The basic law of charges states that like charges repel, unlike charges attract. In this lesson, we will discuss physics experiments that can verify this basic law.
Experiments to verify the law of charges
To investigate what happens when two charges bodies are brought together, we need the following apparatus:
glass rods
silk cloth
Silk Thread
Stand
bunsen burner
polythene rod
duster
To investigate the law of charges, use the following procedure:
Dry glass rod by running it over a Bunsen flame a few times.
rub the dry rod with a silk and then suspend it by a thread on a stand
Dry a second glass rod over bunsen burner and rub it with silk cloth.
Hold the second glass rod close to the first suspended glass rod as shown.
With the glass rod still suspended, bring a polythene rod rubbed with fur close to it as shown.
Observations from experiments on law of charges
when we moved a charged glass rod close to a suspended charged glass rod, we observe them to be to repelling each other.
When a charged polythene rod is moved close to a suspended charged glass rod, they were observed to repel each other.
Explanation
The glass rods were rubbed with the same material and so they acquired same positive charge . The repulsion between them implies that like charges repel each other.
When polythene rod was rubbed with fur, it acquired negative charge. When the charged polythene rod attracts the positively charged glass rod, it shows that opposite charges attracts each other. The above experiment and observations brings us to conclusions on charges with the basic law of charges that states that like charges repel while unlike charges attract.
Atomic structure describes how an atom is built from protons, neutrons, and electrons. At the center of the atom is the nucleus, containing positively charged protons and neutral neutrons. Negatively charged electrons orbit the nucleus in shells, with their negative charge attracting the positive protons to hold the atom together. Atoms are electrically neutral because they have an equal number of protons and electrons.
The nucleus of an atom has a specific number of protons and neutrons. The number of protons in the nucleus is called the atomic or proton number. When the number of protons and the number of neutrons in the nucleus are summed up, the resultant number is known as the mass number. Mass number is also known as the nucleon number.
Different atoms has different mass number. For example, hydrogen atom has mass number of 2, meaning it has 1 neutron and 1 proton in it’s nucleus. A neon atom has mass number as 20 having 10 protons, 10 neutrons and 10 electrons. similarly, helium atom has mass number 4 with 2 protons, 2 neutrons and 2 electrons.
describing the mass number in atomic structure
If a certain atom X has atomic number Z with N neutrons and mass number A, then we can express it as:
$$^{A}_{Z}X \ \ \ where A = Z + N$$
Thus neon, helium and hydrogen atom will be represents as:
where Ne is neon atom, He is the helium atom and H the hydrogen atom.
There exists atoms that have the same atomic number but with different mass numbers. Such atoms are said to be isotopes. For example carbon-14 and carbon-12 has mass number 14 and 12 respectively but both has atomic mass 6.
The two will be represented as shown:
$$^{12}_{6}C \ \ and \ \ ^{12}_{6}C $$
Stability of the nuclear in atomic structure
A nuclear is said to be stable when a ratio of it’s proton to neutron number is 1 or close to 1. that is
$$\frac{\text{mass of proton}}{\text{mass of neutron}}=1$$
As atoms gets heavier, there is a marked deviation from this ratio, with the neutron number exceeding that of protons. This causes the nucleus to be unstable and hence increases chances of the nuclear disintegrating to gain stability. A graph of number neutrons N against number of protons Z for different nucleus is illustrated below.
From the graph, it is observed that the stable nuclides are outside the stability line.
Nuclides above the stability lines have too many neutrons. Such nuclides decays in such a way that the number protons increases.
Nuclides below the stability line have too many protons . Therefore, they decay to decrease the number of protons.
Some applications of C.R.O includes measuring of electrical potential as a voltmeter, television displays and in measuring frequency of signals.
applications of C.R.O as a voltmeter
C.R.O can be used as a voltmeter when it time base circuit is switched off and the voltage to be measured is connected across the Y-plates while the X-plates are earthed.
The vertical displacement of the bright spot on the screen is measured and the sensitivity of the C.R.O is adjusted to determine number of volts per units of displacement along the vertical scale.
