This document discusses direct current (DC) circuits and some key concepts related to them including Ohm's law, Kirchhoff's laws, and applications involving resistors, batteries, and LED circuits. Specifically, it covers: - Ohm's law relating current, voltage, and resistance - Kirchhoff's laws for analyzing multiple coil circuits - Potential divider circuits to calculate voltages and currents - Dark and light activated LED circuits using a light dependent resistor (LDR) - Practice problems applying Kirchhoff's laws to solve for unknown currents

1. Topic 5
Direct Current Circuit (DC)
• Ohm’s law and its application in a DC circuit
• Multiple coil circuit and Kirchhoff’s law
− Total current and voltage in a multiple coil circuit

2. Ohm’s Law
• Ohm's law states that the current through a conductor is
proportional to the voltage across the conductor.
• This is true for many materials (including metals) provided the
temperature (and other physical factors) remain constant.
V ∝ I
V = IR

3. What is the difference in the investigation that produces the two
different graphs?

4. What do the two graphs show?

5. Which one of the graphs show a non-ohmic
conductor? What makes them non-ohmic?

6. Potential dividers Calculate I, V1 and V2
in the circuit.
RT = 200 
V = IRT
8 = I(200)
I = 8/200 A
V1 = IR1
= 8 x 50
200
V2 = IR2
= 8 x 150
200
Vin=8 V
R1 = 50 
R2 = 150 
Potential divider formula
Vout = . R2 .
R1 + R2
Vout
(
(Vin

7. Dark and light activated LED circuit
LDR – Light Dependent Resistor
The resistance of a LDR
decreases with increase in
incident light intensity
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8. Practical 3 : Dark and light activated LED circuit
1. Measure the resistance of the LDR for different light intensity.
2. Draw conclusion from data in 1.
3. Build one of the circuit.
a) LED on at night
b) LED on during the day
4. Measure the following during the morning, afternoon, evening and night:
a) voltage across the
i) base and collector of the transistor
ii) led and resistor(if any)
iii) LDR
b) resistance of the LDR
5. Calculate the voltage across the LDR with the resistance measured in 4b.
6. Compare measured with calculated values of the voltages across the LDR during
the different times in 4.
7. Compare the measured voltages across the LDR and across the base of the
transistor and LED plus resistor (if any).
8. Discuss your findings and relate to theory.

9. Kirchhoff’s Laws

10. First Law
Kirchhoff's first law applies to
currents at a junction in a circuit.
It states that at a junction in an
electrical circuit, the sum of
currents flowing into the junction
is equal to the sum of currents
flowing out of the junction.

11. Exercise – Write the Kirchhoff’s first law
equations.
1 2 3 4
5

12. Second Law
The sum of
voltage rise
in a closed loop
equals
the sum of
voltage drop.
1. Loop passes cell from – to +
2. Loop passes resistor anti
direction of current
1. Loop passes cell from + to -
2. Loop passes resistor follow
direction of current
Write equations for loop 1, 2 and 3.
Loop 1
V1 = I1R1 + I3R3
Loop 2
V2 = I2R2 + I3R3
Loop 3
V1 + I2R2 = I1R1 + V2

13. Use Kirchhoff’s Laws to solve for I1, I2, and I3.
I1= -0.143 A
I2= 0.429 A
I3 = 0.286 A
What does the negative in I1 mean?

14. Use Kirchhoff’s Laws to solve for I1, I2, and I3.
The direction of current and loop is
your choice.
Tip
1. Current direction based on out
of cells.
2. Loop direction based on cell
voltage gain as much as
possible.

15. Find the currents in the circuits below.