Current

Electrons Flow- +

Electrons are negatively charged and therefore flow from negative to positive

e-

CurrentCurrent is the FLOW OF ELECTRONS AROUND A CIRCUIT

In metals, current is carried by electrons

We use an ammeter to

measure current

The unit of current is

the ampere (A)

Current in a Series Circuit

2A 2A

2A

The Current in a Series Circuit is the Same at Every

Point

In a series circuit the bulbs are all the same

brightness

If the bulbs are

of the same

rating!

Current Questions

A B C

1)The current reading for bulb B is 2A. What are the readings for bulbs A and C?

2) Bulb suddenly fails. What happens to bulbs A and C?

What Is the Current at This Point?

6A

If a cell is added the current will

increase and the bulb gets brighter

Brightness of Bulbs in a Series Circuit

If a bulb is added the current will

decrease as there is greater resistance in the circuit and the bulbs get

dimmer

Brightness of Bulbs in a Series Circuit

Current in a Parallel CircuitA PARALLEL circuit has more than one path for the current to flow through

Bulb A

Bulb B

Assuming both bulbs are the Same…they will be the same brightness

Current in a Parallel Circuit

Bulb A

Bulb B

The Advantages of a parallel circuit are:

• If one bulb fails ….the other will remain on

• Both bulbs can be switched on separately

Current in a Parallel Circuit

The total flow of current is equal to the total current from all of the branches

2A

2A

2A

6A 6A

What is the ammeter reading?

3A

3A

9A

3A

Voltage (Potential Difference)

Voltage Is the Driving Force That Pushes the Current Around the Circuit

Potential Difference/ Voltage

This is VOLTA. He studied the driving force that pushes current around a circuit. He gave his name to the unit of Potential Difference

The unit of Potential Difference is the VOLT (V)

Voltage in a Series Circuit

12V

12V The voltage in a series circuit is shared across each component. The sum of the voltages across each component equals the source voltage

4V 4V4V

What Is the Voltage?

?V

8V 8V8V

?V

Voltage in a Parallel Circuit

6V

6V

6V

The voltage is the same across all

components in parallel

What Is the Voltage?

9V

? V

? V

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Complete the meter readings

V1

V2

12V

6A

A2

A1

V3

A3

Which Is the Series and Parallel Circuit

A * Current is THE SAME at any point

* Voltage SPLITS UP over each component

* Current SPLITS UP down each branch

* Voltage is THE SAME across each branch

B

Resistance

Resistance

It can be calculated using Ohm’s Law:

Resistance = Voltage (V)

(Ohms) Current (A)

V

I x R

Resistance is anything in a circuit that restricts the

flow of current

The unit of Resistance is the

Ohm

Summary Table

Current Is the flow of electrons around a circuit

Voltage Is the driving force that pushes the current

around.

Resistance

Is anything in a circuit which slows the flow

down

Calculate the resistance

12V

3A ?

Electrical quantities in an electric circuit are related to each other by:

OHM’S LAW

OHM’S LAW says: If the source voltage remains constant,increasing the resistance in a circuit will cause a decrease

in current flow in that circuit.In mathematical terms it tells us that current flow is

inversely proportional to resistance.

In equation form it says:Voltage (V) = Current (I) x Resistance (R)

orV(in volts) = I (in amps) x R (in ohms)

PotentialIn volts

(joules / coul)

CurrentIn amperes

(coul / second)

ResistanceIn ohms

(volts / amp)

Drop across a resistance

Current passingthrough the

resistor

VOLTAGE

(volts)

Current (I) in amps

I

v

The slope of theLine gives theResistance (Ω)

Factors Affecting Resistance1. Thickness (diameter) of wire

2. Length of wire

3. Type of material

4. Temperature

Electron flow and Resistance

Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter.

Resistance is a measure of how much a material tries to stop electricity passing through it.

CROSS SECTIONAL AREA

-Thin wires have more resistance than thick wires

-Halving (1/2) the cross sectional area of a wire doubles its resistance because there is half as much space for the electrons to move

Resistance - Length of Wire

-Doubling the Length of a wire doubles its resistance because the electrons have twice as far to move

Type of MaterialDifferent materials have different resistances.

Example: A nichrome wire has more resistance than a copper wire of the same size (the atoms in nichrome hold the electrons more tightly than copper atoms)

Temperature

As temperature increases, resistance increases.

Voltage –current for a lamp

V/volts I / amps

0 0

0.5 0.04

1.0 0.08

1.5 0.12

2.0 0.15

2.5 0.18

3.03.5

0.190.20

Plot these results

What do they show?

Current-Voltage Graphs Show Resistance

I

V

A Filament

Bulb

As the temperature of the filament increases, the

resistance increases – hence the curve

Current-Voltage Graphs Show Resistance

I

V

A Diode

Current will only flow through in ONE

DIRECTION

(It has very high resistance in the reverse direction)

There is a Balance….

If you increase the Voltage – then more current will flow

If you increases the Resistance – then less

current will flow

The Light Dependent Resistor

1A

Dark

5A

Light

The resistance of the LDR depends on the amount of light falling on it. Its resistance decreases as the amount of light falling on it increases

The Thermistor

1A 5ACOLD WARM

The resistance of a thermistor depends on its temperature.

Its resistance decreases as the temperature of the thermistor increases