Transcript Electrical Resistance

EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 2-4
Electrical Resistance
Edexcel IGCSE Physics pages 82 to 88
All content applies for Triple & Double Science
June 17th 2012
Edexcel Specification
Section 2: Electricity
c) Energy and potential difference in circuits
describe how current varies with voltage in wires, resistors, metal
filament lamps and diodes, and how this can be investigated
experimentally
describe the qualitative effect of changing resistance on the current in
a circuit
describe the qualitative variation of resistance of LDRs with
illumination and of thermistors with temperature
know that lamps and LEDs can be used to indicate the presence of a
current in a circuit
know and use the relationship:
voltage = current × resistance V = I × R
Resistance
Resistance is the opposition
that an electrical device has
to the flow of electrical
current.
All devices have some
resistance. A resistor is a
device that has a particular
resistance.
a resistor
circuit symbol for
a resistor
Resistance equation
resistance = voltage
current
R = V
I
Where:
voltage is in volts (V)
current is in amperes (A)
resistance is in ohms (Ω)
Also:
V=IxR
and:
I=V
R
V
I
R
Measuring Resistance
The resistance of a
component can be
found by measuring
the current through,
and voltage across,
the component.
Circuit used for measuring the
resistance of an indicator lamp
Question 1
Calculate the resistance of a lamp if a
voltage of 12V causes a current of 3A to
flow through the lamp.
resistance = voltage
current
= 12V / 3A
resistance = 4 ohms (4Ω)
Question 2
Calculate the resistance of a heater if a voltage of
230V causes a current of 200mA to flow through
the heater.
resistance = voltage
current
= 230V / 200mA
= 230V / 0.200A
resistance = 1150 Ω
Question 3
Calculate the voltage across a resistance of
40Ω when a current of 5A is flowing.
V=IxR
= 5A x 40Ω
voltage = 200V
Question 4
Calculate the current flowing through a wire
of resistance of 8Ω when a voltage of 12V is
connected to the wire.
current = voltage
resistance
= 12V / 8Ω
current = 1.5A
Complete:
Answers
V
I
R
20 V
4A
5Ω
200 V
5A
40 Ω
300 V
0.20
6A
50 Ω
8V
500 mA
16 Ω
3 kV
20 A
150 Ω
120 V
4 mA
30 kΩ
Current-voltage graphs
These are used to show how the
current through a component
varies with the applied voltage.
The circuit opposite could be used
to obtain a current-voltage graph
of a resistor.
The variable resistor is used to
apply a range of voltages across
the resistor.
Typical results:
Voltage (V)
0.00 0.30 0.60 0.90
1.20
1.50
Current (A)
0.00 0.04 0.08 0.12
0.16
0.20
The current-voltage graph of a wire or a
fixed resistor at a constant temperature
I
The graph is a straight
line through the origin.
The wire or resistor
obeys Ohm’s law which
states that that the
current is proportional to
the voltage.
V
Choose appropriate words to fill in the gaps below:
current
An electric ________
will only flow around a circuit if there
are no ______
gaps in the circuit.
resistance
All components have __________.
The greater the resistance
smaller is the current for the same applied voltage.
the ________
ohms
Resistance is measured in ______.
resistor is a straight line
A current – voltage graph for a ________
origin
through the _______.
This shows that the current through
proportional to the applied voltage.
the resistor is ___________
WORD SELECTION:
smaller resistor
proportional
gaps
origin resistance ohms current
Filament lamp
The resistance of a filament
lamp increases as the
temperature of the filament
increases.
I
V
Reversing the voltage
(negative values on the graph)
reverses the direction of the
electric current but does not
change the shape of the
curve.
The lamp does not obey
Ohm’s law
Diode
I
Current only flows through a diode
in one direction. (Shown by the
arrow on its symbol)
The diode has a very high
resistance in the reverse direction.
V
A light emitting diode (LED) emits
light when electric current flows.
Thermistor
The resistance of a
thermistor decreases as
the temperature
increases.
The higher temperature
line therefore has a
greater slope than the
lower temperature case.
I
HOT
COLD
V
Light dependent
resistor (LDR)
The resistance of a
light-dependant resistor
decreases as light
intensity increases.
The bright light line
therefore has a greater
slope than the dim light
case.
I
BRIGHT
DIM
V
Choose appropriate words to fill in the gaps below:
The resistance of a filament lamp _________
increases when the lamp
temperature
comes on and the filament rises in ___________.
diode only allows electric current to flow one way. The
A ______
arrow on its circuit
allowed direction is shown by the _______
symbol.
resistance of a thermistor decreases if its temperature is
The ________
decreases if the
increased. The resistance of a LDR _________
light
_______level
is increased.
WORD SELECTION:
temperature arrow decreases increases diode resistance light
Online Simulations
Simple demonstration of the effect of a variable
resistor - Freezeway.com
Battery-Resistor Circuit - PhET - Look inside a
resistor to see how it works. Increase the
battery voltage to make more electrons flow
though the resistor. Increase the resistance to
block the flow of electrons. Watch the current
and resistor temperature change
Light bulb being controlled by a variable
resistor
Electric circuits with resistors - series & parallel
with meters - netfirms
Variable resistor with an ammeter & a voltmeter
Resistance measurement demo - Molecular
Expressions
Ohm's Law - PhET - See how the equation
form of Ohm's law relates to a simple circuit.
Adjust the voltage and resistance, and see the
current change according to Ohm's law. The
sizes of the symbols in the equation change to
match the circuit diagram.
Ohm's Law - Fendt
Simple V-I circuit with a light bulb Freezeway.com
Resistance in a Wire - PhET - Learn about the
physics of resistance in a wire. Change its
resistivity, length, and area to see how they
affect the wire's resistance. The sizes of the
symbols in the equation change along with the
diagram of a wire.
Resistance Wire Simulation - by KT - Designed
for the GCSE Investigation but can also be
used to show the affect of source resistance
and to show power supply maximum power.
Introduction . Worksheets for GCSE
investigation
Conductivity - PhET - Experiment with
conductivity in metals, plastics and
photoconductors. See why metals conduct and
plastics don't, and why some materials conduct
only when you shine a flashlight on them.
BBC AQA GCSE Bitesize Revision:
Calculating resistance
Changing resistance
Filament lamp with IV curve
Thermistors & LDRs
Electrical Resistance
Notes questions from pages 82 to 88
1.
2.
3.
4.
5.
6.
7.
Give the equation for resistance and state the units used. (see
page 83)
Describe how the resistance of a thermistor and LDR can be
varies. (see page 84)
With the aid of a circuit diagram describe an experiment to find out
how the current through a wire varies with the voltage across it.
(see pages 85 and 86)
Sketch graphs showing how current varies with voltage with a (a)
fixed resistor; (b) lamp; (c) diode. (see pages 86 and 87)
What is Ohm’s law? (b) How does a graph show whether or not
Ohm’s law is being obeyed? (see page 85)
Answer the questions on page 88.
Verify that you can do all of the items listed in the end of chapter
checklist on page 88.