Magnetic Effects of Electric Current · Lesson 8 of 8
Chapter Summary and Practice
“Every field, force and fuse returns for one final magnetic reunion.”
• Connect the magnetic effects of current across straight conductors, loops and solenoids. • Review magnetic field lines and the direction rules used throughout the chapter. • Compare solenoids, electromagnets and bar magnets. • Apply Fleming’s Left-Hand Rule to conductors and moving charges. • Review domestic-circuit safety and current calculations. • Solve mixed conceptual, diagram-based and numerical problems.
Detailed Chapter Summary
This chapter connects four ideas: electric current produces magnetic fields, magnetic fields can exert forces on currents, coils can strengthen and shape magnetic fields, and domestic circuits use electrical connections and safety devices to deliver power safely. The sections below organise these ideas for quick revision.
Magnetic Field and Field Lines
A magnetic field is the region around a magnet or current-carrying conductor in which magnetic force can be detected. A compass reveals the field direction because its north pole aligns with the local magnetic field. Magnetic field lines provide a visual representation of both direction and relative strength.
• Outside a bar magnet, field lines emerge from the north pole and enter the south pole. • Inside the magnet, they continue from south to north, so every field line forms a closed curve. • Closely spaced lines represent a stronger magnetic field. • Field lines never intersect because the field has only one direction at any point.
Magnetic Field due to a Current-Carrying Conductor
Oersted’s compass observation established that current in a metallic conductor produces a magnetic field. The compass deflects while current flows, returns when the added field disappears, and deflects oppositely when current direction is reversed. This shows that electricity and magnetism are directly connected.
Magnetic Field due to a Current through a Straight Conductor
A long straight current-carrying conductor is surrounded by concentric circular magnetic field lines. At a fixed distance, increasing current strengthens the field. For unchanged current, moving farther from the conductor weakens the field. Reversing current reverses every field-line arrow without changing the circular pattern.
Current controls both field strength and direction. More current produces a stronger field; greater distance produces a weaker field; reversed current produces the opposite field direction.
Right-Hand Thumb Rule
Hold the straight conductor in the right hand with the thumb pointing along conventional current. The curled fingers show the direction of the magnetic field. Current emerging from the page produces an anticlockwise field, while current entering the page produces a clockwise field.
• A dot represents current coming out of the page, like the tip of an approaching arrow. • A cross represents current entering the page, like the tail of an arrow moving away. • Clockwise and anticlockwise directions must always be read from the stated viewing side.
Magnetic Field due to a Current through a Circular Loop
Every section of a current-carrying circular loop produces a magnetic field. At the centre, these contributions point in the same direction and reinforce one another. Anticlockwise current produces a central field towards the observer; clockwise current produces a field away from the observer. Increasing current or using more turns strengthens the field.
For identical turns carrying the same current, the fields add. An n-turn coil therefore produces n times the field of one turn at the same point under the same conditions.
Magnetic Field due to a Current in a Solenoid
A solenoid is a cylindrical coil containing many closely wound insulated turns. Its field resembles that of a bar magnet, with one north end and one south end. Inside a long solenoid, straight, parallel and equally spaced field lines represent a strong, nearly uniform magnetic field. Reversing current interchanges the poles.
| Arrangement | Essential idea |
|---|---|
| Solenoid | The current-carrying cylindrical coil that produces the magnetic field |
| Soft-iron core | Magnetic material placed inside the coil and magnetised by its field |
| Electromagnet | The controllable magnet formed by the current-carrying coil and magnetised core |
An electromagnet can be switched on or off by controlling current. Its strength can be increased by increasing current, increasing the number of turns or using a suitable soft-iron core.
Force on a Current-Carrying Conductor in a Magnetic Field
A current-carrying conductor placed in an external magnetic field experiences a force. Reversing either current or magnetic field reverses the force; reversing both leaves the force direction unchanged. The force is greatest when current and magnetic field are perpendicular, and the force then acts perpendicular to both.