The number of volts per unit of division is the sensitivity. The voltage can the be determined as:
Voltage = displacement x sensitivity
Sensitivity is adjusted using the Y-gain knob which automatically connects the input signal through an application system.
Amplification system ensures that even very signals are raised to the levels where they cause measurable deflection of the beam.
application of C.R.O as a voltmeter is considered a superior voltmeter compared to convectional ones because of the following reasons:
can measure large voltages without being damaged
can measure both direct and alternating voltages
Has infinite resistance hence takes no current meaning that it rarely interferes with the circuit into which it is being connected.
It responds instantaneously unlike ordinary meters whose meters swings momentarily about the correct reading due to inertial.
Example Problem on usage of C.R.O as a voltmeter
A D.C voltage of 80V when applied to the Y-plates of a C.R.O causes a deflection of the spot as shown in figure below:
(i) Determine sensitivity of the y-gain
(ii) show what will be formed on the screen if an a.c of peak voltage 64V
is fed onto the Plates.
solutions
(i)
spot deflection on the screen from the center of the screen = 5 divisions
Voltage in a.c with be a straight line 4 divisions above the central line and 4 divisions below it as shown below
Example applications of C.R.O
The Y-gain of a CRO has a sensitivity of 500V/div. An a.c voltage of 2500V is connected across the Y-plates. show what is observed on the screen.
solution
Voltage = sensitivity x number of divisions
We need to determine the number of divisions that a signal will be displaced vertically.
$$\text{Number of divisions} = \frac{voltage}{sensitivity} = \frac{2500 V}{500 V / div} = 5 divisions $$
Numberofdivisions=Voltagesensitivity
=2500V500v/div=5divisions
Hence a straight vertical line will be formed on the screen covering 5 divisions above the x-axis and 5 divisions below it as shown
applications of C.R.O to measure frequency of an a.c signal
The signal is fed into the Y_plates of a C.R.O with the time base on. The time base control is then adjusted to give one or more cycles of the input signal on the screen. By adjusting the time base control, we can determine the number of cycles made on the screen from the periodic time T. Frequency can then be calculated from f=1/T.
Example question on applications of C. R.O
Figure below shows a trace on the screen for an signal connected to the Y_plates of a CRO with time base on.
Given that the time base control is 20ms/div and the Y-gain is at 80V/div, determine:
(i) frequency of the a.c signal
(ii) The peak voltage of the input signal
solution (i)
Time base settings is 20ms/div
Number of waves shown on the screen =3.25
7 div is covered by 2.5 waves.
1 wave = 7.0/2.5 = 2.8 divisions
Time taken to complete 1 wave (periodic time) = 2.8 div x 20ms/div = 56ms = 56 x 10-3s
Frequency=1T
And so the frequency will be given by:
Frequency=156∗10−3s
0.01786 x 103 Hz =17.86 Hz
solution (ii)
Y-gain =80div
Approximate deflection as about 2.2 divisions as per the graph
The term “properties of cathode rays” refers to the various physical characteristics and behaviors observed when cathode rays are studied under different conditions.
properties of cathode rays includes:
They travel in a straight line
causes fluoresce or glow to certain materials
they are charged
possesses kinetic energy
pass through thin materials demonstrating their ability to penetrate objects to varying degrees. This depends on the material’s density and the energy of the rays.
ionize gases
The charge-to-mass ratio of cathode rays (electrons) is relatively high. A property used to identify the electron and distinguish it from other particles.
Deflection by Magnetic Fields
Showing that cathode rays travels in a straight line
When an opaque object is placed between the screen and the cathode in the path of the cathode rays, a sharp shadow is cast on the screen.
Cathode rays causes certain substances to glow or fluoresce
Fluoresce materials are materials that glows when electromagnetic energies falls on them. such materials includes zinc sulphide, Fluorescein, Rhodamine, Coumarin, Acridine Orange and Quantum Dots.
when cathode rays falls on screen coated with the fluoresce materials, the fluoresce material glows.
showing that cathode rays are charged
Cathode rays are deflected by both magnetic and electric fields.
