spring constants

Question 7

how are ammeters connected in circuits and what is the resistance of them?

Answer 7

connected in series, designed to be close to zero resistance

Question 8

how are voltmeters connected in circuits and what is the resistance of them?

Answer 8

connected in parallel, designed to have close to infinite resistance so that none of the current flows through them

Question 9

what is the rule for current in series circuits?

Answer 9

stays the same

Question 10

what is the rule for current in parallel circuits?

Answer 10

splits at junctions

Question 11

what is the rule for voltage in series circuits?

Answer 11

splits, the voltage of the cell is equal to the sum of the voltages across the components (not necessarily and equal split)

Question 12

what is the rule for voltage in parallel circuits?

Answer 12

the same, each branch has the same voltage as the cell (so would split if there were more than one component on the same branch)

Question 13

what are the steps for solving circuit analysis questions using Kirchoff’s law’s?

Answer 13

1. draw a diagram
2. find the largest source of EMF
3. draw the loops of current all going from and back to the largest source of EMF
4. pick 2 loops that come from this source of EMF
5. use kirchoff’s first law and a junction to set up an equation
6. use kirchoff’s second law to form another equation
7. use ohm’s law to express voltages in terms of current
8. eliminate one of the unknowns in the equation from kirchoff’s second law using the first equation
9. solve simultaneously

Question 14

what is the rule you must remember for directions in circuits when doing analysis questions using Kirchoff’s Laws?

Answer 14

∑Ɛ = ∑IR

for Ɛ:
If the EMF is in the SAME DIRECTION as the loop (i.e loop goes from positive to negative terminals of cell) = positive value
If the EMF is in the OPPOSITE DIRECTION as the loop = negative value

for potential difference, V:
If the current is in the SAME DIRECTION as the loop = negative value
If the current is in the OPPOSITE DIRECTION as the loop = positive value

Question 15

what is internal resistance?

Answer 15

internal resistance of a source of e.m.f is the resistance to electric current of the materials inside, when current flows energy is transferred to these materials resulting in the p.d dropping

Question 16

what is the difference between the e.m.f and the terminal p.d known as?

Answer 16

the ‘lost volts’

Question 17

define terminal p.d

Answer 17

the potential difference recorded across the terminals of the cell

Question 18

what is the equation that links terminal p.d and e.m.f?

Answer 18

Ɛ - Ir = V, can also be written as…
Ɛ = V + Ir
Ɛ = I (R + r)

where Ɛ = electromotive force
Ir = lost volts
V = terminal p.d
uppercase R is used for external quantities (in this case the resistance of the circuit)
lowercase r is used for internal quantities (in this case the internal resistance)

Question 19

outline an investigation to determine internal resistance of a chemical cell/other source of e.m.f

Answer 19

• set up a circuit with a battery or cell, an ammeter in series, a variable resistor and a voltmeter in parallel
• vary the current by changing the value of the load resistance R using the variable resistor
• measure the voltage for different values of current, record your results in a table and plot a graph of voltage (y axis) against current (x axis)
• remember E = V + Ir, rearranging to get V = -rI + E, means that because its a straight line the y intercept is the EMF and the gradient = -r, multiply gradient by -1 to get r

Question 20

what is a potential divider circuit?

Answer 20

a potential divider circuit uses two resistors in series to split or divide the voltage of the supply in a chosen ratio so that a chosen voltage can be provided to another device or circuit (see page 194 for good diagrams)

Question 21

what is the potential divider equation? what are the two versions?

Answer 21

V1 / V2 = R1 / R2

Vout = R2 / R1 + R2

Question 22

what is the potential divider ratio explained in words?

Answer 22

the proportion of voltage across the second resistor is equal to the proportion of resistance in terms of the total resistance

Question 23

how can you specifically alter the output voltage in a potential divider circuit?

Answer 23

use a variable resistor as the second resistor so you can adjust the resistance ration and therefore the voltage ratio

Question 24

outline the procedure to investigate potential divider circuits using an NTC thermistor, describe how it works…

Answer 24

-an NTC thermistor has a HIGH resistance at low temperatures, but LOW resistance at high temp
-a thermistor can be used in a potential divider circuit to provide a variable output voltage to control a heater
-when the temp is low and therefore the resistance of the thermistor is high, the thermistor has a high output voltage which is enough to switch the heater on
-when the temp is high the resistance of the thermistor decreases meaning it will have a lower output voltage so the heater switches off
-when actually performing the experiment you must place a thermistor in a water bath with a bunsen burner to change the temp
WORKS LIKE A HEAT SENSOR
(note draw a normal potential divider circuit but with a thermistor at Vout)

Question 25

outline the procedure to investigate potential divider circuits using an LDR, describe how it works…

Answer 25

• an LDR has HIGH resistance in dark, but LOW resistance in light
• an LDR can be used in a potential divider circuit to provide a variable output voltage to control street lamps
• when the incident light intensity is low (dark conditions) the resistance of the LDR is high, the LDR has a high output voltage which is enough to switch the street lamp on
• when the incident light intensity is high (bright conditions) the resistance of the LDR decreases meaning it will have a lower output voltage so the street lamp switches off

WORKS LIKE A LIGHT SENSOR
(note draw a normal potential divider circuit but with an LDR at Vout)