Textbook: Resistance and factors that change it | Technical Science for Grade 10 PDF Download

 Page 1


 technical science GRADE 10 325
c hapter 14 r esistance and factors that change it
In Chapter 13 you learned that a potential difference can cause a current in a circuit. But how 
big is that current? In this chapter you will learn that the current can be big or small depending 
on the resistance in the circuit; you will learn about the relationship between voltage, current 
and resistance. You will go on to learn the four factors that affect resistance – length, thickness, 
temperature and the nature of the substance – and conduct an experiment involving these. In 
Chapter 15 you will learn how to connect resistors in series and parallel and how to work out 
the resistance in a circuit.
unit 14.1 conductors and resistance
r esistance
definition:  Resistance is the opposition to the flow of an electric current.  
The unit of resistance is the ohm (O): 1 O = 1 V  A 
-1
 
When you connect a light bulb in a circuit with one cell and one indicator light bulb, that light 
will glow. However, as you add bulbs in series to such a circuit, the indicator light becomes 
dimmer and dimmer. As we add more bulbs in series, the battery finds it harder to push current 
around the circuit, and the current becomes smaller and smaller. Why is this? The reason is that 
the filament wire in each bulb has resistance.
Electric charges in a resistor must push past the electrons of metal atoms, and so they give up 
energy to the metal atoms. The energy makes the resistor hot, and the charges flow slowly 
because they are losing energy. The resistor opposes the flow of charges and we say a resistor 
has resistance.
The higher the resistance, the smaller the current. The lower the resistance, the bigger 
the current.
Good conductors have low resistance, bad conductors have high resistance. We can 
therefore also say that insulators have extremely high resistance – we use insulators because 
they have such high resistance that current will not pass through them.
Good conductors are copper, gold, silver and aluminium, for example. They have very low 
resistance. 
Medium-bad conductors are tungsten, graphite (the black stuff in your pencil) and 
nichrome. They conduct electricity, but not very well. The filament wire in a bulb is made 
of tungsten.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   325 2015/12/17   10:06 AM
Page 2


 technical science GRADE 10 325
c hapter 14 r esistance and factors that change it
In Chapter 13 you learned that a potential difference can cause a current in a circuit. But how 
big is that current? In this chapter you will learn that the current can be big or small depending 
on the resistance in the circuit; you will learn about the relationship between voltage, current 
and resistance. You will go on to learn the four factors that affect resistance – length, thickness, 
temperature and the nature of the substance – and conduct an experiment involving these. In 
Chapter 15 you will learn how to connect resistors in series and parallel and how to work out 
the resistance in a circuit.
unit 14.1 conductors and resistance
r esistance
definition:  Resistance is the opposition to the flow of an electric current.  
The unit of resistance is the ohm (O): 1 O = 1 V  A 
-1
 
