lab report Magnetic field

PHYSICS FOR SCIENTIST AND ENGINEERS LABORATORY EXPERIMENTS

EXPERIMENT 5
(Assignment)

EXPERIMENT 5 – MAGNETIC FIELDS

OBJECTIVES
1. To study the workings of magnetic fields.
2. To determine the North and South poles of magnets and magnetic field lines.
3. To understand the attractive and repulsive forces acting on it.

INTRODUCTION
Electric current is defined as the rate at which charge flows through a surface. As with all quantities defined as a rate, there are two ways to write the definition of electric current.
I =
Δq

Δt and instantaneous current for those with no fear of calculus
I = lim Δq = dq Δt → 0

Δt

dt
The unit of current is the ampere [A], which is named for the French scientist André Marie Ampère(1775-1836).
Since charge is measured in coulombs and time is measured in seconds, an ampère is the same as a coulomb per second.
⎡
⎣
A =
C
⎤
⎦

s

This is an algebraic relation, not a definition. The ampere is a fundamental unit in the International System. Other units are derived from it. Fundamental units are themselves defined by experiment. In the case of the ampere, the experiment is electromagnetic in nature.

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.

Here, we will discuss more on electromagnets. An electromagnet is made from a coil of wire that acts as a magnet when a current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as steel, which greatly enhances the magnetic field produced by the coil.

Around a typical bar magnet - or any magnetized object (like the Earth, for example) - there are lines of magnetic flux. These are said to flow away from the north pole and re-enter at the south pole.

These field lines become evident if iron filings are sprinkled over a sheet of paper underneath which there is a bar magnet. Their direction can be plotted using a small 'plotting compass '.The needle aligns with the N-S field (flux) lines and the needle follows the same pattern as revealed by the iron filings.

[Iron filings that have oriented in the magnetic field produced by a bar magnet]

When two magnets are brought close to each other, the flux lines from both magnets interact. If these flux lines are flowing in the same direction, they will link up and the magnets will attract each other. If they are flowing in opposite directions, they will produce a repulsive force and push away from each other (often taking the magnets with them). The force between them depends on the separation distance and the flux density (magnetic strength) of the magnets used. A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field.

Mapping the magnetic field of an object is simple in principle. First, measure the strength and direction of the magnetic field at a large number of locations (or at every point in space). Then, mark each location with an arrow (called a vector), pointing in the direction of the local magnetic field with its magnitude proportional to the strength of the magnetic field.

An alternative method to map the magnetic field is to 'connect ' the arrows to form magnetic field lines. The direction of the magnetic field at any point is parallel to the direction of nearby field lines, and the local density of field lines can be made proportional to its strength.

Magnetic field lines are like the contour lines (constant altitude) on a topographic map in that they represent something continuous, and a different mapping scale would show more or fewer lines. An advantage of using magnetic field lines as a representation is that many laws of magnetism (and electromagnetism) can be stated completely and concisely using simple concepts such as the 'number ' of field lines through a surface.

QUESTIONS
1. Draw the magnetic field lines shown in the diagrams.
2. Label the North, N and South, S of the magnet bars.

Diagram 1 Diagram 2

Diagram 3 Diagram 4

EXPERIMENT 5 (b) – FLUX DENSITY

OBJECTIVES
1. To study the workings of magnetic flux density.
2. To understand the nature of the magnetic fields produced by several current configurations.
3. To understand the use and significance of solenoids
4. To determine the polarities of the magnets.
.

INTRODUCTION
Magnetic flux most often denoted as Φm, is a measure of the amount of magnetic field passing through a given surface. The magnetic flux through a given surface is proportional to the number of magnetic field lines that pass through the surface. This is the net number, i.e. the number passing through in one direction, minus the number passing through in the other direction. For a uniform magnetic field B passing through a perpendicular area the magnetic flux is given by the product of the magnetic field and the area element. The magnetic flux for a uniform B at any angle to a surface is defined by a dot product of the magnetic field and the area element vector. Where θ is the angle between B and a vector that is perpendicular (normal) to S.
In the general case, the magnetic flux through a surface S is defined as the integral of the magnetic field over the area of the surface.
Where is the magnetic flux, B is the magnetic field,
From the definition of the magnetic vector potential A and the fundamental theorem of the curl the magnetic flux may also be defined as:
Where the closed line integral is over the boundary of the surface and dℓ is an infinitesimal vector element of that contour Σ.
The magnetic flux is usually measured with a flux meter. The flux meter contains measuring coils and electronics that evaluates the change of voltage in the measuring coils to calculate the magnetic flux.
Flux density is measured in Tesla (T) where 1 T = 1 Wbm-2

QUESTIONS
1. Draw the magnetic field produced by a straight wire carrying a current.
Answer:

2. Copy the following diagram and mark in the polarities of the two ends of the coil.
Answer:

3. Copy the following diagram and mark in the compass directions.
Answer:

4. Question 4 take µo = 4π  10-7 N A-2 Calculate the magnetic flux density at the following places:
(a) 2 m from a long straight wire carrying a current of 3 A Answer: At distance r from a long straight wire: Magnetic flux density (B) = oI / 2r = = 3 x 10-7 T

(b) At the centre of a solenoid of 2000 turns 75 cm long when a current of 1.5 A flows Answer:

At the centre of a solenoid: Magnetic flux density (B) = oNI / L = = 5.03 x 10-3 T
5. A solenoid of length 25 cm is made using 100 turns of wire wrapped round an iron core. If the magnetic flux density produced when a current of 2 A is passed through the coil is 2.5 T calculate the permeability (µ) of the core.
Answer:

Magnetic flux density (B) = NI / L 2.5 = x 100 x 2 / 0.25  = 0.0031 T.m/A

6. A Hall probe measures a steady magnetic field directly by detecting the effect of the field on a slice of semiconductor material. A student sets up the circuit below to investigate, using a Hall probe, the factors which determine the magnetic flux density within a long solenoid.

7. Suggest and explain two ways of varying the magnitude of the flux density in the solenoid.
Answer:
Factors affecting field strength are current I and spacing of coils, N coils in length L:

6 A solenoid similar to that shown in the diagram has 100 turns connected in a circuit over a length of 0.50 m. µo = 4π  10-7 N A-2. Calculate the flux density at the centre of the solenoid when a current of 10 A flows.
Answer:

Magnetic flux density (B) = NI / L = = 2.5 mT

References:.
1. Lam Chok Sang, Lim Siang Kee. “Pre-U Text STPM Physics”, Pearson Malaysia Sdn. Bhd P.J
2. David Halliday, Robert Resnick, Jearl Walker. “Fundamentals of Physics(6th edition)”. John Wiley & Sons, Inc. (JWa, JWb).
3. Hugh D. Young, Roger A. Freedman, Lewis Ford. “University Physics(12th edition)”, Pearson Education publishings.

References: . 1. Lam Chok Sang, Lim Siang Kee. “Pre-U Text STPM Physics”, Pearson Malaysia Sdn. Bhd P.J 2. David Halliday, Robert Resnick, Jearl Walker. “Fundamentals of Physics(6th edition)”. John Wiley & Sons, Inc. (JWa, JWb). 3. Hugh D. Young, Roger A. Freedman, Lewis Ford. “University Physics(12th edition)”, Pearson Education publishings.