# Resistance

Chapter overview

1 week

This chapter starts off by explaining the meaning of resistance in an electric circuit. Learners will then look at the use of resistors. This is a revision of some of the concepts covered in Gr 8 Energy and Change when looking at the Energy transfers within an electrical system (Chapter 2). For an easy reference to what learners covered in the previous grade, you can visit the website www.curious.org.za where this content is located online, and navigate to the relevant grade and chapter.

This year, the concept of resistance will be extended by looking at the factors that affect resistance in a resistor, namely:

• the type of material of which the conductor is made
• the thickness of the conductor
• the length of the conductor
• the temperature of the conductor

These will be investigated experimentally. Learners must be able to explain the relationships between these factors and the resistance offered by the resistor. It is not necessary to show experimentally how temperature affects resistance, but the concept must still be covered. CAPS suggests investigating at least one of the other factors. All three investigations have been included in this workbook so that you have the choice as to which one you would like to conduct with your class, or, if time permits, you can conduct all three investigations. Three hours have been allocated to this section in CAPS, so you should have time to perform more than one of the investigations. In the workbook, they have been presented as three separate investigations, but you can also perform them concurrently, or allocate a different investigation to different groups. The groups can then report back to the class on their findings and you can subsequently summarise the effects on the board.

If you only teach Natural Sciences, it is a good idea to check with the Technology teachers to see how these two curriculums complement each other, especially with regard to electricity. Some of the concepts which might be introduced for the first time in Natural Sciences, have already been covered in the Technology curriculum. Knowing what learners have already covered and been introduced to will help make your classes more efficient and more stimulating for learners.

3.1 What is resistance?

This is included as an introduction.

3.2 Uses of resistors (1 hour)

 Tasks Skills Recommendation Activity: Useful resistance Recalling, identifying, describing Suggested Activity: Make your own rheostat Following instructions, predicting, observing, explaining Optional Activity: Comparing a LED to a filament light bulb Describing, drawing, explaining, comparing Suggested

3.3 Factors that affect resistance (2 hours)

 Tasks Skills Recommendation Investigation: How does the material of the resistor affect the resistance? Hypothesising, identifying variables, following instructions, drawing, observing, describing, analysing, concluding CAPS suggested Investigation: How does the thickness of the conductor affect the resistance? Hypothesising, identifying variables, following instructions, drawing, observing, describing, analysing, concluding CAPS suggested Investigation: How does the length of a conductor affect the resistance of the conductor? Hypothesising, identifying variables, following instructions, drawing, observing, describing, analysing, concluding CAPS suggested

• What is resistance?
• What do we use resistors for?
• Does length affect resistance?
• Does temperature affect resistance?
• Does the type of resistor material affect resistance?
• Does the thickness of a resistor affect the resistance?

## What is resistance?

• resistance
• resistor
• electric current
• electric charge
• delocalised
• conductor

A good way to introduce this topic is to act out the following situation with your learners which is explained in the learner's book. You can even just create an imaginary field by drawing a square with chalk on the ground and then a narrow corridor coming off of it. Tell learners to first walk around randomly in the field and then when you signal (indicating that a potential difference has been applied across the wire), they all need to move towards the corridor and get through it. You can make the corridor start off wide and become narrower to further illustrate how the resistance to their movement increases as the corridor becomes narrower. This only demonstrates one of the factors influencing resistance (namely the width of the conductor), but can be used to introduce the idea of resistance.

We have revised the concept of electric current and how electrons move within a conducting wire before introducing the idea of resistance in an electric circuit.

Think about your school break time. All of the learners are outside on the field, sitting in groups and relaxing. Some of you will be moving around the field from group to group as you greet your friends. The school bell rings, signaling the end of break. You all get up and start moving toward the school building. You are all able to move easily because there is a great deal of space but what happens as you enter the corridor of the school building?

Everyone now has to fit through a narrow corridor. Everyone is trying to get to class and so some learners will bump into other learners. As you try to enter your classroom it becomes even more difficult because the doorway is even narrower than the corridor and so only one or two learners can pass through at a time.