When you connect a light bulb in a circuit with one cell and one indicator light bulb, that light 
will glow. However, as you add bulbs in series to such a circuit, the indicator light becomes 
dimmer and dimmer. As we add more bulbs in series, the battery finds it harder to push current 
around the circuit, and the current becomes smaller and smaller. Why is this? The reason is that 
the filament wire in each bulb has resistance.
Electric charges in a resistor must push past the electrons of metal atoms, and so they give up 
energy to the metal atoms. The energy makes the resistor hot, and the charges flow slowly 
because they are losing energy. The resistor opposes the flow of charges and we say a resistor 
has resistance.
The higher the resistance, the smaller the current. The lower the resistance, the bigger 
the current.
Good conductors have low resistance, bad conductors have high resistance. We can 
therefore also say that insulators have extremely high resistance – we use insulators because 
they have such high resistance that current will not pass through them.
Good conductors are copper, gold, silver and aluminium, for example. They have very low 
resistance. 
Medium-bad conductors are tungsten, graphite (the black stuff in your pencil) and 
nichrome. They conduct electricity, but not very well. The filament wire in a bulb is made 
of tungsten.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   325 2015/12/17   10:06 AM
326 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
Semi-conductors you will use are mixtures of silicon or germanium, with small amounts 
of other elements like phosphorus. Their resistance decreases as you increase the pd (voltage) 
across them. They conduct better and better as they get hotter and hotter.
Very bad conductors are plastic, glass and wood, for example. They have such high resistance 
that a battery cannot make any current fl ow through them. We say they are good insulators. 
The plastic covering on electrical wire is the insulator on the wire.
But even good insulators will conduct current if the voltage across them is high enough. 
For example, air will conduct a current of lightning when the voltage between the cloud 
and the ground is millions of volts.
Quick activity: a range of resistivity* from very low to very high 
Draw the diagram below in your notebook, then write the names of these substances at a 
suitable place on the scale: 
nichrome wire; plastic; copper; gold; glass; cooking foil; pencil graphite; dry paper; paper wet 
with salt water; wet human skin; moist air; dry air
Very good conductors: Good conductors: Poor conductors: Very poor conductors:
very low resistivity very high resistivity
 e.g. silver
the reason why materials have different resistivity*
NOTE: When we talk about the tendency of a specifi c material 
to resist the fl ow of current, we use the word “resistivity” 
instead of “resistance”. This is because “resistance” is a quality 
of a specifi c object and it depends on the object’s length, 
thickness and shape. “Resistivity” is a quality of a specifi c 
material – it depends only on the nature of the material and 
not on its length, thickness or shape.
We know that even good conductors, such as copper, have 
some resistance. The reason for this resistance is that electrons 
(which have negative charge, remember) do not move in 
straight lines; they tug on the positive nuclei of the metal 
atoms as they go past and the nuclei tug on them. Remember 
that if the current is 1 ampere, 6,25 × 10
18
 electrons pass 
each point in the resistor every second! That huge number 
of electrons can make the metal atoms vibrate faster as each 
electron pulls on every atom it passes. The result is that 
the metal gets hot.
? ? resistivity is a quantitative 
measure of a material’ s resistance 
to the fl ow of current. It 
depends only on the nature 
of the material and not on the 
material’ s shape or size.
6,25 x 10
18
 electrons is
6 250 000 000 000 000 000 
electrons!
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   326 2015/12/17   10:06 AM
Page 3


 technical science GRADE 10 325
c hapter 14 r esistance and factors that change it
In Chapter 13 you learned that a potential difference can cause a current in a circuit. But how 
big is that current? In this chapter you will learn that the current can be big or small depending 
on the resistance in the circuit; you will learn about the relationship between voltage, current 
and resistance. You will go on to learn the four factors that affect resistance – length, thickness, 
temperature and the nature of the substance – and conduct an experiment involving these. In 
Chapter 15 you will learn how to connect resistors in series and parallel and how to work out 
the resistance in a circuit.
unit 14.1 conductors and resistance
r esistance
definition:  Resistance is the opposition to the flow of an electric current.  
The unit of resistance is the ohm (O): 1 O = 1 V  A 
-1
 
