SPHERICAL MIRRORS

A mirror whose reflecting surface is a part of an imaginary hollow sphere is known as a spherical mirror.

Spherical mirrors are of two types:

(1) Concave mirrors

(2) Convex mirrors

Concave mirror: It is a spherical mirror whose reflecting surface is towards the centre of the imaginary sphere of which the mirror is a part.

Convex mirror: It is a spherical mirror whose reflecting surface is away from the centre of the imaginary sphere of which the mirror is a part.

Example: The inner shining surface of a steel spoon serves as a concave mirror and the outer shining surface of the steel spoon serves as a convex mirror.

ACTIVITY 1

(1)    Take a large shining spoon. Try to view your face in the inner shining surface of the spoon.

(2)    Bring the spoon close to your face. Observe the image.

(3)    Now move the spoon away from you. Observe the image.

(4)    Now try to view your face in the outer shining surface of the spoon.

(5)    Bring the spoon close to your face. Observe the image.

(6)    Now move the spoon away from you. Observe the image.

Terms related to spherical mirror

(1) Centre of curvature (C)

The centre of curvature of a spherical mirror is the centre of the imaginary hollow sphere of which the spherical mirror is a part.  It is denoted by C.

The centre of curvature is not a part of the spherical mirror. The centre of curvature of a concave mirror lies in front of the spherical mirror and centre of curvature of a convex mirror lies at the back of the mirror.

(2) Pole (P)

The pole of a spherical mirror is the centre of reflecting surface of the mirror. It is denoted by the point P

(3) Radius of curvature (R)

The radius of curvature of a spherical mirror is the radius of the imaginary hollow sphere of which the spherical mirror is a part. It is denoted by R

(4) Principal axis

The principal axis of a spherical mirror is the straight line passing through the centre of curvature C and pole P of the spherical mirror, produced on both sides.

(5) Aperture

The aperture of a spherical mirror is the diameter of the reflecting surface of the mirror

(6) Principal focus of a concave mirror (F)

The principal focus of a concave mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the concave mirror actually meet at a point on the principal axis.  It is denoted by the letter F

(7) Principal focus of a convex mirror (F)

The principal focus of a convex mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the convex mirror appears to meet at a point (or appears to diverge from a point)  on the principal axis.  It is denoted by the letter F

(8) Focal Length (f)

It is the distance between the principal focus (F) and pole P of the mirror. It is denoted by letter f.

Relation between focal length (f) and radius of curvature

Provided the aperture of a spherical mirror is much smaller than the radius of curvature, the focal length f can be related to radius of curvature R as

REFLECTION OF LIGHT BY A PLANE MIRROR

Image formed by a point object

Image formed by a finite object

Following are the important characteristics of images formed by a plane mirrors:

(1)  The image formed is always virtual. Such a image cannot be taken on a screen.

(2)  The image formed is always erect.

(3) The size of the image is same as the size of the object.

(4)  The image formed in a plane mirror as far behind the mirror, as the object is in front of the mirror.

(5) The image formed in a plane mirror is laterally inverted i.e. the left side of the object becomes the right side of the image and vice-versa.

LAWS OF REFLECTION OF LIGHT

First Law: The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Second Law: The angle of reflection (r) is equal to the angle of incidence (i).

These laws of reflection are valid for all types of reflecting surfaces. The surface may be smooth or rough. It may be a plane mirror, curved mirror, cylindrical or spherical mirror. The surface may be a spoon or a wall or a book. These laws of reflection are valid for all types of surfaces.

When light falls on smooth surfaces, regular reflection will occur. This type of reflection gives rise to image formation.

When light falls on rough surfaces, irregular reflection will occur. This type of reflection gives rise to scattering of light. Such a surface can be seen from all possible directions.

REFLECTION OF LIGHT

When light travelling in a given medium strikes any surface, a part of the incident light bounces back into the same medium. This phenomenon is called reflection.

Light reflected from a surface

Thus in reflection, the path of light rays changes its direction without any change in the medium of light.

Reflection is of two types:

Regular reflection

When the reflecting surface is smooth and well polished, the parallel rays of light incident on it are reflected parallel in one particular direction. This is known as regular reflection.

The regular reflection gives rise to image formation.

Regular reflection

Irregular reflection

When is reflecting surface is rough, the parallel rays falling on it are reflected in different directions. Such a reflection is known as irregular reflection or diffused reflection. No clear image is formed in case of irregular reflection.

