Download Apply Ohm`s Law - OTEN - Your Decision

Contents
These are samples of learning materials and may not necessarily be
exactly the same as those in the actual course.
Contents
1
Introduction
2
Ohm’s law relationships
3
The Ohm’s law equation
4
Calculating circuit current
6
Calculating resistance
7
Current direction and voltage polarity
Meter connection and use
8
10
Measuring voltage
10
Measuring current
11
Measuring voltage and current simultaneously
12
Connecting to the circuit
12
Range setup
13
Reading analogue scales
13
Zero adjust
17
Digital meters
19
Meter selection
20
Analogue versus digital meters
20
Multimeters
21
Summary
24
Answers
25
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
1
Introduction
Ohm's Law states the relationship between the voltage across a resistor, the
current through a resistor and resistance. Ohm’s law allows us to perform
calculations to determine any one of current, voltage, or resistance from the
other two.
Ohm’s Law is the most extensively used equation in electrical theory.
After completing this topic, you should be able to:
2
•
state the relationship between voltage and current from measured values
in a simple circuit
•
calculate the voltage, current or resistance in a circuit given any two of
these quantities
•
interpret and draw graphs to show relationships of voltage, current, and
resistance
•
explain the relationship between voltage, current and resistance.
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
Ohm’s law relationships
In an electrical circuit, there is a definite relationship between current,
voltage and resistance that was discovered by Georg Ohm. He expressed the
relationships in written form, which became known as Ohm’s law.
In a circuit with a constant resistance, any increase in the applied voltage
will cause a proportional increase in current. This means for example that:
•
doubling the voltage doubles the current,
•
halving the voltage halves the current,
•
quartering the voltage quarters the current, and so on.
This relationship is one of direct proportion and can be written as:
I∝E
Read this as ‘current is proportional to emf’
Now if we maintain a constant supply voltage and vary the circuit
resistance, the circuit current changes in inverse proportion. That is:
•
doubling the resistance halves the current,
•
halving the resistance doubles the current,
•
quartering the resistance increase current four times, and so on.
This relationship is one of inverse proportion and can be written as:
1
I ∝
R
Read this as ‘current is inversely proportional to resistance’
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
3
The Ohm’s law equation
Ohm’s Law states that the current flowing through a resistance is:
•
directly proportional to the potential difference between them
•
inversely proportional to the resistance.
Ohm’s law may be written as:
V
I=
R
where: V = voltage (volt)
R = resistance (ohm)
I = current (ampere)
Written as shown above, the Ohms Law equation expresses the current
resulting from a particular voltage (V) and resistance (R). But you will just
as often be given current (I) and resistance (R) and asked to find voltage (V).
Or you may be given current (I) and voltage (V) and asked to find resistance
(R).
Transposing to find R
Start with the basic equation:
V
I=
R
Multiply both sides of the equation by R:
V
R× I = × R
R
The letter R cancels on right hand side of the equation giving:
I ×R =V
Divide both sides by I:
R ×I V
=
I
I
The letter I cancels on left hand side of equation giving:
V
R=
I
4
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
Transposing to find V
Start with the basic equation:
V
I=
R
Multiply both sides of the equation by R:
V
I×R = ×R
R
Cancel Rs on right hand side:
I ×R =V
Usually it is expressed as:
V = IR
We have three forms for the Ohm's Law relation:
V
I=
R
V
R=
I
V = IR
Note that in any of these forms, the voltage (V) may also be an emf (E).
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
5
Calculating circuit current
Let's use the Ohm's Law equation to calculate current from voltage and
resistance. You may need to do this for example before you connect a
circuit, to check that the current will remain within safe limits.
Example 1
Determine the current in a 5 Ω resistor when 12 V is applied.
Given R = 5 Ω
V = 12 V
I =?
V
I=
R
12
= 5
= 2.4 A
Note: Always express answer as a decimal, not a fraction.
Example 2
If the voltage in Example 2 is reduced to 8 V, find the new current value.
Given R = 5 Ω
V =8V
I =?
V
I=
R
8
=5
= 1.6 A
6
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
Calculating resistance
Example 3
A current of 6 A flows in a 10 Ω resistor when connected to a 60 V supply.
What resistance will reduce the circuit current to 5 A?
Given V = 60 V
I =5A
R=?
V
R=
I
60
= 5
= 12 Ω
Example 4
A variable resistor can have its resistance varied to control the current in
a circuit with a 120 V supply. If the current is to be varied from 25 A to 8 A,
what must be the variation in resistance in the variable resistor?
Given V = 120 V
I max = 25 A or I1 = 25 A
I min = 8 A or I 2 = 8 A
R = ? (variation)
R1 =
V
I1
120
= 25
= 4.8 Ω
R2 =
V
I2
120
= 8
= 15 Ω
Therefore the variation in resistance of the variable resistor is from 4.8 Ω to
15 Ω.
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
7
Check your progress
1
Using Ohm’s law, fill in the blanks in the table below.
Voltage (V) volt
Current (I) ampere
30 V
12 V
3 kΩ
40 m A
600 Ω
4.8 m A
2.2 Ω
330 µ A
12 M Ω
20 V
10 V
Resistance (R) ohm
10 A
2
Calculate the resistance of the lamp in the following circuit.
3
A 24 Ω heating element requires a current of 10 A to produce its specified heat output.
Calculate the required supply voltage for this heater.
22
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
4
Calculate the resistance of a resistor that takes 100 mA when connected to a 10 V
battery.
5
Calculate the voltage across a 4.7 kΩ resistor that has 3.5 A passing through it.
6
In what way does a digital multimeter differ from an analogue multimeter when using
it to measure current?
7
Give one advantage of a digital multimeter compared to an analogue multimeter.
8
Give one disadvantage of a digital multimeter compared to an analogue multimeter.
9
Briefly explain why an analogue meter must be viewed straight on.
10 Briefly explain how to ‘zero’ an analogue ohmmeter.
11 Explain why it is important to set the multimeter to its highest range, especially when
measuring an unknown voltage.
Check your answers with those given at the end of the section.
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
23
Summary
•
Ohm’s law states that the current in any circuit, or part of a circuit, is
directly proportional to the voltage and inversely proportional to the
resistance, that is:
V
I=
R
•
The three equations obtainable from Ohm’s Law and its transpositions
are:
I=
V
R
V = IR An ammeter is an instrument used for measuring electrical
V
R=
I
current flow in a circuit.
24
•
A voltmeter is an instrument used for measuring electrical voltage
between two points in a circuit.
•
A multimeter is a multi-purpose meter that can measure voltage, current
and resistance and has a series of ranges for each function.
•
Ammeters, voltmeters and multimeters are available in analogue and
digital versions.
•
Voltmeters should have as high a resistance as possible, and ammeters
as low a resistance as possible.
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
Answers
Activity 1
1 Ohm's law I =
V
R
2
I=
V
R
10
= 2
=5 A
3
4
The current is halved.
The current is increased by a factor of 4 that is, quadrupled.
5 At minimum resistance, R = 15 Ω and so
V
I=
R
150
= 15
= 10 A
At maximum resistance R = 45 Ω and so
V
I=
R
150
= 45
= 3.33 A
Therefore the current variation is 3.33 to 10 A.
Activity 2
1
Resistance parallel.
2
Ammeter resistance.
3
UEENEEE104A Edn 1 Topic 3 Ohms law
© New South Wales, TAFE NSW 2017, Version 1, April 2013
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