The movement of the learners is very similar to the movement of electrons in an electrical conductor. The field offers a very low resistance to the movement of the learners and so the learners are able to move freely. The corridor has a higher resistance to the movement of the learners because less learners can now pass through the corridor than through the field. The classroom doorway offers the highest resistance as it only allows a few learners through at a time.

How can we use this to illustrate electrical resistance? Let's first revise some concepts about electric current.

Electric charge, or charge, is the physical property of matter that causes it to experience a force when close to other electrically charged matter. There are two types of electric charges - positive and negative. Electrons have a negative charge and protons have a positive charge.

An electric current is the rate of charge flow in a closed, electric circuit. The electrons in an atom are arranged in the outer space around the central nucleus. In metals, the electrons are able to move freely within the metal. The electrons are not associated with a particular atom in the metal. We say electrons in a metal are delocalised. Have a look at the following diagram which shows this.

We can think of the model of a metal as the positive metal ions in fixed positions surrounded by a 'sea' of electrons.

Conducting wire in an electric circuit is made of metal. If we supply it with a source of energy and a complete circuit, then the electrons will all move in the same general direction through the wire to the positive terminal of the battery. This movement of electrons per time through a conductor is the electric current.

Resistance in an electrical circuit opposes the passage of electrons. The unit of measurement for resistance is the ohm, with the symbol Ω.

The ohm gets its name from the German physicist Georg Simon Ohm, who noticed that the potential difference across a conductor and the electric current are directly proportional (Ohm's Law).

Do you remember what an electrical conductor is? Write your own definition below.

An electrical conductor is a type of material or object which allows electric charge to pass through it.

You can also remind learners at this point that electrical insulators are non-conductors as they do not allow electric charge through them.

All electrical conductors have some resistance. Some conducting materials have a particular resistance and are used to add electrical resistance to a circuit. An electrical component which adds resistance to a circuit is called a resistor. Different types of resistors used to add resistance to an electric circuit.

Can you see that there are different coloured bands on the resistors? This isn't just to make them look pleasing to the eye. The coloured bands are actually a code that tells us the strength of the resistance of the resistor.

Resistors are electrical components and have a symbol to represent them in an electric circuit diagram. Do you remember the symbol from Gr 8? Draw it in the space below.

There are two symbols used to represent resistors, but the one most commonly used is the the one using a block.

On a microscopic level, electrons moving through the conductor collide (or interact) with the particles of which the conductor (metal) is made. When they collide, they transfer kinetic energy. This leads to resistance. The transferred energy causes the resistor to heat up. You can feel this directly if you touch a cellphone charger when you are charging a cell phone - the charger gets warm because its circuits have some resistors in them.

## Uses of resistors

• LED
• motor
• variable resistance
• rheostat
• Sankey diagram
• input energy
• output energy

Resistors can be used to control the current in a circuit. Think back to some of the work that you did in Gr 8. If you increase the resistance in a circuit, what happens to the current? Explain your answer.

Discuss this with your class as they might not have conducted these investigations in the previous grade. When the resistance in a circuit increases, the current decreases. Adding more resistance increases the opposition to the flow of charge so it is more difficult for charge to move through the circuit. Therefore there is less current (as current is the rate of flow of charge). We say that the current is inversely proportional to the resistance, meaning as the resistance increases, the current decreases.

Another way in which we can use resistors is to provide useful energy transfers. Do you remember looking at energy transfers in a system in Gr 8? The input energy enters the system and then provides an output energy. Some of the output energy is useful to us, and some is wasted energy. For example, a resistor can be used to transfer electrical energy into light (light bulb) or into heat (kettle element). Energy is wasted as it is lost to the surroundings. Resistors are used to provide useful energy transfers.

# Useful resistance

This activity links back to the work done in Grade 8 Energy and Change. The difference between "useful" and "wasted" energy is highlighted again. The learners should see that resistors can be used to provide useful energy transfers.

Why do we want to resist the movement of electrons? Resistors can be extremely useful. Think about a kettle. If you look inside you will see a large metal coil. Looking inside a kettle.

This metal coil is the heating element. If you plug in and switch on the kettle, the element heats up and heats the water. The element is a large resistor. When the electrons move through the resistor, they release a lot of energy in overcoming the resistance. This energy is transferred to the water in the form of heat. This transfer of energy is useful to us as the thermal energy is used to boil water in the kettle.

