1Physics I Honors

Current and Circuits

Current

2Physics I Honors

Electric Current

• Voltage (which indicates the presence of an electric field) causes charges to move in conductors provided there is a complete path in which the charges can move.

Resistor

CurrentCurrent

Battery

3Physics I Honors

Electric Current• The rate at which electric charges move

through a wire from higher potential to lower potential.

• This works just like gravity. – For water flowing down a hill, the greater the

elevation change (greater change in gravitational potential) the swifter the water flows.

– For electrical current, the greater the electrical potential difference (or voltage) the greater the current.

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Electric Current

• Electric potential (V), or voltage, is a measurement of the energy available at a location– higher voltage means more energy per electron

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Electric Current• I = Q/t• 1 Ampere = 1 Coulomb/sec

– In wires, moving electrons create the current. (Electrons moving in one direction are mathematically equivalent to protons moving in the other direction.)

– Conventional current says positive charges move.• In some materials (liquids, gases, semi-conductors)

positive or negative changes can move.

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CURRENT AND CIRCUITS

TYPES OF CURRENT

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Conventional Current vs. Electron Flow

Conventional current: Direction a positive charge would flowThis is opposite the direction of electron flow in a conductor!

-+

electron flowcurrent

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Alternating Current vs Direct Current

direct current (DC) – electrons flow in one direction only

i.e. batteries

alternating current (AC) – the direction of flow changes (60 cycle in United States)

i.e. power station

Although net electron flow over one cycle is zero, can still do useful work!

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CURRENT AND CIRCUITS

Resistance

Current

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Resistance ()

Opposition to the flow of electrons

Resistance depends on:– Length (L)– Cross-sectional area (A) {diameter}– Conductivity of the material– Temperature (T)

• Ohm’s Law is not always valid!!• Increased temperature increases resistance

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Less Resistance More Resistance

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Resistance ()

• Resistance = Resistivity * Length / Area – R = L/A

– Resistivity depends upon the material.

Current

L = LengthA = Area

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Resistance ()• R = V/I (Resistance = Voltage/Current)

– or V = I * R

It takes one volt to push one amp through one ohm

1 = 1 V/A

V VV

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Resistance ()

V = 9 V

R = 50 R = 100

V=9 V

V = IR I = V/R

I = 9 V /50 ohms = .18 A

V = IR I = V/R

I = 9 V /100 ohms = .09 A

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Current (I)

• Current is the flow of charge.

• Charges flow when there is a potential difference (voltage) and there is a complete path for the charges to flow.

• To maintain a constant flow of charge (current), a continuous potential difference must be maintained.

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Teen dementia test

• 4 electrons pass a point in a circuit every second. What is the current?

• A resistor experiences a current of 2 amps and a voltage of 4 volts. What is the size of the resistor?

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Current and Circuits

Circuits

Schematics

Use of Meters

Safety

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Circuit• A path through which

electrons flow.

• The path is from a power source’s negative terminal, through the various components and on to the positive terminal

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Circuit• Open circuit –

– a break prevents the flow of electricity

• Closed circuit – – electricity can flow

• Short circuit – – the electricity completes a circuit without

going through the load

• Load – – item using electricity

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Schematics - Symbols

wire or conductor

resistor or other load

bulb voltmeter

battery ammeter

switch generator

V

A

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Schematics – Simple Circuit

circuit diagramcircuit diagram

+

-

A+ -

A

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Schematics - Complex Circuit

Construct theConstruct thecircuit diagramcircuit diagram

+

-

A

+ -

A

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Meters - Ammeter

• Measures current.• It is a Galvanometer wired in

parallel to a resister. • Ammeters are connected in

series to the circuit.• Have low resistance

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Meters - Voltmeter

• Measures electric potential…voltage.

• It is a Galvanometer wired in series with a resistor.

• Voltmeters are connected in parallel to the load

• Have high resistance

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Fuses and Circuit BreakersIf an electric circuit gets “overloaded” (too

much current!) fuses or circuit breakers interrupt the flow of current.

Fuses

Circuit Breaker

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Current and Circuits

Series Circuits

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Series Circuit• Only ONE path for the electricity.

– Christmas lights– Batteries in a series (more current storage)

• Total current is constant and voltage decreases with additional resistors (i.e dimmer lights)

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Series - Computing Current

• The current reading will be the same anywhere in the circuit.

I1 = I2 = I3 …

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Series - Computing Resistance

• The total resistance is the sum of the resistances of all of the loads.

RT = R1 + R2 + R3 …

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Series - Computing Voltage• The total electricity (energy) used by the

loads is the sum of the usages of all of the loads.

VT = V1 + V2 + V3 …• All the energy is used before the current

returns to the energy source

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Sample Problem Series CircuitThree 20 resistors are connected in series across

a 120 V generator.

What is the total resistance of the circuit?

RT = R1 + R2 + R3 = 20 + 20 + 20 = 60

What is the current in the circuit?

I = V/RT = 120V / 60 = 2 A

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Current and Circuits

Parallel Circuits

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Parallel Circuits

• two or more paths for the electricity.

