Electrical Circuits Part (2) : dc circuits Lecture 2

3-4: Practical Units

• The three forms of Ohm’s law can be used to define the practical units of current, voltage, and resistance:
• 1 ampere = 1 volt / 1 ohm
• 1 volt = 1 ampere × 1 ohm
• 1 ohm = 1 volt / 1 ampere

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3-4: Practical Units

?

20 V

4 W

I =

20 V

4 W

= 5 A

1 A

?

12 W

V = 1A × 12 W = 12 V

3 A

6 V

?

R =

6 V

3 A

= 2 W

Applying Ohm’s Law

V

I

R

Problem

• Solve for the resistance, R, when V and I are known
• V = 14 V, I = 2 A, R = ?
• V = 25 V, I = 5 A, R = ?
• V = 6 V, I = 1.5 A, R = ?
• V = 24 V, I = 4 A, R = ?

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Kirchhoff’s 1st law

The total current entering a circuit junction equals the total current leaving it.

[conservation of charge]

The sum of the emfs round a loop in any circuit

= the sum of the p.d.s round the loop.

[conservation of energy]

E1 + E2 + E3 + … = I1R1 + I2R2 + I3R3 + …

where I1, I2, I3 … represent currents through the resistances R1, R2, R3 …

Physlets (simulations): ‘Second semester’< ‘DC Circuits’

• ‘Kirchhoff's Loop Rule’
• ‘Applying Kirchhoff's Rules’

Kirchhoff’s 2nd law

A circuit consists of a cell of emf 1.6 V in series with a resistance 2.0  connected to a resistor of resistance 3.0  in parallel with a resistor of resistance 6.0 .

Determine the total current drawn from the cell and the potential difference across the 3.0  resistor.

Kirchhoff’s 2nd law - example

What did you think about the lesson?

• What did you learn?
• What did you love?
• What do you want to learn more about?
• What was boring?
• How would you make the lesson better?

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For More Information

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I thank for attention

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