At this point we should know that current is the time dependent rate at which charge flows. In a circuit, potential difference (emf/voltage) is what drives the charge to flow. So a larger potential difference, would result in a larger current. Resistance is a measure of how difficult it is for charge to flow in a circuit. A circuit with a larger resistance would have a smaller current. IN order to figure out exactly how these quantities are related we could complete two experiments, one where we measure current and change the potential difference, and another where we measure current and change total resistance. If we did this we would find current is directly proportional to voltage and inversely proportional to resistance. This relationship is called Ohm's Law.
Take a look at the simulation below. Notice how the value of current (I) change when you change the value of voltage (V) or resistance (R). Does current double when you double the voltage? Does current halve when you double resistance? If you double resistance and double voltage, does current stay the same? If the answer tothese questions is ,"yes," then we would call this resistor Ohmic. This means it abides by Ohm's Law. A non-Ohmic resistor is one that does not obey this relationship. Notice the graph below. Ohmic resistors have a proportional graph of Current (I) vs Voltage (V) with a slope of Resistance (R). A non-Ohmic resistor would have a non linear graph instead.
Circuits in Series & Parallel
In order for a circuit to have current, it must be a complete loop of conductors with both positive and negative terminals of a battery included along with a resistor to resist the flow of charge. The simplest complete circuit is what we call a circuit in series. A series circuit means the current has only one path which the charge can flow. If we add more resistors in series, we will add the resistance of each resistor to figure out the total resistance of the circuit. The current thorough each resistor must be the same, because the charge could not go anywhere else. While the current is the same, the voltage across each resistor is not.