1. ELECTRICAL CIRCUITS

2. 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.

3. Where does the energy come from? Remember… electric potential… V = PE q In a circuit, a battery gives electrons a push

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

5. Simple Circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. wires cell lamp switch To make the circuit, current must flow through a closed loop via conducting (metal) wires.

6. circuit diagram For our simple circuit: cell lamp switch wires

7. circuit diagrams In circuit diagrams components are represented by the following symbols; cell battery switch lamp buzzer ammeter voltmeter motor resistor variable resistor

8. 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)

9. Electric Current A 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 )

10. 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 current • How much charge is stored in the cell? • How many electrons are stored in the cell? • How long will the cell power the motor?

11. 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)

12. Example • A 25 Ω motor is connected to a 6.0 V battery. • Draw a circuit diagram • Find the current

13. 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)

14. 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) = I2R (2) P = (V/R)V = V2/R (3)

15. Example Find the resistance of a 40 W car headlight designed for a 12 V battery.

16. Paying for Energy (not Power)

17. 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: kW Time: hr Energy: kW•hr

18. 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?

19. Series and Parallel Circuits SERIES CIRCUITS PARALLEL CIRCUITS

20. SERIES 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.

21. 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.

22. measuring current Electric current is measured in amps(A) using an ammeter connected in series in the circuit. A

23. Measuring Current This is how we draw an ammeter in a circuit. A A PARALLEL CIRCUIT SERIES CIRCUIT

24. Measuring Current SERIES CIRCUIT 2A • current is the same at all points in the circuit. 2A 2A PARALLEL CIRCUIT 2A 2A • current is shared between the components 1A 1A

25. 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

26. Measuring Voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter a voltmeter is connected across the components (in parallel) The term Potential Difference is used when talking about voltage.

27. Measuring Voltage This is how we draw a voltmeter in a circuit. V V SERIES CIRCUIT PARALLEL CIRCUIT

28. Measuring Voltage V V V V

29. Series Circuit Voltage is shared between the components 3V 1.5V 1.5V

30. Parallel Circuit Voltage is the same in all parts of the circuit. 3V 3V 3V

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

32. a) 6V 4A A V V A

33. b) 6V 4A A V A V A

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

35. Given that I = 1 A in this circuit, Find: • The resistance of resistor R • Voltage drop across R

36. Find: • Req for this circuit • The current flowing through the circuit • Current through R4