Warm Up 9 2.24.11

1
Warm Up 9 2.24.11

• Hypothesis A 5-gram quantity of salt will
  dissolve faster in 30C water than it will in
  10C water.
• In addition to a hot plate and beakers, which of
  these items are needed to test the hypothesis
  above?
• F Test tube, barometer, thermometer
• G Balance, thermometer, hand lens
• H Stopwatch, microscope, test tube
• J Thermometer, stopwatch, balance

2
Recap

• What is current?
• What is the equation for current?
• When does current flow?

3
Voltage

• A sustained current requires a voltage source to
  maintain the potential difference.
• In our simple circuit what was the voltage
  source?
• So which comes first, current or voltage?
• To measure potential difference you measure
  across the wire or object

4
Voltage

• The voltage is the electric pressure to move
  electrons between the terminals in a circuit.
• Electric pressure is also known as the
  electromotive force (emf).

5
Current and Voltage

• Voltage causes current ( the flow of charge).
• Increasing the voltage will also increase the
  current.
• Hook up your light bulbs.

6
Resistance

• Flowing electrons produce current.
• Does every single electron within an object move
  at the same speed and direction?

7
Resistance

• No!
• Resistance is like a traffic jam, it slows down
  flow of electrons
• What would this do to current?
• All objects have resistance unless they are
  superconductors.

8
Voltage Current and Resistance

• You have 2 green resistor. It provides a set
  amount of resistance.
• You have 1 black diode, it only allows electrons
  to flow 1 way.
• What happens to the brightness of the light when
  you insert a resistor into your circuit?

9
Ohms Law

• More Voltage ? Brighter Light ? More Current
• Less Voltage ? Dimmer Light ? Less Current
• More resistance ? Dimmer Light ? Less Current
• Less resistance ? Brighter Light ? More Current

10
Current and Voltage

• The amount of charge that flows in a circuit
  depends on
• Voltage (potential difference)
• Resistance the property of a material to
• resist, retard the flow of
  charges.

11
Ohms Law

• V IR
• I current (amperes, amps, A)
• V voltage (volts, V)
• R resistance (volts/amps, ohms, ?)

12

• 110 V of AC current passes through a load with a
  current of 2.2 A
• What is the loads resistance?
• I V
• R
• 2.2 A 110 V / R
• R 110 V / 2.2 A
• R 50 ?

13

• Modified from G. Garcia