Magnetic Effects of Electric Current · Lesson 3 of 8
Magnetic Field due to a Current-Carrying Conductor
“The current goes straight, but its magnetic field prefers running in circles.”
• Describe how the magnetic-field direction changes when current direction is reversed. • Explain the concentric field-line pattern around a straight current-carrying conductor. • Relate magnetic-field strength to current and distance from the conductor. • Apply the Right-Hand Thumb Rule to determine magnetic-field direction. • Interpret dots, crosses, clockwise and anticlockwise directions in field diagrams. • Solve directional problems for points above, below or beside a straight conductor.
A bar magnet has a familiar two-pole field pattern, but a straight wire has no visible north or south end. When current flows through the wire, its magnetic field wraps around the conductor in circles. The pattern is controlled by the shape of the conductor, while its direction is controlled by the direction of current.
Activity
Connect a long straight copper wire, two or three cells and a plug key in series. Place the wire parallel to and above a compass needle. If current flows through the wire from north to south, note the direction in which the compass north pole deflects. Then reverse the cell connections so that current flows from south to north and observe again.
The compass deflection reverses when the current reverses. The compass is responding to the field direction, so this observation shows that reversing current reverses the magnetic field around the conductor. Current direction and magnetic-field direction are linked; one cannot be changed without affecting the other.
Keep the wire and compass positions unchanged while reversing only the cell connections. If both current direction and apparatus position change, the cause of the changed deflection becomes unclear.
Magnetic Field due to a Current through a Straight Conductor
To investigate the complete pattern, pass a long, straight, thick copper wire vertically through the centre of a horizontal cardboard sheet. Connect it in series with a battery, key, ammeter and rheostat. When current flows, sprinkle iron filings uniformly on the cardboard and tap it gently. The filings arrange themselves in concentric circles centred on the wire.
Each circle is a magnetic field line in the plane of the cardboard. At any point on one circle, a compass needle lies tangentially to that circle. Moving around the conductor changes the tangent direction continuously, even though the current direction remains fixed. This is why simply saying “the field points left” is incomplete; the field direction depends on the observation point.
Activity
Insert the vertical conductor securely through the cardboard. Keep the rheostat at a chosen setting, close the key only while observing and tap the cardboard gently. Use a compass at selected points to add arrows to the circular pattern. Reverse the current and confirm that every arrow reverses.
The magnetic field becomes stronger when the current increases. In the activity, increasing current produces a larger compass deflection at the same point. This comparison is valid only when the compass remains at the same distance from the wire. Current is the changed quantity, so the increased deflection is attributed to the stronger field.
The field becomes weaker as the observation point moves away from the conductor while current remains unchanged. A compass placed farther from the wire deflects less. The circles also grow larger with distance, so the same magnetic influence is represented as more spread out.
Bpropto rac{1}{r}| Change | What must be kept fixed | Effect on magnetic field |
|---|---|---|
| Increase current | Distance from wire | Field becomes stronger |
| Decrease current | Distance from wire | Field becomes weaker |
| Move closer to wire | Current | Field becomes stronger |
| Move farther from wire | Current | Field becomes weaker |
| Reverse current | Position and current magnitude | Field direction reverses |
Right-Hand Thumb Rule
Hold the straight conductor in your right hand so that the extended thumb points in the direction of conventional current. The way the fingers curl around the wire gives the direction of the circular magnetic field lines. The rule is also called Maxwell’s corkscrew rule: if a corkscrew advances in the direction of current, its direction of rotation gives the field direction.
First identify the conventional-current arrow. Point the right thumb along it. Curl the fingers naturally without rotating the page mentally midway. Read the field direction at the required point. If current points out of the page, the field is anticlockwise; if current points into the page, the field is clockwise.
Diagrams often use a dot to show the tip of an arrow coming out of the page and a cross to show the tail feathers of an arrow going into the page. These symbols describe current direction, not magnetic poles. After identifying the symbol, the right-hand rule gives the circular field direction.
Problem
Current in a vertical wire flows upward. Viewed from above the wire, is the magnetic field clockwise or anticlockwise?
- 1.Given: current flows upward, towards an observer looking from above.
- 2.Point the right thumb upward, towards the observer.
- 3.The curled fingers run anticlockwise as seen by that observer.
- 4.Therefore the field is anticlockwise when viewed from above.
- 5.Check: viewing from below would reverse the apparent sense to clockwise.
Problem
A horizontal power line carries current from east to west. Find the field direction directly below the wire.
- 1.Given: conventional current points west. Required: field direction at a point below the wire.
- 2.Point the right thumb from east towards west.
- 3.Curl the fingers around the line. At the point directly below it, the curled fingers point towards the south.
- 4.Thus the magnetic field directly below the line is towards the south.
- 5.At a point directly above the wire the field would be opposite, towards the north.
Problem
A wire passes perpendicular to a page. A compass placed to the right of the wire points upward because of the wire’s field. Determine whether current enters or leaves the page.
- 1.At the right-hand side of a circular field, an upward tangent belongs to an anticlockwise circle.
- 2.Use the right-hand rule in reverse: curl the right-hand fingers anticlockwise.
- 3.The thumb then points out of the page.
- 4.Therefore conventional current leaves the page and should be represented by a dot.
- 5.Check: current into the page would create clockwise field, which points downward at the right-hand side.
Do not confuse the circular field direction with the straight current direction. Always state the viewing direction when using clockwise or anticlockwise. A reversed observer sees the same physical circulation with the opposite visual sense.
Quiz
What is the shape of magnetic field lines around a long straight current-carrying wire?
At a fixed point, what happens when current increases?
What happens to field strength when a compass is moved farther from the same wire carrying unchanged current?
A cross inside a wire symbol represents current
Current is into the page. Which field direction is seen on the page?
Practice Problems
- Describe the field-line pattern around a straight conductor. Answer: The lines are concentric circles centred on the wire. A compass at any point aligns tangentially to the local circle.
- Current in a wire is doubled while the observation point is unchanged. Predict the field change. Answer: The magnetic field becomes stronger because at fixed distance it increases with current. Its direction remains unchanged because current direction has not reversed.
- A compass is moved from distance r to a greater distance while current is fixed. Predict and explain the change. Answer: Its deflection decreases because the magnetic field around a straight conductor weakens with increasing distance.
- Current emerges from a page. Determine the field at a point on the left of the wire. Answer: Current out of the page gives an anticlockwise field. On the left side, the tangent to an anticlockwise circle points downward.
- A westward current produces a field towards south directly below a wire. Predict the direction at the same point if current reverses. Answer: Reversing current reverses every field direction, so the field directly below becomes northward.
Key Takeaways
• A straight current-carrying conductor is surrounded by concentric circular magnetic field lines. • A compass at any point aligns tangentially to the local field circle. • Increasing current strengthens the field at a fixed distance. • Increasing distance weakens the field for unchanged current. • Reversing current reverses the magnetic-field direction. • The Right-Hand Thumb Rule connects straight current direction with circular field direction. • Clockwise or anticlockwise descriptions must be tied to a stated viewing direction.