• Total current increases and volts remain constant with additional resistors

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Parallel Bulbs

Voltage dropped is the same at each resistance

Current is different

But: total current out of battery = sum of the currents through each bulb

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Parallel - Computing Current

• The sum of the currents through each branch equals the total current

IT = I1 + I2 + I3 …

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Parallel - Computing Resistance

• Req (total resistance) is smaller than any individual resistor;

1/Req = 1/R1 + 1/R2 + 1/R3…

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Parallel - Computing Voltage

• The voltage across each resistor is the same.

V1 = V2 = V3 …

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Sample Problem – Parallel Circuit

Three 20 resistors are connected in parallel and placed across a 120 V potential difference.

What is the equivalent resistance?

1/Req = 1/20 + 1/20 + 1/20 = 3/20 = 1/6.67

Req = 6.67W

20

20

20

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Sample Problem – Parallel Circuit

What is the current through the circuit?

I = V / Req

= 120 V / 6.67

the total current I = 18A

20

20

20

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Sample Problem – Parallel Circuit

What is the current through each branch of the circuit?

For each branch, In = V / Rn = 120V/20 = 6A

20

20

20

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Current and Circuits

Combination Circuits

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Combination

• circuit with both series and parallel paths for the electricity

• The parallel section of the circuit is like a sub-circuit and actually is part of an over-all series circuit.

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Combination - Series/Parallel

• This circuit is composed of two resistance segments: – R1 alone

– R2 and R3 in parallel

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Combination - Series/Parallel

• Resistors R2 and R3 can be combined to

form a single equivalent resistance: R2,3

• This leave a simple series circuit

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Using the series-parallel schematic• What is the equivalent resistance of the

circuit? • first treat the parallel section

– R1 = 3

– R2 = 4

– R3 = 6

– VT = 12 V

• 1/Req = 1/R2 + 1/R3 = 1/4 + 1/6 = 3/12 + 2/12 = 5/12 = 1/2.4

• Req = 2.4 for the parallel portion

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Using the series-parallel schematic

• Then, add all the resistances as if they were in series

RT = R1 + R2,3

= 3 + 2.4 = 5.4

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Using the series-parallel schematic

• What is the current in the circuit?

I = V/R = 12 V / 5.4 = 2.22 A

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Using the series-parallel schematic

• What is the current through each resistance?

• I = V / R

• I2 = V2 / R2

= 5.33 V / 4 = 1.33 A

• I3 = V3 / R3

= 5.33 V / 6 = 0.89 A

• I2,3 = I2 + I3

= 1.33 A + 0.89 A = 2.22 A

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Using the series-parallel schematic• How much voltage is lost at each

resistor?• I = V / R so• V1 = IR1

= 2.22 A * 3 = 6.67 V

• V2,3 = IR2,3

= 2.22 A * 2.4 = 5.33 V

• VT = V1 + V2,3

= 6.67 V + 5.33 V = 12 V

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Parallel/Series• This circuit is composed of

two resistance segments: – R1 and R2 in series

– R3 in parallel

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Current and Circuits

POWER

TRANSMISSION OF ELECTRICAL ENERGY

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POWER

Power is calculated by multiplying current times voltage

P = IV

Resistors use energy.

Rate = Power = I2RV

R

I

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POWER• Energy per unit of time converted into

another form of energy.

• Thermal energy is produced at a rate given by P = I2R

• Power is lost during transmission due to production of heat ( conversion of electrical energy to thermal energy)

54Physics I Honors

Transmission of Power• In transmission of energy, it is important

to maintain the power and reduce the loss of energy.

• Using P = I2R, we must reduce either the resistance or the current.

• Reducing resistance – using wire that is highly conductive and of

large diameter.

55Physics I Honors

Transmission of Power• Reducing the current.

– Looking at P = IV, – to reduce current, increase the voltage to

maintain the same power delivered.– Long distance lines may carry voltages of

more than 500,000 V (that’s half a million volts)

56Physics I Honors

Paying for Electricity

• You pay for electric energy, not power.

• Electric energy is paid for by the kilowatt hour (kWh).

• 1 kWh = (1000 J/s)(3600 s)

= 3.6 x 106 J

• Light bulbs are often sold in wattage. A 100 Watt bulb uses 100 Watts of electric energy in 1 hour.

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Electric PowerA microwave oven is rated at 120 V and 840 W.

a) What is the current drawn?

P = IV I = P/V = 7 A

b) What is the resistance of microwave oven?

P = I2R R = P/I2 = 17.14 c) How much energy is used in 5 minutes?

E = Pt = 840 W * 5 min * 60 s = 2.52 x 105 J

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Injuries through Electricity

CurrentCurrent Effect Effect Fatal?Fatal?

1 mA mild shock no

5 mA painful no

10 mA paralysis of

motor muscles no

20 mA breathing stopsbreathing stops minutes minutes

100 mA100 mA heart stopsheart stops seconds seconds

1000 mA1000 mA serious burnsserious burns instantly instantly