What is the input energy in this system?

Electrical energy.

What is the useful output energy?

Thermal energy.

Look at the photograph of a light bulb on the left. Can you see there is a small coiled wire in the glass bulb? This is called the filament. The filament is made from tungsten wire. This is an element with high resistance. An incandescent light bulb.[link] The tungsten filament glowing brightly.

Incandescent means to emit light as a result of being heated.

When the electrons move through the filament they experience high resistance. This means that they transfer a lot of their energy to the filament when they pass through. Describe the energy transfer taking place.

Electrical energy is transferred to heat and light.

What is the useful energy output and what is the wasted energy output in this light bulb?

Light is the useful output energy and heat is the wasted output energy.

The filament is tightly coiled. Why do you think this is? Discuss this with your class and teacher.

This is an extension question as learners will only cover factors affecting resistance later so discuss this as a class.

This is to fit a longer length of tungsten within a small space to increase the resistance.

The first electric light was made in 1800 by a man called Humphry Davy. He invented an electric battery, to which he connected wires and a piece of carbon. Being a resistor, the carbon glowed and produced light.

The inventor Thomas Edison, experimented with thousands of different resistor materials until he eventually found the right material that allowed the bulb to glow for over 1500 hours.

Look at the following photo of a toaster. An electric toaster.

Can you see the glowing filament inside? Why does the element glow?

The electric current passes through the toaster and the element has a high resistance. Energy is transferred to the particles in the element so that they gain kinetic energy and the temperature of the wire increases. Some of the energy is also transferred as light to the surroundings and the wire glows.

What is the useful output energy in this system?

Heat.

What is the wasted output energy in this system?

Light.

Rheostats are another form of resistor which are commonly used. A rheostat is a device which is able to offer a variable resistance. Rheostats are used in electric circuits where you want to adjust the current, for example in sound equipment to adjust the volume, in dimmer switches for lights and in controlling the speed of motors. Let's look at how rheostats can be used in a circuit. An example of a rheostat.

The rheostat (video).

# Make your own rheostat

If you have rheostats in your laboratory then you may choose to simply demonstrate how the resistance is varied by changing the position of the slider. The position of the slider affects the length of the resistor coil. Because length is a factor which affects resistance, the shorter the coil, the less resistance; the longer the coil, the greater the resistance.

MATERIALS:

• graphite rod or graphite pencil
• torch light bulb
• battery (AA)
• insulated copper conducting wires with crocodile clips
• ammeter

If you do not have a graphite rod then a graphite pencil can be used. Sharpen the pencil on both sides and carve the wood from the pencil at various points along the length of the pencil. This is easier than trying to remove the entire graphite rod from the pencil. The graphite is soft and often breaks into pieces if you try to remove the entire thing.

The ammeter is not strictly necessary. If you do not have one, then the learners can use the brightness of the bulb as an indication of the strength of the current.

INSTRUCTIONS:

Set up a circuit as in the diagram below with the battery, ammeter, light bulb and graphite rod connected in series. Use crocodile clips to attach the wires to each end of the graphite rod.

Does current flow through the circuit? How do you know?

Yes. The light bulb glows and the ammeter has a reading on it.

The crocodile clips are connected on either end of the graphite rod. Predict what you think would happen if you moved the crocodile clips closer towards the center of the piece of graphite.

Learners' predictions will vary. They should refer to how they think the length of graphite will affect the current strength.

Move the crocodile clips closer towards the centre of the graphite rod. What do you observe?

The light should burn brighter and the ammeter reading should increase.

How do you think the length of the graphite connected to the circuit has affected the current strength?

The shorter the length, the smaller the resistance and so the current strength is greater. By changing the length, the resistance has changed.

Draw a circuit diagram to represent this set-up. The symbol for a variable resistor.

The graphite rod was behaving as a rheostat. The resistance of the graphite rod was changed by changing the length connected to the circuit. A dimmer switch often has a dial which can be turned. Turning the dial increases the resistance of the circuit and makes the light dim. Why do you think this happens?

When the resistance is increased, the current decreases, and so the brightness of the bulb decreases.

