Using a Voltmeter

Question 1

how is the potential difference between two points measured in a circuit

Answer 1

• by connecting a voltmeter between those two points

- in parallel

Question 2

what must the voltmeter take from the circuit in order to operate (like every other component)

Answer 2

some current

Question 3

if you have a voltmeter in parallel with a lamp, how would the current flowing into the voltmeter and into the lamp differ

Answer 3

• the lamp would be receiving less current than is provided in total
• as the voltmeter has taken some current (or has had some current travel down its route)
• so the current is split and both are receiving less than the total current

Question 4

how would you symbolically or algebraically write how the current has changed throughout the circuit

Answer 4

• the current before it reaches the parallel loop would be I
• when it reaches the loop, the current traveling into the voltmeter would be i
• so the current flowing through the lamp would be I - i
• becoming I once again after the split current comes together in the series part

Question 5

what is done in order to keep i, the current the voltmeter takes, as small as possible

Answer 5

the voltmeters are designed to have a very high resistance

Question 6

why does it make sense for a voltmeter to have very high resistances in order to the current it takes up to be as little as possible

Answer 6

• R = V / I
• V is remaining constant during the current split
• so if R is really high, then I must be the only thing changing
• and it would be very small
• using V = IR implies the same thing
• as if V is constant and R increases, I must decrease

Question 7

what would the typical resistance of a 20V digital voltmeter be

Answer 7

10M ohms

Question 8

what is different about analogue voltemeters

Answer 8

• they need more current in order to operate

- so they are likely to have resistances in the realm of k ohms