Unit 11.04:

Magnetic Effectof a current

Unit 11.04:Unit 11.04:

Magnetic EffectMagnetic Effectof a currentof a current

Prepared in Dec 1998second editing in March 2000

At the end of this unit you should be able to :

1. describe the pattern of the magnetic field due to currents in a straight wires and in solenoids.

2. state the qualitative variation of the strength of the magnetic field over salient parts of the pattern.

3. describe the effect on the magnetic field of changing the magnitude of the current.

Learning objectivesLearning objectives

OerstedOersted ‘s Experiment‘s Experiment• Let a compass settle in a north-south

direction.• When the switch is closed, the compass

needle will defect.• What conclusion we can make from

this observation ?

• Sprinkle some iron filings on the horizontal card (see the fig.)

• Pass large current through the vertical wire.

• Tapping the card so that the iron filling show the sharp of magnetic field.

• What can you see ?

Finding the shape ofFinding the shape of

magnetic fieldmagnetic field

by iron fillingby iron filling

Magnetic field

RightRight--handhand grip rulegrip rule

Right-hand grip rule

• From the observation of the previous slide.

• We can conclud:Imagine gripping the wire with your right thumb pointing the same way as the current, then your fingers are curling the same way as the magnetic field lines.

The above figs. show the top view of lastslide.• (a) shows the current flowing up and out of

the slide.• (b) shows the current flowing down into

the slide.

• Sprinkle iron filling round the coil.• Pass a large current and tap the card.• The pattern is as shown in the fig.

A Short CoilA Short Coil

by iron by iron fillingfilling

(continue on next slide)

A short coilA short coil

by iron by iron fillingfilling

Finding North or South poleFinding North or South poleby rightby right--hand grip rulehand grip rule

SolenoidSolenoid

Its magnetic field

Forces between two parallel currentForces between two parallel current--carrying conductorscarrying conductors

• When current flowing in the same direction through two conductors.

• Attraction is produced.

• Why ?

Forces between two parallel Forces between two parallel currentcurrent--carrying conductorscarrying conductors

• When current flowing in the different direction through two conductors.

• Repulsion is produced.

• Why ?

• Hand the wire between the poles of a powerful magnet. (see fig.)

• When you press the switch and allow the current flows through the wire.

• The wire will then moves horizontally outward.

• From this observation, what conclusion we can make ?

switchforce

Magnetic forceon a current-carrying conductor

(continue on next slide)

Magnetic forceon a current-carrying conductor

• The wire moves because the magnetic field of the permanent magnet reacts with the magnetic field of the current in the wire.

• Fig. shows the combined field of the magnet and wire.

GCE OGCE O--LevelLevelPast Examination PaperPast Examination Paper

Science (Physics)

All rights go to University of Cambridge Examinations Syndicate and other sources

Nov 1991

14. The diagram shows resistance wire wrappedon to a rod to make an electrical heater.

Which of the following materials would bemost suitable for the rod ?

A polished copper B soft ironC coloured plastics D hard steel

A

Nov 1995O’ level Physics

14. An experiment was carried out using four rods made of different materials which were placed, in turn, in a coil of wire.

rod material rod materialA glass B ironC plastic D steel

D

A large direct current was passed through the coil for a few seconds and was then switched off. As a result one of the rods was permanently magnetised by this experiment. Which one?

GCE O Nov 1997

13. The diagram shows a coil in which there is acurrent.What will happen to the magnetic field when aniron bar is inserted into the bar ?

A It becomes much strong.B It becomes slightly

weaker.C It changes direction. D It stays the same.

A

Nov 199812. The diagram shows a 12 V d.c. Power supply,

connected across a coil of wire with a metal core.The core becomes a stronger magnet when the current is switched on. It remains a magnet after the current is switched off.

Which metal is the core made of ?A aluminiumB copper C soft ironD steel

D

Nov 1990

11. Which diagram shows the magnetic field pattern near a straight wire carrying a current?

A

11.(c) You are given three apparently identical painted metal bars, one steel, one soft iron and one brass. How would you identify them without damaging them in anyway? [2]

(Cont. …) Q. 11 Nov 1995

I may mix a strong magnet with these threebars. The bar does not attract by the magnet is the brass rod.I remove the magnet from the two bars, andthen test each bar with the magnet, the bar that repel the magnet is soft iron bar, and thebar only attract by the magnet is made by steel.

November 1992

6. The diagram below shows two metal rods placed inside a solenoid.When a battery is connected to A and B, thetwo rods rolled away from eachother.(a) Explain this

observation.

(continue on next slide)

Because same end of the ends of two rods have the same type of magnetism, thereforerepel one another,

6(b) Name one material from which the rods could have been made. [1]

(c) The same effect was observed when an alternating voltage was applied between Aand B. Explain this observation. [2]

(Cont. …) Q. 6 November 1992

Soft iron rod or steel rod.

Both rods will lost their magnetism (if theyhad magnetized) when a.c. is supplied to thesolenoid. Therefore, both rod will roll backand contact one another by the action of gravity.

Q. 4 Nov 1990

4.(c) Explain how, using suitable equipment.(i) a direct current may be used to magnetise a

magnet. [2]

Wind insulated copperwire round and round a large iron nail. Then connect the ends of wire to a battery (d.c.). After few seconds, the nailwill magnetize and able to pick up iron nails.

(continue in next slide)

4.(c)(ii) an alternating current may be used to demagnetise a magnet. [2]

(Cont. …) Q. 4 Nov 1990

Wind insulated copper wire round and round magnetized steel nail . Pass alternative current (a.c.) through the coil. Gradually pull the nail out of the coil. The magnetism will then destroyed.