Grade 10: Physics Workshop

 Miscellaneous Topics

Fuse:

• A fuse is an electrical safety device.
• It is designed to protect an electrical circuit from excessive current.
• It consists of a thin metal wire or strip that melts and breaks the circuit when the current exceeds a safe level.
• It prevents overheating, fire, or damage to electrical appliances.

Properties of a Fuse

• Low Melting Point:

The fuse wire is made of materials like tin, lead, or an alloy of tin and lead, which have a low melting point. This ensures that the fuse melts quickly when the current exceeds the safe limit.

• High Resistance:

The fuse wire has a high resistance to produce heat when excess current flows through it, causing it to melt.

• Current Rating:

Every fuse is rated for a specific maximum current. If the current exceeds this limit, the fuse blows (melts) to break the circuit.

Applications of Fuses:

• Protecting household electrical systems.
• Preventing damage to electronic devices like computers and TVs.
• Ensuring safety in industrial electrical circuits.

Short-Circuit:

A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit.

It results in excessive current flow in the power source through the 'short,' and may even cause the power source to be destroyed.

It can occur also when live wire and Neutral wire comes in touch with each other. In this situation, heavy current is drawn from the mains, if not avoided; it will burn the conducting wires and the connected devices.

The FUSE acts as a protecting mechanism. A fuse is a small piece of conductor with a low melting point. It melts in case current exceeds in case of short-circuit or overload.

Overloading

Overloading occurs when the total electrical load connected to a circuit exceeds the circuit's capacity, causing excessive current flow.

Causes

1. Connecting too many appliances to a single circuit.
2. Use of devices with high power consumption.
3. Faulty appliances drawing excessive current.

Key Differences: Short-Circuit vs. Overloading

Aspect	Short-Circuit	Overloading
Cause	Direct contact between live and neutral wires.	Excessive load on the circuit.
Current Path	Current flows through a low-resistance path.	Current flows through the normal path.
Result	Instantaneous surge in current.	Gradual increase in current.

In domestic circuits all the appliances are connected in parallel; and in parallel connection, the total current from the mains will be the sum of the individual current drawn from each of the devices.

Here again the FUSE is used a protecting mechanism.

Earthing or Grounding:

Earthing or Grounding is a safety mechanism. It is used to protect human being from getting an electric shock. The electrical devices with the mechanical body like coolers, Ions and geysers etc. When comes in touch with the Live wire due to some internal fault in device. This makes the body of the equipment at the potential of the Live wire. The Earth is a good conductor of electricity. The person standing on ground if touches the metallic body of the device. The electric current will flow through the human body resulting in electric shock resulting immediate death of the person.

Magnetic Field Around a Current-Carrying Conductor

When an electric current flows through a conductor, it creates a magnetic field around it. The shape and strength of this magnetic field depend on the shape of the conductor, such as a straight conductor or a circular loop.

Magnetic Field Due to a Straight Current-Carrying Conductor

Description:

When current flows through a straight conductor, concentric circular magnetic field lines are formed around the conductor.

Direction:

The direction of the magnetic field is determined by the Right-Hand Thumb Rule:

Point your thumb in the direction of the current, and the curl of your fingers shows the direction of the magnetic field.

Magnetic Field Due to a Circular Current-Carrying Loop

Description:

When current flows through a circular loop, the magnetic field forms concentric circles around each segment of the loop.

At the center of the loop, the magnetic field is strong and almost uniform.

Direction:

Use the Right-Hand Thumb Rule:

Curl your fingers in the direction of the current in the loop, and your thumb points in the direction of the magnetic field at the center.

Solenoid:

A solenoid is a coil of wire wound in the shape of a cylinder, designed to generate a magnetic field when an electric current passes through it. It is widely used in electrical and mechanical systems to produce controlled magnetic effects.

The magnetic field, thus produced, is very much similar to that of a bar magnet. The strength of magnetic field produced by a current carrying solenoid depends upon:

(i) The number of turns in the solenoid: Larger the number of turns in the solenoid, greater will be the magnetic field produced.

(ii) The strength of current in the solenoid: Larger the current passed through solenoid, stronger will be the magnetic field produced.

(iii) The nature of “core material” used in making solenoid: The use of soft iron rod as core in a solenoid produced the strongest magnet.

Force on a Current-Carrying Conductor in a Magnetic Field:

Andre Marie Ampere (1775–1836) suggested that the magnet also exert an equal and opposite force on the current carrying conductor.

