Download File - The Physics Doctor

A –Level Physics:
Electrical Quantities:
Internal Resistance
Objectives:
45. Know the definition of electromotive force (e.m.f.) and understand
what is meant by internal resistance and know how to distinguish
between e.m.f. and terminal potential difference
47. Understand how changes of resistance with temperature may be
modelled in terms of lattice vibrations and number of conduction
electrons and understand how to apply this model to metallic
conductors and negative temperature coefficient thermistors
48. Understand how changes of resistance with illumination may be
modelled in terms of the number of conduction electrons and understand
how to apply this model to LDRs.
Additional skills gained:
• Practical Planning
• Integrating GCSE content
Starter:
Riddle (answer revealed next lesson)
You have one 16-quart pitcher full of water, one empty
nine-quart pitcher, and one empty seven-quart pitcher…
10mins
16
9
7
Using nothing but these three pitchers, can you dividethe water
evenly so that the 16-and nine-quart pitchers are each holding
exactly eight quarts of water?
Internal Resistance
Why is it that when a voltmeter is placed around a 1.5V
battery, it doesn’t read ‘1.5v’?
When a current is flowing through a circuit, the electrons still have
to travel through the cell.
A small portion of the voltage/energy will be lost here because the
electrons have to overcome the internal resistance of the cell!
Because the cell has this internal
resistance, the convention is to show
the cell adjacent to a resistor.
This resistor would have a very low
value.
Power/I.Resistance equations
What does V= in Ohm’s law? So, if Power=Current x
Voltage, what is the formula relationship between Power
and resistance?
1. V= I x R
2. P= I x V (so we can substitute ‘V’ for I x R)
3. P= I x I x R
4. P= I2 x R
As electrons are resisted through a cell, they will dissipate power
(work done in a set time). This is why each coulomb, will not get the
full number of joules (voltage).
So, the EMF is actually
what is left when we
subtract the voltage used
in internal resistance. So
as V=IR, what is the
equation for the EMF of this
circuit?
Power/I.Resistance equations
So the total energy produced by the cell (ε). Is the voltage
used against internal resistance added to the voltage
supplied to the rest of the circuit. So what’s the equation?
ε =IR + Irinternal
So, rearranging:
• IR = ε - Irinternal
• V= ε – Irinternal
So this shows that the voltage
produced at the terminals of the
cell (i.e. outside of the cell) is
effectively the EMF – the voltage
overcoming internal resistance
Typical Exam Q #1
Q1.
A battery is connected to a 10ohm resistor as shown. The e.m.f.
(electromotive force) of the battery is 12V.
(a)
(i) Explain what is meant by the e.m.f. of a battery.
(ii) When the switch is open the voltmeter reads 12.0V and when it is
closed it reads 11.5 V. Explain why the readings are different.
(b) Calculate the internal resistance of the battery.
Typical Exam Q #2
Q2.
A very high resistance voltmeter reads 20V when connected across
the terminals of a d.c. power supply. The high resistance meter is
disconnected and a second voltmeter of resistance 1.0k is then
connected across the supply. The second meter gives a reading of
16V.
(i) State the e.m.f. of the power supply.
(ii) Calculate the current which flows through the second meter.
(iii) Calculate the internal resistance of the power supply.
(iv) Show that the current is equal to 0.080A when the supply is short
circuited.
Typical Exam Q #3
Q3 (a) A steady current of 0.25 A passes through a torch bulb for 6
minutes. Calculate the charge which flows through the bulb in this
time.
(b) The torch bulb is now connected to a battery of negligible internal
resistance. The battery supplies a steady current of 0.25 A for 20
hours.
In this time the energy transferred in the bulb is 9.0 × 104 J. Calculate
(i) the potential difference across the bulb,
(ii) the power of the bulb
Typical Exam Q #4
Q4
(a) A student is given three resistors of resistance 3.0 , 4.0 and 6.0
ohms respectively.
(i) Draw the arrangement, using all three resistors, which will give the
largest resistance.
(ii) Calculate the resistance of the arrangement you have drawn.
(iii) Draw the arrangement, using all three resistors, which will give the
smallest resistance.
(iv) Calculate the resistance of the arrangement you have drawn.
(b) The three resistors are now connected to a battery of EMF 12 V and
negligible internal resistance, as shown in the diagram above
(ii) Calculate the voltage across the 6.0 resistor
Typical Exam Q #5
Q5. In the circuit shown below, the battery, of emf 6.0V, has negligible
internal resistance.
(a) Calculate the current through the ammeter when the switch S is
(i) open
(ii) Closed
(b) The switch S is now replaced with a voltmeter of infinite resistance.
Determine the reading on the voltmeter.
Practice Questions
Grab a laptop and go to the following URL:
http://www.cyberphysics.co.uk/Q&A/KS5/electricity/circuits/circuits.html
For each question:
• Answer the question with DETAILED working out (step by step how
you’re working it out) not just the numbers
• Once completed, then check the solution (bottom right of each
question) and mark/correct
Extension:
http://www.cyberphysics.co.uk/Q&A/KS5/electricit
y/resistivity/index_resistivity.html
40m
Independent Study