Inside the magnetic field, the cathode rays are deflected towards the positive plate showing that they are negatively charged. Remember that opposite charges attract while while same charges repel from the basic law of charges. see the figure below:
When cathode rays passes through magnetic field, they are deflected in the direction determined by Fleming’s left-hand rule. The deflection in magnetic field shows that they are negatively charged as shown in figure below.
Cathode rays have kinetic energy
The deflection of cathode rays in a magnetic field shows they are moving, and therefore possess kinetic energy. By measuring how much the cathode rays bend in the magnetic field, you can calculate their velocity. Using the velocity, you can compute the kinetic energy of the cathode rays.
When cathode rays are suddenly stopped by a metal target, they can produce x-rays. This confirms that they are actually a stream of fast moving electrons.
Exam Questions on properties of cathode rays
Figure 14 shows a cathode ray tube. A metal plate is placed between the anode and the screen.
(I) State with reason what would be observed on the screen when the cathode rays are produced. (2 marks)
(ii) State the effects on the cathode rays produced when the anode is increased. ( 2 marks)
Exam questions on Cathode rays are an important topic in physics and chemistry. They test knowledge on key curriculum areas that includes:
Atomic Structure
Electricity and Magnetism
Properties of Matter
Electron beams
Charge-to-mass ratio (e/m)
Millikan’s oil drop experiment
etc.
below are questions commonly tested in physics examinations on this topic;
1. The figure 1 below represents a cathode ray oscilloscope (C.R.O) (i) Name the parts labeled A and B (2 marks)
Figure 1
A.…….………………………………………………………………………………………… B.…….…………………………………………………………………………………………. ii) What are the functions of parts labeled C and D (2 marks) C ………………………………………………………………………………………………… D …………………………………………………………………………………………………. iii) Explain how electrons are produced. (1 mark) …………………………………………………………………………………………………… ……………………………………………………………………………………………………. iv) Give a reason why the tube is evacuated. (1 mark)
2. State the function of the grid in a cathode ray tube (CRT) (1 mark)
3. State two reasons why the CRO is a more accurate voltmeter than a moving coil voltmeter. (1 mark)
4. Figure 2 shows a cathode ray entering into a region between two charged plates.
Complete the diagram to show the path of the ray in the electric field. (1 Mark)
Cathode rays are streams of negatively charged particles, or electrons, which are accelerated from a cathode to the anode within a vacuum tube by an electrical potential.
These rays travel to the positively charged anode, creating a visible beam. They are also known as electron beams and were instrumental in the discovery of the electron.
Before the electrons are accelerated, they must be extracted from a atoms and be on the surface. Electrons are first extracted from from the nuclei to a metal surface when the metal is heated. The heat energy raises the energy of an electron. This enables it to break loose from the force of attraction of the nuclei. This process where electrons are emitted to the surface is known as thermionic emission.
When a material is heated, its atoms vibrate more vigorously. This thermal energy can be transferred to the electrons in the material. If the temperature is high enough, some electrons gain enough energy to overcome the work function of the material. Work function is the minimum energy needed for an electron to escape from the surface of the material. It escapes into the vacuum or surrounding environment.
Thermionic Emission for cathode rays
In thermionic emission, a cathode is heated from low voltage supply so that electrons can be extracted to the surface. Another higher voltage is then used to accelerates the electrons produced towards an anode. A typical setup used for thermionic is as shown below.
A cathode which is inside the evacuated glass tube is made with mixture of barium oxides and strontium oxide. The resulting metal oxide has a low work function. This means that the minimum energy required to remove an electron from its atom to the surface is low.
In the setup a low voltage of about 6V drives a current through the heating filament which then heats the cathode
Initially the reading on the milliammeter(mA) is zero. When the heater circuit is switched on, some current is observed on the milliammeter after some time. This means that the current circuit between the cathode and the anode has been completed. There is a wide gap between the anode and the cathode. However, when the heater current is switched on, the heater circuit is complete in the gap.