Turning the dial in the opposite direction causes the resistance to decrease and so the light burns brighter. Turning the dial changes the resistance of the rheostat in the switch.

Another device which demonstrates the useful application of resistance is in an LED. LED stands for light emitting diode. A small LED light.

A diode is an electrical component that has a very low resistance to current flow in one direction, and high resistance in the other direction. Therefore, current can only move in one direction.

An LED is a diode because it only allows current to pass through it in one direction. This means that it has to be put into a circuit in a very specific way. LEDs are very sensitive to high currents so when they are connected in a circuit, they need to be protected by a large resistor. The resistor is used to control the current which is allowed to travel through the LED. This is another useful application of resistance.

Many households are choosing to replace incandescent light bulbs with LEDs. Are LEDs a more efficient form of lighting?

Video on drawing a basic Sankey diagram.

# Comparing an LED to a filament light bulb

We can use a Sankey diagram to show how the energy is transferred in a system. This gives us a picture of what is happening and shows the input energy and how the output energy is made up of useful energy (arrow at the top) and wasted energy (arrow going to the bottom). Have a look at the following general example.

Sankey diagrams are named after the Irish Captain Matthew Sankey, who first used this type of diagram in 1898 in a publication on the energy efficiency of a steam engine.

The width of the arrows tell us something in these diagrams. The input energy is the width of the original arrow. The width of both the output energy arrows (useful and wasted) add up to the width of the input arrow. Why do you think this is so?

This is because energy is neither created nor destroyed, but conserved within a system. So the input must equal the output energy in a system.

Sankey diagrams are drawn to scale so that the width of the arrows gives us a visual idea of how much energy is useful and how much is wasted.

QUESTIONS:

The Sankey diagram for an LED is shown below.

1. Describe the energy transfer which takes place in an LED, based on the given Sankey diagram.

2. Is the LED efficient or inefficient? Explain your answer.

1. If 100J of electrical energy is transferred to the LED then the LED transfers 75J of energy as light and 25J of energy as heat.

2. The LED is efficient. The main purpose of the LED is to produce light, most of the energy transferred by the LED produces light and only a small percentage is "wasted" as heat (25%).

The Sankey diagram for an incandescent light bulb is shown below

1. Explain the energy transfers in the incandescent light bulb.

2. Is the incandescent light bulb efficient? Explain your answer.

1. If 100J of energy is transferred to the light bulb then $\text{90}$% of the energy is transferred to the surroundings as "wasted" heat. Only $\text{10}$% of the energy is used to produce light.

2. The incandescent light bulb is inefficient. The purpose of the incandescent light bulb is to produce light. $\text{90}$% of the energy it transfers is wasted and only $\text{10}$% is useful light.

If you are trying to reduce your electricity consumption in order to save money, which light source would you choose? Why?

LEDs as they are more energy efficient. The useful output energy is greater than the wasted output energy.

When we built our own rheostat, we were able to vary the resistance by changing the length of the graphite rod. This tells us that the length of the rod affected the amount of resistance. Let's look at what other factors which affect the resistance of a conductor.

## Factors that affect resistors

What determines the resistance of a component? Let's investigate some of the factors. There are 4 different factors which affect resistance:

• The type of material of which the resistor is made
• The length of the resistor
• The thickness of the resistor
• The temperature of the conductor

### Type of material

It is not necessary to test each of the factors which affect resistance. Testing the type of material is the easiest method for the learners to pursue in class. As an extension, you could have the learners do a small project on how the cross-sectional area of the material affects resistance or how temperature affects resistance.

Conductors can be made of different materials. Do different materials have different resistances?

# How does the material of the resistor affect the resistance?

How can we measure the resistance? Do you remember that in a series circuit, if we increase the resistance, then the strength of the current decreases? This means that we can use the strength of the current in the circuit as an indication of the amount of resistance in the circuit.

We can say that resistance and current in an electric circuit areinversely proportional, because as the one increases, the other decreases, and vice versa.

AIM: To determine whether different types of conducting materials have different resistances.

HYPOTHESIS:

Write a hypothesis for this investigation.

Possible hypotheses are: Different conductors will provide different amounts of resistance in an electrical circuit. The type of material will determine the amount of resistance.

VARIABLES

Which variables would we need to keep constant in an investigation such as this?