When you connect a light bulb in a circuit with one cell and one indicator light bulb, that light 
will glow. However, as you add bulbs in series to such a circuit, the indicator light becomes 
dimmer and dimmer. As we add more bulbs in series, the battery finds it harder to push current 
around the circuit, and the current becomes smaller and smaller. Why is this? The reason is that 
the filament wire in each bulb has resistance.
Electric charges in a resistor must push past the electrons of metal atoms, and so they give up 
energy to the metal atoms. The energy makes the resistor hot, and the charges flow slowly 
because they are losing energy. The resistor opposes the flow of charges and we say a resistor 
has resistance.
The higher the resistance, the smaller the current. The lower the resistance, the bigger 
the current.
Good conductors have low resistance, bad conductors have high resistance. We can 
therefore also say that insulators have extremely high resistance – we use insulators because 
they have such high resistance that current will not pass through them.
Good conductors are copper, gold, silver and aluminium, for example. They have very low 
resistance. 
Medium-bad conductors are tungsten, graphite (the black stuff in your pencil) and 
nichrome. They conduct electricity, but not very well. The filament wire in a bulb is made 
of tungsten.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   325 2015/12/17   10:06 AM
326 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
Semi-conductors you will use are mixtures of silicon or germanium, with small amounts 
of other elements like phosphorus. Their resistance decreases as you increase the pd (voltage) 
across them. They conduct better and better as they get hotter and hotter.
Very bad conductors are plastic, glass and wood, for example. They have such high resistance 
that a battery cannot make any current fl ow through them. We say they are good insulators. 
The plastic covering on electrical wire is the insulator on the wire.
But even good insulators will conduct current if the voltage across them is high enough. 
For example, air will conduct a current of lightning when the voltage between the cloud 
and the ground is millions of volts.
Quick activity: a range of resistivity* from very low to very high 
Draw the diagram below in your notebook, then write the names of these substances at a 
suitable place on the scale: 
nichrome wire; plastic; copper; gold; glass; cooking foil; pencil graphite; dry paper; paper wet 
with salt water; wet human skin; moist air; dry air
Very good conductors: Good conductors: Poor conductors: Very poor conductors:
very low resistivity very high resistivity
 e.g. silver
the reason why materials have different resistivity*
NOTE: When we talk about the tendency of a specifi c material 
to resist the fl ow of current, we use the word “resistivity” 
instead of “resistance”. This is because “resistance” is a quality 
of a specifi c object and it depends on the object’s length, 
thickness and shape. “Resistivity” is a quality of a specifi c 
material – it depends only on the nature of the material and 
not on its length, thickness or shape.
We know that even good conductors, such as copper, have 
some resistance. The reason for this resistance is that electrons 
(which have negative charge, remember) do not move in 
straight lines; they tug on the positive nuclei of the metal 
atoms as they go past and the nuclei tug on them. Remember 
that if the current is 1 ampere, 6,25 × 10
18
 electrons pass 
each point in the resistor every second! That huge number 
of electrons can make the metal atoms vibrate faster as each 
electron pulls on every atom it passes. The result is that 
the metal gets hot.
? ? resistivity is a quantitative 
measure of a material’ s resistance 
to the fl ow of current. It 
depends only on the nature 
of the material and not on the 
material’ s shape or size.
6,25 x 10
18
 electrons is
6 250 000 000 000 000 000 
electrons!
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   326 2015/12/17   10:06 AM
 technical science GRADE 10 327
But why is copper such a good conductor and plastic such a poor conductor? The answer is that 
they are made of different kinds of atoms and molecules. The atoms of a copper wire have lots 
of electrons that can move from atom to atom if they feel an electric force pulling them. But in 
the plastic, almost all the electrons are tightly held in the atoms, and they cannot move from 
atom to atom. You cannot pass a current through a non-conductor; however, as you saw in the 
electrostatics activities, you can remove some electrons from the surface of a non-conductor.
Measuring resistance – two methods
The unit of resistance is the ohm, and its symbol is O.
We use two methods to measure resistance: in the first method we compare current and voltage, 
and in the second method we use the multimeter on the ohms range.
1. the method of comparing current and voltage
Imagine a very long piece of copper wire. If we need 2 volts to push 2 amperes of current 
through the wire, we say that the resistance of that wire is 1 ohm (1 O).
For that piece of wire, we will need 3 volts to push a 3 ampere current through the wire, because 
its resistance is 1 ohm. 
The resistance of that piece of copper wire is 1 ohm and this means that when the voltage 
changes, the current also changes. But if we compare the voltage and the current, we get a 
ratio that is always the same – 1 ohm for that piece of copper wire.
The resistance is a ratio:
 resistance =   
voltage
 
_______
 
current
  
This ratio is called the formula for resistance. 
In symbols:
 R =   
V
 
__
 
I
  
where:
•	 R is resistance measured in ohms (O)
•	 V is voltage measured in volts (V)
•	 I is current measured in amperes (A)
To calculate V, we need V as the subject of the formula:  
 R × I = V
We write:
 V = RI  or V = IR
To calculate I, we need I to be the subject of the formula:
 I =   
V
 
__
 
R
  
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   327 2015/12/17   10:06 AM
Page 4


 technical science GRADE 10 325
c hapter 14 r esistance and factors that change it
In Chapter 13 you learned that a potential difference can cause a current in a circuit. But how 
big is that current? In this chapter you will learn that the current can be big or small depending 
on the resistance in the circuit; you will learn about the relationship between voltage, current 
and resistance. You will go on to learn the four factors that affect resistance – length, thickness, 
temperature and the nature of the substance – and conduct an experiment involving these. In 
Chapter 15 you will learn how to connect resistors in series and parallel and how to work out 
the resistance in a circuit.
unit 14.1 conductors and resistance
r esistance
definition:  Resistance is the opposition to the flow of an electric current.  
The unit of resistance is the ohm (O): 1 O = 1 V  A 
-1
 
