ELECTRICAL ELECTRICAL CIRCUITSCIRCUITS

The CELLThe CELLThe cell stores chemical energy and transfers

it to electrical energy when a circuit is

connected. When two or more cells are connected together we call this a Battery.

The cell’s chemical energy is used up pushing a current round a circuit.

Where does the energy come from?

Remember… electric potential…

V = PEq

In a circuit, a battery gives electrons a push

Electric CurrentPhet – Basic circuit (electron vs. conventional flow)

Analogy: A Ski hill Battery: Chair LiftLoad/Resistor: the Downhill

Short circuit

Simple CircuitsSimple CircuitsHere is a simple electric circuit. It has a cell, a lamp and a switch.

To make the circuit, current must flow through a closed loop via conducting (metal) wires.

cell

lamp

switch

wires

circuit diagramcircuit diagram

cell switch

lamp

wires

For our simple circuit:

circuit diagramscircuit diagramsIn circuit diagrams components are represented by the following symbols;

cell battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

Electric CurrentMeasure of the charge passing in a circuit in a given time

OR

The Rate of Flow of charge in a circuitI = ∆Q /∆t I: Current (Ampere 1 A = 1 C/s)

Q: Amount of charge (C)t: time (seconds)

Electric CurrentA charge (Q) is just a certain number of electrons!

Q = Ne Q: Charge (Coulombs)N: Number of charges e: elementary charge (1.6 x 10-19 C)

(and therefore: I∆t = Ne )

Example

A 1.50 volt AA cell is rated at 0.750 Amp-hours. When connected to a small electric motor 0.250 A of currenta)How much charge is stored in the cell?b)How many electrons are stored in the cell?c)How long will the cell power the motor?

Ohm’s LawResistor/Load: converts electrical energy into other forms of energy. Think of the downhill skiing (or water slide) analogy.

The Voltage drop across a resistor is equal to the current through it multiplied by it’s resistance

V = IR V: Voltage (V)I: Current (Ampere or C/s)R: Resistance (Ohms Ω)

ExampleA 25 Ω motor is connected to a 6.0 V battery. a)Draw a circuit diagramb)Find the current

Measuring Current and Voltage

Ammeter: -Measures current flow-connected in SERIES

Voltmeter: -Measures electric potential gain/drop across a battery (gain) or resistor/load (drop)

Electric PowerP = E/t = Energy transferred/time

And E=qV (Energy, not electric field!)

P = qV/t (and I = Q/t)

P = IV (1)

P = I(IR) = I2R (2)

P = (V/R)V = V2/R (3)

ExampleFind the resistance of a 40 W car headlight designed for a 12 V battery.

Paying for Energy (not Power)

Paying for Energy (not Power)

E = Pt

When calculating the COST of energy, you pay per kW•hr that you use, because the unit of Joules is too small.

Power: kWTime: hrEnergy: kW•hr

ExampleAn electric heater draws 15.0 A on a 120 V line. a) How much power does it use and b) how much does it cost per month (30 days) if it is operated 3.0 hr per day and BC Hydro charges 10.5 cents per kWh?

Series and Parallel CircuitsSeries and Parallel CircuitsSERIES CIRCUITS PARALLEL

CIRCUITS

The current is the same in all parts of the circuitTotal resistance (Rtot) in a series DC circuit is the sum of the individual resistances (Rtot = R1 + R2 + R3 + …)If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.

SERIES CIRCUITS

PARALLEL CIRCUITS

– The current has a choice of routes and is NOT the same in all parts of a DC circuit

– The total current is the sum of the current in each branch of the circuit. Itot = I1 + I2 + I3 + …

– The voltage drop across each resistor is the same (loop rule)– Total resistance (Rtot) is found using this equation:

1/Rtot = 1/R1 + 1/R2 + 1/R3 + ….. – Adding resistors in parallel decreases total resistance– If one bulb ‘blows’ there is still a complete circuit to the other

bulb so it stays alight.

measuring current

Electric current is measured in amps (A)

using an ammeter connected in series in

the circuit.

A

Measuring Current

A A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

Measuring CurrentSERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same at all points in the circuit.

2A 2A

2A

• current is shared between the components

2A2A

1A

1A

Measuring VoltageThe ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter

V

Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current.

Measuring Voltage

Unlike an ammeter a voltmeter is connected across the components (in parallel)

The term Potential Difference is used when talking about voltage.

Measuring Voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUITV

V

Measuring Voltage

VV

V

Series Circuit

1.5V

Voltage is shared between the components

1.5V

3V

Voltage is the same in all parts of the circuit.

3V

Parallel Circuit

3V

3V

Measuring Current & Voltage

Copy the following circuits on the next two slides.

Complete the missing current and voltage readings.

V V

6V4A

A

A

a)a)

V

V

6V4A A

A

A

b)b)

Answers

3V 3V

6V

4A 4A6V

6V

6V4A 4A

2A

2A

4A

a)a) b)b)

Given that I = 1 A in this circuit,Find:a)The resistance of resistor Rb)Voltage drop across R

Find:a)Req for this circuitb)The current flowing through the circuitc)Current through R4