ELECTRICAL CIRCUITS

The CELL The 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 = PE q In a circuit, a battery gives electrons a push

Electric Current Phet – Basic circuit (electron vs. conventional flow) Analogy: A Ski hill Battery: Chair Lift Load/Resistor: the Downhill Short circuit

Simple Circuits Here 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 diagram cellswitchlampwires For our simple circuit:

circuit diagrams In circuit diagrams components are represented by the following symbols; cellbatteryswitchlamp motorammetervoltmeter buzzer resistorvariable resistor

Electric Current Measure of the charge passing in a circuit in a given time OR The Rate of Flow of charge in a circuit I = ∆Q /∆t I: Current (Ampere 1 A = 1 C/s) Q: Amount of charge (C) t: time (seconds)

Electric Current A charge (Q) is just a certain number of electrons! Q = NeQ: Charge (Coulombs) N: Number of charges e: elementary charge (1.6 x C) (and therefore: I∆t = Ne )

Example A 1.50 volt AA cell is rated at Amp-hours. When connected to a small electric motor A of current a)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 Law Resistor/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)

Example A 25 Ω motor is connected to a 6.0 V battery. a)Draw a circuit diagram b)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 Power P = 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) = I 2 R (2) P = (V/R)V = V 2 /R (3)

Example Find 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 kWhr that you use, because the unit of Joules is too small. Power: kW Time: hr Energy: kWhr

Example An 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 Circuits SERIES CIRCUITSPARALLEL CIRCUITS

The current is the same in all parts of the circuit Total 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. I tot = I 1 + I 2 + I 3 + … –The voltage drop across each resistor is the same (loop rule) –Total resistance (Rtot) is found using this equation: 1/R tot = 1/R 1 + 1/R 2 + 1/R 3 + ….. –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 Current SERIES CIRCUIT PARALLEL CIRCUIT current is the same at all points in the circuit. 2A current is shared between the components 2A 1A

Measuring Voltage The ‘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 CIRCUITPARALLEL CIRCUIT V

V Measuring Voltage V V 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

Measuring Current & Voltage Copy the following circuits on the next two slides. Complete the missing current and voltage readings.

V V 6V 4A A A a)

V V 6V 4A A A A b)

Answers 3V 6V 4A 6V 4A 2A 4A a)b)

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

Find: a)R eq for this circuit b)The current flowing through the circuit c)Current through R 4