The materials we test would all need to have the same length and thickness and be kept at the same temperature. The number of batteries used to provide energy in the circuit.

Which variable is the independent variable?

The independent variable is the type of material.

Which variable is the dependent variable?

The dependent variable is the amount of resistance provided by the material, as indicated by the change in current.

This experiment does not measure resistance directly, but rather uses ammeter readings. Later on in this investigation learners are required to draw a bar graph. The bar graph will have ammeter readings (current) on the y-axis.

MATERIALS AND APPARATUS:

• three 1,5 V cells
• insulated, conducting wires with crocodile clips
• conductors of different materials to test
• ammeter
• light bulb

Use whichever metals you have available. Good materials to use are copper (a thicker gauge than those in the normal conducting wires), nickel, nichrome and iron.

METHOD:

1. Set up a circuit with the three cells, ammeter and light bulb connected in series.
2. Test each of the conductors by adding each to the circuit individually. Use crocodile clips to connect each conductor to the circuit, as shown below. A similar setup showing a light bulb, one cell and a piece of copper wire connected in series.
3. Read the ammeter and record the reading for each test material.
4. Draw a bar graph to show your results.

RESULTS:

Draw a circuit diagram of the setup.

Here is an example circuit diagram. The way in which learners represent the connection points for the material to be tested may vary.

Draw a table showing your results.

An example table is shown here:

Table showing the ammeter readings for each type of conducting material tested

 Conducting material Ammeter reading (A) Copper Nickel Nichrome Iron

Learners must provide a heading for the table and each column, and the unit must be in the heading. You can also use Assessment Rubric 4 at the back of your Teacher's Guide for a more detailed assessment.

Draw a bar graph of your results in the space provided.

Learners must provide a heading for the graph, such as 'Graph showing the current in an electric circuit with different types of conducting materials being tested.' The type of material must be on the x-axis and the ammeter reading on the y-axis. The bars must not be touching in this type of bar graph as the data is not continuous. For a more detailed assessment, refer to Assessment Rubric 3.

ANALYSIS AND EVALUATION:

Which material offered the most resistance in the electric circuit? How do you know this?

This will depend on the materials used, but the material which has the lowest ammeter reading, and therefore the lowest current, has the highest resistance.

Which material offered the least resistance in the electric circuit? How do you know this?

This will depend on the materials used, but the material with the highest ammeter reading will have the lowest resistance.

Are there any potential problems with the way in which this investigation was set up, or are there any ways in which you could have improved the design?

Learners' responses will vary but the learners should mention that it is difficult to control the temperature of the conductors and that they should read the ammeter reading immediately upon adding their test material to the circuit.

CONCLUSIONS:

What conclusion can you reach from this investigation?

The type of material from which the resistor is made affects the resistance. Different materials offer different amounts of resistance.

Why must the different conductors have the same length and thickness?

Length and thickness can also affect amount of resistance. We only want to test one variable at a time and therefore other variables must remain unchanged to make sure it is a fair test.

### Thickness of the conductor

When we investigate the thickness of a conductor, we are looking at the cross-sectional area of the wire, called the gauge. This is shown in the following diagram. The cross-sectional area of a wire is indicated by the red circle.

Do you think the thickness of a wire will affect the resistance? Let's do an investigation to find out.

# Investigation How does the thickness of the conductor affect the resistance?

AIM: To determine whether the thickness of the conductor will affect the resistance.

HYPOTHESIS: Write a hypothesis for this investigation.

Possible hypotheses are:

• The thicker the conductor, the smaller the resistance.
• The thinner the conductor, the smaller the resistance.

VARIABLES:

Which variables would we need to keep constant in an investigation such as this?

The wires used must all be of the same material (for example, copper), the same length and the same temperature. The number of batteries used to provide energy in the circuit.

Which variable is the independent variable?

The independent variable is the thickness of the wire.

Which variable is the dependent variable?

The dependent variable is the amount of resistance provided by the different wires, as measured by changes in current.

MATERIALS AND APPARATUS:

• three 1,5 V cells
• insulated, conducting wires with crocodile clips
• conductors of different thickness
• ammeter
• light bulb

A suggestion is to use three equal lengths of copper wire with different thicknesses.