Diffused or irregular reflection

Nature of Light

NATURE OF LIGHT

When we switch on a bulb/tube light, everything in the room becomes visible. When we switch off the bulb/tube light nothing can be seen. So we may conclude that it is the light which makes things visible when it falls on objects.

During the day, it is sunlight which makes things visible to us. The sunlight falling on objects is reflected or scattered and this reflected or scattered light when enter our eyes enable us to see those objects.

Thus,

Light is a form of energy which produces in us the sensation of sight.

What actually is light? or What is it made up of?
It is really a mysterious topic. It is an old topic of debate among scientists and science students. The explanation given by scientists appears to be against our common sense or the way we perceives the world. The theories are mind boggling for a student at the secondary level.

In junior classes, our teachers taught that light is a form of a ray which travels in straight line. A combination of rays forms a beam. A ray falling on a mirror is reflected in such a way that angle of incidence and angle of reflection both are equal. The concept of reflection and refraction can be explained completely on the ray theory of light. 

Interference and diffraction are the characteristics of a wave. It is observed that light also undergoes interference and diffraction supporting that light is also a wave. Scientists also explained reflection and refraction on the basis of wave theory. 

Scientists also found some phenomenon which cannot be explained on the basis of wave theory such as photoelectric effect and compton effect. Several experiments conducted on light proves that light is composed of particles known as photon. 

Now this is very confusing whether to call a light a wave or particle.  It is safer to assume that light has a dual character. It behaves like a wave as well as particle.


Properties of light 
(1)    Light travels in a straight line. This property of light is known as rectilinear propagation of light. This straight line path is usually indicated as a ray of light.
(2)   Nothing can travel faster than light. In vacuum the speed of light is 3 x 10⁸ m/s.
(3)  The speed of light in different medium is different. For example, speed of light in glass is 2 x 10⁸ m/s.
(4)   On entering from one transparent medium (say air) to another (say water) light changes its direction. The extent and way of bending depends upon the optical density of two media.
(5)   According to the modern theory of light, Light has a dual character. It is emitted or absorbed as a particle (called photon), but it propagates as a wave.
(6)     The particle character of light is called PHOTON. When light propagates in the form of wave it consist of electric and magnetic field hence light is also known as an electromagnetic wave.

Phenomena related to light:

(1)    Reflection of light
(2)    Refraction of light
(3)    Dispersion
(4)    Scattering
(5)    Interference
(6)    Difraction
(7)    Polarisation
(8)    Doppler effect

Sample paper 3

1.  What are wind energy farms?

2.  What is meant by potential difference between two points?

3.  How much energy will an electric bulb draw from a 220 V source, if the resistance of the bulb-filament is 1200 ohms?

4. What is electromagnetic induction?

5. In what way can the magnitude of induced current be increased?

6. Calculate the electrical energy consumed by a 1200 W toaster in 20 minutes.

7. Biomass has been used as fuel since ancient times, how has it been modified to function as a more efficient fuel in the recent past.

8. Two identical resistors, each of resistance 20 ohms are connected (i) in parallel (ii) in series, in turn, to a battery of 10V. Calculate the ratio of power consumed in the combination of resistors in the two cases.

9. Two resistors of resistances 3 ohms and 6 ohms respectively are connected to a battery of 6 V so as to have
(a)    Minimum resistance,    (b) minimum current
(i)  How will you connect the resistances in each case?
(ii) Calculate the strength of the current in the circuit in both cases.

10. Explain what is short circuiting and overloading in an electric supply?

11. Why can’t two magnetic field lines cross each other?

12. Draw the magnetic field lines (including field directions) of the magnetic field due to a long straight solenoid. What important property of this field is indicated by this field line pattern? Name any two factors on which the magnitude of the magnetic field due to this solenoid depends.

13. State the rule to determine the direction of a
(i)  Magnetic field produced around a straight conductor carrying current.
(ii) Force experienced by current – carrying straight conductor placed in a magnetic field which is perpendicular to it.
(iii) Current induced in a coil due to its rotation in a magnetic field.

