Apparatus
bar magnet compass
large sheet of paper
Theory
The direction of a magnetic field at a point can be found by placing a small compass at the point. The north pole of the compass points in the direction of the magnetic field at the point.
Method
1. Place the sheet of paper on a horizontal surface. 2. Use the compass to find the N–S direction and mark this direction at the centre of the paper. 3. Place the bar magnet on the paper along the
N–S line marked on the paper with the north pole of the magnet pointing north.
4. Mark on the paper the outline of the magnet and label the poles N and S.
5. Place the compass at a point near the north pole of the magnet. Mark with two points the position taken up by the compass needle. 6. Move the compass to a new position so that
the position of the compass needle follows from the previous position. This is illustrated in figure 8.40.
7. Continue in this way until you reach a position near the south pole of the magnet.
8. Draw a continuous curve through the points you have marked on the paper.
9. Mark with arrows the direction of the mag- netic field at several points along your line. 10. Repeat five times starting from different
positions of the compass.
Analysis
1. At which pole do the magnetic field lines begin?
2. At which pole do the magnetic field lines end? 3. Where is the magnetic field strongest? How is
this shown by the magnetic field lines?
Aim
To map the magnetic field surrounding a long, straight wire carrying an electric current
Apparatus
50 cm length of straight wire
sheet of cardboard approximately 20 cm × 20 cm power supply
variable resistor connecting wire switch
compass
some means of supporting the wire some means of supporting the cardboard
Method
1. Set up the apparatus as shown in figure 8.41. To increase the strength of the magnetic field, a number of loops of wire can be used.
8.2
MAGNETIC FIELD
SURROUNDING
A MAGNET
S
N
(a) Mark first direction of compass.
S
N
(b) Place compass so that the direction follows on from the previous direction.
Figure8.40
8.3
MAGNETIC FIELD
PRODUCED BY
A CURRENT IN
A LONG,
STRAIGHT WIRE
A
CTIC
AL A
CTIVITIES
Figure8.41
2. Connect the power supply so that the con- ventional current flows downwards through the wire.
3. Adjust the voltage of the power supply and the variable resistance so that the current has the value given by your teacher.
4. Place the compass about 5 cm from the wire. 5. Switch on the current and mark the positions
of the ends of the compass on the cardboard. 6. Proceed as in practical activity 8.2, tracing out
the magnetic field line. (Ideally this should return to the starting point to form a closed loop.)
7. Mark the direction in which the north pole of the compass pointed at several places on the magnetic field line.
8. Repeat this a number of times with the initial position of the compass at different distances from the wire.
9. Draw smooth lines of magnetic field through each set of points.
10. Reverse the direction of the current and observe what happens to the compass needle.
Analysis
Show that your result is compatible with the right- hand grip rule.
Questions
1. When the current was coming upwards out of the cardboard, was the direction of the mag- netic field lines around the wire clockwise or anticlockwise?
2. Can you use this to formulate an alternative rule for determining the direction of the mag- netic field surrounding a current-carrying wire?
Aim
To map the magnetic field surrounding a solenoid
Apparatus
solenoid
sheet of cardboard approximately 20 cm × 20 cm scissors connecting wires power supply variable resistor switch compass
Method
1. Connect the apparatus as shown in figure 8.42.
Figure8.42
2. Note the direction of the conventional current around the solenoid.
3. Map the magnetic field around the solenoid using the same method as was used in the previous two practical activities.
4. Reverse the direction of the current through the solenoid. Note what happens to the direction of the magnetic field.
A Compass on cardboard
8.4
MAGNETIC FIELD
OF A SOLENOID
CARRYING A
CURRENT
Iron core A CardboardPR
A
CTIC
AL A
CTIVITIES
Analysis
1. With the first direction of the conventional current, which end of the solenoid was the north pole? Explain.
2. Is this result compatible with the right-hand grip rule for solenoids?
3. Draw a sketch showing how the right-hand grip rule for solenoids applies to your result.
4. What happened to the magnetic field when the direction of the current was reversed?
5. How does the magnetic field produced by a current in a solenoid compare with the magnetic field surrounding a magnet?
Aim
To build an electromagnet and observe its properties
Apparatus
iron rod for core of electromagnet insulated conducting wire for coil power pack
connecting wire variable resistor ammeter small iron nails
Theory
A soft iron core is placed in a solenoid carrying a current and becomes magnetised. When the current is switched off the soft iron core loses its magnetism.
Method
1. Build the electromagnet by winding the con- ducting wire closely from one end of the iron core to the other. To make the electromagnet stronger one or more layers of coils can be wound on top of the first. It is essential that all layers of coils are wound in the same direction around the core.
2. Connect the electromagnet to the power supply as shown in figure 8.43.
Figure8.43
3. Test the magnetism of the electromagnet by observing the attraction of small iron nails to the end of the soft iron rod. The greater the number of iron nails attracted to the rod, the greater is the magnetism.
4. Observe the magnetism of the electromagnet when there is no current.
5. Observe the magnetism of the electromagnet for a range of currents.
6. Observe how much time is taken for the elec- tromagnet to gain and lose its magnetism when the current is switched on and off.
Analysis
1. How did the magnetism change as the current was increased?
2. Was there any delay observed in the gain or loss of magnetism when the current was switched on or off?
3. Was there any magnetism left when the current was turned off?