Electric Current Calculations

Electric Current Calculations

K Warne

V1

R1

A

VT

V2

R2

A1

A2

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Electrical Circuits

A

V1

VoltmeterAmmeter

Resistance

The Ammeter measures the current flowing in the circuit. (Amps A)

The Voltmeter Measures potential difference or voltage in volts. (V)

The Resistance of the Resistor is given in Ohms (Ω).

V1 = V2 V2

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Series Circuits• Adding resistors in series…increases

the total resistance - because all the current flows through all the resistors.

Rt = R1 + R2

• The total potential difference (voltage) is the sum of the potential differences of the resistors – the total potential loss must equal the all the potential lost along the way.

Vt = V1 + V2The potential differences will be

proportional to the resistances.

• The current flowing is the same all over the circuit and would decrease as more resistances are added -

I = I2 = I3

VTA

V1 V2

R1 R2

VT

V1 v2

AVT

A2

A3

3Ω1Ω

12v

VT = 12 V

V1 = 3 VV2 = 9 V

I = ...

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Parallel Circuits• Adding resistors in

parallel…decreases the total

resistance.

1 1 1

Rt R1 R2

• The potential difference (voltage) is EQUAL over the resistances – there is no potential lost between the resistors and the cells.

VT = V1 = V2

• The current flowing is divided between the resistances and would increase as more resistances are added- more routes for the current to flow.

= +

V1

R1

AVT

V2

R2

A1

A2

IT= I1 + I2

Electric Current in a Conductor

Conventional current - positive to negative

Maintaining a current Conductor - closed

circuit

Potential difference

Replacement of charges - SOURCE

+ -

+ -

+ +++

++

+ ++ +

++

++

+ ++ +

++

++

+ ++ +

++

++

+ ++ +

++

++

+ +

e-

e-

e-

e-e-e-

e-

e-e-

e-

e-

e-e-e-

e-

e-e-

e-e-

e-e-+ -e-

e-

e-

<------- electrons “Positive spaces” ------>

Conventional current is the movement of positive “spaces” from + to - in a

conductor.

Conventional current e-

If 160 C of charge flow through the ammeter in 3s what current is flowing?

The current is the number of charges passing any point in one second

Current (I) = Charge(Q)

Time (t)

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Calculating CurrentCalculate the current flowing through the circuit.

+

_Ammeter

A53.3333

If 160 C of charge flow through the ammeter in 3 s what current is flowing?

I = Q/t

= 160/3

=53.3 A

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Current & Resistance

RESISTANCE

• Electrical charge experiences resistance as it moves through a conductor.

• The resistance is due to collisions with particles in the metal atoms and ions.

• The moving charges lose kinetic energy in the collisions which heat up the conductor.

+_

CURRENT: An electrical current is a movement of chargesthrough a conducting material from positive to negative. (?!)

Effects of Current

+_

Electric current generates heat in a conductor.

+_

A small current (0.1A) would have only a few charges flowing.

A large current (15A) would have a large number of charges flowing and generate far more heat.

As a conductor heats up the RESISTANCE INCREASES.

Factors affecting Resistance

Material: Different materials have different resistance.

Length: Increasing the length will increase the resistance.

Temperature: As temp increases – atoms vibrate faster

RESISTANCE INCREASES.

+ -

+ -

A V

1. Material

2. Length

3. Temperature

R

Current, voltage & resistance

We define the unit of resistance; one ohm () is one volt

per ampere.

R = V/I 1 = 1V/1A

+ -

+ -

A V

The ratio: VoltageCurrent

will always give a constant value for any resistor.

If the resistance increases the ratio increases.

We therefore use this ratio to represent the resistance of a resistor.

R

Resistance (R) =Voltage (V)Current (I)

Ohm’s law: The current passing through a resistor is directly proportional to the potential difference across the resistor provided the temperature remains constant.

Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as

other resources, are freely available on my resource sharing website:

www.warnescience.net

(paste into your browser if link above does not work)

Have a look and enjoy!

Keith Warne