METHOD:

1. Assess the lengths of wire that you have and arrange them in order from thickest to thinnest. Label the thickest wire as 1, the next thickest as 2, and so on, so that you can easily record the results.
2. Set up a circuit as in the previous investigation with the three cells, ammeter and light bulb connected in series.
3. Test each of the different wires by adding each to the circuit in turn. Use the conducting wires with crocodile clips attached at the ends to join each conductor to the circuit.
4. Read the ammeter and record the reading for each wire.

RESULTS:

Draw a table showing your results.

An example table is shown here:

Table showing the ammeter readings for each thickness of wire used

 Wires of different thicknesses Ammeter reading (A) 1 (thickest) 2 3 (thinnest)

Learners must provide a heading for the table and each column, and the unit must be in the heading. You can also use Assessment Rubric 4 at the back of your Teacher's Guide for a more detailed assessment.

ANALYSIS AND EVALUATION:

Which thickness of wire offered the most resistance in the electric circuit?

The thinnest wire offers the most resistance.

Which thickness of wire offered the least resistance in the electric circuit?

The thickest wire offers the least resistance.

Are there any potential problems with the way in which this investigation was setup, or are there any ways in which you could have improved the design?

Learners' responses will vary but the learners should mention that it is difficult to control the temperature of the conductors and that they should read the ammeter reading immediately upon adding their wire to the circuit.

CONCLUSIONS:

What conclusion can you reach from this investigation?

The thickness of the wire does affect the resistance. The thinner the wire, the higher the resistance.

Can you accept or reject your hypothesis?

Learner-dependent answer.

The thinner the wire, the more resistance it offers. Thicker wires offer less resistance. This is easy to understand if you think back to the example of all the learners filing back into the classrooms after break. If the corridor is narrow (or thin) then it is harder for all the learners to move through. A very wide corridor would be easier to move through as it offers less resistance. This is the same in a conducting wire. A thinner wire is more difficult for electrons to move through than a thicker wire.

Optional, online simulation:

If you have access to the internet for your students then you can do the activities listed in this subsection. The simulation referred to in the activity can be found here http://www.ktaggart.co.uk/physics/Simulations/EJS/ResistanceWire.html

Alternatively, the PhET simulation for investigating resistance in a wire listed in the visit box can also be used.

The simulation opens by showing the value of the resistance of a 50 cm length of 26 SWG constantan wire as being 1,585 ohms. This resistance is given to three decimal places and shown in the yellow area of the display screen. There is a reset button at the top of the screen which will set the simulation back to the resistance of a 50 cm length of 26 SWG constantan wire.

There are various sliders on the screen where you can adjust the length, material type and cross-sectional area of the resistor. Take some time to familiarise yourself with the simulation before allowing the learners to use the simulation. Here is an example of the investigation you could do.

Investigation: How does the diameter or cross-sectional area of the wire affect the resistance of the wire ?

MATERIALS AND APPARATUS:

METHOD:

1. Open the simulation.
2. Change the diameter of the resistor according to the table below. Do not change any other factors.
3. Write down the resistance (shown in the yellow block on the screen) which corresponds to each wire diameter.
4. Draw a graph of your results.

RESULTS:

 Wire diameter (mm) Cross-sectional a rea of wire (mm 2 ) Resistance (ohms) 1 2 3 4

Draw a graph of wire diameter versus resistance.

CONCLUSIONS:

As the diameter of the wire increases, the resistance decreases. In fact, if we double the cross-sectional area then the resistance halves. This means that resistance is inversely proportional to the area of the wire.

Learn about the resistance in a wire with this simulation. Change its length and area to see how they affect the wire's resistance. http://phet.colorado.edu/en/simulation/resistance-in-a-wire

### Length of the conductor

In each of the previous investigations, we have used the same length for each conductor. It was a controlled variable. Let's now investigate how the length of a conductor affects the resistance.

# How does the length of a conductor affect the resistance of the conductor?

Watch a video on this investigation on length of a resistor.

HYPOTHESIS: Write a hypothesis for this investigation.

A possible hypothesis is: The longer the length of the conductor, the higher the resistance.