A current-carrying conductor when placed in a magnetic field, it experiences a force due to the interaction between the magnetic field and the current flowing through the conductor.

Direction of Force:

The direction of the force is given by the Fleming's Left-Hand Rule:

• Stretch your thumb, forefinger, and middle finger of your left hand so they are mutually perpendicular.
• Forefinger points in the direction of the magnetic field (B).
• Middle finger points in the direction of the current (I).
• Thumb points in the direction of the force (F)

As observed that the rod will displace i.e. the rod will experience a force, when it is placed in magnetic field, in a perpendicular direction to its length.

The direction of the exerted force will be reversed if the direction of current through the conductor is reversed.

If one changes the direction of field by inter changing the two poles of the magnet, again the direction of exert force will change.

Therefore, the direction of exerted force depends on

(a) Direction of current

(b) Direction of magnetic field lines.

Atmospheric Refraction

Atmospheric refraction refers to the bending of light as it passes through the Earth's atmosphere due to variations in air density. The atmosphere consists of layers with different densities and refractive indices. As light travels through these layers, it refracts (bends) gradually.

Causes of Atmospheric Refraction

1. Variation in Air Density:

The density of air decreases with altitude.

Higher density near the Earth's surface causes greater refraction compared to the upper atmosphere.

2. Refractive Index Gradient:

The refractive index of air changes with temperature, pressure, and humidity.

Cooler, denser air closer to the ground has a higher refractive index than warmer, less dense air above.

Effects of Atmospheric Refraction

1. Twinkling of Stars:

Light from stars passes through different layers of the atmosphere, undergoing multiple refractions.

The continuous bending and scattering of light cause stars to appear as though they are twinkling.

2. Apparent Position of Stars:

Due to refraction, stars appear higher in the sky than their true position.

The shift is more pronounced near the horizon.

3. Advanced Sunrise and Delayed Sunset:

Atmospheric refraction allows us to see the sun about 2 minutes earlier during sunrise and 2 minutes longer during sunset.

The light from the sun bends around the Earth's curvature, making it visible even when it is below the horizon.

 Scattering of Light

Scattering of light refers to the redirection of light in different directions when it encounters particles, molecules, or irregularities in its path. This phenomenon explains many natural optical effects in the Earth's atmosphere, such as the blue color of the sky and the reddish hues of sunrise and sunset.

Causes of Scattering

1. Interaction with Particles:

Light interacts with atmospheric particles like dust, water droplets, and gas molecules.

The extent of scattering depends on the size of the particles compared to the wavelength of light.

2. Wavelength Dependency:

Shorter wavelengths (blue and violet) scatter more than longer wavelengths (red and orange) due to their smaller size.

Examples of Scattering in Nature

1. Blue Sky:

Shorter wavelengths (blue) scatter more effectively than longer wavelengths (red), making the sky appear blue during the day.

2. Red Sunrise and Sunset:

During sunrise and sunset, sunlight passes through a thicker layer of the atmosphere.

Shorter wavelengths scatter out, leaving longer wavelengths (red and orange) to dominate.

3. Clouds Appearing White:

Water droplets and ice crystals scatter all wavelengths equally, making clouds appear white.

 

Dispersion of Light

Dispersion of light refers to the splitting of white light into its constituent colors (spectrum) when it passes through a medium like a prism. This phenomenon occurs because different colors of light have different wavelengths and refract (bend) by varying amounts.

White Light Composition:

• White light is a combination of seven colors: violet, indigo, blue, green, yellow, orange, and red (VIBGYOR).

Refractive Index Dependence:

• Each color has a different wavelength.
• The refractive index of a medium varies slightly for each wavelength.
• Shorter wavelengths (violet, blue) refract more than longer wavelengths (red, orange).

Prism Effect:

• A glass prism causes light to refract twice—once when it enters the prism and once when it exits.
• Due to differing refractive indices for different wavelengths, the light splits into a spectrum.

Key Points

• Red Light: Longest wavelength, bends the least.
• Violet Light: Shortest wavelength, bends the most.

Worksheet:

1.    Q.  a) (i) State the rule used to find the force acting on a current carrying conductor placed
|            in a magnetic field.

       (ii) Given below are three diagrams showing entry of an electron in a magnetic field. 
             Identify the case in which the force will be 1) Maximum and 2) Minimum   
             respectively.  Give reason for your answer.

