Extra - Resistance

• Natuurkunde / Scheikunde
• Middelbare school
• havo
• Leerjaar 2

Extra: Resistance

Extra: Resistance

Resistance?

Why is this going well?

1. The outside is made of plastic
2. Only a very small amount of current passes through you
3. The screwdriver is only in one hole at a time

Goals

1. After the lesson you will know a formula to calculate the resistance with the voltage and the current.

2. After the lesson you will know the difference in resistance between conductors and insulators. 

When is there current

• Current is moving charge
  

• Something completely stuck cannot move
  

• In some materials this cargo can move easily, in others it cannot 
  

Conductors

• Charge is loose
  
• Charge can move
  
• Conducts electricity 

Isolators

• Charge is stuck
  
• Charge can't move
  
• Doesn't conduct electricity 

Conductors?

Insulator?

What determines how large the amperage will be? 

Resistance

What happens to the amperage if we put two solid blocks in the middle?

1. The amperage increases
2. Amperage decreases (but not 0)
3. The amperage remains the same
4. The amperage becomes 0

Resistance

What happens to the current if we make the path less wide?

1. The amperage increases
2. Amperage decreases (but not 0)
3. The amperage remains the same
4. The amperage becomes 0

Resistance

Resistance

• Resistance tells us how much a particular electrical component opposes current flow. (How much resistance it offers to current flow)

• Symbol: R (Resistance)
  
• Unit: Ω (Ohm) 

Conductors

• Charge is loose
  
• Charge can move
  
• Conducts electricity 
• Low resistance

Isolators

• Charge is stuck
  
• Charge can't move
  
• Doesn't conduct electricity 
• High resistance

Formula

Resistance is how much voltage we need for a given amperage 

R=​I​​U​​

The unit of resistance is:

1. Volt
2. Amps
3. Watts
4. Ohm

Example

Calculate the resistance of a pocket calculator that has a 1.35 V battery, with a current of 0.200 mA.

Calculate the resistance of a pocket calculator that has a 1.35 V battery, with a current of 0.200 mA.

Given:

U = 1.35 V

I = 0.200 A

Asked:

The resistance (R)

Formula:

R = U / I

Calculation:

R = 1.35/0.200 = 6.75 Ω

Answer:

The resistance = 6.75 Ω

A current of 0.3 A is measured at a voltage of 18 V. Calculate the resistance.

A current of 0.3 A is measured at a voltage of 18 V. Calculate the resistance. 

Given:

U = 18 V

I = 0.3 A

Asked:

The resistance (R)

Formula:

R = U / I

Calculation:

R = 18/0.3 = 60 Ω

Answer:

The resistance = 60 Ω

The following variables are measured in an electrical circuit:
The amperage is 12 amps, the resistance is 3.5 ohms.

Calculate the voltage.

The following variables are measured in an electrical circuit:

The amperage is 12 amps, the resistance is 3.5 ohms.

Calculate the voltage. 

Given:

I = 12 A

R = 3.5 Ω

Asked:

The resistance (R)

Formula:

R = U / I  --> U = R x I

Calculation:

U = 3.5 x 12 = 42V

Answer:

The voltage = 42V

HomeWork

Read the (PDF)-file: "2tH_ Resistance" (see attachment in Magister and/or Magister ELO)

Make the following exercises (in the same document): 

• 13, 14c, 15, 17