This is because cathode the electrons produced by the cathode are able to move to the anode across the gap. How do they do that?
The hot cathode emits electrons which are then attracted to to the anode due to the 12V accelerating potential. This acceleration ensures electrons are able to move across the gap hence making the circuit complete. This setup allows us to observe thermionic emission. It helps explain the cathode rays inside the evacuated glass tube we call a cathode ray tube.
Production of cathode rays
Cathode rays are usually produced in cathode ray tube (CRT).
A cathode ray tube
Electrons produced at the cathode by thermionic emission are accelerated towards a fluorescent screen. This fluorescent screen connected to the anode which is connected to the positive terminal of an extra high tension (E.H.T) source. When cathode rays are stopped by the fluorescent screen, the kinetic energy of the cathode causes the screen to glow.
The reason the tube is evacuated is to ensure electrons do not collide with gaseous particles before reaching the screen. If electrons collides with other particles, ionization of the gas in the tube occurs. This is found to be causing different observations from the one intended. Collision with some other particles can cause electrons to loose their energy and probably not able to reach the screen. The movement of cathode rays in a cathode ray tube is illustrated in the table figure below:
Questions
What is thermionic emission
Illustrate production of cathode rays using a suitable diagram.
Explain why it is important for a cathode ray tube to be evacuated.
This lenses operates from the principles of light refraction(bending) when the medium of transmission changes. Questions on thin lenses test several important physics concepts. These include:
1. Refraction of Light
Bending of light when it passes from one medium to another
How curved surfaces (lenses) change the direction of light rays.
Application of Snell’s Law (conceptually, even if not directly calculated).
II . Image Formation
Understanding how images are formed by:
Convex (converging) lenses
Concave (diverging) lenses
A learner must know:
When images are real or virtual
if images formed upright or inverted
Whether image formed is
magnified or diminished
III. Ray Tracing Principles
Use of the three principal rays:
Ray parallel to the principal axis
Rays through the optical centre
Ray through the focal point
This tests geometric understanding of light behavior.
iv. Lens Formula (Mathematical Relationship)
Tests:
Algebraic manipulation
Substitution and rearrangement
Use of correct sign convention
v. Magnification
M=uv
or
Tests:
Proportional reasoning
Understanding of image size relationships
vi. Graphical Skills
Some questions require:
Scale diagrams
Plotting graphs
Determining focal length from graphs
vii. Experimental Skills
Students may be tested on:
Determining focal length experimentally
Sources of error
Drawing conclusions from results
viii. Concept of Proportionality
Especially in experiments relating object distance, image distance, and focal length.
Some examination Questions on Thin Lenses
Question 1 (Thin lenses)
(a) State the meaning of the term “principal focus” as applied in thin lenses. (1 mark)
(b) you are provided with the following apparatus to determine the focal length of a lens:
-a biconvex lens and lens holder.
-a lit candle
-a white screen.
-a metre rule
(i) Draw a diagram to show how you would arrange the above apparatus to determine the focal length of the lens (1 mark)
(ii) Describe the procedure you would follow. (1 mark)
(iii) state two measurements that you would take (2 marks)
(iv) Explain how the measurements in (iii) would be used to determine the focal length. (2 marks)
(c) An object is placed 30 cm infront of a concave lens of focal length 20 cm. Determine the magnification of the image produced. (4 marks)
Question 2
(a) state 2 applications of convex lenses (2 marks)
(b) A form 4 student did an experiment on thin lenses by varying the object distance and recording the corresponding image distance for the formed by the lenses in an attempt to determine the focal length.
Object distance u (cm)
Image distance V (cm)
UV (cm2)
(U+V)cm
15
30
20
20
25
16.7
30
15
40
13.3
60
12
On the grid provided, plot the graph of uv against u+v (5 marks)
High voltage power transmission systems are designed to transport electrical power over long distances while reducing power losses caused by resistance in transmission cables. By increasing voltage and lowering current, power companies can deliver electricity more efficiently, safely, and reliably. This process plays a major role in modern power grids and helps ensure stable electricity supply across cities and rural areas.