MATERIALS AND APPARATUS:

• piece of resistance wire (110cm) long
• ammeter
• two 1,5 V cells
• metre ruler
• tape
• insulated copper conducting wires

The wire must be without any insulation. Copper wire often has varnish as insulation and therefore will not work. Nichrome or constantan wire work well (between 28 and 32 SWG - the SWG rating indicates the cross-sectional area of the wire). Other wires can also be used. The length is a suggestion only. If you are only able to obtain smaller lengths, the learners will take fewer readings. If you do not have an ammeter then use a light bulb as an indicator of current strength. You want about 3 V for this circuit so any combination of cells which provide 3 V would work, or a low voltage power supply.

VARIABLES:

Which variables would we need to keep constant in an investigation such as this?

The wires used must all be of the same material (for example, copper). The number of batteries used to provide energy in the circuit.

Which variable is the independent variable?

The independent variable is the length of the wire.

Which variable is the dependent variable?

The dependent variable is the amount of resistance provided by the different length of the wire, as measured by changes in current.

METHOD:

1. Tape the resistance wire to the metre ruler. Make sure the wire is stretched flat and that the numbers on the ruler are still visible.
2. Assemble a circuit according to the following diagram.

3. Use the flying lead and touch it to the resistance wire at the 1 m mark. Record the ammeter reading.
4. Use the flying lead and touch it to the resistance wire at the 0,9 m mark. Record the ammeter reading.
5. Move the flying lead in 10 cm intervals until you have 10 readings. Record the ammeter reading each time.

RESULTS:

The results obtained in this investigation will depend on the type of material chosen and the SWG rating of the wire chosen.

Record your results in the following table.

 Length of wire (m) Ammeter reading (A) 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1

Draw a graph to show the relationship between the length of the resistor and the ammeter readings.

The graph should have the independent variable (length) on the x-axis and the dependent variable (ammeter reading) on the y-axis. The graph should show that as the length of the resistance wire increases, the ammeter reading decreases. It is unlikely to be a straight line. Learners must provide a heading, such as "Graph showing the change in current in a circuit as the length of a conducting wire is varied."

As an extension, learners can rather calculate the resistance using the formula R = V/I and then plot resistance versus length directly.

CONCLUSIONS:

Look at your table and graph. What conclusion can you draw?

There is a relationship between the length of the resistor and the current strength. Increasing the length of the conductor decreases the current strength.

What is causing the decrease in current strength?

Current is affected by resistance. If the current has decreased, it must mean that the resistance has increased.

What can you conclude about the relationship between the length of the resistor and the resistance of the resistor?

Increasing the length of the resistor increases the resistance of the resistor.

The length of the resistor affects how much resistance it offers to the circuit. The longer the resistor, the more resistance it has. The shorter the resistor, the less resistance it has.

Optional, online simulation:

If you have access to the internet for your students then you can do an online simulation. The simulation referred to in the activity can be found here http://www.ktaggart.co.uk/physics/Simulations/EJS/ResistanceWire.html

Alternatively, the PhET simulation for investigating resistance in a wire listed in the previous visit box can also be used.

The simulation opens by showing the value of the resistance of a 50 cm length of 26 SWG constantan wire as being 1,585 ohms. This resistance is given to three decimal places and shown in the yellow area of the display screen. There is a reset button at the top of the screen which will set the simulation back to the resistance of a 50 cm length of 26 SWG constantan wire.

There are various sliders on the screen where you can adjust the length, material type and cross-sectional area of the resistor. Take some time to familiarise yourself with the simulation before allowing the learners to use the simulation. Here is an example of the investigation you could do.

Investigation: How does the length of the wire affect the resistance of the wire ?

MATERIALS AND APPARATUS:

METHOD:

1. Open the simulation.
2. Change the length of the resistor according to the table below. Don't change any other factors.
3. Write down the resistance (shown in the yellow block on the screen) which corresponds to each wire length.
4. Draw a graph of your results.

RESULTS:

 Wire length (cm) Resistance (ohms) 10 20 30 40 50 60 70 80 90 100

Draw a graph of wire length versus resistance

CONCLUSIONS

As the length of the wire increases, the resistance increases. In fact, if we double the length then the resistance doubles. This means that resistance is directly proportional to the length of the wire.

