Lesson 4 — current, voltage and resistance

• Science
• Middelbare school
• havo
• Leerjaar 2

Class layout – Science

Class layout – Science

Chapter 9 Electricity

This lesson

• Recap
• series and parallel
• Current, voltage & resistance
• Ohms law

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B

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D

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B

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B

What is current?

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B

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D

The amount of electric charge flowing by

The electric charge flowing by per second

The amount of electrons going through a wire

The amount of time electricity is flowing through a circuit

What is current?

A

B

C

D

The amount of electric charge flowing by

The electric charge flowing by per second

The amount of electrons going through a wire

The amount of time electricity is flowing through a circuit

What is the unit of current?  

(Example: the unit of distance is meters)

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B

C

D

Amperes, symbol = I

Coulomb, symbol = Q

Coulombs, symbol = C

Amperes, symbol = A

What is the unit of current?  

(Example: the unit of distance is meters)

A

B

C

D

Amperes, symbol = I

Coulomb, symbol = Q

Coulombs, symbol = C

Amperes, symbol = A

1A =...

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B

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0.001 mA

1mA

1000 mA

0.1 mA

1A =...

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B

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D

0.001 mA

1mA

1000 mA

0.1 mA

A

B

C

D

A

B

B

B

Link

 Currnet is measured with an ammeter

ammeter

Electricity is: a flow of free electrons flowing from a negatively charged pole to a positively charged pole.

This flow of electrons is called: electric current. 

• The symbol for electric current is: I
• The unit of electric current is ampere (A)

1 ampere stands for 1 coulomb flowing by per second (1 coulomb per second).

• 1 coulomb stands for 6.24 x 10¹⁸ electrons
• Example: on average a current of 1,5 A flows through phone charging cable while it is charging. This mean 1,5 coulombs are flowing through that cable per second.

The formula to calculate current is: 

I=​t​​Q​​

current=​time​​charge​​

Practice problem

In a school classroom there are approximately 9 LED lamps. At the end of the day after all the lamps have been turned off someone came back, because they forgot something. They turned the lamps on and took 2,5 minutes to find what they were looking for. Through one LED lamp flows a current of 30 mA. Calculate how much electric charge went through these 9 lamps during that time.

Series and parallel circuits

Current (I):

• The amount of charge flowing by per second (coulombs/ second).
• Expressed in amperes (A)

Voltage (U):

• The difference in electrical potential energy between two points in a circuit.
• Expressed in volts (V)

Resistance (R):

• The amount something resists the flow of current.
• Expressed in ohms (Ω)

Ohm's law

Current=​Resistance​​Voltage​​

I=​R​​V​​

quantity	unit
Current (I)	Ampere (A)
Voltage (U)	Volt (V)
Resistance (R)	Ohm (Ω)

Practice exercise

A) An average refrigerator works on a voltage of 120 Volts. A normal resistance for a refrigerator can be 45 Ohm. Calculate the current flowing through a refrigerator that has these properties.

B) What would the resistance on a fridge that has the same voltage have to be if it has a current of 2.4 Ampere flowing through it?

3 lamps are connected to a battery in a parallel circuit. The battery supplies the circuit with 2.4 Ampere. 

Because of its resistance, the second lamp gets 2/3 of the current supplied to the circuit. The rest of the current gets divided evenly over the other two lamps. 

The voltage over the battery is 6 Volts.

Calculate the resistance of each lamp.