chapter 10 - electrical circuits

Question 1

mr duddles circuit top tips

Answer 1

• write everything on the diagram
• apply V= IR
• for series - current is same- emf splits (largest R gets most) - overall R ^ (Rt = R1 + R2)
• for parallel- current splits - voltage is the same - overall R decreases (1/Rt = 1/R1 + 1/R2)
• dudds law - Rt = Vt/ It

Question 2

internal resistance

Answer 2

the resistance of the battery due to the materials its made from and causes energy to be wasted in the form of heat (results in lost V)

Question 3

3 circuits - ammeter lamp battery voltmeter
1 - wire not connected
2 - all connected properly
3 -another wire connected across the lamp

Answer 3

1 - if no current flows voltmeter shows emf of battery
2 - when current flows you lose some of the emf to the internal resistance of the battery
3 - if a component is short circuited high I flows through the low R wire not the component so we lose more emf when high I flows

Question 4

in series emf = (in terms of internal resistance)

Answer 4

emf = output voltage (terminal pd) + lost volts
E = V + Ir

Question 5

experiment to measure r

Answer 5

• set up circuit with battery voltmeter ammeter and variable resistor
• plot I and V(out)
• y intercept = emf
• gradient = r
  V = Ir + E
  y = mx + c

Question 6

internal resistance of a car battery

Answer 6

I = E/ (R+r)
starter motor has low R
E is about 15V
starter motor requires a large I
r needs to be low to supply the high I

Question 7

internal resistance of a school power pack

Answer 7

I = E/ (R+r)
E is up to 12V
r I large (+circuit breaker) to prevent dangerously high I
eg is some idiot connects a low R wire causing it to short circuit

Question 8

max P out

Answer 8

max P out from a cell is when the load R (R of a component) = internal resistance
R = r

Question 9

E in series and parallel

Answer 9

series
- total E = E1 + E2…
- if connected backwards subtract emf
parallel
- total E = E

Question 10

r in series and parallel

Answer 10

series
- total r = r1 + r2…
parallel
- total r = r/2 (if 2 identical)
- 1/rt = 1/r1 + 1/r2…

Question 11

efficiency

Answer 11

proportion of useful energy/ power out from the total energy/power in

Question 12

efficiency equations

Answer 12

%efficiency = useful Eout/ total Ein *100
% efficiency = useful Pout/ total Pin *100

Question 13

useful Pout =

Answer 13

Pin - Pwasted

Question 14

cells get hot due to

Answer 14

power losses in r

Question 15

battery charger

Answer 15

• when a battery goes flat emf reduces over time
• when you connect a charger with a higher emf to the batter - this forces current into the battery to charge is

Question 16

potential dividers

Answer 16

• change the output voltage
• if you have a fixed power supply eg a battery not every component wants that voltage

Question 17

3 ways to calculate V out

Answer 17

1 - find I = Vt/R1+R2
- then Vout = IR2
2 - use ratios
- R1/R2 = V1/V2
3 - use potential dividers eq
- Vout = Vt ( R2/(R1 +R2))
- assumes voltmeter has infinite resistance

Question 18

why is Vout reduced in potential dividers

Answer 18

• parallel combinations of the resistor and voltmeter has less R
• supply V splits in the same ratio as the R
• ∴ the parallel combination gets a smaller share of the 12V

Question 19

potentiometer

Answer 19

• varies voltage
  + vary output V from a fixed supply (eg battery) from 0 - maxV
  (see diagrams)

Question 20

rheostat

Answer 20

• can be connected as a variable resistor - good if a large I needs to be varied
• or can be connected as a potentiometer to vary V is currents are very small

Question 21

sensor circuits

Answer 21

• can use an LDR or thermistor in the potential divider
• to supply a particular value of V to heating/air con/ lighting
• in response to environmental change
  (we use the potential divider to control another circuit)

Question 22

why a sensor circuit can turn on heating when temp gets too low

Answer 22

• the R of thermistor increases when cold
• since supply V splits in the same ratio as R the thermistor gets a bigger share
• enough to turn on heating