Transcript parallel resistors

What we will do today
• Complete and correct check test 5.
• Look at resistors in parallel.
• Define a formula for the total
resistance of resistors in parallel.
Revision (READ)
Resistors in series:
• What was the relationship between
the total resistance, Rt, and three
resistors in series, R1, R2 and R3?
• Rt = R 1 + R2 + R3
Combining resistors (Read)
• We are now going to investigate the
effect of four resistors in a parallel
circuit.
• What apparatus will be needed?
• What will our circuit diagram look
like?
Resistors in Parallel
• We will measure three resistors separately
and then measure them when combined:
Resistor
R1 (on its own)
R2 (on its own)
R3 (on its own)
RT (all together)
Resistance ()
Resistors in Parallel
• From our results it is hard to see a
relationship.
• However, if we look carefully we can
see a pattern develop when the
relationship is reciprocal eg. 1/R.
Resistors in Parallel
• Construct the following table:
1
R1
1
1
1 +1 + 1
R2 R3 R1 R2 R3
1
RT
Resistors in Parallel
• We can now see that the relationship
for resistors in parallel is as follows:
1
=
1
+
1
+
1
Rt
R1
R2
R3
• The total resistance is always smaller
than the smallest resistor.
Example 1
• Find RT below
Example 1
• What to input into
your calculator:
• How to lay out in
the exam:
Example 2
• Find RT below
Example 2
• What to input into
your calculator:
• How to lay out in
the exam:
General Rule
•
In a series circuit, Rt is always
bigger than the biggest resistor.
•
In a parallel circuit, Rt is always
smaller than the smallest resistor.
What if we have both?
•
If we have both parallel and series
resistors in a circuit;
1. Work out any resistors that are in
series in the same branch.
2. Then work out resistors in parallel
and replace them with the value of
one resistor.
3. Work out the resistors in series.
Example 1: both parallel and
series
•
Find RT
•
Example 2: both parallel and
series
Find RT
What have we learned today?
•
For a parallel circuit:
1
=
1
+
1
+
1
Rt
R1
R2
R3
• If we have resistors in series AND
parallel:
1. Work out Rt of parallel resistors.
2. Then work out Rt of series
resistors.
2003, Qu: 11
• C
2004, Qu: 8
• C
2006, Qu: 10
• D
2004, Qu: 8
• C
2008, Qu: 25(b)(i)
2009, Qu: 25(b)(i), (ii)
2010, Qu: 24(a), (b)
2005 Qu: 25