A simple circuit consists of a resistor R, a capacitor C charged to a potential Vo, and a switch that is initially open but then thrown closed. Immediately after the switch isthrown closed, the current in the circuit is

1. Vo/R.2. zero.3. need more information

The circuit shown at the left below has been rendered operable using the setup photographed at the right. Closing the switch by pushing it to the left connects a capacitor and the two light bulbs across the battery as shown in the circuit. This particular circuit uses a 3-volt battery, two 1.5-volt light bulbs, and a 1-Farad capacitor.

When the switch is closed, connecting the circuit, what would happen? (1) both lamps will light. (2) only the upper lamp will light. (3) only the lower lamp will light. (4) neither lamp will light.

If one of the lamps were to light, it would: (1) go on and stay on at the same brightness. (2) go on brightly and decrease its intensity to a constant zero level. (3) go on dimly and increase its intensity to a constant bright level.

When the switch is closed by pushing it to the left the capacitor charges. The current produced starts large and decreases exponentially, so the light bulbs go on brightly and their intensity decreases exponentially to zero.

What happens after the capacitor is fully charged, and the switch is then closed to the right, completing the right side of the circuit as seen in the drawing at the right above. When the switch is closed, connecting the circuit, what will happen? (1) both lamps will light. (2) only the upper lamp will light. (3) only the lower lamp will light. (4) neither lamp will light.

If one of the lamps were to light, it will: (1) go on and stay on at the same brightness. (2) go on brightly and decrease its intensity to a constant zero level. (3) go on dimly and increase its intensity to a constant bright level.

Example: Answer the following questions for a charging capacitor based on the circuit given.a) What is the time constant for this circuit?b) How long does it take for the capacitor to become half-filled?c) How much current is in the circuit at the instant when the capacitor is half-filled?d) What happens to the answers for each part if the resistance is changed to 2 kW?

CR

C = 5 mFR = 20 We = 10 V

a)RC 0.1ms

b)

1t

RCq t Q e

12

tQQ e

11

2

t

e

1

2

t

e

1ln2

t

69 s

c)

0

t

I t I e

069t

I s I e

t

eR

0.25A

d) RC 0.01s

1t

RCq t Q e

11

2

t

e

1

ln2

t

6.9ms

0

t

I t I e

06.9t

I ms I e

t

eR

2.5mA

Electricity and Magnetism

Magnetism

CH 29: MAGNETIC FIELDS

We have previously discussed electric fields from stationary charges, and we have begun to discuss moving charges as current moving through a wire.

We are now going to examine the electric field present from a moving charge.

When a positive charge is stationary it has an electric field directed away from the charge in all directions. When this charge begins moving the electric field gets distorted. The result of this distortion in the electric field is a Magnetic Field.

Magnetic Field – a field effect that is observed for moving charges.

We typically discuss magnetic fields as the effects from magnets. There are two different categories of magnets:

1. Permanent magnets2. Electromagnets

Permanent Magnets – Materials that have a permanent magnetic field.Where does this magnetic field come from, if magnetic fields require moving charges?

The motion of electrons around a nucleus.

Each atom has a certain magnetic orientation, and groups of atoms with the same magnetic orientation are called magnetic domains.

When a majority of the magnetic domains in a material are aligned, the material becomes a permanent magnet.

Most materials cannot maintain a permanent alignment of magnetic domains.