Baby-Quiz

1. Which of the following affects the resistance of a wire: diameter, type of material, length, or temperature? Explain.

2. Which circuit offers the greater resistance to to battery, two bulbs in series or two bulbs in parallel? Why?

3. If a string of five bulbs in series is added in parallel to a single-bulb circuit, you find that the brightness of the original bulb does not change. Has the current through the battery increased, decreased, or stayed the same? Has the resistance of the circuit increased, decreased, or stayed the same? Explain.

4. What happens to the power supplied to a battery if the resistance connected to the battery is doubled? Why/

Magnets and Magnetic Fields

Important: do not confuse the magnetic poles and electric charges!

If a bar magnet is broken into two pieces, how many magnetic poles are there?

Magnetic Field Lines

Source of Magnetism

Electric current produces a magnetic field!!!

Direction of Magnetic Field Produced by a Current-Carrying

Wire: Right-Hand Rule

Grasp the wire with your right hand so that your thumb points in the direction of the conventional (positive) current; then your fingers will encircle the wire in the direction of the magnetic field.

Force on an Electric Current in a Magnetic Field.

Important: the direction of the force is always perpendicular to the direction of the current and also perpendicular to the direction of the magnetic field.

Another Right-Hand Rule(for the Force)

Force on Electric Current in a Magnetic Field

Maximum F=IlB

Units of Magnetic Field

1 Tesla (T) = 1 N/A m

1 Gauss (G) = 10-4 T

Force on an Electric Charge Moving in a Magnetic Field

Maximum F=qvB

Right-Hand Rule Again

Important: right-hand rule defines direction of a force acting on a positive charge. For negatively charged particles the force is in exactly the opposite direction.

Don’t forget this when you are answering questions about electrons!!!

Proton’s Path in a Uniform Magnetic Field

A proton travels at velocity v in a plane perpendicular to a magnetic field. Describe its path.

Conceptual Example: A Spiral Path

What is the path of a charged particle if its velocity is not perpendicular to the magnetic field?

Magnetic Field Due to a Straight Wire

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r

IB

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27

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Magnetic Field of a Solenoid

When you look down the axis of a solenoid so that the current circulates in the counterclockwise direction, are you looking in the direction of the magnetic field or opposite of it? Explain.

Force between Two Parallel Wires

Each current produces magnetic field that is “felt” by the other wire so that each must exert a force on the other.

Parallel current in the same directions attract each other, antiparallel currents repel.

Parallel current in the same directions attract each other, antiparallel currents repel.

An electric circuit and telephone lines. Two wires carrying currents in opposite direction are twisted together. How does this reduce the magnetic fields surrounding the wire?