Hold the thumb, forefinger and middle finger of the left hand mutually perpendicular. The forefinger shows magnetic field, the middle finger shows conventional current, and the thumb shows force or motion.
| Moving particle | Direction used as conventional current |
|---|---|
| Positive charge | Same as the direction of motion |
| Electron | Opposite to the direction of motion |
A perpendicular magnetic force can change a particle’s direction without changing its speed. Velocity and momentum change because both include direction; mass and speed remain unchanged.
The interaction of current and magnetic fields is used in motors, generators, loudspeakers, microphones and measuring instruments. Magnetic behaviour also supports Magnetic Resonance Imaging used for medical diagnosis.
Domestic Electric Circuits
The domestic supply described in this chapter is 220 V alternating current at 50 Hz. It passes through an electricity meter, main fuse and main switch before dividing into separate circuits. Appliances are connected in parallel so that each receives the full supply voltage and can be switched independently.
| Wire or component | Main function |
|---|---|
| Live wire | Carries the supply to an appliance |
| Neutral wire | Provides the normal return path |
| Earth wire | Provides a low-resistance safety path from exposed metal |
| Main fuse | Interrupts excessive current by melting |
| Main switch | Connects or disconnects the house installation |
| Parallel branches | Provide full voltage and independent control |
• Earthing helps keep a leaking metal body near earth potential. • A fuse melts because of Joule heating when current exceeds a safe value. • A 5 A circuit is used for lower-power loads, while higher-power appliances require a suitably rated circuit such as 15 A. • Never replace a fuse with an incorrectly rated thick wire.
Short-circuiting occurs when live and neutral make direct low-resistance contact, producing a sudden large current. Overloading occurs when the combined demand of appliances exceeds the safe circuit rating. Both may cause excessive heating, but their immediate causes are different.
Calculate current using I = P/V, then compare it with the circuit rating. If the calculated current is greater than the rating, the circuit is overloaded and the protective fuse should interrupt the supply.
| Key term | Essential meaning |
|---|---|
| Magnetic field | Region in which magnetic force can be detected |
| Magnetic field lines | Directed representation of field direction and relative strength |
| Solenoid | Cylindrical coil of many closely wound insulated turns |
| Uniform magnetic field | Field with the same magnitude and direction throughout a region |
| Electromagnet | Controllable magnet formed with a current-carrying coil and magnetic core |
| Short-circuiting | Direct low-resistance contact between live and neutral |
| Overloading | Current demand greater than the safe circuit rating |
Relationship And Rule Review
Point the right thumb in the direction of conventional current through a straight conductor. Curled fingers show the magnetic-field direction. For a loop, curl the fingers with current and the thumb shows the central field and the loop’s north-facing direction.
Hold the left forefinger, middle finger and thumb mutually perpendicular. Forefinger gives magnetic field, middle finger gives conventional current and thumb gives force or motion.
When a conductor moves in a magnetic field and the direction of induced current is required, hold the right thumb, forefinger and middle finger mutually perpendicular. Forefinger gives magnetic field, thumb gives conductor motion and middle finger gives induced current. Do not confuse this generating rule with the left-hand force rule.