14. Differentiate between AC and DC. Write one advantage of AC over DC.

Domestic Electric Circuit

Domestic Electric Circuit

1.     Household electric supply consists of 3 types of wire. Live wire (red insulation cover), neutral wire (black insulation cover) and earth wire (green insulation cover)

2.    The potential difference between neutral and live wire is 220V.

3.    In household circuit system, two type of circuit are used. One of 15 A for appliances with higher power ratings and the other of 5 A ratings for bulbs etc.

4.    In domestic circuit, different appliances are connected in parallel combination. This ensures that if one appliance is switched ‘on’ or ‘off’, the others are not affected.

5.    The earth wire is used as a safety measure, especially for those appliances that have a metallic body.

Working of earth wire 

The metallic body is connected to the earth wire, which provides a low resistance-conducting path for the current. It ensures that any leakage of current to the metallic body of the appliance will flow to the earth only and the user may not get a severe shock.
Watch the video below to more explanation on working of earth wire.

Short circuit

It means that the two wires live and neutral have come in contact with each other. This may happen either due to their insulation have been damaged or due to a fault in the appliance. In such a case, the resistance of the circuit decreases to a very small value. According to Ohm’s law, the current increases enormously. It may results in spark at the place of short circuit, which may even cause fire. Sometimes, the current also increase due to overloading of the circuit.

Overloading

It is a situation when too many appliances are connected in the same circuit such that the overall current (sum of all the current used by all appliances) exceeds the current carrying capacity of the connecting wires. The wires cannot withstand such a high current and melt and may cause fire. 

The electric fuse

Electric fuse is used as safety device for the protection of electric circuits and appliances due to short circuiting or overloading of the circuit.. The electric fuse is a piece of wire having a very low melting point and high resistance.When a high current flows through the circuit due to short circuit or overloading, the fuse wire gets heated and melts. The circuit is broken and current stops flowing thus save the electric circuit and appliance form damage.

Capacities of fuse wire

The fuse for domestic purposes are rated as 1A, 2A, 3A, 5A, 10A and 15A.         

Characteristics of fuse wire

1.    Low melting point
2.    High resistance 

Fuse wire are made up of pure tin or made of an alloy of copper and tin.              

AC and DC

Alternating Current and Direct Current

Alternating Current (AC)

It is a current whose magnitude changes continuously and direction changes periodically.

The source of AC is AC generator.

In India mostly current is supplied in AC form. The frequency of AC in India is 50 Hz. It means AC changes direction every 1/100 of a second. In complete cycle of AC, it changes its direction twice.

Direct Current (DC)

It is a current whose magnitude is always constant and such a current flows in one direction only.

The source of DC are DC generators, cells, batteries etc

Advantage of AC over DC

1.  An alternating current (AC) can be transmitted to long distances without much power loss where as if direct current (DC) is supplied to long distance that most of its energy is wasted in the form of Joule’s heat.

2.  An alternating current (AC) can be step-up and step-down i.e. the voltage can be increased or decreased with the help of transformers whereas a DC can not step-up or step-down.

Electromagnetic Induction

Electromagnetic Induction

The process, by which a changing magnetic field in a conductor induces a current in it, is called Electromagnetic Induction.

Activity 1

1.    Take a coil of wire having many turns.

2.     Connect it to a sensitive galvanometer.

3.     Take a bar magnet and move the north pole of the magnet towards the coil.

Observation: you will observe that galvanometer will give a deflection in one direction (say left).

4.    Now move the north pole of the magnet away from the coil.

Observation: you will observe that galvanometer now give a deflection in opposite direction (now right)

**Similar effect will be observed if we use the south pole of the magnet in the above activity. But the direction of deflection will be reversed.

**Similar effect will be observed if we keep the magnet stationary and move the coil towards or away from the coil.

**When the coil and the magnet are both stationary, there is no deflection in the galvanometer.

Thus whenever there is a relative motion between the coil and the magnet, it induces a current in the coil.

Summary of the activity

POSITION OF THE MAGNET	DEFLECTION IN THE GALVANOMETER
Magnet at rest	No deflection in galvanometer
Magnet moves towards the coil	Deflection in galvanometer in one direction
Magnet is held stationary at same position (near the coil)	No deflection in galvanometer
Magnet moves away from the coil	Deflection in galvanometer but in opposite direction
Magnet is held stationary at same position (away from the coil)	No deflection in galvanometer
Magnetic is held stationary inside the coil	No deflection in galvanometer

Activity 2

1.    Take two different coils of copper wire having large number of turns (say 50 and 100 turns respectively). Insert them over a non-conducting cylindrical roll

2.    Connect the coil-1, having larger number of turns, in series with a battery and a plug key.

3.    Also connect the other coil-2 with a galvanometer as shown.

4.    Plug in the key. Observe the galvanometer.

Observation: You will observe that the needle of the galvanometer instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil-2.

