magnetic field
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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 Both electricity and magnetism are related to charge
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?
Right Hand Rule
If v is your straight fingers, and you curl your fingers in the direction of B, F is your thumb
F
v
B
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 ()
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 ()
The force can be written as:F = qvB sin
Charged Particle in Field
q
vB
Magnetic Field
We can use the expression for the force to write an expression for the magnetic field:
B = F/qv sin We will often use a smaller unit, the gauss (G)
Typical bar magnet ~ Earth’s magnetic field ~
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
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
Next Time
Read 28.6-28-10 Problems: Ch 28, P: 9, 15, 16, 32, 46
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?
A) It increases
B) It decreases
C) It depends on C
D) It stays the same
E) None of the above
Over which time range does the charge on a capacitor increase the least (t=0 is uncharged)
A) 0 to 1B) 1 to 2C) 2 to 3D) 3 to 4E) 4 to 5
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?
A) increase, increaseB) increase, decreaseC) decrease, decreaseD) decrease, increaseE) You can’t tell without knowing the voltage of the
battery
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