Physics - Chapter 20

• Current - the movement of charge, the rate at which charge flows (Amperes)
• Conventional current - the direction that a + charge would flow
• negative charges move in the direction opposite to the electric flow; electronic flow
• Drift velocity (Vd) - the average velocity of the free charges
• drift velocity - I=anAvd
• Ohm's law holds for OHMIC substances
• The resistivity of all materials depends on temperature
• resistivity increases with increasing temp
• 2 types of current: alternating and direct
• Direct current (DC) - the flow of electric charge in only one direction
• Alternating current (AC)- the flow of. electric charge that periodically reverses direction
• Thermal hazards - occurs when excessive electric power causes undesirable effects such as starting a fire in the wall of a house
• shock hazard - occurs when electric current passes through a person
• shock hazards are dependent on

  • the amount of current I
  • path taken by the current
  • duration of shock
  • frequency of current
• depolarization occurs when a stimulus makes the membrane permeable to Na+ ion
• repolarization follows as the membrane again become permeable to Na+ and K+ moves from high to low concentration
• in the long term active transports slowly maintains the concentration differences but the cell may fire hundreds of times in rapid succession without seriously depleting them
• electrocardiogram (ECG) - record of the voltages created by the wave of depolarization and subsequent polarization of the heart
• P-wave - generated by by the depolarization and contraction of the atria as they pump blood into the ventricles
• QRS complex - created by the depolarization of the ventricles as they pump blood to the lungs and the body.
• T wave - generated by the repolarization of the ventricles
• infarcts - regions of damaged heart tissue that reflect electrical waves