5.    Disconnect coil-1 from the battery.  Observe the galvanometer.

Observation: You will observe that the needle momentarily moves, but to the opposite side. It means that now the current flows in the opposite direction in coil-2.

Remark on activity 1 and 2

Thus form activity 1 and activity 2 it is clear that we can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient in most situations to move the coil in a magnetic field.

The induced current is found to be the highest when

the direction of motion of the coil is at right angles to the magnetic field. In this situation, we can use a simple rule, Fleming's Right Hand Rule, to know the direction of the induced current.

Fleming’s Right Hand Rule

Hold the forefinger, the central finger and the thumb of the right hand perpendicular to each other so that the forefinger indicates the direction of the field, and the thumb is in the direction of motion of the conductor. Then, the central finger shows the direction of the current induced in the conductor.

Force on a current carrying wire

FORCE ON A CURRENT CARRYING CONDUCTOR PLACED IN A MAGNETIC FIELD

Any current carrying conductor when kept in magnetic field experiences a force. 
Watch a youTube video below to observe how a current carrying wire experiences a force when kept in a magnetic field. 


Video courtesy : Learn n hv fun YouTube Channel (A volunteer member of the website)

The direction of force is given by FLEMING’S LEFT HAND RULE.

ACTIVITY

1.   Take a small aluminium rod AB.
2.   Suspend it horizontally with the help of connecting wires from a stand.
3.   Place a strong horseshoe magnet in such a way that the rod is between the two poles with the field directed upwards.
4.   When current is passed in the rod from B to A, the rod gets displaced towards left.
5.   On reversing the direction of the current, the rod gets deflected towards right.

The deflection in the rod is caused by the force acting on the current carrying rod when placed in a magnetic field.

The displacement of the rod is largest (or the magnitude of the force is the highest) when the direction of current is at right angles to the direction of the magnetic field. In such a condition we can use a simple rule to find the direction of the force on the conductor.

FLEMING’S LEFT HAND RULE

Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger that of the current, then the thumb will point towards the force or direction of motion of the conductor.

FORCE ON A MOVING CHARGE PARTICLE IN A MAGNETIC FIELD

A current carrying conductor experiences a force when placed in a magnetic field. As current is simply flowing of charges, it implies that moving charged particles also experiences a force in a magnetic field.

The direction of the force on a moving positive charge is given by Fleming’s Left hand rule (discussed above).

Application of force experienced when placed in a magnetic field

Devices that use current-carrying conductors and magnetic fields include electric motor, loudspeakers, microphones and measuring instruments.

Solenoid

SOLENOID

A coil of many circular turns of insulated copper wire wrapped closely in the form of a cylinder is called a solenoid.

A solenoid

MAGNETIC FIELD DUE TO CURRENT IN A SOLENOID

1.   The magnetic field due to a solenoid is very much similar to that of a bar magnet. The pattern of the magnetic field lines around a current-carrying solenoid is similar to that of bar magnet. Just like a bar magnet, one end of the solenoid behaves as a magnetic north pole, while the other behaves as the South Pole. 
2.   The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.

PRACTICAL USE OF SOLENOID

A strong magnetic field produced in a solenoid can be used to magnetize a piece of magnetic material when it is placed within the coil, which is carrying electric current.

ACTIVITY

1.   Take a iron nail.

2.   Wrap a coil of insulated copper wire on it.

3.   Connect the coil to a battery through a switch.

4.   As the current is passed through the coil, the nail, which acts as a core inside the solenoid, gets magnetized.

The magnet so formed is called an electromagnet.

ELECTROMAGNET

An electromagnet consists of a long coil of insulated copper wire wound on a soft iron core.

TEMPORARY MAGNET

If the core of the solenoid is taken of soft iron and electric current is passed through the solenoid, the soft iron core is temporarily magnetized which means when the current is switched off soft iron loses its magnetic properties. An electromagnet is a temporary magnet.