When you connect a light bulb in a circuit with one cell and one indicator light bulb, that light 
will glow. However, as you add bulbs in series to such a circuit, the indicator light becomes 
dimmer and dimmer. As we add more bulbs in series, the battery finds it harder to push current 
around the circuit, and the current becomes smaller and smaller. Why is this? The reason is that 
the filament wire in each bulb has resistance.
Electric charges in a resistor must push past the electrons of metal atoms, and so they give up 
energy to the metal atoms. The energy makes the resistor hot, and the charges flow slowly 
because they are losing energy. The resistor opposes the flow of charges and we say a resistor 
has resistance.
The higher the resistance, the smaller the current. The lower the resistance, the bigger 
the current.
Good conductors have low resistance, bad conductors have high resistance. We can 
therefore also say that insulators have extremely high resistance – we use insulators because 
they have such high resistance that current will not pass through them.
Good conductors are copper, gold, silver and aluminium, for example. They have very low 
resistance. 
Medium-bad conductors are tungsten, graphite (the black stuff in your pencil) and 
nichrome. They conduct electricity, but not very well. The filament wire in a bulb is made 
of tungsten.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   325 2015/12/17   10:06 AM
326 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
Semi-conductors you will use are mixtures of silicon or germanium, with small amounts 
of other elements like phosphorus. Their resistance decreases as you increase the pd (voltage) 
across them. They conduct better and better as they get hotter and hotter.
Very bad conductors are plastic, glass and wood, for example. They have such high resistance 
that a battery cannot make any current fl ow through them. We say they are good insulators. 
The plastic covering on electrical wire is the insulator on the wire.
But even good insulators will conduct current if the voltage across them is high enough. 
For example, air will conduct a current of lightning when the voltage between the cloud 
and the ground is millions of volts.
Quick activity: a range of resistivity* from very low to very high 
Draw the diagram below in your notebook, then write the names of these substances at a 
suitable place on the scale: 
nichrome wire; plastic; copper; gold; glass; cooking foil; pencil graphite; dry paper; paper wet 
with salt water; wet human skin; moist air; dry air
Very good conductors: Good conductors: Poor conductors: Very poor conductors:
very low resistivity very high resistivity
 e.g. silver
the reason why materials have different resistivity*
NOTE: When we talk about the tendency of a specifi c material 
to resist the fl ow of current, we use the word “resistivity” 
instead of “resistance”. This is because “resistance” is a quality 
of a specifi c object and it depends on the object’s length, 
thickness and shape. “Resistivity” is a quality of a specifi c 
material – it depends only on the nature of the material and 
not on its length, thickness or shape.
We know that even good conductors, such as copper, have 
some resistance. The reason for this resistance is that electrons 
(which have negative charge, remember) do not move in 
straight lines; they tug on the positive nuclei of the metal 
atoms as they go past and the nuclei tug on them. Remember 
that if the current is 1 ampere, 6,25 × 10
18
 electrons pass 
each point in the resistor every second! That huge number 
of electrons can make the metal atoms vibrate faster as each 
electron pulls on every atom it passes. The result is that 
the metal gets hot.
? ? resistivity is a quantitative 
measure of a material’ s resistance 
to the fl ow of current. It 
depends only on the nature 
of the material and not on the 
material’ s shape or size.
6,25 x 10
18
 electrons is
6 250 000 000 000 000 000 
electrons!
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   326 2015/12/17   10:06 AM
 technical science GRADE 10 327
But why is copper such a good conductor and plastic such a poor conductor? The answer is that 
they are made of different kinds of atoms and molecules. The atoms of a copper wire have lots 
of electrons that can move from atom to atom if they feel an electric force pulling them. But in 
the plastic, almost all the electrons are tightly held in the atoms, and they cannot move from 
atom to atom. You cannot pass a current through a non-conductor; however, as you saw in the 
electrostatics activities, you can remove some electrons from the surface of a non-conductor.
Measuring resistance – two methods
The unit of resistance is the ohm, and its symbol is O.
We use two methods to measure resistance: in the first method we compare current and voltage, 
and in the second method we use the multimeter on the ohms range.
1. the method of comparing current and voltage
Imagine a very long piece of copper wire. If we need 2 volts to push 2 amperes of current 
through the wire, we say that the resistance of that wire is 1 ohm (1 O).
For that piece of wire, we will need 3 volts to push a 3 ampere current through the wire, because 
its resistance is 1 ohm. 
The resistance of that piece of copper wire is 1 ohm and this means that when the voltage 
changes, the current also changes. But if we compare the voltage and the current, we get a 
ratio that is always the same – 1 ohm for that piece of copper wire.
The resistance is a ratio:
 resistance =   
voltage
 
