Chapter 19 DC Circuits. 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)
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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
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
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
19.2 Resistors in Series • The current through each resistor is the same • The voltagedepends on the resistance. • The total or net resistance is the sum of the separate resistances
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
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
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
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
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
19.5 Circuits Containing Capacitors in Series • Capacitors in series have the same charge • Placing capacitors in series results in a smaller capacitance
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.
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
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
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 cords • Stay away from power poles, wires and green power boxes • Keep cords and wires away from heat and water • Don’t overload sockets • Unplug appliances when not using them
19.8 Ammeters and Voltmeters • Ammeter measures current • Voltmeter measures voltage • Either type may be analog or digital
Connecting meters in circuits • Ammeters must be connected in series • Should have as low a resistance as possible, for the least disturbance
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
References • Zitewitz. Physics: Principles and Problems. 2004 • Giancoli, Douglas. Physics: Principles with Applications 6th Edition. 2009.