PHYSICS S.S. ONE (FIELD)

 SECOND TERM: WEEK 4

TOPIC: -                FIELD

INSTRUCTIONAL MATERIALS: -                  magnet, nail

REFERENCES BOOK: -                                       

1.FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2. M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publish             

1.       INTERNET

PREVIOUS KNOWLEDGE: - The Students have been familiar with thermometer.

INSTRUCTIONAL OBJECTIVES: -      At the end of the lessons students should be able to: -

1.       Define field

2.       Give examples of force field

3.       Explain why two bodies of different masses when released from rest at the same point simultaneously, fall to the ground at the same time.

4.       Explain electric lines of force

5.        State the properties of field line.

CONTENT: -

FIELD

Field is defined as a region of space under the influence of some physical agency such as gravitation, magnetism and electricity.

Force fields are defined as forces whose sources do not require contact with the body to which they are applied. We identified such force fields as gravitational force, electric force, magnetic force and electromagnetic force.

There are two classes of force fields: scalar field and vector field.

A scalar field is one that has only magnitude but no direction e.g. temperature, energy and density.

A vector field is a field that has both magnitude and direction e.g. gravitational, magnetic and electric fields.

Gravitational  Field

The up and down movements of objects on the earth’s surface are subject to the influence of the gravitational field of the earth. The earth attracts every object existing in the earth’s gravitational field. This attraction is called gravitational attraction and its effect is to change the velocity of objects under its influence i.e. to accelerate such objects. In the absence of air resistance or friction all bodies fall with the same acceleration irrespective of their masses.

                Newton’s law of universal gravitation states that the gravitational force of attraction between any two masses, m1, m2 is directly proportional to the product of the masses, and inversely proportional to the square of the distance (r ) between them.

F =  , G is the gravitational constant (= 6.67x10-11Nm2Kg-2)

Example

Calculate the force of attraction between two small objects of mass 10Kg and 50Kg respectively which are 10cm apart. Take G as 6.67x10-11Nm2Kg-2.

Solution

F =  =  = 3.33x10-6N

Magnetism and Magnetic Field

The ability of a magnet to attract magnetic substances is called magnetism. A magnet attracts a piece of iron, nickel and cobalt. Such substances that can be attracted by magnet are known as magnetic substances.

The pole of a magnet is the portion of the magnet where its magnetic attraction appears to be strongest. Like or similar pole of magnets repel one another, unlike or dissimilar pole attract one another.

Magnetic Field is the region or space around a magnet in which the influence of the magnet can be felt or detected. Magnetic field is a force field. Magnetic force can be felt at a distance. It influences an object even when not in contact with it. It’s also a vector field.

The magnetic line of force of a magnet is defined as the line along which a free N-pole would tend to move if placed in the field or a line such that the tangent to it at any point gives the direction of the field at that point. Thus the direction of the line of force at any point is taken as the direction in which a free N-pole placed at that point would tend to move     

                               

ELECTRIC FIELD

An electric field is a region of space where a charged body experiences an electric force.

An electric field is the space around a charged body where another charged body would be acted on by a force. These fields are represented by lines of force. This line of force also called an electric flux line points in the direction of the force. The electric field is represented by the symbol E. The SI unit of the electric field is newton per coulomb which is equal to volts per meter.

The direction of an electric field at any point is given by direction of the force acting on a small positive charge placed at that point. The arrow shows the direction of motion of a small positive charge placed at each such point and hence at the direction of the field line at such point.

ELECTRIC LINE OF FORCE

This is an imaginary line drawn in an electric field in such a way that the direction at any point (or the direction of the tangent) gives the direction of the electric field at such a point. It can also be defined as the path which an isolated small positive charge would follow if placed in the field.

There are two types of electric lines of force:

(a)    Uniform field: the field lines are straight.

(b)   Variable field:   field lines are curved.

Electric field patterns

Just like in magnetic fields, the closeness of the electric field-lines of force is the measure of the field strength. Their direction is always from the north or positive to the south or negative.

The field about an isolated positive charge is radically outward because a positive charge placed at any point around it is repelled outward along a line through the two charges.

The field about a negative charge is radially inwards.

 The electric field is represented by the imaginary lines of force. For the positive charge, the line of force come out of the charge and for negative charge the line of force will move towards the charge. The electric fields for positive and negative charges are shown below

Consider a unit charge Q placed in a vacuum. If another charge q is placed near Q then according to Coulomb law(it states that  the electric force between two point charges, q1 and q2 seperated by a distance r is directly proportional to the product of the charges and inversely proportional to the square of the distance between the charges), the charge Q apply a force on it. The charge Q produce an electric field around it, and when any other charge is placed near it, then the electric field of Q apply force on it.

The electric field produced by the charge Q at a point r is given by  where Q – unit charge, r- distance between the charges. A charge Q applies the force on a charge q is expressed by   . The charge q also apply an equal and opposite force on the charge Q.

PROPERTIES OF FIELD LINES

1.       Lines of force start only on positive charges and end only on negative charges.

2.       Lines of force do not cross each other or intersect each other.

3.       In a uniform field, the lines of force are straight, parallel and uniformly spaced.

4.       Lines of force indicate the direction of the electric field. The field point in the direction tangential to the lines of force at any point.

5.       The lines of force are continuous in any region with free charges.

6.       The lines are drawn such that the electric field is proportional to the number of lines crossing unit area perpendicular to the lines.

7.       The field is strong when the lines are close together, and it is weak when the field lines move apart from each other.

8.       If the charge is single, then they start or end at infinity.

PRESENTATION

Step 1: The teacher introduces the new topic to the student

EVALUATION

The teacher assesses the students with these questions:

1.       Define field

2.       Give examples of force field

3.       Explain why two bodies of different masses when released from rest at the same point simultaneously, fall to the ground at the same time.

4.       Explain electric lines of force

5.        State the properties of field line.

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