Magnetic field and Oersted's experiment
A moving charge or current makes a magnetic field around it. Oersted found this in 1820, when a compass needle turned near a current-carrying wire.
What Oersted saw
In 1820 the Danish physicist Hans Christian Oersted noticed something new. A compass needle near a straight wire turned when a current flowed in the wire. The needle set itself at right angles to the wire, along a circle drawn round it. When the current was reversed, the needle turned the other way. The turn was larger for a larger current, or when the needle was closer to the wire. Iron filings sprinkled on a card round the wire formed concentric circles (circles with the wire at their common centre). Oersted concluded that moving charges, that is currents, produce a magnetic field around them. This was the first link found between electricity and magnetism.
Activity: repeat Oersted's experiment
You need a magnetic compass, a long straight copper wire, a 1.5 V cell, a plug key and a rheostat. A rheostat is a variable resistance that keeps the current safe.
- 1Place the compass on the table and let the needle settle along north-south.
- 2Hold the straight wire just above the compass, parallel to the needle.
- 3Join the wire, the key, the rheostat and the cell in series. Close the key for a few seconds only.
- 4Observe: the needle turns away from north-south. Note which way its N pole moves.
- 5Reverse the cell connections and close the key again. Observe: the needle turns the opposite way.
- 6Now place the wire below the compass, with the current as in step 3. Observe: the needle again turns the opposite way. Result: a current produces a magnetic field whose direction depends on the direction of the current.
What is a magnetic field?
A magnetic field is the region round a magnet or a current where a moving charge feels a force and a compass needle is turned. It is a vector field, written B: it has a size and a direction at every point. Its direction at a point is the direction in which the N pole of a compass points there. A charge at rest feels no magnetic force, and a charge at rest makes no magnetic field. The size of B is defined by the force on a moving charge, F = qvB sin θ, which you will meet as the Lorentz force. Take a charge of 1 C moving at 1 m/s at right angles to the field. If it feels a force of 1 N, the field is 1 tesla (T). The tesla is a large unit, so the gauss (1 G = 10⁻⁴ T) is also used. Magnetic fields obey superposition: the field of several sources is the vector sum of their separate fields.
Direction of the field round a straight wire
- Right-hand thumb rule: hold the wire in your right hand with the thumb along the current. Your curled fingers show the direction of the field lines.
- The field lines are concentric circles in a plane perpendicular to the wire.
- A dot ⊙ means a current or field coming out of the page, towards you. A cross ⊗ means one going into the page.
- For a current coming out of the page (⊙), the circles run anticlockwise. For a current going into the page (⊗), they run clockwise.
- The field gets weaker farther from the wire, so the circles are drawn farther apart as they get bigger.
Typical magnetic fields (orders of magnitude, for interest only)
| Where | Field B (T) |
|---|---|
| Surface of a neutron star | 10⁸ |
| Strong laboratory magnet | 1 |
| Small bar magnet | 10⁻² |
| Earth's surface | 10⁻⁵ |
| Human nerve fibre | 10⁻¹⁰ |
| Interstellar space | 10⁻¹² |
How to draw the field round a straight wire
- Draw a small circle to show the wire end-on. Put a dot inside it for current out of the page, or a cross for current into it. Label it I.
- Draw three or four circles round it, with the gaps increasing outwards.
- Put arrows on each circle: anticlockwise for ⊙, clockwise for ⊗.
- Mark a point P on one circle and its distance r from the wire. Draw B at P as an arrow along the tangent.
- Write the heading: 'Magnetic field lines of a straight current-carrying wire'.
An electrician's clamp meter measures the current in a wire without cutting it. It senses the magnetic field that the current makes round the wire, just as Oersted's compass did.
1 marks: “What is the SI unit of magnetic field? Express 1 gauss in this unit.” Tesla (T). 1 G = 10⁻⁴ T.
2 marks: “Describe Oersted's experiment. What did it show?” A compass needle near a straight wire turns when current flows; it sets at right angles to the wire. Reversing the current reverses the turn. Conclusion: an electric current produces a magnetic field around it.
2 marks: “A long straight wire carries a current vertically upwards. Find the direction of its magnetic field at a point due east of the wire.” Right-hand thumb rule: thumb up, so the field circles anticlockwise seen from above. At a point to the east, that circle runs towards the north. B points north.
Thumb along the current, fingers curl with the field. Dot ⊙ comes out of the page, cross ⊗ goes into it.