| Question asks for | Use |
|---|---|
| Field around a current-carrying conductor | Right-Hand Thumb Rule |
| Force or motion of a conductor carrying current in a field | Fleming’s Left-Hand Rule |
| Induced current when a conductor moves through a field | Fleming’s Right-Hand Rule |
| Current drawn from appliance power and voltage | I = P/V |
| Often confused ideas | Correct distinction |
|---|---|
| Current direction and electron motion | They are opposite; positive-charge motion agrees with conventional current |
| Field direction and force direction | For the perpendicular case, force is perpendicular to both field and current |
| Clockwise field and clockwise current | A straight wire has circular field around straight current; a loop has circular current producing axial central field |
| Solenoid and electromagnet | A solenoid is the coil; adding a magnetised core produces an electromagnet |
| Earth and neutral | Neutral is the normal return path; earth is a protective leakage path |
| Short circuit and overload | Short circuit is low-resistance live-neutral contact; overload is excessive total demand |
| Fuse and earth wire | Fuse interrupts excessive current; earth provides a safe low-resistance path from metal casing |
| Field-line density and line count | Relative spacing within a drawing indicates strength; lines are not physical objects |
Diagram-Based Revision
For a bar magnet, verify that outside arrows run north to south, inside arrows return south to north, lines are closest near the poles and no lines cross. For a straight conductor, replace the two-pole pattern with concentric circles and use current direction to place arrows. For a circular loop, show the axial central field and curved external return. For a solenoid, show nearly parallel internal lines and a bar-magnet-like external pattern.
In force diagrams, label field from north to south and distinguish dot or cross current before applying the left-hand rule. In domestic-circuit diagrams, trace live and neutral as supply rails, confirm that appliances form separate parallel branches, place the fuse in the live path and connect exposed metal to earth.
Avoid treating field lines as physical threads, forgetting that they continue inside a magnet, applying a hand rule without first identifying current, using electron motion as current, assuming parallel appliances share the voltage, or comparing watts directly with amperes. Convert units and interpret every final direction or current value.
Chapter-Level Practise
Problem
Current emerges from a page through a straight conductor. State the field direction and predict the change if current is doubled.
- 1.A dot represents current towards the observer.
- 2.The Right-Hand Thumb Rule gives an anticlockwise magnetic field.
- 3.At fixed distance, B is proportional to I.
- 4.Doubling current therefore doubles the field magnitude under the same conditions, while direction remains anticlockwise.
Problem
A conductor carries current southward in an eastward magnetic field. Determine the force direction, then state what happens if both current and field reverse.
- 1.Use conventional current southward and magnetic field eastward.
- 2.Apply Fleming’s Left-Hand Rule with forefinger east and middle finger south.
- 3.The thumb points upward, so force is vertically upward.
- 4.Reversing only one direction would reverse force. Reversing both makes two directional reversals, so force remains upward.
Problem
A 1.2 kW heater and two 110 W lamps operate at 220 V. Can they share a 5 A circuit?
- 1.Convert heater power: 1.2 kW = 1200 W.
- 2.Total lamp power = 2 × 110 W = 220 W.
- 3.Total power = 1200 + 220 = 1420 W.
- 4.Current I = P/V = 1420/220 = 6.45 A approximately.
- 5.Since 6.45 A exceeds 5 A, the circuit would be overloaded and is unsuitable for these loads together.
Quiz
Which property of magnetic field lines prevents them from crossing?
A current into the page produces which field around a straight wire?
Which coil produces the stronger field under identical conditions?
Which rule gives force on a current-carrying conductor?
An electron moves east. What is the conventional-current direction?
Why are appliances connected in parallel?
Which condition best describes short-circuiting?
A 2 kW load at 220 V draws approximately
Practice Problems
- Draw magnetic field lines around a bar magnet and explain every arrow. Solution: Outside the magnet, arrows emerge from north and enter south. Inside, they return from south to north, forming closed curves. Lines should be denser near poles and must not intersect.
- A straight wire carries upward current. State how to determine the field at any surrounding point. Solution: Point the right thumb upward and follow the curled fingers. The field is tangent to concentric circles around the wire; its exact direction depends on the selected point and viewing direction.
- Current into a page produces a clockwise field. Find the field direction at a point above the wire. Solution: At the top of a clockwise circle, the tangent points to the right.
- Current in a straight conductor triples while distance doubles. Compare the field using the chapter relationships. Solution: B is proportional to I/r, so the factor is 3/2. The new field is one and a half times the original field, with unchanged direction if current direction is unchanged.
- A three-turn coil produces field 3B at its centre. Current is reversed. State the new field. Solution: Its magnitude remains 3B if current magnitude is unchanged, but its direction reverses.
