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Resistance is measured in ohms and has different cumulative effects when added to series and parallel circuits. The potential difference, or voltage (ΔV), across an energy source is the potential energy difference (ΔE) supplied by the energy source per unit charge (q) (ΔV = ΔE/q). The electric potential difference across a resistor is the product of the current and the resistance (ΔV = I R). In physics, only ohmic resistors will be studied. When potential difference vs. current is plotted for an ohmic resistor, the graph will be a straight line and the value of the slope will be the resistance. Since energy is conserved for any closed loop, the energy put into the system by the battery must equal the energy that is transformed by the resistors (loop rule). For circuits with resistors in series, this means that λV battery = ΔV₁ + ΔV₂ + ΔV₃ + .... The rate of energy transfer (power) across each resistor is equal to the product of the current through and the voltage drop across each resistor (P = ΔV I) and P battery = IΔV₁ + IΔV₂ + IΔV₃ + ... = IΔVbattery. Equations should be understood conceptually and used to calculate the current or potential difference at different locations of a parallel, series or mixed circuit. However, the names of the laws (e.g., Ohm’s law, Kirchoff’s loop law) will not be assessed. Measuring and analyzing current, voltage and resistance in parallel, series and mixed circuits must be provided. This can be done with traditional laboratory equipment and through computer simulations.
Once a circuit is switched on, the current and potential difference are experienced almost instantaneously in all parts of the circuit even though the electrons are only moving at speeds of a few centimeters per hour in a current-carrying wire. It is the electric field that travels instantaneously through all parts of the circuit, moving the electrons that are already present in the wire. Since electrical charge is conserved, in a closed system such as a circuit, the current flowing into a branch point junction must equal the total current flowing out of the junction (junction rule).