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Magnetic Field

Magnetic Field. PH 203 Professor Lee Carkner Lecture 15. Electricity and Magnetism. Magnets exert a force on two types of objects: Both of these forces are due to the same fact: Magnetic fields produce a force on moving charges Moving charges produce a magnetic field

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Magnetic Field

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  1. Magnetic Field PH 203 Professor Lee Carkner Lecture 15

  2. Electricity and Magnetism • Magnets exert a force on two types of objects: • Both of these forces are due to the same fact: • Magnetic fields produce a force on moving charges • Moving charges produce a magnetic field • Both electricity and magnetism are related to charge

  3. Vectors • A magnet produces a magnetic field (B) • The moving particle has a velocity (v) • All three quantities are vectors • What is the relationship between them? • i.e., if the B field points one way and the charge is moving another way, what is the direction of the force?

  4. If v is your straight fingers, and you curl your fingers in the direction of B, F is your thumb Right Hand Rule v F B

  5. Vector Conventions • The force on a negative particle is opposite that of a positive one • Vectors going into the page are represented with a cross (X), vectors going out of a page are represented with a dot ()

  6. Magnetic Force Magnitude • The magnitude of the magnetic force depends on 4 things: • The magnitude of the charge (q) • The angle between the v and B vectors (f) • The force can be written as: F = qvB sin f

  7. Charged Particle in Field B v f q

  8. Magnetic Field • We can use the expression for the force to write an expression for the magnetic field: B = F/qv sin f • We will often use a smaller unit, the gauss (G) • Typical bar magnet ~ • Earth’s magnetic field ~

  9. Crossed Fields • Electric force: in direction of field • If the E and B field are at right angles to each other, the forces will be in opposite directions

  10. Velocity Selector • How could we get the forces to cancel out? • If we “tune” B until the particle is not deflected, we can find the velocity

  11. Next Time • Read 28.6-28-10 • Problems: Ch 28, P: 9, 15, 16, 32, 46

  12. A resistor R and capacitor C are connected to a battery. If the resistor is replaced with a resistor of 2R, what happens to the time needed to charge the capacitor? • It increases • It decreases • It depends on C • It stays the same • None of the above

  13. Over which time range does the charge on a capacitor increase the least (t=0 is uncharged) 0 to 1t 1t to 2t 2t to 3t 3t to 4t 4t to 5t

  14. Consider a simple circuit consisting of a battery and resistor. What will happen to the current if a voltmeter is used to measure the voltage through the resistor? What will happen to the current if a ammeter is used to measure the current through the resistor? • increase, increase • increase, decrease • decrease, decrease • decrease, increase • You can’t tell without knowing the voltage of the battery

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