Electricty

AP Physics Review

Object gain or lose electrons to become charged.

Opposite charge attract likes repel. Any charge will attract a neutral object.

When touching electrons are transferred between conductors.

All charges are measured in Coulombs (C) and must be a multiple of the charge on an electron 1.6x10-19C

Remember protons do not move out of the nucleus, it is only the transfer of electrons that cause a charge.

Charge

The electrostatic force between two charged objects is determined by there charges(q) and the distance between them.

We can use this to find the net force on a charge due to other charges. Use sum of the forces methods as previously discussed. Direction must be interpreted from the charges. Typically a negative force is attractive and positive is replusive.

Coulomb’s Law

A electric field tells us the force that a charge would experience at a location around a charged object.

The density of the electric field lines determine the relative strength of the force at that location.

Field lines always point in the direction that a positive test charge would move at that location.

Voltage or POTENTIAL DIFFERENCE is the amount of energy per unit charge (J/C) at a location in the field.

E-Fields and Voltage

Current (I) is the amount of charges that passes a point every second I = q/t . Units are Amps or C/s

A circuit must have a source of potential difference (voltage or EMF), an energy user, and a complete path for the electrons to travel through.

Ammeters measure current through a branch of a circuit and must be wired in series. They have a low resistance so not to use any energy as the current flows through.

Voltmeter must the drop in potential difference around an energy user. They must be wired in parallel around the user. They have a high resistance to keep all of the current from flowing through the voltmeter instead of the device.

Circuits

Resistance uses the energy supplied by the potential difference. Can be converted to heat or light etc.

This is due to the electrons interfering with the molecules in the resistor.

Resistance can be found using the equation:

Resistivity is a property of the material that the resistor is made of.

As temperature increases resistance increases as well.

Remember electrons are lazy.

Resistance

V= IR This holds true for most circuits and most

resistors. For a specific resistor, as the potential

difference increases, the current passing through also increases.

Ohm’s Law

P = VI units are wattsRemember this can be rearranged many ways

using ohm’s law V=IR Since P=W/t and W is synonymous with

energy we can say Energy = Pt or Energy = VIt Units are J

Power and Energy

Junction Rule – Total current into a junction must equal the total current out of a junction

Loop Rule – Around any closed loop in a circuit, the total potential difference added to the loop by a power source must equal the total potential difference used by the components of the loop.

These are two very important rules for solving series and parallel circuits.

Kirchoff’s Laws

Only one path for current. No junctions means that the current stays constant through out.

Resistors in series are added together to find the total resistance.

Drops in potential difference can be added together to find the total or terminal voltage.

Series Circuits

A parallel circuit has a path for each energy user in the circuit. Since each user is on its own loop, according to kirchoff, each gets the terminal voltage.

Because there are many junctions, the total current is sum of the current passing through each user.

Resistance is tricky. The more resistors added in parallel, the smaller the total resistance becomes.

Parallel Circuits

Complex circuits combine sections in series and some in parallel.

The standard series and parallel rules apply to each of these sections.

The best method is to find the equivalent resistance of each parallel section and treat the whole thing as a series circuit.

Once you have found total resistance, current, and voltage, you can start to expand the circuit to solve for unknowns at each resistor using the proper series or parallel laws depending on the location in the circuit.

Complex Circuits