Experiment with this simulation by changing the wire diameter and length to see the effect each has on the resistance. http://www.ktaggart.co.uk/physics/Simulations/EJS/ResistanceWire.html

Longer wires have more resistance than shorter wires. Let's take a close-up look at the filament of an incandescent light bulb.

Tungsten is also known as wolfram. It is a chemical element with the symbol W and atomic number 74. A close-up photograph of the tungsten filament in an incandescent light bulb.

You can see that the filament is made up of coils of tungsten wrapped up tightly. We want to fit a very long wire into a small space. The electrons have to travel through this very long, high-resistance wire. How is this more beneficial compared to having a shorter wire? Discuss this with your class.

A longer wire increases the resistance. The electrons travelling through the wire therefore transfer a lot of energy to the wire in the form of heat and light. A shorter wire would not provide as much heat and light.

### Temperature of the conductor

We are not going to investigate this factor as it is difficult to control the temperature of a wire in an investigation.

The last factor which affects resistance is the temperature of the conductor. The hotter a resistor becomes the more resistance it has. The atoms of the conductor vibrate much faster when they are hot due to the increase in kinetic energy. This makes it more difficult for the electric current to move through. Cold resistors offer less resistance to the circuit.

Factors affecting resistance.

• Resistance is the opposition to electric current in a circuit.
• A resistor is an electrical component used to add resistance to an electrical circuit.
• Resistance can be useful. For example, the filament in a light bulb and a toaster have a high resistance.
• There are four factors which influence the amount of resistance of a conductor: type of material, length, thickness and temperature.
• Different materials will offer different amounts of resistance.
• Longer length resistors will offer more resistance than shorter resistors.
• Thicker resistors offer less resistance than thinner resistors.
• Hot resistors offer more resistance than cold resistors.

Concept Map

Complete the following concept map to summarise what you have learnt about resistance in this chapter. For example, when looking at the factors that affect resistance, you need to describe the relationship by completing the sentences.

Teacher's version

# Revision questions

There are many useful applications of resistance. Give two examples of appliances which require large resistances in order to function. [2 marks]

Kettle, stove top plate, light bulb, heaters.

Look at the following photograph of an electric toaster. An electric toaster.
1. Do you think the element in the toaster has a low or high resistance? Explain your answer. [2 marks]

2. Explain the energy transfers which take place within the heating element of the toaster. [3 marks]

3. Is there wasted energy in this system? If so, what is it and why can we consider it 'wasted energy'? [2 marks]

• The element has a high resistance as it is opposing the electric current which causes the wire to heat up and glow.
• The electrical energy is transferred to the heating element. The heating element has a high resistance and so a lot of energy is transferred. The element glows and warms up. The heating element becomes very hot and transfers that energy to the bread to toast it.
• The light given off by the element can be considered as wasted energy as this is not used to toast the bread.

List the factors which affect the amount of resistance in a resistor [4 marks]

• length of material
• type of material
• temperature of resistor
• cross-sectional area of resistor

The pictures below show two pieces of the same type of metal wire with the same diameter.

Which piece has the higher resistance? Explain why. [2 marks]

Wire B has the higher resistance as it is longer. The longer the resistor, the higher the resistance as the electrons have further to move through the wire.

The pictures below show cross sections of two pieces of the same type of metal wire. The pieces are the same length but have different diameters.

Which piece has the lower resistance? Explain why. [2 marks]

Wire A has the lower resistance. It has a thicker diameter and so there is less resistance as there is more space for the electrons to move through the wire.

Look at the image of a stove top heating element. The heating element offers a large resistance to the flow of electric current.

1. Why is the heating element in the shape of a coil? [2 marks]

2. What is the input energy in this system? [1 mark]

3. What is the output energy? [2 marks]

4. Is all of the energy transferred to the heating element useful? [2 marks]

1. This increases the surface area for heating. It allows a longer resistor in a smaller area to increase the efficiency of the stove.

2. Electrical energy.

3. Light and heat.

4. The purpose of the heating element is to heat food. Most of the energy transferred to the element is used to heat the element. This means that most of the energy is useful. The energy which is used to produce light is not useful, but it is a small amount compared to the heat.

Total [26 marks]