Download Electric Current and Circuits

Electric Current & Circuits
What is Current?
Electric current is a flow of electric charge
 Actually electrons flow away from – and
toward +
 Symbol of current is I
 Unit is the ampere (A) (coulomb/sec)

Batteries
Batteries produce charge continuously from
chemical reactions
 Consist of two dissimilar metals in an
electrolyte (liquid, paste, or gel)
 Gives potential Energy (volt)
 1 volt= 1 joule/coulomb

Current is Flow of Charge in a
Conductor
 I = DQ/Dt

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
 Current flows through the
wires from one terminal
of the battery to another

Courtesy http://www.uce.ac.uk/education/research/cript/electricity%20book/water%20model%20electric%20circuit.htm
Current Must Flow in a
Continuous Loop
If there is a break anywhere in the loop
circuit is OPEN. No current flows.
 If no break circuit is CLOSED. Current
flows.

What Really Happens
Potential difference between terminals of
battery sets up an electric field in the wire
and just outside parallel to it
 Free electrons leave negative terminal of
battery, pass through circuit and re-enter
battery at positive terminal

Ohm’s Law
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Current flow is proportional to voltage
Inversely proportional to resistance
Resistance is constant of proportionality
I is current
V
=IR
 I = V/R
R=V/I
Ohm’s Law
Ohm’s Law V = IR
What happens to current if you increase V?
 What happens if you increase R?

UNITS
 Voltage
Volt (V)
 Current
Amperes (A)
 Resistance Ohm()
Resistance

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

Superconductivity

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Resistance of certain materials
becomes zero at low temperatures
Niobium-titanium wire at 23K
Yttrium-Barium-Copper-Oxygen at 90K
Bismuth-strontium-calcium copper oxide
Can make strong electromagnets that do not
require power
Japanese Maglev Train goes 329 mph
AC Versus DC
DC is direct current.
 Steady
 Comes from battery or power supply
 AC is alternating current that changes from
+ to – and vica versa 60 times a second (in
the U.S.)
 Sine wave with frequency of 60 Hz

Where it Comes From
Power
Power = Current x Voltage
 Power = energy/time = QV/time = IV
 Unit: watt
 Power = Joule/sec
 Energy = Power x time

Examples
What power does a 6 amp toaster operating
at 120 volts use?
 What power is used by a 120 volt motor
with an operating resistance of 10 ohms?
 What current is drawn by a 100 watt
headlamp on a car (12 volt)? What is its
operating resistance?

Cost of an Electric Heater

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 = 120V)
Will it Blow?

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

Power = energy transformed/time = QV/t
P = IV unit: watt
Since V = IR
P = IV = I2R = V2/R
In power transmission, why is high voltage
advantageous?
Series Resistive Circuit
Full current goes through all circuit
components
Batteries in Series

When batteries or other sources of potential are
connected in series, the total potential difference is
the algebraic sum of the separate potentials.

6V + 6V = 12V

Another example: a 9 volt radio battery consists
of 6 1.5 volt cells in series.
Batteries in Parallel

The voltages do not add but more current is
available
Parallel Resistive Circuit

Same voltage across all circuit elements
IT = I1 + I2 + I3 +
VT = V1 = V2 = V3
1/RT = 1/R1 + 1/R2 + 1/R3 +
Solving Circuits
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Can have both series and parallel parts
Find equivalent resistances starting from point
furthest away from battery
Use Kirchhoff's Rules
 Voltages around a closed loop add to zero
(conservation of energy)
 Sum of currents entering a junction equals sum
of currents leaving (conservation of charge)
Find all Possible Currents and
Voltages
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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
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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
 Emf is not a force
 Real batteries have internal resistance r
 Terminal voltage is less than emf when
internal resistance is accounted for
 Vab = E - Ir

Capacitors in Parallel
Total charge is sum of charges on individual
capacitors
 Q = Q1 +Q2 + Q3 = C1V +C2V + C3V
 Q = CT V
 CTV = C1V +C2V + C3V
 CT = C1 + C2 + C3

Capacitors in Series
Charge same on each capacitor
 Q = CT V
 V = V1 + V2 + V3
 Q/CT = Q/C1 + Q/C2 + Q/C3
 1/CT = 1/C1 + 1/C2 +1/C3

Ammeters and Voltmeters
Ammeters have low resistance and are
placed in series
 Voltmeters have high resistance and are
placed in parallel
 Multimeter measures current, voltage and
resistance

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?

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
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Hold the leads of a multimeter in both hands. Make
sure the meter is on the 2000k 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 120Volts

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Example 120V/1,200,000  = 0.0001 A = 0.1ma (record in
your notes)
Repeat with wet hands
Shock Hazards
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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!
 When the current stops, the heart usually
starts beating again
 But larger currents also cause burns and
tissue damage, especially with voltages
around 500-1000 volts

Meet “Old Sparky”
Electric chair used at
Sing Sing prison in
New York
Up to 2450 volts AC
was used
Why AC is more deadly

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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