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Chapter 5- Ohm’s Law Landstown High School Governors STEM & Technology Academy • Electric circuits – The path that the current follows is called an electric circuit. – All electric circuits consist of: • A voltage source. • A load. • A conductor. 2 • Three types of circuits – Series circuit 3 – Parallel circuit 4 – Series-parallel circuit 5 Open and Closed Circuits • Closed circuit • Open circuit 6 • Current flow can be varied by: – Changing the voltage applied to the circuit. • Voltage increases, current increases. • Voltage decreases, current decreases. – Changing the resistance in the circuit. • Resistance increases, current decreases. 7 • OHM’S LAW The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit. Current Voltage Resistance 8 Finding the Current (I) Value E I R I = current in amperes. E = voltage in volts. R = resistance in ohms. 9 Finding the Voltage (E) Value E= I x R I = current in amperes. E = voltage in volts. R = resistance in ohms. 10 Finding the Resistance (R) Value R = E/I I = current in amperes. E = voltage in volts. R = resistance in ohms. 11 Rules • In a series circuit, the same current flows throughout the circuit. IT = IR1 = IR2 = IR3 . . . = IRn • The total voltage in a series circuit is equal to the voltage drop across the individual loads in the circuit. ET = ER1 + ER2 + ER3 . . . + ERn 12 3 Formulas • E = I x R (Voltage = Current multiplied by Resistance) • R = E / I (Resistance = Voltage divided by Current) • I = E / R (Current = Voltage Divided by Resistance) 13 • The total resistance in a series circuit is equal to the sum to the individual resistances in the circuit. RT = R1 + R2 + R3 . . . +Rn 14 • In a parallel circuit, the same voltage is applied to each branch in the circuit. ET = ER1 = ER2 = ER3 . . . = ERn • The total current in a parallel circuit is equal to the sum of the individual branch currents in the circuit. IT = IR1 + IR2 + IR3 . . . + IRn 15 • To determine unknown quantities in a circuit: – Draw a schematic of the circuit. – Label all known quantities. – Solve for equivalent circuits. – Redraw the circuit. – Solve. 16 • Kirchhoff’s Law – In 1847 G. R. Kirchhoff extended Ohm’s law with two important statements. – Kirchhoff’s current law: • The algebraic sum of all the currents (I) entering and leaving a junction is equal to zero. • IT = I1 + I2 + I3 – Kirchhoff’s voltage law • The algebraic sum of all the voltages (E) around a closed circuit equals zero. • ET = E1 + E2 + E3 17 • In summary: – Electric circuit • Voltage source • Load • Conductor – Current flow • Negative to positive • Varied by changing the voltage or the resistance. – Current path • Series • Parallel • Series-parallel 18 19