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