Current Electricity

Electric Current and Charge
• We consider electric current as the rate of flow of charge. In other words, an electric current flowing determines the amount of charge passing a given point in one second.
• When one coulomb of charge passes a given point in one second the corresponding current is one ampere (A)
• If the electric current is I (in amperes), then the amount of charge Q (in coulombs) which flows past a point in time t (in seconds), is given by
• Q = I t
• I = Q/t
Example-1• When the starter motor of a car is switched on for 0.5 s, 16 C of charge passes through the wires in the motor. Assuming the charge is uniformly transmitted over the time, how large is the electric current?
Solution• Data: Q = 16 C, t = 0.5 s
• Formula: I = Q/t = 16/0.5 = 32 A
• Answer: Hence the current is 32 A
Measuring Electric Current
• To measure the size of an electric current, an ammeter can be used. The ammeter must be connected in series to the circuit.
• For proper connection, note that the electric current (not the flow of electrons which is in the opposite direction to that of conventional current) must flow into the ammeter by the positive (+ or red) terminal and leave by the negative ( - or black) terminal.
• If it is connected the other way round, the pointer will deflect slightly below the zero mark. Check your connections if this happen during an experiment.
• If the circuit consists of only one loop, it does not matter where the ammeters are placed in the circuit. Since the same current flows through the simple circuit, they will all measure the same current.
• Some ammeters allow us to measure different ranges of current. Figure shows such an ammeter. Study the diagram carefully. Can you see how the ammeter is connected and read?
Summary• An electric current is the rate of flow of charge.
• Current = charge/time
• I = Q/t
• One coulomb is that amount of charge which flows past a given point in one second when the current is one ampere.
• An ammeter is used to measure current and must be connected in series in the circuit.