electricity. electricity: the movement of “excited” charged particles electric current: what the...

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Electricity

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Electricity

• Electricity: the movement of “excited” charged particles

• Electric current: what the moving charged particles create– Like water moving through pipes

• Electric circuit: the path an electric current follows– Like pipes and utilities that use water.

• Conductors: materials in which electric current flows easily– Usually metals– Graphite is a form of carbon, but it is a conductor

• Insulators: materials that do NOT allow electric current to flow easily

Measuring Electricity• Voltage: measures the difference

in energy between two places in the circuit.– Measured in volts (V)

• Black – • Red +

• Measure V across two spots on a closed circuit

Batteries

• Batteries pump energy (potential energy) into a circuit

• The more batteries, the more energy (voltage)– Each battery is 1.5 V (carbon-zinc batteries)

• How much voltage will 12 batteries have?

• How many batteries do you need if you have a an alarm clock that requires 9 V?

Voltage is a difference in energy between two points.

Closing the switch establishes a potential difference (voltage) and an electric field in the circuit.

• Electrons flow in a net direction away from the (-) terminal.

High Potential

Low Potential

Conventional Current• By tradition,

direction in which “positive charges” would flow.

• Direction is opposite of electron flow.

While the switch is open:• Free electrons (conducting electrons) are

always moving in random motion.

• The random speeds are at an order of

106 m/s.• There is no net movement of charge across a

cross section of a wire.

What occurs in a wire when the circuit switch is closed?

What occurs in a wire when the circuit switch is closed?

• An electric field is established instantaneously (at almost the speed of light, 3x108 m/s).

• Free electrons, while still randomly moving, immediately begin drifting due to the electric field, resulting in a net flow of charge.

• Average drift velocity is about 0.01cm/s.

Electric Current:

• The flow of electric charges.

Conventional current has the direction that the (+) charges

would have in the circuit.

Voltage Drop

• Voltage drops across something that uses energy (light bulb, heater, etc)– Energy drops because work is done

Measuring Electricity• Current (I)

– Measured in amperes, or amps (A)

• Measure in an open circuit– Electricity must flow through

the meter to measure current

Resistor: something that uses energy in a circuit

Potentiometers

• Potentiometers: a variable resistor.

Dimmer switches are variable resistors.

When you turn the resistance down, the light burns brightly.

When you turn the resistance up, the bulb is dimly lit.

Circuit Diagrams

• Draw a diagram of a circuit that contains two batteries, a switch, a resistor, and a light bulb.

• Draw a diagram of a circuit that contains a battery, a switch, a potentiometer, and two light bulbs (in that order).

Did you know…

• Voltage is painful– More volts, the greater the shock– A bolt of lightning can have more

than 1,000,000 volts

• Current kills– Small amounts of current can be

deadly– (especially through the heart)

Resistance• Resistance (load): something that opposes the

flow of electricity– Energy is used (work is done)

• Abbreviate with R• Unit is Ohm (Ω)

• Measured outside of a circuit

Examples

• Determine the current in a circuit that has a resistance of 40 Ω and a voltage of 1.5 V

Examples

• Determine the voltage in a circuit that has a resistance of 10 Ω and a current of 5 mA