When current flows through the coil, it experiences a torque
Ammeter:
Device used to measure current in a circuit
Connected in series with the circuit
Low resistance to ensure minimal voltage drop
Voltmeter:
Device used to measure potential difference across a component
Connected in parallel with the component
High resistance to prevent significant current flow
Principle of Operation
Moving Coil Galvanometer:
Force experienced by the coil due to magnetic field is given by the equation: F=BILsin(θ)
Torque experienced by the coil is given by the equation: τ=BILsin(θ)⋅N⋅A
The restoring torque is proportional to the current passing through the coil
Ammeter:
The galvanometer is converted into an ammeter by connecting a shunt resistor in parallel
The shunt resistor allows only a fraction of the total current to pass through the galvanometer
Voltmeter:
The galvanometer is converted into a voltmeter by connecting a series resistor
The series resistor limits the current passing through the galvanometer
Equations for Ammeter
Equation for Current through Ammeter:
The current passing through the shunt resistor: Ishunt=Rshunt/NshuntAI−IG
The current passing through the galvanometer: IG=NshuntRg+RshuntIRshunt
Total current passing through the ammeter: I=Ishunt+IG
Example:
A 0.1 Ω shunt resistor with 10 turns and 1 mm² area is connected to a galvanometer having a resistance of 50 Ω.
If the current is 2 A, calculate the current passing through the galvanometer.
Solution: IG=10×1×10−6×50+0.12×0.1=0.00198,A
Equations for Voltmeter
Equation for Voltage across Voltmeter:
The voltage across the series resistor: Vseries=Ig(Rseries+Rg)
The voltage across the galvanometer: IgRg=RseriesRseries+RgVseries
Total voltage across the voltmeter: V=Vseries+IgRg
Example:
A voltmeter is constructed using a galvanometer with a resistance of 50 Ω. A series resistor of 1000 Ω is connected.
If a voltage of 5 V is applied across the voltmeter, calculate the voltage across the galvanometer.
Solution: IgRg=10001000+50×5,V=5.25,V
Potential Energy of a Dipole
A dipole consists of two equal and opposite charges separated by a distance.
Definition of potential energy: The work done in bringing the two charges from infinity to their current position.
Equation for potential energy of a dipole: U=−rpE
Interpretation:
When the dipole is aligned with an external electric field, work is done to bring the charges closer together.
The potential energy is negative, implying that energy is released when the dipole interacts with the field.
Equations for Potential Energy of a Dipole
Equation for potential energy of a dipole with a point charge:
If the dipole is aligned perpendicular to the electric field, the equation becomes: U=−r2pE
Equation for potential energy of a dipole with an electric dipole moment:
If the dipole is at an angle θ with the field, the equation becomes: U=−pEcos(θ)
Example:
An electric dipole with a dipole moment of 10 C.m is placed in an electric field of magnitude 100 N/C.
If the angle between the dipole and field is 30 degrees, calculate the potential energy of the dipole.
Solution: U=−10×100×cos(30)=−500,J
Torque on a Dipole
A dipole experiences a torque when placed in an external electric field.
Equation for torque on a dipole: τ=pEsin(θ)
Interpretation:
The torque tends to align the dipole with the electric field.
The torque is maximum when the dipole is perpendicular to the field.
Example:
An electric dipole with a dipole moment of 5 C.m is placed in an electric field of magnitude 200 N/C.
If the angle between the dipole and field is 45 degrees, calculate the torque experienced by the dipole.
Solution: τ=5×200×sin(45)=500,N.m
Potential Energy of a System of Point Charges
A system of point charges can also possess potential energy.
Equation for potential energy of a system of point charges: U=4πϵ01j>i∑rijqiqj
Interpretation:
The potential energy is positive, indicating the potential energy required to bring the charges together.
Example:
Three charges q1 = 2 µC, q2 = -3 µC, and q3 = 5 µC are located at the vertices of an equilateral triangle with sides of 2 m.
Find the potential energy of the system of point charges.
Solution: U=4πϵ01(22×(−3)+22×5+2(−3)×5),J
Capacitors in Series
Capacitors connected in series have the same charge across them.
Equation for equivalent capacitance of capacitors in series: Ceq1=∑Ci1
Example:
Two capacitors with capacitance values of 5 µF and 10 µF are connected in series.
Calculate the equivalent capacitance of the system.
Solution: Ceq1=51+101=103
Capacitors in Parallel
Capacitors connected in parallel have the same potential difference across them.
Equation for equivalent capacitance of capacitors in parallel: Ceq=∑Ci
Example:
Two capacitors with capacitance values of 5 µF and 10 µF are connected in parallel.
Calculate the equivalent capacitance of the system.
Solution: Ceq=5+10=15,μF
Factors Affecting Resistance
Factors that affect resistance:
Length: Longer the wire, higher the resistance
Cross-sectional area: Smaller the area, higher the resistance
Temperature: Higher the temperature, higher the resistance
Material: Different materials have different resistivities
Resistivity: Intrinsic property of a material that determines its resistance
Ohm’s Law
Ohm’s law relates current, voltage, and resistance.
Equation: V=IR
Interpretation:
Voltage (V) is directly proportional to current (I) if the resistance (R) is constant.
Voltage is inversely proportional to resistance if the current is constant.
Current is directly proportional to voltage if resistance is constant.
Resistors in Series
Resistors connected in series have the same current flowing through them.
Equation for equivalent resistance of resistors in series: Req=R1+R2+…
Example:
Two resistors with resistance values of 5 Ω and 10 Ω are connected in series.
Calculate the equivalent resistance of the system.
