3T circuits 5.2

• Natuurkunde
• Middelbare school
• vwo
• Leerjaar 3

What about today?

1. Discuss the homework from §1

2. What is resistance?

3. Creating the formula ourselves

4. How about varying resistance?

5. Work on some exercises (+ discussing difficult ones)

Questions about the homework?

Learning objectives

• You can do calculations using the relation between resistance, voltage and current.
• You can explain the influence of temperature on the resistance.
• You can explain the difference between an 'ohmic resistor', an 'NTC' and an 'LDR'.

What is resistance?

Let's try to find a relation between the voltage (U) and current (I)!

The graph

Ohm's law

In words:

"The voltage and current are directly proportional."

R=​I​​U​​

Resistance      in [Ω]

Voltage in [V]

Current in [A]

We just used a resistor to get a graph with a
straight line. Is this true for every device? (So does
every device have a straight line as well?)

Yes

No

I dunno

Results of the light bulb

Types of resistors

Ohmic resistor:  The resistance is always the same, so a straight                                     line in the U-I-diagram.

Light bulb: The resistance increases when the temperature                                  increases (so if both U and I increase).

NTC: The resistance decreases when the temperature increases

LDR: The resistance will be lower when more light falls on it.

LDR

NTC

A LED is connected to the electrical grid in
your home. You measure a current of
0.25 A. Calculate the resistance of the LED.

Let's work on some exercises

Read: p. 75-78

Make: 2, 3, 4, 6, 7