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Charge_to_Mass_0212_handout

# Charge_to_Mass_0212_handout - The Charge to Mass Ratio(e/m...

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PHYS 0212 e/m Ratio of the Electron 1 The Charge to Mass Ratio ( e/m ) Ratio of the Electron This experiment has two parts: 2. Measure the e/m ratio of the electron. 1. Map the magnetic field due to Bar magnet Solenoid Set of Helmholtz coils First a little about magnetic fields: N S A magnet always has a north (N) and south (S) pole. No one has ever observed a “monopole”, either a north or south pole alone. PHYS 0212 e/m Ratio of the Electron 2 S N A magnet produces a magnetic field which can be represented by magnetic field lines. 1. Magnetic field lines always form loops. 2. They point away from the north pole and towards the south pole. 3. The number of lines is arbitrary but is proportional to the field strength in a given region of space. Region of strong magnetic field. Region of weak magnetic field. PHYS 0212 e/m Ratio of the Electron 3 If a magnet is allowed to move freely in the magnetic field of a second magnet, it will align with the magnetic field lines. S N S N A compass is a device that shows the direction of a magnetic field. The needle is basically just a bar magnet that is free to rotate. When it is placed in an external magnetic field the N pole will point towards the S pole of the field. Basically, a compass will tell you which way the magnetic field lines point. PHYS 0212 e/m Ratio of the Electron 4 The Earth has a magnetic field like that of a bar magnet. Given that the north pole of a compass needle points towards the north geographic pole , which of the following is the correct orientation of the Earth’s magnetic field? S N (a) (b) S N

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PHYS 0212 e/m Ratio of the Electron 5 Out of Page Into Page An electrical current in a wire produces a magnetic field. I Wire B G B G The direction of the field is given by the Right-Hand rule for electrical currents . 1) Put the thumb of your right hand in the direction of the current. 2) Curl your fingers around the wire. The magnetic field lines will wrap around the wire in the direction of your fingers. What happens if we reverse the direction of the current? I Wire Out of Page Into Page B G B G PHYS 0212 e/m Ratio of the Electron 6 What happens if we bend the straight wire into a loop? I The field lines inside the loop are all going the same direction, pointing out of the page in this example. PHYS 0212 e/m Ratio of the Electron 7 The magnetic field of the loop looks just like the magnetic field of a bar magnet. S N I This is called an electromagnet. What happens if we put many loops together (with the current going the same direction in each loop)?
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