Solution: Req=5+10=15,Ω
Resistors in Parallel
Resistors connected in parallel have the same potential difference across them.
Equation for equivalent resistance of resistors in parallel: Req1=R11+R21+…
Example:
Two resistors with resistance values of 5 Ω and 10 Ω are connected in parallel.
Calculate the equivalent resistance of the system.
Solution: Req1=51+101=103
Kirchhoff’s Laws
Kirchhoff’s laws are used to analyze complex electrical circuits.
Kirchhoff’s first law (KCL):
The algebraic sum of currents entering any junction in a circuit is zero.
Kirchhoff’s second law (KVL):
The sum of potential differences around any closed loop in a circuit is equal to zero.
Example:
In a circuit, three currents enter a junction: 2 A, 3 A, and 5 A. Determine the current leaving the junction.
Solution: Since current entering and leaving a junction must balance, the current leaving the junction is 10 A.
Series and Parallel Connection of Cells
Cells connected in series have the same current flowing through them.
The total voltage of the series-connected cells is the sum of the individual voltages.
Cells connected in parallel have the same potential difference across them.
The total voltage of the parallel-connected cells is equal to the voltage of any one of the cells.
Example:
Three identical cells with individual voltages of 1.5 V each are connected in series and parallel.
Calculate the total voltage in each case.
Solution: In the series connection, the total voltage is 4.5 V (1.5 V + 1.5 V + 1.5 V). In the parallel connection, the total voltage is 1.5 V.
Electric Power
Electric power is the rate at which electrical energy is converted into other forms of energy.
Equation for electric power: P=IV
Interpretation:
Electric power is directly proportional to current and voltage.
Power is the rate of energy transfer or energy consumed per unit time.
Example:
In a circuit, a current of 3 A flows through a resistor with a voltage drop of 5 V. Calculate the power dissipated by the resistor.
Solution: P=3×5=15,W
Joule’s Law of Heating
Joule’s law describes the heat produced in a conductor due to electric current.
Equation for heat produced (Joule heating): H=I2Rt
Interpretation:
The heat produced is directly proportional to the square of the current, resistance, and time.
Heat is dissipated as a result of the flow of electric current.
Example:
A current of 2 A flows through a resistor of 5 Ω for 5 minutes. Calculate the heat produced by the resistor.
Solution: H=22×5×5×60=12000,J
Focal Length of a Spherical Mirror
Focal length is a property of a spherical mirror that determines the focusing power.
Equation for focal length of a spherical mirror: f1=v1+u1
Interpretation:
Focal length is the distance between the mirror and its focal point.
The distance of the object (u) and image (v) from the mirror are used to calculate the focal length.
Example:
A spherical mirror forms an image at a distance of 10 cm when the object is placed at a distance of 15 cm from the mirror. Calculate the focal length of the mirror.
Solution: f1=101+151,f=30,cm
Lens Formula
Lens formula relates the object distance, image distance, and focal length of a lens.
Equation for lens formula: f1=v1−u1
Interpretation:
The object distance (u) and image distance (v) are used to calculate the focal length (f).
The focal length can be positive or negative depending on the lens type.
Example:
A lens forms an image at a distance of 30 cm when the object is placed at a distance of 20 cm from the lens. Calculate the focal length of the lens.
Solution: f1=301−201,f=−60,cm
Magnetic Effects of Electric Current
Magnetic field due to a current-carrying conductor
Right-hand rule to determine the direction of the magnetic field
Magnetic field strength is directly proportional to the current and inversely proportional to the distance from the conductor
Magnetic field inside a solenoid
Force on a current-carrying conductor in a magnetic field
Magnetic Force on a Current-Carrying Conductor
Equation for the force on a current-carrying conductor in a magnetic field: F=BILsin(θ)
Interpretation:
The force is perpendicular to both the magnetic field and the current direction
The magnitude of the force increases with the current, magnetic field strength, and length of the conductor
The force can be determined using the right-hand rule for direction
Magnetic Force on a Moving Charge
Equation for the magnetic force on a moving charge in a magnetic field: F=qvBsin(θ)
Interpretation:
The force is perpendicular to both the velocity and the magnetic field
The magnitude of the force increases with the charge, velocity, and magnetic field strength
The force can be determined using the right-hand rule for direction
Magnetic Field Due to a Circular Loop
Equation for the magnetic field at the center of a circular loop carrying current: B=2Rμ0I
Interpretation:
The magnetic field is directly proportional to the current and inversely proportional to the radius of the loop
The direction of the magnetic field can be determined using the right-hand rule
Force Between Two Parallel Current-Carrying Conductors
Equation for the force between two parallel current-carrying conductors: F=2πdμ0I1I2L
Interpretation:
The force is attractive if the currents are in the same direction and repulsive if they are in opposite directions
The magnitude of the force increases with the currents, length of the conductors, and decreases with the distance between them
Electromagnetic Induction
Faraday’s law of electromagnetic induction
Induced EMF and induced current in a conducting loop
Lenz’s law
Factors affecting the magnitude of induced EMF
Applications of electromagnetic induction (generators, transformers)
Faraday’s Law of Electromagnetic Induction
Faraday’s law states that the induced EMF in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.
Moving Coil Galvanometer, Ammeter, and Voltmeter Moving Coil Galvanometer: Device used to detect current in a circuit Consists of a coil placed in a magnetic field When current flows through the coil, it experiences a torque Ammeter: Device used to measure current in a circuit Connected in series with the circuit Low resistance to ensure minimal voltage drop Voltmeter: Device used to measure potential difference across a component Connected in parallel with the component High resistance to prevent significant current flow