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# Circuits Rapid Review

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By McGraw-Hill Professional
Updated on Feb 12, 2011

For a more thorough review, refer to these concepts:

### Rapid Review

• Current is the flow of positive charge. It is measured in amperes.
• Resistance is a property that impedes the flow of charge. Resistance in a circuit comes from the internal resistance of the wires and from special elements inserted into circuits known as "resistors."
• Resistance is related to current and voltage by Ohm's law: V = IR.
• When resistors are connected in series, the total resistance equals the sum of the individual resistances. And the current through one resistor equals the current through any other resistor in series with it.
• When resistors are connected in parallel, the inverse of the total resistance equals the sum of the inverses of the individual resistances. The voltage across one resistor equals the voltage across any other resistor connected parallel to it.
• The V-I-R chart is a convenient way to organize any circuit problem.
• Kirchoff 's junction rule says that any current coming into a junction will leave the junction. This is a statement of conservation of charge. Kirchoff 's loop rule says that the sum of the voltages across a closed loop equals zero. This rule is helpful especially when solving problems with circuits that contain more than one battery.
• Ammeters measure current, and are connected in series; voltmeters measure voltage, and are connected in parallel.
• When capacitors are connected in series, the inverse of the total capacitance equals the sum of the inverses of the individual capacitances. When they are connected in parallel, the total capacitance just equals the sum of the individual capacitances.
• A capacitor's purpose in a circuit is to store charge. After it has been connected to a circuit for a long time, the capacitor becomes fully charged and prevents the flow of current.
• A capacitor gains or loses charge exponentially. The "time constant" of an RC circuit is equal to the resistance times the capacitance, and gives a characteristic time for the charging or discharging to occur.

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