19.3 Electrical Resistance and Ohm’s Law

1. Key Question: How are voltage, current, and resistance related? 19.3 Electrical Resistance and Ohm’s Law

2. 19.3 Electrical resistance • Resistance measures how difficult it is for current to flow.

3. 19.3 Electrical Resistance • The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage. • The more resistance the circuit has, the less current that flows.

4. 19.3 Measuring resistance • Set the meter to measure resistance (W). • Set the black and red leads on opposite ends of the objects.

5. 19.3 The ohm • Resistance is measured in ohms (W). • One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.

6. 19.3 Ohm's law • German physicist Georg Ohm experimented with circuits to find an exact mathematical relationship between voltage, current and resistance. • Ohm's law can be used to predict any one of the three variable if given the other two.

8. 19.3 Calculate current • A light bulb with a resistance of 2 ohms is connected in a circuit that has a single 1.5-volt battery. • Calculate the current that flows in the circuit. • Assume the wires have zero resistance.

9. 19.3 The resistance of electrical devices • The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω). • Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.

10. 19.3 Changing resistance • The resistance of many materials, including those in light bulbs, increases as temperature increases. • A graph of current versus voltage for a light bulb shows a curve. • A device with constant resistance would show a straight line on this graph.

11. 19.3 Electrical Conductivity • The electrical conductivity describes a material’s ability to pass electric current.

12. 19.3 Conductors and insulators • A material such as copper is called a conductor because it can conduct, or carry, electric current. • Materials that insulate against (or block) the flow of current are classified as electrical insulators. • Some materials are neither conductors nor insulators. • These materials are named semiconductors.

13. 19.3 Resistors • Electrical components called resistors can be used to control current. • Resistors have striped color codes to record their "values" (writing on them is difficult).

14. 19.3 Potentiometers • Potentiometers are a type of "variable" resistor that can change from low to high. • They are wired so that as you turn the knob, it changes the distance the current has to flow.

15. Application: Hybrid Gas/Electric Cars

16. 20.1 Series and Parallel Circuits • In series circuits, current can only take one path. • The amount of current is the same at all points in a series circuit.

18. 20.1 Adding resistances in series • Each resistance in a series circuit adds to the total resistance of the circuit. Rtotal = R1 + R2 + R3... Total resistance (ohms) Individual resistances (W)

20. Light bulbs, resistors, motors, and heaters usually have much greater resistance than wires and batteries. 20.1 Total resistance in a series circuit

21. How much current flows in a circuit with a 1.5-volt battery and three 1 ohm resistances (bulbs) in series? 20.1 Calculate current

23. Each separate resistance creates a voltage drop as the current passes through. As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy Ohm’s law is used to calculate the voltage drop across each resistor. 20.1 Voltage in a series circuit

25. 20.1 Series and Parallel Circuits • In parallel circuits the current can take more than one path. • Because there are multiple branches, the current is not the same at all points in a parallel circuit.

27. 20.1 Series and Parallel Circuits • Sometimes these paths are called branches. • The current through a branch is also called the branch current. • When analyzing a parallel circuit, remember that the current always has to go somewhere. • The total current in the circuit is the sum of the currents in all the branches. • At every branch point the current flowing out must equal the current flowing in. • This rule is known as Kirchhoff’s current law.

29. 20.1 Voltage and current in a parallel circuit • In a parallel circuit the voltageis the same across each branch because each branch has a low resistance path back to the battery. • The amount of current in each branch in a parallel circuit is notnecessarily the same. • The resistance in each branch determines the current in that branch.

30. 20.1 Advantages of parallel circuits Parallel circuits have two big advantages over series circuits: 1. Each device in the circuit sees the full battery voltage. 2. Each device in the circuit may be turned off independently without stopping the current flowing to other devices in the circuit.

31. 20.1 Short circuit • A short circuit is a parallel path in a circuit with zero or very low resistance. • Short circuits can be made accidentally by connecting a wire between two other wires at different voltages. • Short circuits are dangerous because they can draw huge amounts of current.

32. Two bulbs with different resistances are connected in parallel to batteries with a total voltage of 3 volts. Calculate the total current supplied by the battery. 20.1 Calculate current

33. 20.1 Resistance in parallel circuits • Adding resistance in parallel provides another path for current, and more current flows. • When more current flows for the same voltage, the total resistance of the circuit decreases. • This happens because every new path in a parallel circuit allows more current to flow for the same voltage.

35. A circuit contains a 2 ohm resistor and a 4 ohm resistor in parallel. Calculate the total resistance of the circuit. 20.1 Adding resistance in parallel circuits