Download 5: Electric Current

5: Electric Current
5.2 Electric Circuits
Resistor Combinations
Experiment: Resistance in series and parallel
circuits.
• Use a voltmeter and ammeter or Ohmmeter to
determine the resistance of a range of components.
• Then connect them in series or in parallel pairs
and investigate the overall resistance.
Resistors in Series
- Current (I) is equal in both
- The supply voltage is shared
across the resistors
R1
R2
Vs = V1 + V2
IRe = IR1 + IR2 (Re = total equivalent resistance)
but I cancels out giving…
Re = R1 + R2
for resistors in series
I1
Resistors in Parallel
- Total current (I) is shared
between the two resistors
- The voltage across each is
equal (V)
so...
I
I2
R1
R2
I = I1 + I2
but... I = V
R
V =V + V
Re R1 R2
V cancels giving...
1 =1 + 1
Re R1 R2
for resistors in parallel
E.g. Calculate the equivalent resistance of these
resistor combinations:
1200Ω
120Ω
750Ω
360Ω
1200Ω
E.g.2.Put these in increasing order of magnitude
X
Y
Z
The Potential Divider Circuit
We know that for resistors in series, the supply voltage is
shared between the individual resistors.
Demo:
1. Measure V and the voltages
across the two resistors.
1 kΩ
V1
2 kΩ
V2
V
2. Predict how the voltages
would change if you
swapped the 1kΩ resistor
for a 4kΩ resistor?
Conclusion:
The ratio of the resistances is the
same as the ratio of the voltages
Q. Using V=IR, explain why the bigger resistor will always take a larger voltage.
A circuit like this is known as a potential divider
because it divides up the total p.d. supplied by the
cell.
The voltage across one of the resistors can then be
used as an output supply to an external device or
circuit.
E.g.
Assuming voltage V is shared across
the two resistors…
V = I R (where R = equivalent resistance)

V = I (R1 + R2)

I= V
R1 + R2
I
R1
V
R2
Vout
but… Vout = IR2
so…
Vout = V R2
R1 + R2
This is called
the potential
divider formula
E.g.
a. Determine the total resistance and hence the current in
the circuit.
b. Determine the output voltage in the circuit.
c. If a bulb was connected across the output, how would
this affect the output voltage?
I
a. R = 15Ω so… I = 0.8A
b. V = 7.2V
6Ω
V
12V
9Ω
Vout
c. A parallel section would be
created, thus reducing the
total resistance between the
output terminals and thus also
reducing the output voltage.
Extension: If the bulb has resistance 18Ω, determine the new Vout?
( Demo / Experiment
- LDR, 2.2kΩ, 2 cells, voltmeter )
Sensors and potential dividers
1. Strain gauge
I
Strain gauge
V
R2
Vout
The diagram below shows part of
an F1 car suspension. A strain
gauge is stuck on the underside of
strut X and wired into the circuit
shown.
Explain what a decrease in
measured output voltage would
indicate.
X
2. Automatic light switch
The electronic switch will turn
on the lights (on another
circuit) if the voltage rises
above a certain fixed value.
I
R1
V
LDR
Electronic
switch
Vout
i. What happens to the LDR
resistance when the light level
falls?
ii. So what happens to Vout?
iii. What happens next?
The Potentiometer
A potentiometer is a variable potential divider.
The slider can be moved to alter
the ratio of R1 to R2
I
A
R1
V
B
R2
Vout
C
Q. At which positions would the slider enable the bulb to
i. be brightest?
ii. be off (Vout= 0)?
EMF and Internal Resistance
EMF
For components that put energy into a circuit, the
p.d. across them is referred to as the Electro Motive
Force (EMF). Therefore…
The EMF of a cell is the amount of work done by the cell per
Coulomb of charge passing through the circuit.
However the voltage measured across the terminals
of the cell (the terminal p.d.) is often less than the
EMF.
Internal Resistance
Demo / Experiment:
I
1. Set up the circuit and
measure the voltage
across the cell.
V
2. Add one, then two, then
three bulbs in parallel.
What happens to V?
Observations and Conclusion:
The terminal p.d. decreases as the resistance of the
circuit decreases. This indicates that the cell must have
resistance itself, taking a gradually larger share of the
EMF (this is now like a potential divider circuit).
Internal Resistance
Terminal voltage
All cells (and other sources of EMF) have an
internal resistance, r (effectively in series). This is
why cells heat up during use.
I
Lost voltage across r = Ir
r
R
V
EMF
Thus…
EMF = Ir + V
but… V = IR
Experiment:
so… EMF = I (r +R)
Aim: Determine the internal resistance of a single cell.
Subtitle
Text
Subtitle
Text
I
V
Subtitle
Text
Subtitle
Text
Subtitle
Text
Subtitle
Text