Current is the flow of electrons (or charge) per unit time:
Where ‘Q’ is the charge, and ‘t’ is the time in seconds.
Ohm’s Law: the voltage of a circuit is the product of the current and the resistance:
Where ‘V’ is the voltage, ‘I’ is the current, and ‘R’ is the resistance.
Resistance, Resistivity: the resistance of a metal/wire/material is the following equation:
Where ‘ρ’ is the resistivity of that substance, ‘L’ is the length of the substance (or wire, which is usually used for this type of problem), and ‘A’ is the cross-sectional area of the said wire.
As the equation shows, the longer the wire, the more resistance; the greater the cross-sectional area of the wire, the less resistance!
RESISTANCE:
Measured in ‘ohms’ (Ω), the resistance can be either in series or parallel. If in series, the path of the current can only go through that resister (there is no other path for the current to go). However, if the resisters are in series, then the current can go through any resister or wire that the current decieds to go through.
RESISTERS IN SERIES:
Rtotal = R1 + R2 + R3 ….
RESISTERS IN PARALLEL:
You must always calculate the resitance with keeping in mind whether the resisters are in series or in parallel!
Kirchoff’s Laws:
Kirchoff has two very important laws to solve for any circuit problem that may be on the MCAT:
1. The sum of all current (i) entering a junction equals the current leaving that junction. Another way to say this rule is that the sum of current in a junction equals zero.
2. The total voltage across any circuit equals zero (
Using the above two laws, it is possible to solve for practically any circuit problem on the MCAT.
Anmeter: this measure current.
Votage Meter (Galvanic Meter): as this sounds, this measures voltage.
CAPACITORS:
Capacitors store charge, and release it when the orginal voltage across the circuit is stopped (such as by a battery). When the original emf is stopped, the capacitor acts as the new emf, and releases its charge across the circuit.
***The equations for capacitors are exactly the opposite than for resistors:***
CAPACITORS IN SERIES:
CAPACITORS IN PARALLEL:
Ctotal = C1 + C2 + C3
The strength of capacitors (basically, the ability for the capacitor to store charge) is dependent on a variety of factors:
1. the cross sectional area of the metal plate
2. the distance between the plates
3. the material between the plates
The ‘capacitance’ (the ability to store charge) of a capacitor is calculated as such:
Which tells you that the capacitance is the charge divided by the voltage of that charge. Meaning, the amount of charge flowing in directly proportional to the capacitance, while the voltage drop across the plates is inversly proportional to the capacitance. This is because if the voltage across increases, the less the capacitor would be able to store before it has to release the built-up charge.
This tells you that the voltage is the product of the electric field strength and the distance between the plates. As you increase either one, you increase the voltage (which, according to the equation before) thereby decreasing the capacitance.
The equation above tells you that the capacitance of a material is porportional to both the ‘dielectric’ (the ability of the inside material to insulate the plates, thereby increasing the ability for the capacitor to store the charge before releasing it) and the area of the plates. However, it is inversly porportional to the distance (as we saw in the equation before as well).
The potential energy of a capacitor can be calculated in the following equation:
Where ‘Q’ is the charge buildup and ‘V’ is the voltage potential. With the above equation, it is possible to move things around by substituting ‘Q’ with ‘CV’ and so forth.
Source: http://phasing.org/2010/03/14/electrical-circuits/
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