_______
 
current
  
This ratio is called the formula for resistance. 
In symbols:
 R =   
V
 
__
 
I
  
where:
•	 R is resistance measured in ohms (O)
•	 V is voltage measured in volts (V)
•	 I is current measured in amperes (A)
To calculate V, we need V as the subject of the formula:  
 R × I = V
We write:
 V = RI  or V = IR
To calculate I, we need I to be the subject of the formula:
 I =   
V
 
__
 
R
  
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   327 2015/12/17   10:06 AM
328 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
2. the method of reading an ohm-meter
Your multimeter has a resistance range, too. It may be 
marked with an ohm symbol, O.
A.  Turn the rotary switch until the arrow points to 200 O.
B. Plug the red lead into the V O mA socket. Touch 
the probes or croc clips together: you should hear 
a faint beeping noise and the display should show 
approximately zero ohms resistance. 
C. Switch off the current through the nichrome wire. This 
is important, because you cannot measure resistance 
while a circuit is connected to battery or power supply.
D. Connect the red and black leads to the ends of the 
nichrome wire.
E. Think: What is the resistance of that piece of 
nichrome wire, according to the ohm-meter?
Worked examples: calculating the resistance of a conductor
Here are two examples of how we calculate the resistance of a conductor.
1. If the voltage across a piece of nichrome wire is 6 volts, and this causes a current of 2 ampere 
through the wire, what is the resistance of the wire?
 Solution
 Given V = 6 V; I = 2 A
 Unknown resistance
 Formula R =   
V
 
__
 
I
  
   =   
6
 
__
 
2
  
   = 3 O
2. If the voltage across a glowing bulb is 1,4 volts and the current is 0,5 ampere, what is the 
resistance of the bulb?
 Solution
 Given V = 1,4 V; I = 0,5 A
 Unknown resistance
 Formula  R =   
V
 
__
 
I
  
   =   
1,4
 
___
 
0,5
  
   = 2,8 O
No current from 
the battery!
Figure 14.1 Measure the resistance in 
the nichrome wire.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   328 2015/12/17   10:06 AM
Page 5


 technical science GRADE 10 325
c hapter 14 r esistance and factors that change it
In Chapter 13 you learned that a potential difference can cause a current in a circuit. But how 
big is that current? In this chapter you will learn that the current can be big or small depending 
on the resistance in the circuit; you will learn about the relationship between voltage, current 
and resistance. You will go on to learn the four factors that affect resistance – length, thickness, 
temperature and the nature of the substance – and conduct an experiment involving these. In 
Chapter 15 you will learn how to connect resistors in series and parallel and how to work out 
the resistance in a circuit.
unit 14.1 conductors and resistance
r esistance
definition:  Resistance is the opposition to the flow of an electric current.  
The unit of resistance is the ohm (O): 1 O = 1 V  A 
-1
 