In this article, you will learn how high voltage power transmission works, why power losses occur during transmission, the importance of transformers in reducing energy wastage, and the dangers associated with high voltage lines. You will also explore practical examples and calculations that explain how electrical engineers minimize power loss in transmission systems.
Whether you are a physics student, engineering learner, teacher, or simply curious about electricity transmission, this guide will help you clearly understand the science behind high voltage power transmission and power losses.
Electricity generated at power stations must travel long distances before it reaches homes, industries, schools, and businesses. To make this possible efficiently, electrical power is transmitted using high voltage transmission lines. High voltage transmission helps reduce energy losses and ensures reliable delivery of electricity over large distances.
Electricity is generated at power stations.
Voltage is stepped up for efficient long-distance transmission.
High voltage reduces power losses in transmission cables.
Substations step the voltage down before distribution to consumers such as homes, industries, schools, and businesses.
What is High Voltage Transmission?
High voltage transmission is the process of carrying electrical power over long distances using very high voltages such as 110 kV, 220 kV, 400 kV, or even 765 kV. Transmitting electricity at high voltage reduces the amount of energy lost as heat in the transmission cables.
Power stations usually generate alternating current (AC) electricity at voltages between 11 kV and 25 kV. Before the electricity is transmitted, transformers are used to step up the voltage to much higher levels, typically between 132 kV and 400 kV.
The electricity is then transmitted through overhead power lines to substations, where the voltage is stepped down for safe distribution to consumers.
Different consumers require different voltage levels:
Heavy industries may require voltages above 30 kV
Light industries may use around 10 kV
Homes and domestic users usually require 240 V
power transmission cables
How Power is Distributed
The transmission and distribution process follows these stages:
Electricity is generated at the power station.
A transformer steps up the voltage for long-distance transmission.
Electricity travels through high voltage transmission cables.
Substations step down the voltage.
Electricity is distributed to homes, industries, and businesses at suitable voltage levels.
Why High Voltage Reduces Power Loss
Electrical cables have resistance, and this resistance causes some electrical energy to be lost as heat during transmission.
The power loss in transmission lines is given by:
P=I2R
Where:
P = power loss
I = current flowing through the cable
R = resistance of the cable
From the formula, power loss increases when current increases. Therefore, reducing current reduces energy losses.
Since electrical power is also given by:
For the same amount of power, increasing the voltage allows the current to decrease. This is why electricity is transmitted at very high voltages and low currents.
Methods Used to Reduce Power Losses
Power companies use several methods to minimize losses during transmission:
1. Stepping Up Voltage
Transformers increase the voltage before transmission. Higher voltage means lower current and therefore lower power loss.
2. Using Thick Transmission Cables
Thicker cables have lower resistance, which reduces heat losses.
3. Using Good Conductors
Transmission cables are made using materials with low electrical resistance.
Why Aluminum is Preferred for Transmission Cables
Aluminum is commonly used in transmission lines because:
It is a good conductor of electricity
It is lightweight
It is cheaper than many other conducting materials
Dangers of High Voltage Transmission
Although high voltage transmission is efficient, it also presents several risks:
Electric shock if cables fall or hang too low
Fires caused by loose or damaged cables
Strong electric fields that may affect nearby communities
Lightning strikes causing surges in electrical systems
Cables touching during strong winds and causing sparks or fires
Proper maintenance and safety measures are therefore very important.
Example Problem
A transmission cable has a resistance of 100 Ω and carries electricity at 10 kV with a current of 1.0 A. The voltage is stepped up to 18 kV using a transformer.
Determine the power loss after stepping up the voltage.
Solution
Assuming the transformer is 100% efficient:
Step 1: Use the transformer power relationship
VpIp=VsIs
Substituting the values:
Primary voltage = 10,000 V
Primary current = 1.0 A
Secondary voltage = 18,000 V
10000×1.0=18000×Is Is=1800010000 Is=0.556 A
Step 2: Calculate Power Loss
Using:P=I2R P=(0.556)2×100 P≈30.9 W
Without Stepping Up the Voltage
P=(1.0)2×100 P=100 W
This shows that stepping up the voltage significantly reduces power loss.
