Chapter 22 & 23Chapter 22 & 23

Electricity & CircuitsElectricity & Circuits

Ohm’s LawOhm’s Law

Ohm’s Law - Ohm’s Law - The resistance does not The resistance does not depend on the voltage or the current flowing depend on the voltage or the current flowing through the circuit.through the circuit.

∆∆V = I RV = I R

∆∆V V = Potential difference or Voltage drop = V, = Potential difference or Voltage drop = V, voltsvolts

I I = Current = A, amps = Current = A, amps RR = Resistance = Ω, ohms= Resistance = Ω, ohms

Charge in circuitsCharge in circuits

q = I tq = I t

Coulomb = C = A s or A = C/sCoulomb = C = A s or A = C/s

Elementary charge 1.6 X 10Elementary charge 1.6 X 10-19-19 C C

Power Power Power Power is the rate of doing work. Power is the is the rate of doing work. Power is the

rate at which energy is transferred. It is rate at which energy is transferred. It is measured in Watts.measured in Watts.

P= P= ∆E∆E P P = Power = W = J/s= Power = W = J/s tt

P = I ∆VP = I ∆V

P = IP = I2 2 RR P = P = (∆V)(∆V)22

RR

MeasurementsMeasurements

Ohmeter measures resistance in ohms (Ω)Ohmeter measures resistance in ohms (Ω)– Hook up in parallel outside circuitHook up in parallel outside circuit

Voltmeter measures voltage in volts (V)Voltmeter measures voltage in volts (V)– Hook up in parallel with circuit runningHook up in parallel with circuit running

Ammeter measures current in amps (A)Ammeter measures current in amps (A)– Hook up in series. Remove a wire in the circuit Hook up in series. Remove a wire in the circuit

and place ammeter in its place. and place ammeter in its place.

Series and Parallel CircuitsSeries and Parallel Circuits

Series Circuit – one current pathSeries Circuit – one current path

The Equivalent Resistance is greater than the The Equivalent Resistance is greater than the largest resistorlargest resistor

RRTT = R = R1 1 + R+ R22 + ... + ... The current is the same in each element since The current is the same in each element since

there is only one current paththere is only one current path IITT = I = I11 = I = I22 = ... = ... The total voltage is the sum of the individual The total voltage is the sum of the individual

voltages and should equal the voltage of the voltages and should equal the voltage of the batterybattery

∆∆VVTT = ∆V = ∆V11+ ∆V+ ∆V22+... +...

Parallel Circuit – Parallel Circuit – more than one pathmore than one path

The Equivalent Resistance is less than the The Equivalent Resistance is less than the smallest resistorsmallest resistor

1/R1/RTT = 1/R = 1/R11 + 1/R + 1/R22 + … + … The total current is the sum of the The total current is the sum of the

individual currentsindividual currents IITT = I = I11 + I + I22 + ... + ... The voltage drop is the same across each The voltage drop is the same across each

resistor because they each have a path resistor because they each have a path from the batteryfrom the battery

∆∆VVTT = ∆V = ∆V11 = ∆V = ∆V22 = ... = ...

Adding a Resistor to a CircuitAdding a Resistor to a Circuit

Add a resistor in seriesAdd a resistor in series 1) total resistance increases1) total resistance increases 2) therefore, current must go down2) therefore, current must go down

Add a resistor in parallelAdd a resistor in parallel 1) total resistance decreases1) total resistance decreases 2) therefore, current must go up2) therefore, current must go up

Solving Series and Parallel CircuitsSolving Series and Parallel Circuits Calculate the total resistance. Calculate Calculate the total resistance. Calculate

each parallel branch resistance then add the each parallel branch resistance then add the remaining series branches. Make simplified remaining series branches. Make simplified equivalent circuit drawings in order to solve equivalent circuit drawings in order to solve the circuits.the circuits.

Calculate the total currentCalculate the total current Use the total current as the current to the Use the total current as the current to the

series part of the circuit to calculate its series part of the circuit to calculate its voltage drop.voltage drop.

Calculate the parallel voltage drop by Calculate the parallel voltage drop by subtracting the series drop from the total subtracting the series drop from the total voltage drop.voltage drop.

Use a chart to track what is Use a chart to track what is known/unknown. known/unknown.