The chapter on "Electricity" is assigned a weightage of \(7\) marks, highlighting its significance in the overall curriculum. Understanding this chapter will enhance to know how basic electrical components connect and their function.
Also to prepare effectively for related exam questions. It is essential to grasp the various types of electrical circuits, components effects, and their applications. Focusing on these concepts will greatly benefit both academic performance and practical application in physics.
- \(1\) mark - Two questions
- \(5\ \)mark - One question
Learning outcomes:
-
Understanding key concepts: Grasp the basic principles of electric current, voltage, and resistance.
-
Applying Ohm’s Law: Use the relationship between voltage, current, and resistance to solve problems.
-
Analyzing circuits: Distinguish between series and parallel circuits and analyze their behaviour.
-
Calculating power and energy: Compute electrical power and energy consumption in various circuits.
-
Heating effect of electric current: Understand the heating effect of electric current and its applications (e.g., electric heaters, fuse).
-
Mathematical applications: Solve problems related to Ohm’s Law and circuit analysis.
Electricity formulae - At a glance:
| Physical quantity | Symbol | Definition | Formula | SI Unit | Unit Symbol |
| Electric charge | \(Q\) or \(q\) | Basic fundamental properties of all matter | \(Q\ =\ ne\) | \(coulomb\) |
\(C\)
|
| Electric current | \(I\) | Rate of flow of electric charge through a conductor | \(I\ =\ \frac{Q}{t}\) | \(ampere\) | \(A\) |
| Potential difference | \(V\) | Work done to move unit charge between two points | \(V\ =\ \frac{W}{Q}\) | \(volt\) | \(V\) |
| Ohm's law | - | States that current is directly proportional to voltage at constant temperature | \(V\ =\ IR\) | \(volt\) | \(V\) |
| Resistance | \(R\) | Opposition offered by a conductor to the flow of current | \(R\ =\ \frac{V}{I}\) | \(ohm\) | \(\Omega\) |
| Resistivity | \(\rho\) | Intrinsic property of a material that opposes current flow | \(\rho\ =\ \frac{RA}{L}\) | \(ohm-metre\) | \(\Omega\ m\) |
| Resistance of a conductor | \(R\) | Resistance depends on material, length, and area of the conductor | \(R\ =\ \frac{\rho\ L}{A}\) | \(ohm\) | \(\Omega\) |
| Equivalent resistance in series | \(R_s\) | Total resistance when resistors are connected end-to-end | \(R_s\ =\ R_1\ +\ R_2\ +\ R_3\ +\ . . .\) | \(ohm\) | \(\Omega\) |
| Equivalent resistance in parallel | \(R_p\) | Combined resistance when resistors are connected across the same voltage | \(\frac{1}{R_p}\ =\ \frac{1}{R_1}\ +\ \frac{1}{R_2}\ +\ \frac{1}{R_3}\ +\ . . .\) | \(ohm\) | \(\Omega\) |
| Joule's law of heating | \(H\) | Heat generated when current flows through a resistor |
\(H\ =\ I^2Rt\)
\(H\ =\ VIt\)
\(H\ =\ \frac{V^2t}{R}\)
|
\(joule\) | \(J\) |
| Electric Power | \(P\) | Rate at which electrical energy is consumed or converted |
\(P\ =\ VI\)
\(P\ =\ I^2R\)
\(P\ =\ \frac{V^2}{R}\)
|
\(watt\) | \(W\) |
| Electrical energy | \(E\) | Energy consumed by an electrical appliance when it operates | \(E\ =\ P\ \times\ t\) | \(watt-second\) | \(Ws\) |
Common mistakes to avoid:
- Always use SI units and write the correct unit in the final answer.
- If your calculated equivalent resistance in parallel is greater than the smallest resistor, check your calculation!
- If the graph is \(I\) on the \(y-axis\) and \(V\) on the \(x-axis\), then slope is \(1/R\), not \(R\).
- Convert all units to SI before calculating.
Important constants and conversions:
| Quantity | Definition | Relation |
| Electronic charge | Charge carried by a single electron | \(e\ =\ -\ 1.6\ \times\ 10^{-16}\ C\) |
| \(1\ coulomb\) | The quantity of charge transferred in one second by a current of one ampere | \(1\ C\ =\ 6.25\ \times\ 10^{18}\ electrons\) |
| \(1\ ampere\) | Current due to flow of one coulomb charge per second | \(1\ A\ =\ 1\ C/s\) |
| \(1\ volt\) | Potential difference when one joule of work is done per coulomb | \(1\ V\ =\ 1\ J/C\) |
| \(1\ ohm\) | Resistance allowing one ampere current at one volt | \(1\ \Omega\ =\ 1\ V/A\) |
| \(1\ watt\) | Power when one joule of energy is used per second | \(1\ W\ =\ 1\ J/s\) |
| \(1\ kilowatt\) | Commercial unit of power | \(1\ kW\ =\ 1000\ W\) |
| \(1\ kilowatt-hour\) | Commercial unit of electrical energy | \(1\ kWh\ =\ 3.6\ \times\ 10^6\ J\) |
Practical applications of the heating effect of electric current:
- Appliances such as electric irons, heaters, kettles, toasters, and ovens work on the principle of Joule's heating, where electrical energy is converted into heat.
- The heating effect is also used in electric bulbs. A thin tungsten filament, with a very high melting point (\(3380^{\circ}C\)), becomes white-hot and produces light when current passes through it.
- The bulb is filled with nitrogen and argon gases to increase the life of the filament. Most of the electrical energy is converted into heat, while only a small portion is emitted as light.
Electric fuse - An application of Joule's heating:
- An electric fuse is a safety device that works on the principle of Joule's heating.
- It protects electrical circuits and appliances by breaking the circuit when excessive current flows.
- A fuse is connected in series with the appliance and contains a thin wire made of a metal or alloy such as aluminium, copper, iron, or lead with a suitable melting point.
- When the current exceeds the safe limit, the fuse wire heats up, melts, and stops the flow of current.
Last-minute check: