# Electric Current Circuits What is Current Electric current

- Slides: 47

Electric Current & Circuits

What is Current? Electric current is a flow of electric charge n Actually electrons flow away from – and toward + n Symbol of current is I n Unit is the ampere (A) (coulomb/sec) n

Batteries produce charge continuously from chemical reactions n Consist of two dissimilar metals in an electrolyte (liquid, paste, or gel) n Gives potential Energy (volt) n 1 volt= 1 joule/coulomb n

Current is Flow of Charge in a Conductor n I = DQ/Dt n Example: A steady current of 4. 0 amperes flows in a wire for 3 minutes. How much charge passes through the wire? 720 Coulombs

Current Flows in an Electric Circuit A continuous conducting path is called a circuit n Current flows through the wires from one terminal of the battery to another n Courtesy http: //www. uce. ac. uk/education/research/cript/electricity%20 book/water%20 model%20 electric%20 circuit. htm

Current Must Flow in a Continuous Loop If there is a break anywhere in the loop circuit is OPEN. No current flows. n If no break circuit is CLOSED. Current flows. n

What Really Happens Potential difference between terminals of battery sets up an electric field in the wire and just outside parallel to it n Free electrons leave negative terminal of battery, pass through circuit and re-enter battery at positive terminal n

Ohm’s Law n n Current flow is proportional to voltage Inversely proportional to resistance Resistance is constant of proportionality I is current n V = I R n I = V/R R=V/I

Ohm’s Law

Ohm’s Law V = IR What happens to current if you increase V? n What happens if you increase R? n

UNITS n Voltage Volt (V) n Current Amperes (A) n Resistance Ohm( )

Resistance n Measures how much we use the current. Resistance of a metal wire itself: Silver has lowest resistivity Copper is almost as low Gold and Aluminum low too n

Superconductivity Resistance of certain materials becomes zero at low temperatures n Niobium-titanium wire at 23 K n Yttrium-Barium-Copper-Oxygen at 90 K n Bismuth-strontium-calcium copper oxide n Can make strong electromagnets that do not require power n Japanese Maglev Train goes 329 mph n

AC Versus DC DC is direct current. u Steady u Comes from battery or power supply n AC is alternating current that changes from + to – and vica versa 60 times a second (in the U. S. ) u Sine wave with frequency of 60 Hz n

Where it Comes From

Power = Current x Voltage n Power = energy/time = QV/time = IV n Unit: watt n Power = Joule/sec n Energy = Power x time n

Examples What power does a 6 amp toaster operating at 120 volts use? n What power is used by a 120 volt motor with an operating resistance of 10 ohms? n What current is drawn by a 100 watt headlamp on a car (12 volt)? What is its operating resistance? n

Cost of an Electric Heater n How much would it cost to operate a 1500 watt electric heater continuously for one month if the power company charges $. 13 per kwh? (b) What is the resistance of this heater (assume V = 120 V) n

Will it Blow? n A 1200 watt hair dryer, a 6 amp pump motor, and a 250 watt TV are operating on the same 20 amp circuit. How many 100 watt light bulbs could be turned on without overloading the circuit (and blowing the fuse or tripping the breaker? )

Electric Power = energy transformed/time = QV/t P = IV unit: watt Since V = IR P = IV = I 2 R = V 2/R n In power transmission, why is high voltage advantageous?

Series Resistive Circuit Full current goes through all circuit components

Batteries in Series n When batteries or other sources of potential are connected in series, the total potential difference is the algebraic sum of the separate potentials. n 6 V + 6 V = 12 V n Another example: a 9 volt radio battery consists of 6 1. 5 volt cells in series.

Batteries in Parallel n The voltages do not add but more current is available

Parallel Resistive Circuit n Same voltage across all circuit elements IT = I 1 + I 2 + I 3 + VT = V 1 = V 2 = V 3 1/RT = 1/R 1 + 1/R 2 + 1/R 3 +

Solving Circuits n n n Can have both series and parallel parts Find equivalent resistances starting from point furthest away from battery Use Kirchhoff's Rules u Voltages around a closed loop add to zero (conservation of energy) u Sum of currents entering a junction equals sum of currents leaving (conservation of charge)

Find all Possible Currents and Voltages n n Around loop voltage “drops” add up to battery voltage – use to find certain voltages by subtraction Keep applying Ohm’s Law At junction, currents divide up in inverse proportion to resistance they “see” If you have two loops with batteries or wires that cross you may need to apply Kirchhoff's Laws formally to obtain simultaneous equations

Circuit Lab n n n 1. Make two different series circuits, start simple! - Draw them - Measure V, I and R across any connection you make - Calculate Vtot, I tot and Rtot for entire circuit 2. Make two different parallel circuits, repeat above procedures. 3. Make two combination circuits, repeat above procedures.

EMF and Terminal Voltage Battery is said to be a “seat” of electromotive force or emf n Emf is not a force n Real batteries have internal resistance r n Terminal voltage is less than emf when internal resistance is accounted for n Vab = E - Ir n

Capacitors in Parallel Total charge is sum of charges on individual capacitors n Q = Q 1 +Q 2 + Q 3 = C 1 V +C 2 V + C 3 V n Q = CTV n CTV = C 1 V +C 2 V + C 3 V n CT = C 1 + C 2 + C 3 n

Capacitors in Series Charge same on each capacitor n Q = CTV n V = V 1 + V 2 + V 3 n Q/CT = Q/C 1 + Q/C 2 + Q/C 3 n 1/CT = 1/C 1 + 1/C 2 +1/C 3 n

Ammeters and Voltmeters Ammeters have low resistance and are placed in series n Voltmeters have high resistance and are placed in parallel n Multimeter measures current, voltage and resistance n

Solve for Vt, Rt, It, and Pt

Solve for Vt, Rt, It, and Pt

Solve for Vt, Rt, It, and Pt

Solve for Vt, Rt, It, and Pt

Solve for Vt, Rt, It, and Pt

The Shocking Truth Electric Shock Science

What About You? n Turn to a partner and tell of a time you received an electric shock. If you know the voltage involved, tell that

Measuring Body Resistance n n Hold the leads of a multimeter in both hands. Make sure the meter is on the 2000 k scale (reads up to 2 million ohms) The reading will be in thousands of ohms. ie a reading of 673 means 673, 000 ohms Divide the reading into 120 Volts u n Example 120 V/1, 200, 000 = 0. 0001 A = 0. 1 ma (record in your notes) Repeat with wet hands

Shock Hazards n n 1 ma (milliampere) can be felt A few ma can be painful More than 10 ma causes muscles to contract – you may not be able to let go! And, you may not be able to breathe and could die. Artificial respiration could revive you. Above 50 -70 ma (AC) through the heart for one second may cause heart muscles to contract irregularly (ventricular fibrillation) and not pump effectively. Fibrillation is very hard to stop

Using a Defibrillator Could Get Your Heart to Pump Properly Again

Strange But True Larger currents, around 1 Amp stop the heart completely! n When the current stops, the heart usually starts beating again n But larger currents also cause burns and tissue damage, especially with voltages around 500 -1000 volts n

Meet “Old Sparky” Electric chair used at Sing prison in New York Up to 2450 volts AC was used

Why AC is more deadly n n n Human body acts as if it had capacitance in parallel with its resistance This provides an additional path for current to flow Also AC is more likely to cause fibrillation R C

Common Causes of Shocks faulty wiring, electrical overloads, damaged cords, sagging power lines, exposure to power lines, failure to de-energize, lack of warning or shielding devices

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