Undergraduate
  1. Classical mechanics  1.1. dynamics  1.1.1. Motion in one dimension  1.1.2. Motion in two dimensions  1.1.3. projectile motion  1.1.4. Relative speed  1.1.5. Uniform circular motion  1.1.6. Non-uniform circular motion  1.1.7. Reference frames and transformations  1.2. Newton's Laws of Motion  1.2.1. First law of motion  1.2.2. Second law of motion  1.2.3. Third Law of Motion  1.2.4. Applications of Newton's Laws  1.2.5. Friction and its types  1.2.6. Free Body Diagram  1.2.7. Constraints and pseudo-forces  1.3. Work and Energy  1.3.1. work done by the force  1.3.2. Work–energy theorem  1.3.3. Kinetic energy  1.3.4. Potential energy in classical mechanics  1.3.5. Conservative and non-conservative forces  1.3.6. Energy Conservation  1.3.7. Power and efficiency  1.4. Speed and collisions  1.4.1. Linear momentum  1.4.2. Conservation of momentum  1.4.3. Impulse and impact force  1.4.4. Elastic and inelastic collision  1.4.5. Center of mass and speed  1.4.6. Rocket Propulsion  1.5. Rotational motion  1.5.1. Torque and Angular Momentum  1.5.2. Moment of inertia  1.5.3. Rotational Kinematics  1.5.4. Rotational Energy  1.5.5. Rolling Motion  1.5.6. Precession and gyroscopic motion  1.6. Gravitational force  1.6.1. Newton's law of universal gravitation  1.6.2. Gravitational potential energy  1.6.3. Orbital mechanics  1.6.4. Kepler's laws of planetary motion  1.6.5. Gravitational field and potential  1.7. Fluid mechanics  1.7.1. Pressure and Pascal's Principle  1.7.2. Buoyancy and Archimedes' principle  1.7.3. Fluid Dynamics and Bernoulli's Principle  1.7.4. Viscosity and Poiseuille's law  1.7.5. Surface tension and capillarity  1.8. Oscillations and waves  1.8.1. Simple Harmonic Motion  1.8.2. Damped and driven oscillations  1.8.3. Coupled oscillations and common modes  1.8.4. Wave properties and types  1.8.5. Sound Waves and the Doppler Effect  1.8.6. Wave interference and superposition
  2. Electromagnetism  2.1. Electrostatics  2.1.1. Electric charge and properties  2.1.2. Coulomb's law  2.1.3. Electric field and electric potential  2.1.4. Gauss's Law  2.1.5. Capacitance and Dielectric  2.1.6. Electric dipole and potential energy  2.2. Electric circuits  2.2.1. Current and Resistance  2.2.2. Ohm's law  2.2.3. Kirchhoff's Laws  2.2.4. RC Circuit  2.2.5. AC Circuits and Reactance  2.2.6. Electric power and energy  2.3. Magnetism  2.3.1. Magnetic Fields and Forces  2.3.2. Ampere's Law  2.3.3. Magnetic dipole moment  2.3.4. Biot-Savart law  2.3.5. Magnetic Materials and Hysteresis  2.4. Electromagnetic induction  2.4.1. Faraday's Law  2.4.2. Lenz's law  2.4.3. Self and mutual induction  2.4.4. Transformers and Inductive Circuits  2.5. Maxwell's equations  2.5.1. Gauss's law for electricity  2.5.2. Gauss's law for magnetism  2.5.3. Faraday's law of induction  2.5.4. Ampere-Maxwell law  2.5.5. Electromagnetic waves
  3. Thermodynamics  3.1. Laws of Thermodynamics  3.1.1. Zeroth law of thermodynamics  3.1.2. First law of thermodynamics  3.1.3. Second Law  3.1.4. Third Law  3.2. Heat and work  3.2.1. Heat transfer (conduction, convection, radiation)  3.2.2. Thermal expansion  3.2.3. Heat engines and refrigerators  3.2.4. Carnot cycle and efficiency  3.3. Statistical mechanics  3.3.1. Maxwell–Boltzmann distribution  3.3.2. Entropy and Probability  3.3.3. Partition function  3.3.4. Fermi–Dirac and Bose–Einstein statistics
  4. Optics  4.1. Geometrical Optics  4.1.1. Reflection and Refraction  4.1.2. Mirrors and lenses  4.1.3. Optical Instruments  4.1.4. Fermat's principle  4.2. Wave optics  4.2.1. Interference in wave optics  4.2.2. Diffraction  4.2.3. Polarization  4.2.4. Coherence and holography
  5. Quantum mechanics  5.1. Wave–particle duality  5.1.1. Blackbody radiation in wave–particle duality in quantum mechanics  5.1.2. Photoelectric effect  5.1.3. Compton scattering  5.1.4. De Broglie wavelength  5.2. Schrödinger Equation  5.2.1. Time-independent Schrödinger equation  5.2.2. Particle in a box  5.2.3. Quantum Tunneling  5.2.4. Potential wells and obstacles  5.3. Quantum States  5.3.1. Wave function  5.3.2. Quantum operators  5.3.3. Heisenberg uncertainty principle  5.3.4. Angular momentum and spin
  6. Relativity  6.1. Special relativity  6.1.1. Lorentz transformations  6.1.2. Time expansion and length contraction  6.1.3. Relativistic energy and momentum  6.2. General relativity  6.2.1. Principle of Equivalence  6.2.2. Schwarzschild metric  6.2.3. Gravitational waves  6.2.4. Black holes and event horizons
  7. Solid state physics  7.1. Crystal structure  7.1.1. Bravais lattices  7.1.2. X-ray diffraction  7.1.3. Band theory  7.1.4. Phonons and lattice vibrations  7.2. Electrical and Magnetic Properties  7.2.1. Conductors, Semiconductors and Insulators  7.2.2. Superconductivity  7.2.3. Hall effect
  8. Nuclear and particle physics  8.1. Atomic Structure  8.1.1. Atomic Model  8.1.2. Nuclear binding energy  8.2. Radioactivity  8.2.1. Alpha, beta, gamma decay  8.2.2. Half life  8.2.3. Nuclear Fission and Fusion  8.3. Particle physics  8.3.1. Standard Model  8.3.2. Quarks and Leptons  8.3.3. antimatter  8.3.4. Fundamental interactions
  9. Astrophysics and cosmology  9.1. Stellar evolution  9.1.1. Star formation  9.1.2. Black holes and neutron stars  9.1.3. White dwarfs and supernovae  9.2. Cosmology  9.2.1. The Big Bang Theory  9.2.2. Dark matter and dark energy  9.2.3. Cosmic microwave background