PERMANENT MAGNET

If the core of the solenoid is taken of carbon steel, chromium steel, cobalt and tungsten steel and certain alloys like Nipermag (alloy of iron, nickel, aluminium and titanium) and ALNICO (alloy of Aluminium, nickel and cobalt) and a strong electric current is passed through the coil then these materials become permanently magnetized.

Uses of permanent magnets

Such permanent magnets are used in microphones, loudspeakers, electric clocks, ammeter, voltmeter and speedometer, etc.

Circular coil

MAGNETIC FIELD DUE TO A CURRENT CARRYING CIRCULAR WIRE

Using Right Hand thumb rule, the magnetic field lines at every point of the circular wire are in the form of concentric circles with wire as the center. These magnetic field lines become larger and larger as we move away from the wire. Just at the center of the coil, magnetic field lines are almost straight. By applying Right hand rule, it is easy to check that every section of the wire contributes to magnetic field lines in the same direction within the loop.

Factors on which the magnetic field at the centre of the circular loop depends upon

1.   Directly proportional to the current flowing in the loop. 

2.   Inversely proportional to the radius of the circular wire.

3.   Directly proportional to the number of turns of the circular loop. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

Magnetic field due to current carrying straight conductor

Magnetic field around a current carrying straight conductor

Activity

1. Take a straight wire AB and pierced it through a horizontal cardboard such that wire AB is vertical.

2.   The ends of the wire AB are connected to a battery.

3.   Place some iron fillings on the cardboard.

4.   Switched the key on.

5.   Gently tap the cardboard.

6.   The iron fillings arrange themselves in concentric circles around the wire.

7.   This shows that magnetic field lines are concentric circles. The circles become larger and larger as we move away from the wire.

Watch a related video from YouTube.

Direction of field lines- Right Hand thumb rule

The direction of field lines due to a current carrying wire can be determined by using Right Hand Thumb Rule (Or Right Hand Grip Rule). 

Imagine that you are holding a current carrying wire in your right hand such that the thumb is stretched along the direction of the current, then, the fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

Factors on which the magnetic field at a distance from the straight wire depends on

1.   Directly proportional to the current flowing in the wire.

2.   Inversely proportional to the distance from the wire.

Magnetic field and field lines

Oersted’s Experiment

(Relation between electricity and magnetism)

The first evidence of any connection between Electricity and magnetism was established by Hans Christian Oersted. He accidentally discovered that as he laid a wire carrying an electric current near a magnetic compass needle, it got deflected as if acted upon by a magnet.

This observation led to the discovery that when current passes through a conductor, magnetic field is produced around it.

Magnetic field

The space around a magnet or a current carrying conductor, in which the force of attraction or repulsion can be experienced, is called a magnetic field.

Demonstration of magnetic field lines (Iron-filings pattern)

1.  Take a bar magnet and placed it on a cardboard.

2.  Sprinkle some iron-fillings around the magnet.

3.  Tap the cardboard gently.

4.  Iron fillings arrange themselves in a pattern as shown in figure.

Conclusion: This pattern demonstrates that under the influence of magnetic field, the fillings align themselves along the magnetic field lines.

Tracing of magnetic field lines of a bar magnet

1.   Place a bar magnet on a sheet of paper.

2.   Bring the compass near the north pole of the magnet.

3.   The needle will deflect such that its south pole points towards North Pole of the bar magnet.

4.   Mark the position of two ends of needle.

5.   Move the compass so that its south end occupies the position previously occupied by the north end.

6.   Again mark the new position of ends of needle.

7.   Repeat step 5 and 6 till you reach the south pole of the magnet.

8.   Join the points marked to get a smooth curve, which represents a field line.

Magnetic field lines

Magnetic field lines are the imaginary lines used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction of the magnetic field at a point is given by the direction that a north pole placed at that point would take.

Properties of Magnetic field lines

1.   Outside the bar magnet, the magnetic field lines originate from the north pole of a magnet and end at its south pole.

2.   Inside the bar magnet, field lines move from South Pole of the magnet to the North Pole.

3.   Magnetic field lines always form closed curves.

4.   The regions, where field lines are closer, the field is strong and the regions, where the field lines are farther apart, the field is weak.

5.   The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.

6.   The magnetic field lines never cut each other. In case, two field lines intersect each other at a point, then it will mean that at the point of intersection, the magnetic needle would point in two different directions, which is not possible.