Practice Question
A generator produces 750 kW at a voltage of 15 kV. The voltage is stepped up to 125 kV and transmitted through cables with a resistance of 500 Ω.
Assuming the transformers are 100% efficient, calculate:
The current produced by the generator
The current flowing through the transmission cables
The voltage drop across the cables
The power lost during transmission
The power reaching the substation
Conclusion
High voltage transmission is an essential part of modern electrical power systems. By transmitting electricity at high voltage and low current, power companies minimize energy losses and improve efficiency over long distances.
Transformers play a key role in stepping voltage up for transmission and stepping it down for safe use by consumers. Despite the dangers associated with high voltage lines, proper design and maintenance make power transmission both efficient and reliable.
Current and electricity are core topics in physics, encompassing electric current, circuits, resistance, and the behavior of electrons. Key areas that can be tested include the nature of electric current. Another key area is Ohm’s law, Series and parallel circuits, the heating effect of electric current among others
Questions
Figure 1 shows four identical bulbs connected to a 15 volt battery whose internal is negligible.
Figure 1
Determine the reading of the voltmeter V. (2 marks).
2. Figure 14 shows a circuit in which a battery. a switch , a bulb, resistor P, a variable resistor Q. a voltmeter V and two ammeters A1 and A2 of negligible resistance are connected.
P has a resistance of 10 Ω. When the switch is closed A1 and A2 reads 0.10 A and the voltmeter reads 1.5 V.
(a) Determine;. (i) the current passing through P; (3 marks).
(ii) the resistance of the bulb (2 marks).
(b) The variable resistor Q is now adjusted so that a larger current flows through A2 . (i) State how this will affect the resistance of the bulb (1 mark)
(ii) Explain your answer in (b)(i). (3 marks)
(c) A house has one 100W bulb, two 60W bulbs and one 30W bulb. Determine the cost of having all the bulbs switched on for 70 hours,. given that the cost of electricity is 40 cents per kilowatt hour. (3 marks).
3. (a) Define current stating its S.I units. (2 mark)
(b) A battery circulates charges round a circuit for 1.5 minutes. If the current is held at 2.5 Amperes, what quantity of charge passes though the wire? (2 marks)
4. Figure 2 shows arrangement of three capacities of 10µF, 2µF and 5µF.
Determine the effective capacitance. (3 marks)
5.(a) Figure 8 shows a graph of potential difference V (volts) against a current I(amperes) for a certain device.
From the graph:
(i) State with a reason whether or not the device obeys ohms law. (2 marks)
(ii) determine the resistance of the device at ;
(I) I =1.5 A (2 marks)
(II) I = 3.5 A (2 marks)
(iii) From the results obtained in (ii) state how the resistance of the device varies as the current increases. ( 1 mark)
(iv) State the cause of this variation in resistance. (1 mark)
5(b) Three identical dry cells each of e.m.f 1.6 V are connected in series to a resistor of 11.4 ohms. A current of 0.32A flows in the circuit. Determine:
(i) The total e.m.f of the cells (1 mark)
(ii) The internal resistance of each cell; (3 marks)
6. Figure 6 below shows an electric generator. The points P and Q are connected to a cathode ray oscilloscope (CRO).
Figure 6
Sketch on the axes provided the graph of the voltage output as seen on the CRO, given that when t=0 the coil is at the position shown in the figure. (2 marks).
7. A 60 W bulb is used continuously for 36 hours. Determine the energy consumed. Give your answer in kilowatt hour (kWh). (3 marks)
8. Figure 8 shows the cross-section of a dry cell. Use the information on the figure to answer questions 4 and 5.
Figure 8
Name the parts labelled A and B. (2 marks)
8 (b) State the use of the manganese(IV) oxide in the cell. (1 mark).