Undergraduate → Electromagnetism → Electric circuits ↓


Current and Resistance


In electrical circuits, it is essential to understand the concepts of current and resistance to understand how electric current flows and how various components within the circuit interact. These concepts form the foundational knowledge in electromagnetism, an important branch of physics. This lesson will discuss current and resistance in depth, explaining their meaning, relationship, importance, and real-world applications using visual and textual examples.

What is electric current?

Electric current, often simply called current, is the flow of electric charge through a conductor. This charge is usually carried by moving electrons in a wire. The unit of electric current is the ampere (A), which indicates the amount of charge passing a point in a circuit per unit time.

Formula of electric current:

I = Q/t

Where:

  • I is the current in amperes (A)
  • Q is the charge in coulomb (C)
  • t is the time in seconds (s)

Visualize an electric current

Consider a simple analogy for the flow of water in a pipe. Imagine the electrons as water molecules. Electric current is similar to the flow of water through a pipe. If the flow rate is high, it equates to high current. Let's visualize this:

In the figure above, the blue line represents the wire, and the spinning circle represents the electron. As the electron moves, it shows the flow of electric current.

What is electrical resistance?

Resistance is a measure of how much a material opposes the flow of electric current. It slows down the movement of free charges, just like friction. The unit of resistance is the ohm (Ω).

Formula of resistance:

R = V/I

Where:

  • R is the resistance in ohms (Ω)
  • V is the voltage on the conductor in volts (V)
  • I is the current flowing through the conductor in amperes (A)

Visualize electrical resistance

Using our water flow analogy, resistance can be compared to a narrow section of a pipe that restricts water flow. Imagine if the pipe has an obstruction or narrowing at some point, which slows down the flow. Here's a visual representation:

The grey portion represents increased resistance, where the speed of the stream slows down, similar to how water slows down when it hits a narrow pipe section.

Ohm's law

Ohm's law is a fundamental relationship between current, voltage, and resistance in an electrical circuit. It can be used to calculate how any of these parameters affects the other. It is given as:

V = I * R

This formula shows that the voltage (V) across a resistor is equal to the product of the current (I) flowing through it and its resistance (R).

Ohm's law helps predict how changes in resistance or voltage will affect the current within a circuit, making it an important tool for engineers and physicists.

Example of Ohm's law

Consider a circuit with a 9-volt battery and a 3 ohm resistor. We can find the current flowing through the resistor using Ohm's law.

I = V/R

 I = 9V / 3Ω = 3A

Therefore, the current flowing in the circuit is 3 amperes.

Obstructions 9V Battery

In the above example, the two red circles represent electrons moving through the circuit, indicating the flow of current within the conductor, and the resistor experiences opposition according to its resistance.

Factors affecting resistance

There are many factors that affect resistance, including:

  • Materials: Different materials have different resistivities. For example, metals usually have low resistance due to their free electrons, while insulators such as rubber have high resistance.
  • Length: Longer wires have more resistance than shorter wires, because electrons face more opposition as they flow through the material.
  • Area of cross-section: A wider conductor has lower resistance because more electrons can flow through it at once, which is equivalent to a wider highway allowing more cars to pass through it.
  • Temperature: Higher temperatures increase resistance in most conductors because atoms vibrate more rapidly, impeding electron flow.
thin wire thick wire

The figure above shows two wires: a thin wire and a thick wire. The thicker wire naturally allows more electrons to flow due to its larger cross-sectional area, which makes it have a lower resistance than the thinner wire.

Simple Circuit Analysis Using Ohm's Law

Let us consider a simple circuit analysis example by calculating current, voltage or resistance wherever required using Ohm's law. Suppose we have a circuit in which a bulb is connected to a 12V battery and the resistance of the bulb is 6 ohms.

For the present:

I = V/R = 12V / 6Ω = 2A

Here, the current flowing in the circuit is 2 amperes.

Visual circuit diagram

6Ω bulb 12V Battery

In this view, the bulb is shown as the load in the circuit, and current flows continuously, determined by the battery voltage and the resistance of the bulb.

Applications of Current and Resistance

Household wiring: Household wiring systems are intricately designed to balance current and resistance, ensuring safe power distribution. For example, wires are selected based on their ability to carry current without overheating.

Electronic devices: Smartphones, computers, and other electronic gadgets rely on circuit boards with carefully calibrated resistance to ensure their components function optimally.

Heating elements: Appliances such as toasters and electric stoves use high-resistance wires or ribbons to produce heat when electric current is passed through them.

Conclusion

Understanding current and resistance is paramount to efficiently and safely perform desired operations using electricity. Analyzing circuits using fundamental principles such as Ohm's law helps us predict and manipulate electrical behavior to our advantage. The interplay between various parameters such as voltage, current, and resistance unravels the nuances of electronics, making current and resistance cornerstone topics in the study of electromagnetism and physics.


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Current and Resistance

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