Electric current
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Electric Current. Section 1-3. Intro: Take out a sheet of paper for today’s notes Draw the following diagram and label the parts. Open Switch. Conductor. Closed Switch. +. Battery. Load. -. A light is an example of a load. Fuse. Basic Circuit. Resistor. Open Switch.

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Electric Current

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Electric Current

Section 1-3


Intro:Take out a sheet of paper for today’s notesDraw the following diagram and label the parts

Open Switch

Conductor

Closed Switch

+

Battery

Load

-

A light is an example of a load

Fuse

Basic Circuit

Resistor


Open Switch

Intro:Take out a sheet of paper for today’s notesDraw the following diagram and label the parts

Closed Switch

Conductor

+

Battery

Load

-

Fuse

Resistor

Write in your notes: A circuit must be closed to have electrons flow (current) and the load on


Lights on Lights off

An open circuit is off


Lights on Lights off

A closed circuit is on


What electrons really do

  • Electrons move slowly bouncing around in many directions as they move in one direction overall.

Draw the picture above and label it electron drift

Overall Direction

Electron Drift


  • Heat is always a side effect of electric current (because of electron drift).

Overall Direction

Electron Drift


  • A potential difference (V) between two points creates an electric field

  • An electric field moves through a circuit setting electrons in motion


Direct Current (DC)

  • Electrons move in one direction

    • The positive terminal provides the electrons


Alternating Current (AC)

  • Electrons constantly change directions


Alternating Current (ac)

  • In USA, direction changes 60 times per second


  • Conventional way to draw current

    • Current moves from the positive to negative terminal

Load

Battery

Closed Switch


Equation Sheet

  • Take out your equation sheet and find todays equations

OHM’S LAW


Write these equations in your notes

Ohm’s Law

V=IR

P=IV


  • What do the following variables stand for and what are their units?

    Draw this table in your notes


  • What do the following variables stand for and what are their units?

Volts or (V)

Voltage

Amperes or (A)

Current

Ohms or (Ω)

Resistance

Watts or (W)

Power


  • For any electronic device (load) to work there must be a current

  • Electrical Current (I): Continuous flow of electrons

Ohm’s Law

V=IR


  • For a current you need a conductive path

  • And a voltage (potential difference)

    • More electrons in one area than another

-

+

e

Wire

e

e

e

e

e

e

e

e

e

e

e

electron

e


e

e

e

e

e

e

e

  • What happens when there is no more potential difference (voltage)?

  • To create this difference, work must be done adding electrons by a charge pump

e

e

e

e

e

e

e

e

e

e

e

e

electron

e


e

e

e

e

e

e

e

Charge Pumps (sources of pumping electrons):

  • Photocell - light energy into electrical

    Ex: solar cell

  • Battery - chemical energy into electrical

  • Generator – mechanical energy (motion) into electrical

    Example: turbine


  • Electric energy (from charge pumps) can be converted into:

  • Heat

  • Light

  • Sound


Another factor effecting current

  • Resistance (R) – an opposition to the passage of electrons

  • If the wire was thinner would the resistance go up or down?

Ohm’s Law

V=IR

e

Wire

e

e

e

e

e

e

e

e

e

e

e

electron

e


Another factor effecting current

  • Resistance (R) – an opposition to the passage of electrons

  • If the wire was shorter would the resistance go up or down?

Ohm’s Law

V=IR

e

e

e

e

e

e

e

e

e

e

e

Wire

e

electron

e


Factors affecting resistance of a wire

  • Thickness: the thicker the wire the lower the resistance

  • Length: The shorter the wire the lower the resistance

  • Type of metal: gold, silver, and copper have the lowest resistance

  • Temperature: The lower the temperature, the lower the resistance


Questions

V=IR

  • Rearrange this equation for current (I)

  • How is voltage (V) and current (I) related?

  • How is Resistance (R) and current (I) related?

Directly (if V increases, I increases)

Inversely (if R increases , I decreases)


P=IV

Power (P) is the rate of energy use (change in energy)

Power is also related to current and voltage by this equation.


Problem Set 1

  • What is the resistance of an electric frying pan that draws 12 amps of current when connected to a 110v circuit?

  • How much current is drawn by a 23Ω lamp when a voltage of 12v is applied?

  • What is the voltage of a battery if it produces a current of 0.75 amps in a 12Ω resistor?


Problem Set 1

  • What is the resistance of an electric frying pan that draws 12 amps of current when connected to a 110v circuit?


Problem Set 1

2. How much current is drawn by a 23Ω lamp when a voltage of 12v is applied?


Problem Set 1

3. What is the voltage of a battery if it produces a current of 0.75 amps in a 12Ω resistor?


Effects of current on the body

  • 0.001a (1 ma) – barely felt

  • 0.005a (5 ma) – painful

  • 0.010a (10 ma) – muscles contract

  • 0.015a (15 ma) – loss of muscle control

  • 0.100a (100 ma) – can be fatal if the current goes through the heart


Safety Notes

  • Electricians often put their hand in their pocket to avoid current going through heart (just in case)

  • Electricians touch a wire they think is not live with the back of their hand so they are not stuck clinching the wire if they were mistakenly wrong


Modified Equations

V=IRP=IV

Add to your equation sheet:

P = I2R

P=IV

IR substituted for V

V=IR


Modified Equations

V=IRP=IV

Add to your equation sheet:

P = I2R

P=IV

substituted for I


Problem Set 2

  • An appliance draws 13a when connected to a 110 v circuit.

    a) what is the power of the appliance

    b) What is its resistance

    2. What is the power of a light bulb that has a resistance of 190 ohms in a 120 v circuit?


Problem Set 2

  • An appliance draws 13a when connected to a 110 v circuit.

    a) what is the power of the appliance

    b) What is its resistance


Problem Set 2

2. What is the power of a light bulb that has a resistance of 190 ohms in a 120 v circuit?


“Power” companies sell us energy. The unit they use for energy is the kilowatt hour.

Physics challenge:

What is the smallest denomination coin you could use to pay for the energy used by a 60w bulb burning for 8 hours? (at a cost 16.0412 cents per kilowatt hour)

Add to your equation sheet


“Power” companies sell us energy. The unit they use for energy is the kilowatt hour.

Physics challenge:

What is the smallest denomination coin you could use to pay for the energy used by a 60w bulb burning for 8 hours? (at a cost 16.0412 cents per kilowatt hour)

Start by converting 60W to kW

60W

1 kW

= 0.060 kW

1000 W


“Power” companies sell us energy. The unit they use for energy is the kilowatt hour.

Physics challenge:

What is the smallest denomination coin you could use to pay for the energy used by a 60w bulb burning for 8 hours? (at a cost 16.0412 cents per kilowatt hour)

kW


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