9. A 4 ohms resistor is connected in series to a battery of e.m.f 6.0 V and negligible internal . Determine the power dissipated by the resistor (2 marks)
10. State the reason why electrical power is transmitted over long distances at very high voltages .(1 mark)
Electric current and potential difference represents two phenomenon that depends on each other to exist in electricity concepts. An electric current is the rate of flow of charge through a conductor. Current flows when there is a potential difference between two points in a conductor. Electric current is measured in amperes by an instrument called ammeter. An ammeter is an electrical instrument used to measure the current flowing through a circuit. The ammeter is designed to be connected in series with the circuit. This ensures that the current flows through the ammeter, allowing it to accurately measure the amount of electrical current.
There are two types of ammeters:
Instruments used in experiments of electric current and potential difference
Analog Ammeter: This uses a needle or pointer to indicate the current on a scale.
The figure below shows an analog ammeter ammeter common in school laboratories.
An ammeter
2. Digital Ammeter: This displays the current measurement on a digital screen. It provides a digital readout of the electrical current. Digital ammeter allows one to choose a scale of measurement in amperes (A), milli-amperes (mA), or micro-amperes (µA). It uses a numerical display rather than a moving needle or pointer.
A Digital ammeter
Electric current flows between two points in a closed path due to a potential difference between those two points. Sometimes the flowing current can be too small to me measured by an ammeter. A more sensitive instrument may therefore be required to measure small currents.
A millimeter is an instrument used to measure current in terms of one in thousand of an ampere. A milliammeter measures current in terms of milli-amperes.
$$1 \ milli-ampere(MA) = \frac{1}{1000} Amperes$$
Much smaller currents can be measured by a micro- ammeter. A micro-ammeter measures current in terms of micro-ampere.
An ammeter has very low electrical resistance. Therefore it is connected in series with the instrument whose current passing through need to be measured. When connecting an ammeter in the circuit, ensure it is done correctly. The correct procedure is such that current enters the ammeter through positive terminal and exits through the negative terminal. If connected such that convectional current enters through negative terminal, the ammeter may get damaged.
The figure below shows the ammeter connected in series with the bulb. The convectional current flowing through the bulb also flows through the ammeter.
correct ammeter connection
The figure below shows wrong ammeter connection. Note that the positive terminal of the ammeter is connected to the negative terminal of the cell.
wrong ammeter connection
Before connecting the ammeter in a circuit, confirm that it’s pointer is at zero mark on the scale. Otherwise, use the zero adjusting screw to move it to the correct position. Most of ammeters has two scales. An appropriate scale should be selected to safeguard the coil from damaged if current passing exceeds its capacity. For example an ammeter can have a scale of (0-3)A or (0-5)A. The figure below shows an ammeter dashboard with two scales; (0-5)A and (0-2.5)A.
If a scale of (0 – 5) A is selected, the meter can read up to 5 A. With such a scale, 10 divisions represents 1.0 A. For a (0-2.5) A scale, ten divisions will represent 0.5 A meaning each division is 0.05 A. From the diagram, the reading on the ammeter is 2.45 A while reading (0-5) A or 1.225 while reading the (0 -2.5) A.
Electric current and potential difference: using a voltmeter
while investigating electric current and potential difference, we need to measure potential difference across various components in the circuit. A voltmeter is always connected across the device (parallel to the device) which the voltage is to be measured. The figure below shows voltmeter connected across the bulb in parallel arrangement.
Voltmeter is connected in series because it is an instrument with high resistance to the flow of current. Therefore, It takes no current from the component across which the voltage is to be measured.
The positive terminal of the voltmeter is connected to the point where convectional current is entering a component. Its negative terminal is connected to the point where the current is leaving the component.
One should ensure that the pointer is exactly on the zero mark before connecting the voltmeter. If pointer is not at zero, the pointer should be adjusted to zero by the screw.
potential difference
Work must be done to move an electric charge through a conductor. The device that produces energy to do this work is called a source of electromotive force (e.m.f). The source may be a battery, which converts chemical energy to electrical energy, or a generator, which converts mechanical energy to electrical energy. When the battery does the work of pumping charges through a conductor or an electrical device, an electrical potential difference(p.d) develops between its end. This potential difference is measured in volts using the voltmeter.
