Circuits and Ohm’s Law

• Objectives: 1. Gain an understanding of Ohm’s Law.

2. Compare and contrast the movement of current through a series and parallel circuit based on the circuits power supply and resistance.

CurrentCurrent (I) – the amount of charge that flows by an area in a unit of time.Current flows from the positive (+) terminal to the negative (-) terminal of a battery. Electrons flow from – to +Measured in Amperes or Amps with an Amp meter.

A

Voltage

• Voltage (V) - the difference in energy per unit of charge. It is caused by an unbalance of charge and is the push that drives electrical current. Without it, electrons will not flow.

• Measured in Volts (V) with a Voltmeter

• Symbol V

Resistance

• Resistance (R)- Resistance is the value assigned to a specific conductor that indicates its degree of resistance to the passage of a current.

• Measured in Ohms (Ω) = 1 volt/1 amp• Different objects resist the flow of current more or

less• The less resistance, the faster the current will flow.• The more the e- want to flow (voltage), the faster

the current is.

OHM’S LAW

• The amount of current (speed of charge flow) depends on how badly the e- want to get to the other terminal (voltage) and what is in their way to slow them down (resistance).

• better known as Ohm’s law

I = V/Ror

V=IR

S-e-r-i-e-s C-i-r-c-u-i-t-s

• When circuit elements are arranged in such a way that there is no branching, as in the next side, the various devices are said to be connected in series.

• The basic property of this kind of circuit is that the current is the same in all parts of the circuit.

• IT = I1 = I2 = I3 = ….

All electricity must go thru all 3 resistors

If one resistor is turned off, all electricity is off

The more resistors, the higher the total resistance

Each resistor uses some of the total voltage of the power source

Each resistor affects the others

150 Volts

R= 50Ω R= 50Ω

R= 50Ω

Each resistor slows down the electricity (current)

Thus Resistance Total equals the sum of all resistors

RT = R1 + R2 + R3 …. = 50Ω + 50Ω + 50Ω = 150Ω

IT = I1=I2=I3 …. I = VT/RT = 150V/150Ω = 1 Amp through out the circuit

VT = V1 + V2 + V3 … V1 = I1R1 = 1 Amp x 50Ω = 50V

Therefore VT = 50V + 50V + 50V = 150V which is the voltage the battery can provide.

Parallel Circuits

• A circuit of this kind, where the current divides into two or more branches, is called a parallel circuit.

• The total current is the sum of the currents through the individual components

• VT = V1 = V2 = V3 = ……

Electricity may go thru any of the 3 resistors

If one resistor is turned off, the others still work

The more resistors (branches) that are on, the lower the total resistance

Each resistor has the total voltage of the power source

Each resistor has its own circuit

R= 50Ω

R= 50Ω

R= 50Ω

V=150V Current is given 3 different routes to take (thus RT is less than the individual resistances)

1/RT = 1/R1 + 1/R2 + 1/R3 + ….. 1/50Ω + 1/50Ω + 1/50Ω = 3/50Ω ; RT = 16.7Ω

VT = V1 = V2 = V3 = ….. 150V

IT = I1 + I2 + I3 + ……. I1 = V1/R1 = 150V/50Ω = 3 Amps ; 3Amps + 3Amps + 3 Amps = 9 Amps = IT

Ohm’s Law Practice

• Complete the OHM’s Law Practice handout• Use your notes.• Show your work.