Physics A - AS Module 3

Current Electricity

Electric Current and Electric Charge

• When an electric current flows in a circuit, electric charge moves around the circuit.
  
• The bigger the current, the bigger the amount of charge which passes each point in the circuit each second.
  
• The longer the time that the current flows for, the bigger the amount of charge which passes.
  

The Coulomb (C)

• The Coulomb is the unit of electric charge.
  
• If a current of 1 Amp flows for 1 second, 1 Coulomb of electric charge will pass.
  
• If 2 A flows for 1 second, 2 C of charge will pass.
  
• If 1 A flows for 3 seconds, 3 C of charge will pass.
  
• If 2 A flows for 3 seconds, 6 C of charge will pass.
  

The relationship is:

Q = I x t

charge = current x time

Q is Electric Charge measured in Coulombs (C)

I is Electric Current measured in Amps (A)

t is time measured in seconds (s)

A simple way to express this is as follows

The triangle reveals the realtionship you want.

I = Q/t

t = Q/I

Q = I x t

Charge and Electric Current

Current is the rate of flow of electric charge.

I = dQ/ dt

Current I is measured in Amperes (Amps).

Electric Charge Q is measured in Coulombs

The gradient of a graph of Q against t is the current I.

The area under a graph of I against t is the charge.

Electrical Potential Difference

Think of this as electrical potential ENERGY difference.

Potential Difference is the energy converted from Electrical Energy per Coulomb of electrical charge which passes.

When 1 Coulomb of Electrical Charge moves through 12 V, 12 J of energy are converted.

Potential Difference = Electrical Work Done / Charge Passed

V = W/Q

OR W = Q x V

Power

Power is the energy transferred in each second.

Electrical power is the electrical energy transferred each second.

Power= Energy Transferred ÷ Time Taken

P = E/ t

The Units of Power are Joules per Second ( J/s)

1 Joule per second is called a Watt (W)

1 J/s = 1 W

But also

Energy Transferred = Charge x Voltage

E = Q x V

And so

Electrical Power = (Charge x Voltage)÷ Time Taken

P = (Q x V) / t

But

Charge / time taken = Electric Current

Q/t = I

And so the expression for Electrical Power becomes

Power = Current x Voltage

P = I x V