Electricity - Basic Electricity - Basic ideas…ideas…

Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______.

Resistance is anything that resists an electric current. It is measured in _____.”

(Words: volts, amps, ohms, voltage, ammeter, voltmeter)

Charge (Q)Charge (Q)As we said, electricity is when electrons move around a circuit and carry energy with them. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). We can work out how much charge flows in a circuit using the equation:

Q

TI

Charge = current x time

(in C) (in A) (in s)

Charge on electron = 1.6x10-19COne coulomb = I amp per second

Example questionsExample questionsCharge (C) Current (A) Time (s)

5 2

0.4 1

20 0.5

50 250

3 60

1) A circuit is switched on for 30s with a current of 3A. How much charge flowed?

2) During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for?

3) A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

More basic ideas…More basic ideas…If a battery is added the current will ________ because there is a greater _____ on the electrons

If a bulb is added the current will _______ because there is greater ________ in the circuit

Current in a series Current in a series circuitcircuit

If the current here is 2 amps…

The current here will be…

The current here will be…

And the current here will be…

In other words, the current in a series circuit is THE SAME at any

point

Current in a parallel Current in a parallel circuitcircuit

A PARALLEL circuit is one where the current has a “choice of routes”

Here comes the current…

And the rest will go down here…

Half of the current will go down here (assuming the bulbs are the same)…

Current in a parallel Current in a parallel circuitcircuit

If the current here is 6 amps

The current here will be…

The current here will be…

The current here will be…

And the current here will be…

Voltage in a series Voltage in a series circuitcircuit

V

V V

If the voltage across the battery is 6V…

…and these bulbs are all identical…

…what will the voltage across each bulb be? 2V

Voltage in a series Voltage in a series circuitcircuit

V

V

If the voltage across the battery is 6V…

…what will the voltage across two bulbs be?

4V

Voltage in a parallel Voltage in a parallel circuitcircuit

If the voltage across the batteries is 4V…

What is the voltage here?

And here?

V

V4V

4V

SummarySummaryIn a SERIES circuit:

Current is THE SAME at any point

Voltage SPLITS UP over each component

In a PARALLEL circuit:

Current SPLITS UP down each “strand”

Voltage is THE SAME across each”strand”

An example question:An example question:

V1

V2

6V

3A

A1

A2

V3

A3

Georg Simon Ohm 1789-1854

ResistanceResistance

Resistance is anything that will RESIST a current. It is

measured in Ohms, a unit named after me.

The resistance of a component can be calculated using Ohm’s Law:

Resistance = Voltage (in V)

(in ) Current (in A)

V

RI

An example question:An example question:

V

A

1) What is the resistance across this bulb?

2) Assuming all the bulbs are the same what is the total resistance in this circuit?

Voltmeter reads 10V

Ammeter reads 2A

Current-voltage graphsCurrent-voltage graphs

I

V I

V

I

V

1. Resistor 3. Diode

2. Bulb

Explain the shape of each graph

Voltage - Current Graphs: Voltage - Current Graphs: DiodeDiode

Only lets current flow in one direction. Forward biased – forward bias: high resistance (initially gives small current) at ≈ 0.6V, resistance decreases rapidly (current increases)

reverse bias: high resistance (gives ~ zero or slightly negative current. At breakdown, resistance ~ zero (and very large current)I

V

Voltage - Current Graphs: Voltage - Current Graphs: Filament LampFilament Lamp

• As Voltage increases current increases.

• Current heats the filament• Therefore resistance increases• Same applies in negative direction

I

V

Current/Voltage Graphs: Current/Voltage Graphs: Ohmic conductorOhmic conductor

• Straight line through origin – constant gradient

• V and I increasing proportionally

I

V

Resistors in SeriesResistors in Series

• Suppose all the resistors could be replaced with one resistor of equivalent resistance RT

VT = IRT and VT=V1+V2+V3 Eqn1

V1=IR1 V2=IR2 V3=IR3

• Therefore substituting into 1• I RT = IR1 + IR2 + IR3 ( I cancels )• Therfore in series:

RT = R1 + R2 + R3

Resistors in ParallelResistors in Parallel

I = I1+I2+I3 Equation 1• Suppose all the resistors could be replaced with one resistor of

equivalent resistance RT. I=V/RT

• I1 = V/R1 I2 = V/R2 I3 = V/R3

Substituting into Equation 1

• V/RT = V/R1 + V/R2 + V/R3 Cancelling V

• Therefore 1/RT = 1/R1 + 1/R2 + 1/R3

Questions Questions

1. Find the combined resistance of two resistors of 4Ω and 6Ω connected in a) series b) parallel.

2. Find the combined resistance of three resistors of 4Ω and 5Ω and 20Ω connected in a) series b) parallel.

3. What is the combined resistance of 5 resistors, each of value 10Ω, connected in a) series and b) parallel.

4. What resistance must be connected in parallel with a resistor of 16Ω to give a combined resistance of 3.2Ω.

ResistivityResistivity

• The resistance of a piece of conductor depends on length, area, temp and material. Assuming temp is constant

• R = Resistance (Ω)• L = Length (m)• A = Area (m2)

• Ρ is a constant ofProportionalityResistivity units are Ωm.

A

LR

AR

LR

1

A

LR

A

LR

AR

LR

1

Table of ResistivitiesTable of Resistivities

Material Resistivity(Ωm)

Silver 1.6x10-8

Copper 1.7x10-8

Aluminium 2.7x10-8

Silicon 2300

Glass 1010 - 1014

Polystyrene 1015

DC and ACDC and AC

DC stands for “Direct Current” – the current only flows in one direction:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)

1/50th s

240V

V

V

Time

T

Power and fusesPower and fuses

Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by:

P

IVPower = voltage x current

in W in V in A

Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

Power and fusesPower and fuses

Copy and complete the following table:

Appliance Power rating (W)

Voltage (V) Current needed (A)

Fuse needed (3, 5

or 13A)

Toaster 720 240

Fire 2000 240

Hairdryer 300 240

Hoover 1000 240

Computer 100 240

Stereo 80 240

Energy and chargeEnergy and charge

The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around:

W

QV

Energy transferred = charge x voltage

(in J) (in C) (in V)

Electromotive force and Electromotive force and Internal Internal ResistanceResistance

• Let an amount of energy E be supplied when a charge Q passes through the source. We call the energy supplied per unit charge the e.m.f. of the source Є.

Є = E/Q

Where E is measures in Joules, J Q is measured in

coulombs, C

Electromotive Force and Internal Electromotive Force and Internal ResistanceResistance

• For the dry cell battery, as the current is increased, the reading of the voltmeter across the battery decreased.

• Missing Voltage is proportional to Current

• E.M.F. of battery (ε – V) proportional current

– (ε – V) = Ir ( r is the internal resistance of the cell. Chemicals inside the battery offer some resistance to current flow)

– ε = V + Ir = IR + Ir

– therefore ε = I(R+r)

Example questionsExample questions1) In a radio circuit a voltage of 6V is applied and a

charge of 100C flows. How much energy has been transferred?

2) In this circuit the radio drew a current of 0.5A. How long was it on for?

3) A motor operates at 6V and draws a current of 3A. The motor is used for 5 minutes. Calculate: a) The motor’s resistance, b) the charge flowing through it, c) the energy supplied to it

4) A lamp is attached to a 12V circuit and a charge of 1200C flows through it. If the lamp is on for 10 minutes calculate a) the current, b) the resistance, c) the energy supplied to the bulb.