Electric current and circuit

Modern Physics

1. Electrical Current & Circuits

2. Indicators and Objectives PS-6.6: Explain the relationships among voltage, resistance, and current in Ohm’s law. PS-6.9: Compare the functioning of simple series and parallel circuits. Explain the relationship between voltage, resistance and current in an electrical circuit—including units for each Predict energy transformations in a circuit using voltage, resistance, and current Compare/contrast series and parallel circuits in terms of structure, function, and changes in each.

3. Section 7.2: Electric Current Static Electricity : build up of charges that pass QUICKLY to another object Electric Current : continuous flow of charges through a conductor

4. Static Electricity Objects can acquire a static electric charge through: Friction (when an object whose electrons are loosely held rubs against another object) Conduction (when an object with an excess of electrons touches a neutral object) Induction (a neutral object acquires a charge from a charged object close by without contact being made)

5. Friction

6. Induction

7. Conduction Charging by contact!

8. Electricity and Voltage - Electricity is the flow of electrons (-) Charges (-) flow from HIGH voltage areas to LOW voltage areas Voltage is like electrical pressure that pushes and pulls charges Voltage Difference : the push/pull that causes charges to move and is measured in volts (V)

9. Voltage Voltage is created by a chemical cell (battery) when it changes chemical energy to electrical energy by a generator when it changes mechanical energy to electrical energy by a solar cell when it changes light energy to electrical energy.

10. Voltage and Current When a wire connects the terminals of a battery or generators, then the voltage will push and pull electrons through a conductor. One terminal has extra electrons thus a negative charge. The other terminal has a deficit of electrons and thus a positive charge. Electrons in the wire are pushed by the negative terminal and pulled by the positive terminal through the wire Circuit : a closed, conducting path For changes to flow, the wire must always be connected in a circuit Electric Current : the flow of charges through a wire or any conductor. Measured in Amperes (A=Amps) Current is almost always the flow of electrons What happens if we break the circuit?

11. Check for Understanding What is voltage? How is voltage generated? (3 ways) What is current?

12. Resistance Resistance : the tendency for a material to oppose the flow of electrons Changes electrical energy into thermal energy and light Ex: lightbulb filament Resistance is measured in Ohms (Ω)

13. What Affects Resistance?

14. IV. Control the Flow A voltage difference causes the charges to flow Flow of charges= current (Amps or A) Electrical resistance restricts the movement of charges Resistance = current Pressure = current (Voltage Difference)

15. Ohm’s Law Current = voltage difference Resistance I = V/R or V=IR I= current (units = A, amps) V= voltage (units = V, volts) R = resistance (units = ohms Ω) Tutorial

16. Sample Problem Problem #4 A circuit's resistance is 22.41 ohms when connected to a 12.6 volt battery. Find the current in milliAmpere

17. Given : R = 22.41 Ω V = 12.6 v I = ? I =  V        R = 12.6V ÷ 22.41 A = 0.5622 A or 562mA

18. Problem #5 A transistor radio uses 0.24 milliamps of current when it is operated by a 1.50 volt battery. What is the resistance of the radio's circuit?

19. Given: I = 0.24 mA V = 1.50 v R = ? R= V/I = 1.50v ÷ 0.00024A = 6,250 OR 6.3x10 3 ohms

20. A motor circuit has a resistance of 28.6 ohms. If the current through it is 0.844 amps, what is the potential difference in this circuit?

21. V = I R Given: R = 28.6 ohms I = 0.844 A V = ? = 0.844 A x 28.6 ohms = 24.1 v

22. Problem #7 A lamp is plugged into 120 volts. What is the lamp's resistance if the current is 75 milliamps?

23. Given: V = 120 v R = ? I = 75 mA R= V÷ I = 120V ÷ 0.075 A = 1.6 x 10 3 ohms or 1.6 kilo

24. Section 3 – Electrical Energy I. Series Circuit : the current has only one loop to flow through things are wired one right after the other If one thing (bulb) goes out every thing goes out If the circuit is broken the entire flow of current stops

25. Series Circuit Current is the same at each point in the circuit When another resistor (light bulb) is added in series, the total resistance increases. When resistance increases, current will decrease. Decreased current means dimmer light.

26. II. Parallel Circuit : contains two or more branches for current to move through current splits up to flow through the different branches because all branches connect the same two points of the circuit – the voltage difference is the same in each branch more current flows through the branches that have the lower resistance

27. Household Electrical Safety In a house, many appliances draw current from the same circuit If more appliances are connected to a circuit, more current will flow through the wires More current in wires = more heating in the wires More heat causes insulation on wires to melt, which increases chances of fire To protect a house from this, all household circuits have a: Fuse, or Circuit breaker

28. Household Circuits: Fuse: a small piece of metal that melts if the current becomes too high Circuit Breaker : contains a small piece of metal that bends when it gets hot bending causes a switch to flip and opens the circuit

29. Let’s Compare Series and Parallel Circuits Series Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit _______________________ Adding resistance in series ______________________ Parallel Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit _______________________ Adding resistance in parallel _______________________

30. Let’s Compare Series and Parallel Circuits Series Circuits 1 path(s) for current Current is the same at every point Voltage drops at each resistor Break in circuit stops all current Adding resistance in series decreases total current (dimmer light bulbs) Parallel Circuits multiple path(s) for current Current can be different in each branch Voltage same across each resistance Break in circuit does not affect other bulbs Adding resistance in parallel increases total current