Ch. 5 - Calculations

• Science
• Middelbare school
• vwo
• Leerjaar 2

Science - Electricity

Calculations

You need to be able to:

• Making calculations with the formula of power:
• P = U x I
  

• Making calculations with the formula of resistance:
• R = U / I
  

• Making calculations with the formula of Electrical Energy:
• E = P x t
  

Variable	Symbol	Unit	Symbol
Voltage	U	Volt	V
Current	I	Amps	A
Power	P	Watt	W
Resistance	R	Ohm
Energy consumption	E	kiloWatthour	kWh

Ω

Julia measures a current of 0.8 A at a voltage of 12 V in a random circuit.
Calculate the resistance in this circuit.

P = U x I

R = U / I

E = P x t

Explanation

Julia measures a current of 

0.8 A at a voltage of 12 V in a random circuit. Calculate the resistance in this circuit.

• g:   U = 12 V I = 0.8 A
• a:   R = ?
• f:    R = U/I
• c:   R = 12 / 0.8 = 15 Ω

P = U x I

R = U / I

E = P x t

Donna reads a voltage of 4.50 V and a resistance of
300 Ohms in a circuit.
Calculate the current (in mA) in this circuit.

P = U x I

R = U / I

E = P x t

P = U x I

R = U / I

E = P x t

Explanation

Donna reads a voltage of 

4.50 V and a resistance of

300 Ohms in a circuit.

Calculate the current (in mA) in this circuit.

• g:   U = 4.5 V       R = 300 Ω
• a:   I = ?
• f:    R = U/I    -->  I = U / R
• c:   I = 4.5 / 300 = 0.015 A
• 0.015A = 15 mA

P = U x I

R = U / I

E = P x t

A lamp is been lit all evening, about 5.5 hours. The lamp is plugged in the mains socket and the current through the lamp is 400 mA.

Calculate the energy consumption from this lamp and the costs of the use of this lamp if 1 kWh = 22 cents

P = U x I

R = U / I

E = P x t

Explanation

A lamp is been lit all evening, about 5.5 hours. The lamp is plugged in the mains socket and the current through the lamp is 400 mA.

Calculate the energy consumption from this lamp and the costs of the use of this lamp if 1 kWh = 22 cents 

• g:   U = 230 V        t = 5.5 h 
•        I = 400 mA = 0.4 A
• a:   E = ?
• f:    E = P x t
•        But missing the P
•        So: first P = U x I

• c:  P = 230 x 0.4 = 92 W
•      E = 0.092 x 5.5 = 0.51 kWh
•      0.51 kWh x 22 = 11.13 cent

P = U x I

R = U / I

E = P x t

John reads a voltage of 12V. In addition,
the circuit has a total resistance of 30 Ohms. Calculate the power in this circuit.

P = U x I

R = U / I

E = P x t

Explanation

John reads a voltage of 12V. In addition, the circuit has a total resistance of 30 Ohms. Calculate the power in this circuit.

• g:   U = 12 V       R = 30 Ω
• a:   P = ?
• f:    P = U x I
•        But missing the I
•        So: first I = U / R

• c:   I = 12 / 30 = 0.4 A
•        P = 12 x 0.4 = 4.8 W

P = U x I

R = U / I

E = P x t

Camil has a lightbulb with a power of 50W. This bulb needs to be plugged into the mains socket, otherwise is won't work.
Calculate the resistance of this bulb

P = U x I

R = U / I

E = P x t

Explanation

Camil has a lightbulb with a power of 50W. This bulb needs to be plugged into the mains socket, otherwise is won't work. 

Calculate the resistance of this bulb

• g:   P = 50 W      U = 230 V 
• a:   R = ?
• f:    R = U / I
•        But missing the I
•        So: first I = P / U

• c:   I = 50 / 230 = 0.22 A
•        R = 230 / 0.22 = 1045 Ω

P = U x I

R = U / I

E = P x t