Electricity Review

Physics 115

Fall Semester, 1998

S. T. Thornton

Batteries always deliver the same voltage, not the same current.

The current through a battery depends on the circuit to which it is connected.

Bulbs have resistance.

Adding more bulbs in series increases the resistance.

Adding more bulbs in parallel decreases the resistance.

The greater the compass deflection, the greater the current.

The brighter the bulb, the greater the current flowing through the bulb.

Electrons are what actually move in a circuit.

Electrons move (and cause current) when they are pushed. What pushes them?

A battery (having oomph or voltage) pushes the electrons through the circuit.

I like to think of the electrons as being repelled out of the negative end of the battery and attracted to the positive end of the battery.

This allows me to determine which way the electrons move.

How do the capacitors stop the electron flow?

The electrons move to one of the two plates and stop. They repelled electrons on the other plate which causes electron flow to continue through the circuit.

As the electrons pile up on the first plate, those electrons will repel the next few electrons that arrive there. Because of this repulsion, the current will decrease. As more electrons arrive, the repulsion increases, and the current continues to decrease until finally, no more electrons can reach the plate. This happens when the capacitor has exactly the voltage of the battery.

If we then disconnect the battery, what do we have in the capacitor?

If we connect the capacitor to a bulb, the electrons are able to move away from the plate (in the opposite direction they came) and pass through the bulb. As soon as some electrons leave, there is less repulsion from the remaining electrons, and the current decreases.

This continues until the current goes finally to zero, and the bulb goes out.

What would happen if we put an uncharged capacitor between a battery and a charged capacitor that had been charged with the battery?

Think about it.

Answer: no current, because the new capacitor feels equal "pushes" from both directions: the battery on one side and the charged capacitor on the other side!

The current will always move more towards the path of least resistance in a complex circuit.

The following are notes on the text:

Coulomb's Law

F = kq₁q₂/r²

The coulomb force is proportional to the product of the two charges.

The coulomb force gets smaller as the distance between them increases (1/r²)

Like charges repel.

Unlike charges attract.

Understand why an uncharged object is attracted to a charged object (induced polarized charge).

Charges produce electric fields. These are similar to gravitational fields. An electric field due to a single small charge is radial - lines moving out from the charge.

A second charge will feel a force when placed in an electric field, just like a mass feels a force when placed in a gravitational field.

Electric current (I) is a measure of the flow of electric charge per unit time.

I = q/t unit is ampere (A)

1 A = 1 coulomb/s or 1 C/s

Current is measured with ammeters.

The resistance (R) of a circuit element (bulb, wire, battery, etc.) limits the current through a circuit.

The voltage (V) determines the amount of work for a given charge to pass through an electric field.

Read the text to learn more about potential difference or voltage.

The unit of voltage is the volt, V.

We have a relation between V, I, and R.

V = IR or I = V/R

If we connect it to a battery pack having 4-1.5 V batteries, we have 6 V. What is the current?

= 0.15 A

Notice that we always use SI units!

Many circuits can be understood by V = IR, called Ohm's Law.

However, many circuit elements do not obey this law. (light bulbs do)

Electric power is the product of VI, and is the rate of doing work.

P = W/t = qV/t = VI

Magnetism

Magnets have North and South poles. Compasses have N and S written on them. If they represent poles, then our geographic north pole is actually a magnetic south pole in order for the N on the compass to be attracted towards it. Complicated, huh??

Like poles repel.

Unlike poles attract.

Only dipoles seem to exist in nature.

Magnets have magnetic fields around them. See the text for examples of magnetic field lines.

Magnets exist because of domains which are tiny areas of magnetic materials.

Magnets are affected by moving electric charges.

Moving charges are also affected by magnetic fields.

We can build small electric motors using the interaction between magnetic fields and electric current. We are able to do work with them.

Permanent magnets are made of ferromagnetic materials.

Ferromagnetic materials have special structure that enhances their magnetic effects.

We can increase the effects of currents producing magnetic fields by using iron.