Answer

(i) Fleming's left-hand rule is used to find the direction of force acting on a current carrying conductor, placed in a magnetic field.

According to this rule: Stretch the thumb, fore finger and middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of magnetic field and the middle finger in the direction of current, then the thumb will point in the direction of motion or force acting on conductor.

(ii) (1) Force on electron is maximum in case (i) because here direction of motion of electron is at right angles to the magnetic field.

(2) Force on electron is minimum in case (iii) because here direction of motion of electron is along the direction of the magnetic field.

Additionally,

Formula for Force

The magnitude of the force (F) on the conductor is given by:

F=ILB sin θ

Where:

F = Force on the conductor (Newtons, N)

I = Current in the conductor (Amperes, A)

L = Length of the conductor in the magnetic field (meters, m)

B = Magnetic field strength (Tesla, T)

θ = Angle between the conductor and the magnetic field

             Special Cases:

If θ = 0∘ or 180∘
The force is zero because the current is parallel to the magnetic field.

If θ = 90∘:
The force is maximum because the current is perpendicular to the magnetic field.

Or

b) (i) Draw the pattern of magnetic field lines of

       (1) a current carrying solenoid

       (2) a bar magnet

   (ii) List two distinguishing features between the two fields.

(1) The pattern of magnetic field lines of a current carrying solenoid is shown below:

(2) The pattern of magnetic field lines of a bar magnet is shown below:

(ii) Distinguishing features between:

Magnetic field lines of a current carrying solenoid	Magnetic field lines of a bar magnet
If we cut a solenoid into two halves, the magnetic field strength of the halves gets decreased.	When we cut a bar magnet into two halves, the magnetic properties do not change and both act as a magnet.
The poles of a solenoid can be altered and hence the direction of magnetic field lines.	In case of a bar magnet, it is fixed.

1.      Q. Give reasons for the following:

(a) Danger signals installed at airports and at the top of tall buildings are of red colour.

(b) The sky appears dark to the passengers flying at very high altitudes.

(c) The path of a beam of light passing through a colloidal solution is visible.

(a) Red colour has the longest wavelength and it deviates the least, hence, danger signals installed at airports and at the top of tall buildings are of red colour so that they can be easily seen from a distance.

(b) Scattering of light takes place because of the particles present in the atmosphere. At high altitude, the atmospheric medium is rarer so the scattering of light taking place is very low. Hence, the sky appears dark to passengers flying at high altitudes.

(c) The path of a beam of light passing through a colloidal solution is visible due to scattering of light by colloidal particles and this is known as Tyndall effect.

Q. (A) (i) Why is an alternating current (A.C.) considered to be advantageous over direct current (D.C.) for the long-distance transmission of electric power?

Efficiency in Transmission:

• A.C. can be easily transmitted over long distances with minimal energy loss by using transformers to step up (increase) the voltage and step down (decrease) the current.
• Higher voltage reduces the heat generated due to resistance in transmission lines, improving efficiency.

Transformability:

• A.C. voltage can be stepped up or down using transformers, which is not possible with D.C. in a simple, cost-effective manner.

Cost and Maintenance:

• The generation and distribution of A.C. is more economical and requires less complex infrastructure compared to D.C.

(ii) How is the type of current used in household supply different from the one given by a battery of dry cells?

Household Supply:

• Provides alternating current (A.C.), which periodically reverses its direction and changes magnitude.
• In India, the frequency of A.C. is 50 Hz (i.e., it reverses direction 50 times per second).

Battery of Dry Cells:

• Provides direct current (D.C.), which flows in one constant direction and has a steady magnitude.

(iii) How does an electric fuse prevent the electric circuit and the appliances from a possible damage due to short circuiting or overloading.

Working Principle:

• A fuse is a thin wire made of a material with low melting point (e.g., tin or lead alloy).
• It is connected in series with the circuit.

Action during Overloading or Short Circuiting:

1. Overloading:

When too many appliances are connected to a circuit, the current exceeds the safe limit.

2. Short Circuiting:

Happens when live and neutral wires come in direct contact, causing an abrupt surge in current.

In both cases, the excessive current generates heat, causing the fuse wire to melt and break the circuit. This prevents damage to the appliances and reduces the risk of fire or other hazards.

