Download Potential Dividers (Revision)

Potential Dividers (Revision)
Notes p.8
There are a number of ways to deal with potential
dividers.
Easy Ratios
(do this in your head – not on your notes!)
-
12 V
+
10W
10W
V1
V2
Here V1 = 6V and
The potential
differences V1 and
V2 are in the same
ratio as the
resistors. And add
up to 12!
V2 = 6 V
Have another go …
Notes p.8
1. Easy Ratios
-
12 V
+
20W
10W
V1
V2
Here V1 = 8V and
V2 = 4 V
Notice the 10W did NOT change but its voltage
did!
And another go in your head (not on notes) …
Easy Ratios
-
6V
+
100W
20W
V1
V2
Here V1 = 5V and
V2 = 1 V
2. Difficult Ratios (by ratios)
-
6V
+
140 W
15 W
V1
V2
Here
V1 = 140 / 155 of 6V
= 140 / 155 x 6
= 5.42 V
You can still apply
ratios here if you
are confident
enough.
V2 = 15 / 155 of 6V
= 140 / 155 x 6
= 0.58 V
2. Difficult Ratios (by ratios)
-
6V
What you are
doing here is
+
140 W
15 W
V1
V2
Where
V1 = R1 x Vs
RT
and
V2 = R2 x Vs
RT
RT = total resistance
Vs = supply voltage
2. Difficult Ratios (Using V = I R)
By this method you
+
6V
will usually have to
calculate current
first from a given
140 W
15 W
potential difference
and its
V1
V2
corresponding
resistance.
I = Vs /RT or
I = V1 /R1
or
I = V2 /R2
Here… I = Vs /RT = 6 / 155 = 0.0387A
2. Difficult Ratios (Using V = I R) cont …
-
6V
+
140 W
15 W
V1
V2
V1 = I R1
or V2 = I R2
Then use the
current to find
the unknown
potential
differences with
V = IR.
or
Vs = I RT
Here… V1 = I R1 = 0.0387 x 140 = 5.42 V
V2 = I R2 = 0.0387 x 15 = 0.58 V
3. Potentiometer
Calculate the Voltmeter
reading when the slider is
at:
+12V
5W
20W
B
0V
A
V
a) Point A
0V
b) Point B
12V
c) Midway between A and
B?
6V
d) 3/4 of the way up from
A? 3/4 x 12 = 9 V
Bridging Across Potential Dividers
Consider this “potential divider” circuit:
12V
A
1kW
X
2kW
B
What’s the “potential” at X?
4V as 8V is used up in the 2kW
resistor so there’s 4V left for
the 1kW .
What other resistor combinations between A and
B will give a potential of 4V between the resistors.
12V
Examples …
A
4kW
X
8kW
B
4kW
B
0V
A
2kW
Y
If X and Y above are joined together …
NO CURRENT FLOWS between them because
the “potential difference” across XY is 0V!
What’s the potential difference across XY in the
following circuits:
12V
A
4kW
X
4kW
B
4kW
B
V
A
2kW
Y
A
12V
4kW
X
8kW
B
2kW
B
V
A
10 k W
Y
Problems sheet:
“Calculations on Wheatstone Bridge Circuits”
Then Problems booklets p.10, Q. 16 -20.