When you connect a light bulb in a circuit with one cell and one indicator light bulb, that light 
will glow. However, as you add bulbs in series to such a circuit, the indicator light becomes 
dimmer and dimmer. As we add more bulbs in series, the battery finds it harder to push current 
around the circuit, and the current becomes smaller and smaller. Why is this? The reason is that 
the filament wire in each bulb has resistance.
Electric charges in a resistor must push past the electrons of metal atoms, and so they give up 
energy to the metal atoms. The energy makes the resistor hot, and the charges flow slowly 
because they are losing energy. The resistor opposes the flow of charges and we say a resistor 
has resistance.
The higher the resistance, the smaller the current. The lower the resistance, the bigger 
the current.
Good conductors have low resistance, bad conductors have high resistance. We can 
therefore also say that insulators have extremely high resistance – we use insulators because 
they have such high resistance that current will not pass through them.
Good conductors are copper, gold, silver and aluminium, for example. They have very low 
resistance. 
Medium-bad conductors are tungsten, graphite (the black stuff in your pencil) and 
nichrome. They conduct electricity, but not very well. The filament wire in a bulb is made 
of tungsten.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   325 2015/12/17   10:06 AM
326 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
Semi-conductors you will use are mixtures of silicon or germanium, with small amounts 
of other elements like phosphorus. Their resistance decreases as you increase the pd (voltage) 
across them. They conduct better and better as they get hotter and hotter.
Very bad conductors are plastic, glass and wood, for example. They have such high resistance 
that a battery cannot make any current fl ow through them. We say they are good insulators. 
The plastic covering on electrical wire is the insulator on the wire.
But even good insulators will conduct current if the voltage across them is high enough. 
For example, air will conduct a current of lightning when the voltage between the cloud 
and the ground is millions of volts.
Quick activity: a range of resistivity* from very low to very high 
Draw the diagram below in your notebook, then write the names of these substances at a 
suitable place on the scale: 
nichrome wire; plastic; copper; gold; glass; cooking foil; pencil graphite; dry paper; paper wet 
with salt water; wet human skin; moist air; dry air
Very good conductors: Good conductors: Poor conductors: Very poor conductors:
very low resistivity very high resistivity
 e.g. silver
the reason why materials have different resistivity*
NOTE: When we talk about the tendency of a specifi c material 
to resist the fl ow of current, we use the word “resistivity” 
instead of “resistance”. This is because “resistance” is a quality 
of a specifi c object and it depends on the object’s length, 
thickness and shape. “Resistivity” is a quality of a specifi c 
material – it depends only on the nature of the material and 
not on its length, thickness or shape.
We know that even good conductors, such as copper, have 
some resistance. The reason for this resistance is that electrons 
(which have negative charge, remember) do not move in 
straight lines; they tug on the positive nuclei of the metal 
atoms as they go past and the nuclei tug on them. Remember 
that if the current is 1 ampere, 6,25 × 10
18
 electrons pass 
each point in the resistor every second! That huge number 
of electrons can make the metal atoms vibrate faster as each 
electron pulls on every atom it passes. The result is that 
the metal gets hot.
? ? resistivity is a quantitative 
measure of a material’ s resistance 
to the fl ow of current. It 
depends only on the nature 
of the material and not on the 
material’ s shape or size.
6,25 x 10
18
 electrons is
6 250 000 000 000 000 000 
electrons!
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   326 2015/12/17   10:06 AM
 technical science GRADE 10 327
But why is copper such a good conductor and plastic such a poor conductor? The answer is that 
they are made of different kinds of atoms and molecules. The atoms of a copper wire have lots 
of electrons that can move from atom to atom if they feel an electric force pulling them. But in 
the plastic, almost all the electrons are tightly held in the atoms, and they cannot move from 
atom to atom. You cannot pass a current through a non-conductor; however, as you saw in the 
electrostatics activities, you can remove some electrons from the surface of a non-conductor.
Measuring resistance – two methods
The unit of resistance is the ohm, and its symbol is O.
We use two methods to measure resistance: in the first method we compare current and voltage, 
and in the second method we use the multimeter on the ohms range.
1. the method of comparing current and voltage
Imagine a very long piece of copper wire. If we need 2 volts to push 2 amperes of current 
through the wire, we say that the resistance of that wire is 1 ohm (1 O).
For that piece of wire, we will need 3 volts to push a 3 ampere current through the wire, because 
its resistance is 1 ohm. 
The resistance of that piece of copper wire is 1 ohm and this means that when the voltage 
changes, the current also changes. But if we compare the voltage and the current, we get a 
ratio that is always the same – 1 ohm for that piece of copper wire.
The resistance is a ratio:
 resistance =   
voltage
 
