EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 2-4

Electrical ResistanceEdexcel IGCSE Physics pages 82 to 88

June 17th 2012

All content applies for Triple & Double Science

Edexcel Specification

Section 2: Electricityc) Energy and potential difference in circuitsdescribe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentallydescribe the qualitative effect of changing resistance on the current in a circuitdescribe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperatureknow that lamps and LEDs can be used to indicate the presence of a current in a circuitknow and use the relationship: voltage = current × resistance V = I × R

ResistanceResistance 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 equationresistance = voltage

current

R = V I

Where:voltage is in volts (V)current is in amperes (A)resistance is in ohms (Ω)

Also:

V = I x R

and:

I = V

R V

RI

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 / 3Aresistance = 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 / 0.200Aresistance = 1150 Ω

Question 3

Calculate the voltage across a resistance of 40Ω when a current of 5A is flowing.

V = I x R= 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 = voltageresistance

= 12V / 8Ωcurrent = 1.5A

AnswersV I R

20 V 4 A

5 A 40 Ω

300 V 0.20 50 Ω

8 V 500 mA

3 kV 150 Ω

4 mA 30 kΩ

5 Ω

200 V

6 A

16 Ω

20 A

120 V

Complete:

Current-voltage graphsThese 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

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.

I

V

Choose appropriate words to fill in the gaps below:

An electric ________ will only flow around a circuit if there are no ______ in the circuit.

All components have __________. The greater the resistance the ________ is the current for the same applied voltage. Resistance is measured in ______.

A current – voltage graph for a ________ is a straight line through the _______. This shows that the current through the resistor is ___________ to the applied voltage.

resistance currentresistor ohmsgaps originsmaller

WORD SELECTION:

proportional

resistance

current

resistor

ohms

gaps

origin

smaller

proportional

Filament lamp

The resistance of a filament lamp increases as the temperature of the filament increases.

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

I

V

Diode

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.

A light emitting diode (LED) emits light when electric current flows.

I

V

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

V

HOT

COLD

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

V

BRIGHT

DIM

Choose appropriate words to fill in the gaps below:

The resistance of a filament lamp _________ when the lamp comes on and the filament rises in ___________.

A ______ only allows electric current to flow one way. The allowed direction is shown by the _______ on its circuit symbol.

The ________ of a thermistor decreases if its temperature is increased. The resistance of a LDR _________ if the _______level is increased.

resistancedecreasesarrow diode lightincreases

WORD SELECTION:

temperature

resistance

decreases

arrow

diode

light

increases

temperature

Online SimulationsSimple 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 resistorElectric circuits with resistors - series & parallel with meters - netfirms Variable resistor with an ammeter & a voltmeterResistance 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 ResistanceNotes questions from pages 82 to 88

1. Give the equation for resistance and state the units used. (see page 83)

2. Describe how the resistance of a thermistor and LDR can be varies. (see page 84)

3. 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)

4. Sketch graphs showing how current varies with voltage with a (a) fixed resistor; (b) lamp; (c) diode. (see pages 86 and 87)

5. What is Ohm’s law? (b) How does a graph show whether or not Ohm’s law is being obeyed? (see page 85)

6. Answer the questions on page 88.7. Verify that you can do all of the items listed in the end of chapter

checklist on page 88.