Current

1
Current

• Current the rate of motion per unit of time.
• Electric Current the rate of flow of a charge
  through a cross-sectional area of a conductor.
• a. SI unit ampere (A) referred to amp.
• 1A 1C/s
• b. Equation symbol is I

2
Factors Affecting Current

• NOTE The more charge that moves the greater the
  current.
• Cross sectional surface area. Increase the area
  increases the current.
• Type of conductor. Al v. Cu v. Ag
• Temperature of conductor Increase temperature
  decreases current.

3
Factors Affecting Current

• 4. Distance to move. The greater the distance the
  less the flow.
• 5. Amount of electric force. Potential
  difference. Increase potential difference means
  to have greater force therefore greater flow.
  High to Low
• NOTE charge is defined in terms of () charge
  movement.

4
Factors Affecting Current

• NOTE Increase in drift causes an increase in
  collision and increase in temperature resulting
  in a decrease in current.

5
Questions

• 1. How does the current change if the number of
  charge carries increases.
• ans current increases
• 2. How does the current change if the time
  interval during which a given number of charge
  carries pass the cross sectional area increases
• ans current decreases

6
Equation

• I ?Q/?t
• I charge passing C/s Ampere
• change in time

7
Potential Difference

• The potential difference in a circuit is the
  amount of work done to move a charge in an
  electric field.
• The standard metric unit on electric potential
  difference is the volt, V after Alessandro
  Volta.
• One Volt is equivalent to one Joule per Coulomb.

8
Power

• Power is the rate of flow.
• P IV
• I electric current measured in c/s A
• V potential difference, V E/q
• I q/t and V E/q
• P (q/t) (E/q) E/t rate flow of an electric
  current, POWER
• SI unit for power is Watt, W J/c

9
Power Sampler

• An electric vac is connected to a 120 V outlet is
  rated at 3 A. What is the power requirements of
  the motor?
• P IV (3 c/s)( 120 J/c) 360 j/c 360 W
• You have two cordless power drills both connected
  to a 12V battery. One is rated at 6 A and the
  other at 18 A. Which has a greater requirement on
  the motor?
• P IV (12V)(18A) 216 W (72W)
• HW page 594 1-5.

10
Resistance

• Impedes the rate of flow (current)
• Resistance in mostly constant
• Ohms Law at a constant resistance, the
  potential difference (voltage) is direct to the
  current. Increase the voltage (the force)
  increases the current.
• Ohms Law W Dv / I
• Note Table 22-1 page 595

11
Effects on Resistance

• Increase length Decrease current Increase
  resistance.
• Increase cross section Increases current
  Decrease resistance.
• More ? less conductive Decreases current
  Increase resistance.
• Increase temperature Increases current
  Decreases resistance
• Increase collision Decrease current Increase
  resistance

12
Problem

• What is the rate of flow of an electric current
  moving in a conductor that has a resistance of
  3.5 O connected to a 1.5 V battery?
• V 1.5 v R 3.5 W I ?
• W v/I I v/W 1.5 v/ 3.5 W
  ans 0.43 A
• TLS Problem Set Ohms Law

13
Sources for Electric Current

• 1. The 1 source of all energy in the world is
  the
• battery
• a. wet cell (car battery)
• b. dry cell (flashlight)

14
Types of Electric Current AC v. DC

• Direct current (dc) the charge moves in one
  direction only.
• a. current flows from high to low in the
  battery
• b. flow () ? (-) in a battery b/c of the
  potentail difference at the terminal.
• c. current flow () to () in the circuit
• () end is high potential and the (-) is low
  potential so the flow is high to low.
• water does not run up hill

15

• 2. Alternative current (ac) the source of the
  potential is changing sign. There is NO net
  motion of a charge. The charge vibrates at a
  point
• a. household current 60 Hz means every one
  second the current changes direction 60 times

16
Power

• Mechanical power is the work done in a period of
  time
• Electric power is the rate of flow determined by
  the force pushing. SI unit is watt (W)
• P I?V
• Power current x potential difference
• If current, I V/R then P ?V ?V ?V2
• R R

17
Thermal Energy

• Conversion of power to energy with respect to
  time.
• E Pt
• Energy with respect to flow Sub P I2R
• E I2Rt
• Energy with respect to voltage Sub P V2/R
• E (V2/R)t
• NOTE 5000J / 1oC change

18
Power Sampler

• The PowerMate 3200 is a hair dryer that when
  connect across a 120 V outlet in your home pulls
  3200 W of power. What is the resistance of this
  dryer?
• V 120 V P 3200 W R ?
• P V2/R then R V2 / P
• R 120 V2 / 3200 W
• R 4.5 O

19
Power in Time

• Power measures the rate in which energy is used
  in J/s or Watts.
• At a power of 10 J/s, how much energy is used in
  10 s?
• 10 J/s x 10 s 100 J
• If it cost 10 per joule, how much did it cost?
• 100 J _at_ 0.1 10
• OR
• Energy Cost P?t express in kW/h
• TLS Problems 23-31 textbook, pp 603-605

20
Electric Circuit

• A pathway of flow of electric current through a
  conductor.
• Defined by Ohms Law
• R V/I
• Resistance is inverse to current
• Resistance is direct to force (voltage)
• Two type of general circuits based on the
  arrangement of resistors.
• Series and Parallel

21
Circuit Diagram

• Shows or represents the pathway with symbols.
• Flow is from the () terminal to the (-)
  terminal.
• NOTE figure 22-6 page 597.

22
Series Circuit

• When the resistors are arranged in series.
• Total current individual current across the
  resistor
• IT I1 I2 I3 .
• Total potential difference (V) SUM of each
  volt across each resistor
• VT V1 V2 V3
• Total resistance SUM of each resistance
  across each resistor
• RT R1 R2 R3

23
Series Circuit Sampler

• You are given a 3 cell battery at 2 V per cell
  connected to an ammeter and a series of 3
  resistor. R1 6 O , R2 3O and R3 3O
• Draw and label the circuit diagram.
• Since each cell is 2V, what is the total voltage
  of this battery?
• What is the voltage drop across each resistor?
• What is the total current ?
• What is the total voltage?

24
Series Circuit Sampler

• You have a 12V battery connected to 3 resistors
  set in series. R1 10 O , R2 25O, and R3 5
  O.
• Draw a circuit diagram.
• What is the total resistance through the circuit?
• What is the voltage of each cell in a 4 cell
  battery?
• What is the total current through this circuit?
• What is the voltage drop across each resistor?

25
Parallel circuits

• When the resistors are arranged side by side .
• Total current SUM of the individual
  current
• IT I1 I2 I3
• Total potential difference is the same over
  each resistor
• VT V1 V2 V3 ..
• Total resistance SUM of the recepical of
  each resistor
• 1/RT 1/R1 1/R2 1/R3

26
Parallel Sampler

• A parallel series of three resistors is connected
  to a 6 V battery. R1 3O, R2 2O, and R3 10O.
  The circuit contains an ammeter and a voltmeter.
• Draw the circuit diagram.
• What is the voltage drop over the 2nd resistor?
• What is the total resistance?
• What is the current over the 3rd resistor?
• What is the total current?
• If there are 4 cells in the battery set in
  series, what is the voltage of each cell?