Potential Difference and the Voltmeter
A lack of “pumping” charges through a conductor indicates that there is no potential difference between two points. The potential difference (p.d) between two points A and B (Vab)of a conductor is defined as the work done in moving a unit charge from point B to A.
in other words:
$$\text{Potential difference} = \frac{\text{work done W(in joules)}}{\text{charge moved Q (in coulombs)}}$$
$$V_{AB} = \frac{W}{Q}$$
where: (VAB) = potential difference across AB.
(W) = work done (in joules)
(Q) = charge moved (in coulombs)
From the equation, one volt is equal to one joule per coulomb.
Measuring potential difference
The voltmeter measures potential difference. In laboratories, moving coil voltmeters are commonly used, although today many of these instruments are replaced by digital voltmeters.
(a) Analogue voltmeter
(b) Digital voltmeter
Please note that instruments like fuel gauge and speedometers are essentially voltmeters.
Example
In moving a charge of 10 coulombs from point B to point A, 120 joules of work is done. What is the potential difference between A and B?
solution:
$$p.d = \frac{W}{Q} = \frac{120}{10} = 12V$$
Using a Voltmeter
A voltmeter is always connected across (in parallel to) the device whose voltage is to be measured. consider the diagram below.
Voltmeter connected across the bulb
lab setup for bulb connected across the battery
Because the voltmeter has a high resistance to the flow of current, it draws very little current from the component.
The positive terminal of the voltmeter is connected to the point where conventional current enters the component, while the negative terminal is connected to the point where the current leaves the component.
Experiment To Investigate the Current and Voltage in a Parallel Circuit Arrangement
Apparatus
Two 1.5 V cells
3 identical bulbs
3 ammeters
4 voltmeters
Switch
Connecting wires
Procedure
Connect the circuit as shown in figure below.
Current and Voltage in Parallel
Switch on the circuit and take the readings on the ammeters A1, A2,A3 and A4.
Switch off the circuit and disconnect the ammeters.
Connect the bulbs and the voltmeters as shown in figure
Take the readings on V1,V2,V3 and V4.
Observation
Reading on A1 = Reading on A2 + Reading on A3 = Reading on A4
Reading on V1 = Reading on V2 = Reading on V3 = Reading on V4.
Conclusion
When components are connected in parallel:
The sum of the currents in parallel circuits is equal to the total current. The total current entering a junction equals the total current flowing out.
The same voltage drops across each of the components.
Example 2
Find the current passing through L1 in figure below, given that 0.8 A passes through the battery, 0.28 A through L2, and 0.15 A through L3.
Solution
Current through battery = Current through L1 + Current through L2 + Current through L3
0.8 = I1+I2+I3
0.8=I1 + 0.28+0.15
0.8 =I1 + 0.43
Therefore:
I1 = 0.8A – 0.43A = 0.37A
Experiment To Investigate Current and Voltage in Series Arrangement
Apparatus
4 voltmeters
3 torch bulbs (2.8 V)
Bulb holder
Switch
Connecting wires
Two cells
Procedure
Connect the circuit as shown in figure below
Switch on the circuit and record the readings on the meters.
Switch off the circuit and disconnect the ammeter.
Connect the bulbs, ammeter, and voltmeters as shown in figure below.
Switch on the circuit and record the readings on the meters.
observations
The reading of current by the ammeters A1 and A2 and A3 is the same.
The total voltage drops across the bulbs (V1+v2+v3) equals to the total voltage v4 across the terminals of the battery.
please note that the observations will remain true even when the bulbs are not identical.
conclusions
In a series arrangement, the same current flows through each component.
The sum of the voltage drops across the components is equal to supply voltage
Summary
A voltmeter measures potential difference.
Potential difference is the work done per unit charge.
Voltmeters are connected in parallel across components.
In parallel circuits, current splits while voltage remains the same.
In series circuits, current remains the same throughout the circuit.
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