Electric current and direct-current circuits A flow of electric charge is called an electric current.

Electric current and direct-current circuits

When electric charge flows through a closed path and returns to its starting point the path is called an electric circuit.

Electric current and direct-current circuits When electric charge flows through a closed path in one direction the path is called a direct-current circuit.

Electric current and direct-current circuits When electric charge flows through a closed path and periodically reverses direction the path is called a alternating-current circuit.

Electric current and direct-current circuits A battery produces a difference in electric potential between its terminals through chemical reactions.

Electric current and direct-current circuits The symbol for a battery is

Electric current and direct-current circuits The terminal designated + corresponds to the higher potential, while the terminal designated by a – corresponds to the lower potential.

Electric current and direct-current circuits By convention we say that the direction of the current is the direction in which a positive charge would move.

Figure 21-4 Direction of Current and Electron Flow

Electric current and direct-current circuits The electromotive force (emf) (ξ) is the potential across the terminals (voltage) of a battery under ideal conditions.

Electric current and direct-current circuits The charges that actually move through a conductor, are electrons.

Electric current and direct-current circuits In a real conductor there is always some resistance to electron flow, and a potential difference is necessary to keep them flowing.

Electric current and direct-current circuits Ohm’s Law relates the potential(V), resistance (R)and current (I)in a circuit

Electric current and direct-current circuits Ohm’s Law

Electric current and direct-current circuits Ohm’s Law

Electric current and direct-current circuits Ohm’s Law

Electric current and direct-current circuits Ohm’s Law Unit for resistivity is

Electric current and direct-current circuits When an electric charge moves across a potential difference the potential energy changes by the amount

Electric current and direct-current circuits SI unit; watt, W

Electric current and direct-current circuits Other expressions for electric power

Electric current and direct-current circuits Other expressions for electric power

Electric current and direct-current circuits Resistors in a series are connected end to end.

Example 21-5 Three Resistors in Series

Electric current and direct-current circuits The equivalent resistance for resistors in series is just the sum of the individual resistances

Electric current and direct-current circuits For the example given

Electric current and direct-current circuits Each of the resistors connected in series has the same current going through it.

Electric current and direct-current circuits Resistors connected in parallel are connected across the same potential difference.

Example 21-6 Three Resistors in Parallel

Electric current and direct-current circuits The equivalent resistance for resistors in parallel is calculated by adding the reciprocal values of the individual resistors.

Electric current and direct-current circuits This gives the reciprocal of the equivalent resistance

Electric current and direct-current circuits For the example given

Electric current and direct-current circuits The current going through individual resistors connected in parallel is not necessarily the same.

Electric current and direct-current circuits The sum of the currents will be equal to the current calculated for the individual resistors.

Electric current and direct-current circuits For circuits that contain resistors connected both in series and in parallel, we first calculate the equivalent resistances.

Electric current and direct-current circuits We then treat the result as if it were just another resistor in series. Ex.21-7 on page 693.

Example 21-7 Combination Special

Electric current and direct-current circuits The sum of the voltage drops in a circuit must be equal to the voltage applied to the circuit.

Figure Capacitors in Parallel

Electric current and direct-current circuits The equivalent capacitance for capacitors in parallel is just the sum of the individual capacitances

Electric current and direct-current circuits For the example given

Electric current and direct-current circuits The sum of the individual charges on the capacitors is equal to the charge on the equivalent capacitor.

Figure Capacitors in Series

Electric current and direct-current circuits The equivalent resistance for capacitors in series is calculated by adding the reciprocal values of the individual capacitors.

Electric current and direct-current circuits This gives the reciprocal of the equivalent capacitance.

Electric current and direct-current circuits For the example given

Electric current and direct-current circuits Active example 21-3 p 700.

Electric current and direct-current circuits Kirchoff’s rules 1. The sum of the currents entering a junction, must equal the sum of the currents leaving that junction (result of charge conservation).

Electric current and direct-current circuits Kirchoff’s rules 2. The algebraic sum of the potential differences around a closed loop is zero. The potential increases in going from the negative to the positive terminal of a battery, and decreases when crossing a resistor in the direction of the current. (energy conservation).

Electric current and direct-current circuits Batteries – all non-ideal batteries have an internal resistance. The voltage measured across the terminals of a battery will be less with current flowing than without current flowing.

Electric current and direct-current circuits Ammeters are connected in series with the part of the circuit being tested. The ideal resistance of an ammeter is 0 

Electric current and direct-current circuits Voltmeters are connected in series with the part of the circuit being tested. The ideal resistance of a voltmeter is ∞ 