Week 4 · Chapter 2 Magnetic Effects of Electric Current and Measuring Instruments (A): Oersted’s observation, the field of a straight wire (flux density B = μI/2πd, Ampere’s right-hand rule) and of a circular loop (B = μNI/2r, the right-hand screw rule, the loop as a magnetic dipole) · 3 lessons
1
Oersted’s observation and the field around a straight wire
Physics Sec 3 Term 1, Chapter 2: pp. 31–35
GoalSay what Oersted’s compass observation shows, and give the direction of the magnetic field round a straight wire with the right-hand rule.
New words
magnetic field · the region round a magnet or a current where a magnetic force acts on a compass needle or on another currentfield line · a line drawn so that the way it points at each place is the way a compass north pole points thereright-hand rule · thumb along the conventional current, curled fingers show the way the field goes round
Explanation
The scientist Oersted saw that a compass needle placed beside a wire turns when a current flows in the wire, and goes back when the current is switched off. This shows that an electric current is surrounded by a magnetic field. For a long straight wire the field lines are circles round the wire, in planes at right angles to it. The circles are crowded close to the wire, where the field is strong, and spread out farther away, where it is weak. A larger current gives a stronger field everywhere. The direction follows the right-hand rule of Ampère: point the thumb of your right hand along the conventional current and your curled fingers show the way the field goes round. Reverse the current and the field reverses. Do not repeat Oersted’s wire-and-battery experiment: a wire joined straight across a cell gets hot. Use the story and the drawing instead.
Worked examples
Place
Where the compass north pole points
east of the wire
north
north of the wire
west
west of the wire
south
south of the wire
east
A vertical wire carries a current upward. Look down on it from above, with east to the right and north up the page. The field goes anticlockwise round the wire. Ignoring the Earth’s own field, a compass north pole east of the wire points north, north of the wire it points west, west of the wire it points south, and south of the wire it points east.
Reverse the current so that it flows downward. Seen from above, the field now goes clockwise. East of the wire the north pole points south, and north of the wire it points east.
Place
I (A)
d (cm)
I ÷ d
A
3
2
1.5
B
6
6
1
C
3
6
0.5
Three places are compared: A is 2 cm from a wire carrying 3 A; B is 6 cm from a wire carrying 6 A; C is 6 cm from a wire carrying 3 A. The field grows with the current and falls with the distance, so compare I ÷ d: A has 3 ÷ 2 = 1.5, B has 6 ÷ 6 = 1, C has 3 ÷ 6 = 0.5. The order from strongest to weakest is A, B, C.
Class activity
With a grown-up, place a small compass on a table and notice which way its needle points. Bring a fridge magnet slowly towards it from the side and see what happens; then turn the magnet round and look again. Keep the magnet away from phones and bank cards. Then, on paper, draw the story of Oersted: a compass under a wire, once with the current off and once with it on, showing the needle each time. This is a drawing only.
Practice
1
The current in a vertical wire flows downward. Seen from above, is the field clockwise or anticlockwise?
Clockwise: thumb down, curled fingers go clockwise when seen from above.
2
The current in a vertical wire flows upward. Ignoring the Earth’s field, which way does a compass north pole point at a place south of the wire?
The field goes anticlockwise seen from above, so south of the wire it points east.
3
Rank by field strength: P is 4 cm from 2 A, Q is 4 cm from 4 A, R is 16 cm from 4 A.
I ÷ d: P = 2 ÷ 4 = 0.5, Q = 4 ÷ 4 = 1, R = 4 ÷ 16 = 0.25. The order is Q, P, R.
4
What does the needle do when the current is switched off, and why?
It goes back to its usual north–south direction, because the field of the wire disappears and only the Earth’s field is left.
