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Introduction

The study of electrical circuits is essential to understand the technology that uses electricity in the real-world. This includes electricity being used for the operation of electronic devices like computers.

Ohm's law

Definition of ohm's law

Experiment : ohm's law

Aim:

In this experiment we will look at the relationship between the current going through a resistor and the potential difference (voltage) across the same resistor.

Method:

  1. Set up the circuit according to the circuit diagram, starting with just one cell.
  2. Draw the following table in your lab book.
    Voltage, V (V) Current, I (A)
    1,5
    3,0
    4,5
    6,0
  3. Get your teacher to check the circuit before turning the power on.
  4. Measure the current.
  5. Add one more 1,5 V cell to the circuit and measure the current again.
  6. Repeat until you have four cells and you have completed your table.
  7. Draw a graph of voltage versus current.

Results:

  1. Does your experimental results verify Ohm's Law? Explain.
  2. How would you go about finding the resistance of an unknown resistor using only a power supply, a voltmeter and a known resistor R 0 ?

Activity : ohm's law

If you do not have access to the equipment necessary for the Ohm's Law experiment, you can do this activity.

Voltage, V (V) Current, I (A)
3,0 0,4
6,0 0,8
9,0 1,2
12,0 1,6
  1. Plot a graph of voltage (on the x -axis) and current (on the y -axis).

Conclusions:

  1. What type of graph do you obtain (straight line, parabola, other curve)
  2. Calculate the gradient of the graph.
  3. Do your experimental results verify Ohm's Law? Explain.
  4. How would you go about finding the resistance of an unknown resistor using only a power supply, a voltmeter and a known resistor R 0 ?

An important relationship between the current, voltage and resistance in a circuit was discovered by Georg Simon Ohm and is called Ohm's Law .

Ohm's Law

The amount of electric current through a metal conductor, at a constant temperature, in a circuit is proportional to the voltage across the conductor. Mathematically, Ohm's Law is written:

V = R · I .

Ohm's Law tells us that if a conductor is at a constant temperature, the current flowing through the conductor is proportional to the voltage across it. This means that if we plot voltage on the x -axis of a graph and current on the y -axis of the graph, we will get a straight-line. The gradient of the straight-line graph is related to the resistance of the conductor.

Phet simulation for ohm's law

Ohmic and non-ohmic conductors

As you have seen, there is a mention of constant temperature when we talk about Ohm's Law. This is because the resistance of some conductors changes as their temperature changes. These types of conductors are called non-ohmic conductors, because they do not obey Ohm's Law. As can be expected, the conductors that obey Ohm's Law are called ohmic conductors. A light bulb is a common example of a non-ohmic conductor. Nichrome wire is an ohmic conductor.

In a light bulb, the resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we increase the supply voltage in a real lamp circuit, the resulting increase in current causes the filament to increase in temperature, which increases its resistance. This effectively limits the increase in current. In this case, voltage and current do not obey Ohm's Law.

The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, which increase a lot in temperature (up to about 1000 C, and starting from room temperature) the change is quite large.

In general non-ohmic conductors have plots of voltage against current that are curved, indicating that the resistance is not constant over all values of voltage and current.

Experiment : ohmic and non-ohmic conductors

Repeat the experiment as decribed in the previous section. In this case use a light bulb as a resistor. Compare your results to the ohmic resistor.

Using ohm's law

We are now ready to see how Ohm's Law is used to analyse circuits.

Consider the circuit with an ohmic resistor, R . If the resistor has a resistance of 5  Ω and voltage across the resistor is 5 V, then we can use Ohm's law to calculate the current flowing through the resistor.

Ohm's law is:

V = R · I

which can be rearranged to:

I = V R

The current flowing through the resistor is:

I = V R = 5 V 5 Ω = 1 A

The resistance of the above resistor is 10  Ω and the current going through the resistor is 4 A. What is the potential difference (voltage) across the resistor?

  1. It is an Ohm's Law problem. So we use the equation:

    V = R · I
  2. V = R · I = ( 10 ) ( 4 ) = 40 V
  3. The voltage across the resistor is 40 V.

Got questions? Get instant answers now!

Ohm's law

  1. Calculate the resistance of a resistor that has a potential difference of 8 V across it when a current of 2 A flows through it.
  2. What current will flow through a resistor of 6  Ω when there is a potential difference of 18 V across its ends?
  3. What is the voltage across a 10  Ω resistor when a current of 1,5 A flows though it?

Questions & Answers

which gas is ideal gas
ANAMIKA Reply
how to calculate emf when you are given two circuits with different external resistance, current but no internal resistance?
Andiswa Reply
you have to find the current of whole both circuits nd add em...
Amanzialwandle
calculate the distance between two charge +4nc and -3nc if electrostatic is 0.005N
Malaza Reply
use the coulombs law equation where there is F u substite by 0.005 then u cross multiple
Neldo
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motheo Reply
what is charge
motheo
why do we need to add oil to an engine
Kamohelo
we need to add all to an engine 2 lubricate the bearings and the shops to avoid friction that will lead to overheating and knocking
Easyas
charge is a unit of matter that expresses the extent to which it has more or less electrons than protons
Easyas
how to calculate the distance between Q1 and point P if you are given distance between point P and Q2
Lwando Reply
how do we calculate the magnitude of the force between two objects
Ntombi Reply
we use Newtons law ...fnet=ma
Fikile
how do you calculate the enery change in enthalpy reaction, can someone explain it to me guys
Buyie Reply
∆H(enthalpy change)= Hproducts- Hreactants.... use this formula, H stands for enthalpy which is regarded as energy
Andiswa
atomic model john dalton
Alliieyza Reply
Hlw,, I want to join.
crippah
what is a normal force
tumelo Reply
normal force is the line that is perpendicular to the surface
Thabo
normal force is the force exerted to an object by the surface , perpendicular to the surface
Fhatani
I want to join
Londiwe
better explanation of FARADAY'S LAW OF INDUCTION
Kgaugelo
what is the formular of energy
Wilka Reply
E=f\q
Wonder
E=hf
Mondli
doesnt it like depend on the type of energy
Benjamin
what does (1s2 2s2 and 2p6) mean?
Ryan Reply
That is Albau Diagram I guess!! it represent number of electrons in each energy orbit!! the numbers 1s, 2s&2p, 3s and so on represent energy orbit and then the other numbers after s & p represent number of electrons
Andiswa
That is SP notation
Kgaugelo
What are the differences between pd and emf?
Santosh Reply
How to calculate magnitude of Friction force
Tanaka Reply
in Newton
Obakeng
yeh
Obakeng
in newton
Fikile
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crippah
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Fikile
First of all in wat Grade ae u..?
crippah
grde 11
Fikile
Oka nd hw is the properties of a substance afected by the intermoleculaer forces...
crippah
hi guys I m in grade 11 I need help with electrostatics
Serati
what is newton
Ndamulelo Reply
look for the x components n y components then after y add the x components separate n y separate then u use the Pythagoras theorem 2 find the resultand
Abigail Reply
Thank you very much
Luvuyo
how do I find 🔍 the critical angle
Ntandokazi
angle of incident and angle of reflection must always approach normal line thats where u will see 90° which is critical point
Tumelo

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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