Q. Why should an ammeter have low resistance?

An ammeter is an instrument used to measure the electric current flowing through a circuit. It is connected in series with the circuit, and for accurate measurements, it should have low resistance. Here's why:

Reasons Why an Ammeter Should Have Low Resistance

1. Minimize Voltage Drop:

When an ammeter is connected in series, the current flowing through the circuit also flows through the ammeter.

If the ammeter has high resistance, it would cause a significant voltage drop across it, reducing the voltage available to the rest of the circuit. This would alter the actual current in the circuit, leading to inaccurate measurements.

2. Preserve Circuit Conditions:

The purpose of an ammeter is to measure current without disturbing the circuit's normal operation.

A high-resistance ammeter would significantly increase the total resistance of the circuit, thereby decreasing the current, which defeats the purpose of the measurement.

3. Avoid Power Loss:

Power dissipated in a resistor is proportional to its resistance (P = I²R).

If the ammeter has high resistance, it would dissipate more power, making the device inefficient and possibly overheating the circuit.

Conclusion

An ammeter with low resistance ensures:

• Accurate measurement of current.
• Minimal disturbance to the circuit.
• Efficient operation with negligible power loss.

For practical purposes, an ideal ammeter is considered to have zero resistance.

Q. Why should voltmeter have high resistance?

A voltmeter is an instrument used to measure the potential difference (voltage) across two points in a circuit. It is connected in parallel with the component whose voltage is to be measured. For accurate measurements, a voltmeter should have high resistance. Here's why:

Why Should a Voltmeter Have High Resistance?

1. Minimize Current Draw:

A voltmeter connected in parallel creates an alternate path for the current.

If the voltmeter has low resistance, a significant amount of current will flow through it, altering the current in the circuit and affecting the voltage across the component being measured.

High resistance ensures that only a negligible amount of current flows through the voltmeter, minimizing its impact on the circuit.

2. Preserve Circuit Conditions:

The purpose of a voltmeter is to measure the potential difference without changing the behavior of the circuit.

A high-resistance voltmeter prevents any significant current diversion, preserving the circuit's normal operation.

3. Accurate Measurement:

                        A voltmeter with high resistance ensures that the voltage drop across its terminals is
                        primarily due to the potential difference of the component being measured, leading to
                        accurate readings.

Conclusion

A voltmeter with high resistance ensures:

• Accurate voltage measurements.
• Minimal impact on the circuit's current.
• Proper functioning of the circuit during measurement.

For practical purposes, an ideal voltmeter is considered to have infinite resistance.

 Q. a). An electric iron consumes energy at a rate of 880 W when heating is at the maximum rate and 330W when heating is at the rate minimum. If the source voltage is 220 V, calculate the current and resistance in each case.

1.      Power formula:

P =V x I

Where:

P is power (in watts, W),

V is voltage (in volts, V),

I is current (in amperes, A).

Rearranging for current:

I = P / V​

2.      Ohm’s Law:

V = I x R

Rearranging for resistance:

R = V / I

Case 1: Maximum Heating Rate

·         Power, P = 880 W

·         Voltage, V = 220 V

·         Current:

I = P / V = 880 / 220 = 4 A

Resistance:

R = V / I = 220 / 4 = 55 Ω  

Case 2: Minimum Heating Rate

·         Power, P = 330 W

·         Voltage, V = 220 V

Current:

I = P / V = 330 / 220 =1.5 A

Resistance:

R = V / I = 220 / 1.5 ≈ 146.67 Ω

b) What is heating effect of electric current?

The conductor offers resistance to the flow of current, due to this resistance the electrical energy is converted to heat energy. The conductor becomes hot as a result of it.

When an electric current flows through a conductor (like a wire), the free electrons in the conductor collide with atoms and other electrons. These collisions cause the kinetic energy of the electrons to be converted into thermal energy, which raises the temperature of the conductor.

 

c) Find an expression for the amount of heat produced when a current passes through a  
    resistor for some time.

V = W/Q        -------(1)

W = V x Q     -------(2)

I   = Q/t          -------(3)

From (3). We have

Q = I x t         -------(4)

From (2) & (4), We have

W = V x I x t -------(5)

Energy (E) is the capacity to do work. Therefore Equation (5) becomes-

E = V x I x t -------(6)

From ohms’ law,

V = I x R       -------(7)

From (6) & (7)

E = I² x R x t -------(8)

The energy consumed is transformed into heat energy. Therefore equation (8) becomes-

                        H = = I² x R x t ------(9)