- Explain why a solenoid has a nearly uniform internal field. Solution: Closely spaced turns produce reinforcing fields. In the long central region their field lines are straight, parallel and equally spaced, representing nearly constant magnitude and direction.
- Viewed from one solenoid end, current is clockwise. Identify that pole and predict the effect of reversing current. Solution: The viewed face is south. Reversing current makes it north and reverses the complete field direction.
- Field points right and current points into the page. Find the force. Solution: Use the left forefinger to the right and middle finger into the page. The thumb points downward, so force is downward.
- A proton moves west in an upward magnetic field. Determine force. Solution: Current follows proton motion westward. Applying Fleming’s Left-Hand Rule with current west and field up gives force towards north.
- An electron and proton move in the same direction through the same magnetic field. Compare force directions. Solution: Conventional current follows proton motion but is opposite electron motion. Their magnetic forces are therefore in opposite directions.
- State which properties of a freely moving charged particle may change when magnetic force bends its path without changing speed. Solution: Velocity changes because direction changes, and momentum changes for the same reason. Mass and speed remain unchanged.
- Explain why a metal-bodied appliance is earthed. Solution: If live current leaks to the casing, earth provides a low-resistance path and keeps the casing near ground potential, reducing shock risk and helping an overcurrent device disconnect the fault.
- Differentiate short-circuiting and overloading with one cause of each. Solution: Short-circuiting is direct low-resistance contact between live and neutral, often from damaged insulation. Overloading is excessive total demand, often from too many high-power appliances on one circuit.
- A 880 W appliance and a 220 W appliance operate at 220 V. Find total current and assess a 5 A circuit. Solution: Total power = 1100 W. I = 1100/220 = 5 A. This equals the stated rating, so no additional load should be added and the circuit must be correctly protected.
- A 2.2 kW heater operates at 220 V. Find current and select between 5 A and 15 A circuits. Solution: 2.2 kW = 2200 W. I = 2200/220 = 10 A. This exceeds 5 A but is below 15 A, so the appropriately protected 15 A circuit is required.
- A learner says a fuse and earth wire perform the same job. Correct the statement. Solution: A fuse melts and opens the live circuit when current is excessive. An earth wire provides a low-resistance path from exposed metal during leakage. They cooperate but perform different functions.
- Choose the correct rule for three situations: field around a wire, force on a powered conductor and induced current in a moving conductor. Solution: Use the Right-Hand Thumb Rule, Fleming’s Left-Hand Rule and Fleming’s Right-Hand Rule respectively.
- Explain one connection linking a straight wire, circular loop and solenoid. Solution: Each field is produced by electric current. Bending the straight conductor into a loop makes contributions reinforce at the centre; arranging many loops as a solenoid produces a strong, nearly uniform internal field.
- Construct a safe domestic-circuit explanation using meter, main fuse, main switch, parallel branches and earth. Solution: Supply passes through the meter, main fuse and main switch before distribution. Appliances form independently switched parallel branches across live and neutral. Exposed metal bodies connect to earth for leakage protection.
- A student reverses both current and magnetic field in a force experiment and expects force to reverse twice. Evaluate the statement. Solution: Each individual reversal changes force direction. Two reversals cancel, so the final force direction is the same as the original.
• I can draw and interpret magnetic field lines without intersections. • I can apply the Right-Hand Thumb Rule to straight conductors and loops. • I can compare field strength when current, distance or turn count changes. • I can explain uniform field inside a solenoid and the formation of an electromagnet. • I can use Fleming’s Left-Hand Rule and correctly reverse electron motion to conventional current. • I can distinguish live, neutral and earth wires. • I can distinguish short-circuiting from overloading and explain fuse action. • I can calculate appliance current from power and voltage and compare it with a circuit rating.
Previous · Lesson 7
Domestic Electric Circuits
Next
End of chapter