_______
 
current
  
This ratio is called the formula for resistance. 
In symbols:
 R =   
V
 
__
 
I
  
where:
•	 R is resistance measured in ohms (O)
•	 V is voltage measured in volts (V)
•	 I is current measured in amperes (A)
To calculate V, we need V as the subject of the formula:  
 R × I = V
We write:
 V = RI  or V = IR
To calculate I, we need I to be the subject of the formula:
 I =   
V
 
__
 
R
  
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   327 2015/12/17   10:06 AM
328 chapter 14 RESISTANCE AND FACTORS THAT CHANGE IT
2. the method of reading an ohm-meter
Your multimeter has a resistance range, too. It may be 
marked with an ohm symbol, O.
A.  Turn the rotary switch until the arrow points to 200 O.
B. Plug the red lead into the V O mA socket. Touch 
the probes or croc clips together: you should hear 
a faint beeping noise and the display should show 
approximately zero ohms resistance. 
C. Switch off the current through the nichrome wire. This 
is important, because you cannot measure resistance 
while a circuit is connected to battery or power supply.
D. Connect the red and black leads to the ends of the 
nichrome wire.
E. Think: What is the resistance of that piece of 
nichrome wire, according to the ohm-meter?
Worked examples: calculating the resistance of a conductor
Here are two examples of how we calculate the resistance of a conductor.
1. If the voltage across a piece of nichrome wire is 6 volts, and this causes a current of 2 ampere 
through the wire, what is the resistance of the wire?
 Solution
 Given V = 6 V; I = 2 A
 Unknown resistance
 Formula R =   
V
 
__
 
I
  
   =   
6
 
__
 
2
  
   = 3 O
2. If the voltage across a glowing bulb is 1,4 volts and the current is 0,5 ampere, what is the 
resistance of the bulb?
 Solution
 Given V = 1,4 V; I = 0,5 A
 Unknown resistance
 Formula  R =   
V
 
__
 
I
  
   =   
1,4
 
___
 
0,5
  
   = 2,8 O
No current from 
the battery!
Figure 14.1 Measure the resistance in 
the nichrome wire.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   328 2015/12/17   10:06 AM
 technical science GRADE 10 329
activity 1 calculation exercises
1. Calculate the resistance of the piece of nichrome wire in 
Figure 14.2. Give your answer in ohms.
2. We can put 3 cells in the circuit, or 2 cells, or 1 cell. By 
changing the numbers of cells in Figure 14.2, we can put 
different voltages across the nichrome wire. Calculate 
the resistance of that piece of nichrome wire when:
a) the voltmeter reads 6 volts and the ammeter reads 
0,5 amperes
b) the readings are 3,6 volts and 0,3 amperes
c) the readings are 1,2 volts and 0,1 amperes
d) the readings are 2,4 volts and 0,2 amperes
3. What do you notice in the resistances in Question 2? 
This quantity is the resistance of that piece of nichrome 
wire you see in Figure 14.2.
4. Light-emitting diodes (the small red, green, white or yellow lights you see on electronic 
equipment) are also called LEDs. If you use an LED in a circuit, you must connect a resistor 
in series with it so that the current through the LED is 0,02 ampere or less. If the circuit uses 
a 9 volt battery, what is the ohm value of the resistor?
5. You measure a current of 0,01 ampere through a resistor of 470 ohms. What is the potential 
difference V across the resistor?
6. A resistor of 5 ohms is connected to a 1,5 volt cell. What current does this potential 
difference cause? 
resistors let us control current in electrical equipment 
Inside electrical equipment like a radio, you 
will see many small coloured cylinders. Look at 
Figure 14.3. Many of these coloured cylinders 
are resistors. The radio designer* chose 
the right resistors to let small currents go to 
some parts of the radio and big currents to 
other parts. 
These resistors are made from carbon, and the 
factory carefully cuts away some carbon until the resistor has a 
certain resistance, which stays constant*. 
The factory then puts coloured rings on each resistor to show 
how much resistance it has. They use a code which you will 
fi nd in the Resource Pages.
Figure 14.2 Calculate the resistance in 
the nichrome wire.
0,4 ampere
4,8 volts
Figure 14.3 Resistors in a circuit board
? ? See Glossary.
TechSci_G10-LB-Eng-DBE3_9781431522842.indb   329 2015/12/17   10:06 AM