Chapter 19 DC Circuits

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19.1 EMF and Terminal Voltage

Electric circuits need a battery or generator to transform one type of energy (chemical, mechanical, light, etc.) into electric energy

These devices are called sources of electromotive force, emf (ε epislon)

A Battery is a nearly constant voltage source, but does have a small internal resistance, which reduces the actual voltage from the ideal emf

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19.1 EMF and Terminal Voltage

Internal resistance can not be removed

Points a and b are the terminals and the terminal voltage is Vab= Va – Vb

When no current is drawn from the battery terminal voltage = emf and Vab=ε

As a result, when current flows the actual voltage will be lower

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19.2 Resistors in Series and in Parallel

A series connection has a single path from the battery, through each circuit element in turn, then back to the battery.

If you break the connection at any point in the circuit all will stop working

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19.2 Resistors in Series

The current through each resistor is the same

The voltage depends on the resistance.

The total or net resistance is the sum of the separate resistances

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19.2 Resistors in Parallel A parallel connection splits the current into different paths

If you disconnect one device the others will not be interrupted since there is more than one path for the current to follow

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19.2 Resistors in Parallel

The voltage across each resistor is the same

The total current is the sum of the currents across each resistor

The resistance for a parallel circuit is

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19.3 Kirchhoff’s Rules Some circuits are too complicated to be broken down into series and parallel connections For these type of circuits we use Kirchhoff’s rules Devised in mid- 1800’s by G.R Kirchhoff

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19.3 Kirchhoff’s Rules Kirchhoff’s First Rule or Junction rule is based on conservation of electric charge

At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction

Or, whatever charge goes in must come out

Kirchhoff’s second or Loop rule is based on conservation of energy

The sum of the changes in potential around a closed path of a circuit is zero

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19.5 Circuits Containing Capacitors in Parallel

Capacitors can be placed in series or parallel

Capacitors in parallel have the same voltage across each one

Placing capacitors in parallel increases the capacitance

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19.5 Circuits Containing Capacitors in Series

Capacitors in series have the same charge

Placing capacitors in series results in a smaller capacitance

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19.6 RC Circuits – Resistor and Capacitor in Series

RC circuits have capacitors and resistors in the same circuit

Windshield wipers, camera flashes, pacemakers, etc.

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19.7 Electric Hazards

How much current is “bad”? Most people can feel a current of about 1mA

Currents >10mA cause severe muscle contraction; may not be able to release faulty appliance or wire. Can cause respiratory paralysis and death

Current 80mA to 100mA will cause ventricular fibrillation (V-fib). If lasts long will cause death

Larger current can also cause severe burns

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Why do we get shocked?

A person receiving a shock has become part of a complete circuit

Normal circuit breakers protect equipment and buildings from overload, but much lower currents are still dangerous to us

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Some ways to protect yourself

The safest plugs are those with three prongs; they have a separate ground line.

Don’t use frayed or damaged cordsStay away from power poles, wires and green power boxes Keep cords and wires away from heat and waterDon’t overload socketsUnplug appliances when not using them

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19.8 Ammeters and Voltmeters Ammeter measures current

Voltmeter measures voltage

Either type may be analog or digital

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Connecting meters in circuits

•Ammeters must be connected in series•Should have as low a resistance as

possible, for the least disturbance

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•Voltmeters are connected in parallel•Its leads are connected to points around

the point to be measured•The larger the resistance of the

voltmeter the less it affects the circuit

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References Zitewitz. Physics: Principles and Problems.

2004 Giancoli, Douglas. Physics: Principles with

Applications 6th Edition. 2009.

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