(1) Physics Purpose: Materials: Theory: We investigate the elementary properties of magnetism using the following four magnets:

Transcription

1 Department of Physics and Geology The study of magnetic fields Physics 2402 Purpose: We investigate the elementary properties of magnetism using the following four magnets: 1. a compass needle 2. a bar magnet 3. a solenoid 4. the Earth. Materials: Frederiksen Power Supply 0-24V AC/DC Extech Instruments AC/DC Clamp Meter Pasco Scientific Milli-Amp Meter Magnet Kit 2 Compasses Soleniod One Meter Stick One Ruler Theory: Magnets produce a magnetic field in the space around them. This field interacts with other magnetic fields to produce forces of attraction and repulsion. The law of magnetism states that like poles repel each other and unlike poles attract each other. If a magnet is allowed to freely rotate, one end will always point towards geographic north. This is because the earth generates a magnetic field which interacts with the magnetic field of the magnet. The end of the magnet that points toward geographic north is called the north-seeking pole and the opposite end is called the south-seeking pole. Usually these ends are simply called the north and south poles. All magnets have a north and a south pole. If you break a bar magnet in half, each half still behaves like a complete (but weaker) magnet. You can continue breaking the pieces in half and never isolate a single pole. This suggests that atoms themselves are magnets. A compass is a small magnet which is balanced and whose north and south poles are marked. It can be used to detect the magnetic fields of other magnets or the magnetic field of the earth. A solenoid also becomes a magnet when a current I flows through it, and the field lines are quite similar to the ones of a bar magnet. Inside the solenoid the magnetic field is nearly constant, and its magnitude is given by (1)

2 Here N is the number of windings of the solenoid, L its length, and is the so-called magnetic permeability of free space. 0 =4 x10-7 (2) Experimental Procedure: Part 1 1) Find bar magnet in Magnet Kit Storage box. 2) Place bar magnet on a blank sheet of paper. 3) Trace the bar magnet on the paper. Make sure you denote which end is North and South. 4) Place the compass on the blank sheet of paper a few cm from the magnet 5) Slide the compass on the paper around the magnet denoting direction of the needle as shown in the figure. 6) Make several arrows denoting direction of the field of both North and South ends of the bar magnet. 7) Use a straight edge to draw lines from the arrows into the bar magnet trace. Where the lines cross the most is the MAGNETIC POLE of the bar magnet 8) Measure the length of the bar magnet with ruler. This is the PHYSICAL LENTH of the bar magnet. 9) Measure the length of the magnetic poles you found in step 7. This is the MAGETIC LENGTH. Table 1 Physical Length= cm Magnetic Length= cm

3 Experimental Procedure: Part 2 1) Use the compass to find the magnet north of Earth. 2) Tape a meter stick to the desk so that it is in line with the Earth s magnetic pole, or pointing North. 3) Place the bar magnet on the meter stick with the SOUTH pole of the bar magnet pointed towards the Earth s magnetic north pole 4) Slide the compass away from the bar magnet until the compass needle points in perpendicular direction. This distance is the null point where both fields cancel each other out. 5) Record this distance between the compass and the North Pole of the bar magnet in Table 2 as d 1 6) Record this distance between the compass and the South Pole of the bar magnet in Table 3 as d 2 7) At the null point B M = B E. If we take B E = 3.5x10-5 Tesla (T) for the magnetic field of the Earth, we can use this equation below to calculate the pole strength P of the bar magnet. 8) Calculate the Pole Strength from d 1 9) Calculate the Pole Strength from d 2 10) Calculate the Difference between d 1 and d 2 11) Calculate the difference between in Pole Strengths B e = 3.5x10-5 Tesla (T) Table 2 Where L is the magnetic length of the bar magnet and d=d 1 and µ 0 = 4πx10-7 d 1 = cm P= Table 3 d 2 = cm P= Table 4 % difference in distance= cm % difference in P=

4 Experimental Procedure: Part 3 Table 5 1) Use a compass to find the Earth s magnetic field 2) Place the bar magnetic perpendicular to the Earth s magnetic field as show in the figure. 3) Slide the compass away from the pole of the bar magnet until the compass needle points to B M = 0.5 B E. 4) Record this distance between the compass and the North Pole of the bar magnet in Table 5 as d 3 5) Record this distance between the compass and the South Pole of the bar magnet in Table 5 as d 4 d 3 = cm d 4 = cm % difference in distance= cm Experimental Procedure: Part 4 1) Set up the Frederiksen Power Supply 0-24V AC/DC and plug the unit into the wall socket. 2) Plug in one blue/black wire into the -DC port and attach the other end using an alligator clip to attach to the solenoid terminal/

5 Table 6 3) Plug in one red wire into the +DC port and attach the other end to the 500port on the Pasco Scientific Milli- Amp Meter 4) Plug in on red wire into the (-) port on the Pasco Scientific Milli-Amp Meter and the other end using an alligator clip to the solenoid terminal. 5) Turn on the Frederiksen Power Supply 0-24V AC/DC and adjust the voltage knob until the amperage gauge reads 300mA. 6) Use the compass to find out which end of the solenoid is the north pole of the solenoids magnetic field 7) Align the solenoid opposite to the Earth s magnetic field. 8) Reduce the current slowly to while paying close attention to the Pasco Scientific Milli-Amp Meter. 9) Find the current at which the solenoid magnetic field and Earth s magnetic field cancel out. 10) Calculate the magnetic field of the solenoid B s using the equations in the THEORY section. The solenoid has 330 windings and a length of 15cm. 11) Compare with the theoretical value for the Earth s magnetic field given above. I= ma Table 7 B s = x 10-5 Tesla % error = Questions: 1) Why would there be a difference between d 1 and d 2. 2) Why would there be a difference in pole strength in the bar magnet? 3) How does a current create a magnetic field?