Direct Current Circuits

1
Direct Current Circuits

• Direct Current (DC) and circuits
• Direct Current the DIRECTION of the current in a
  circuit does not change with time. The amplitude
  of the current may or may not change with time.
• The opposite, the Alternating Current (AC) the
  DIRECTION of the current in a circuit changes
  with time. The amplitude of the current usually
  changes with time as well, but may also stay
  constant.
• Battery operated circuits have usually DC.

2
Basic components

• Basic components and their symbols
• Wire connects other components in the circuit,
  without any resistance. Symbol
• Switch when it is closed, a circuit (a loop for
  the current) is formed. Symbol
• Resistor the component that follows the Ohms
  Law .
• Symbol
• Capacitor the component that is defined through
  .
• Symbol
• Battery the device that
• provides the electric potential
• difference in a circuit, that
• converts other type of energy
• into electric. Symbol
• Inductor will be discussed later part of this
  semester.
• Transistor not in the scope of this course.

or

3
The battery

• The electromotive force (emf, E) of a battery
• The capacitance of a battery
• This is often specified as Amp?hour or a
  variation of this unit (like mA?h)
• The internal resistance and the terminal voltage

The external force (mechanical or chemical) that
provides the potential difference is called the
electromotive force. For a battery, this emf
comes from chemical reactions.
4
The battery

• The internal resistance and the terminal voltage

The batterys emf is 1.50V and its internal
resistance is 0.0336 ohm. What is the terminal
voltage with a 5 ohm resistor load?
Question how do you measure the emf?
5
The battery

• Maximum efficiency of the battery and the maximum
  power the battery can deliver.

The efficiency is defined as
emf, E
R, the load
With a load R, the power delivered to the load is
6
The battery
7
The simplest circuit and the electric potential
in a circuit

• The simplest circuit
• The electric potential in the loop

8
Ammeter, Voltmeter and a DMM

• Ammeter
• Voltmeter
• DMM digital multi-meter (ammeter, voltmeter,
  ohmmeter, )

• Ammeters measure current.
• Always connect the ammeter in
• the circuit and the current flows
• through the ammeter.
• The internal resistance of an
• ammeter is very small.

• Voltmeters measure voltage.
• Always connect the voltmeter
• across the component of which
• the voltage over it is to be
• measured.
• The internal resistance of an
• voltmeter is very large.

9
Resistors in a circuit

• One resistor in a circuit ? Ohms Law.
• Two resistors connected in
• Series
• Parallel

10
Resistors in a circuit

• Current, voltage and power division over two
  resistors when they are in
• Series
• Parallel

11
Resistors in a circuit

• Three resistors connected in
• Series
• Parallel

12
Resistors in a circuit

• Mixed connections

13
More examples
What is the total current I, and the current,
voltage and power over each resistor?
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More examples
15
The Kirchhoffs Rules

• The Kirchhoffs rules
• The loop rule
• The sum of the changes in electric potential
  across all components around any complete loop of
  a circuit is zero.
• The junction rule
• The total current flowing into a junction
• equals the total current flowing out of
• a junction.
• Steps in solving a problem

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More examples
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One more example
What current flows through the 5.00 O resistor?
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One more example