Electricity Part 2: Electric Current

Definition of current Electric current is the amount of charge moving through a surface in a time interval I = current  Q = amount of charge that passes point.  t = time for charge to pass by.

Current Positive or negative charges Convention: positive charge carriers Same current

Units of Current 1 Ampere (Amp) = 1 Coulomb/second

Electric currents only flow in wires A.True B.False

Examples of Currents Aurora Aurora Coronal mass ejection Coronal mass ejection Switch opening Switch opening

3 electron beams in a color TV

Tokamak Fusion Experiments JET discharge JET discharge JET discharge

Simple circuit When a charged particle passes through the battery, it gains energy. When the particle passes through the light bulb it gives up the energy as heat.

Ohm’s Law V = IR V= Voltage of the Battery. I=current in circuit. R=Resistance in the bulb/resistor. (Depends on materials and geometry.)

Units of Resistance R = V/I (volts/amps) R = V/I (volts/amps) By definition, 1 Ohm = 1 volt/amp, By definition, 1 Ohm = 1 volt/amp,or 1  = 1 V/A.

Water system analogy for circuit Pressure  voltage Rate  current Restriction  resistance

Water system analogy for circuit Water  charge Pipe  wire Restriction  resistance Pump  battery (emf) Without closed loop flow stops

Water circuit analogy

Example problem How many amps of current would flow in a light bulb that has a resistance of 60  if it is connected to a 12 V battery.

Current through 60  resistor with 12 V potential A.0.2 A B.5 A C.720 A D A

Example problem How many amps of current would flow in a light bulb that has a resistance of 60  if it is connected to a 12 V battery.

Power in a circuit When a charge Q passes through the battery it gains an amount of energy E = Q×V (This is the amount of work the battery does on the charge.)

If the charge takes an amount of time  t to pass through the battery, the battery supplies a power of (does work at a rate of)

The power supplied by the battery must be dissipated in the resistor. We also know the V = IR. Power dissipated in resistor

The power supplied by the battery must be dissipated in the resistor. We also know the V = IR. Power dissipated in resistor

Example Which light bulb has a higher resistance, a 100 W bulb or a 60 W bulb?

How much current does a 1500 W hairdryer draw? A.12.5 A B.0.08 A C.9.6 A D.216 A

Which light bulb has a higher resistance: a 100 W or a 60 W bulb? A.100 W bulb B.60 W bulb C.Both have the same resistance D.Neither