Ohm s Law Book page Syllabus 2.10

Transcription

1 Ohm s Law Book page Syllabus 2.10

2 What s wrong with this circuit diagram?

3 Task 2 Sketch a simple series circuit containing a cell and a bulb. On your circuit diagram, show an ammeter and voltmeter being used to measure the current through the bulb and the voltage across it.

4 Task 3 The two bulbs are identical. A1 is 4 A. So what are A2, A3 and A4? A 1 A A 4 4 A 3 3

5 Task 4 V2 is 4 V. So what are V1 and V3? V 1 V 2 V 3

6 Task 5 The voltage across the battery is 6 V. The voltage across bulb B is 4 V. What are the voltages across bulbs A and C?

7 Aim Know how to graphically represent ohmic conductors Describe the effects of changing resistance in a circuit Know how to investigate resistance in a circuit Key words Ohmic conductor Ohm s Law Filament lamp Diode

8 Have you ever wondered The bigger picture How can resistance be useful in electrical appliances?

9 How is resistance useful? Adjustments in colour and volume in electrical appliances are made by changing the current and the voltage in the circuits of the appliance The results are achieved by altering the resistance

10 What affects resistance? The resistance of a wire depends on several factors: material thickness length temperature What experiments could be used to test how length and thickness affect resistance?

11 Measuring Resistance We make use of Ohm s Law which states that the resistance is constant and that R = V, providing I the temperature is constant AA VV

12 Aim Planning Section of Your Write up Investigate how the resistance of a wire changes with length. Hypothesis Explain with diagrams, the factors that affect resistance and how they affect it. In particular make a hypothesis with a reason about length. Method Begin your method, explaining how you will carry out the experiment and, in particular how you will keep it a fair test, etc.

13 Investigation Length (cm) Voltage (V) Current (A) Resistance (Ω) Plot a graph of length vs resistance What is the relationship between the two variables?

14 How does length affect resistance? What general pattern did your results show? When the length of the wire increases, resistance increases. Resistance is caused by electrons colliding with metal ions. When the length of the wire is increased, the electrons have to travel further, so more collisions will occur.

15 A thicker wire has more paths, allowing more electrons to pass through without collision The Effect of Thickness So the thicker the wire, the lower the resistance

16 The Effect of Heating As a wire gets hotter, its atoms vibrate more. This means that they create more of a barrier to the flow of electrons So the hotter the wire, the larger the resistance. Cool Hot

17 Ohm s Law The current that flows through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant Non ohmic conductor Ohmic conductor Can you match them? Filament lamp diode

18 Resistance true or false?

19 Plenary The resistance of a wire is 10W. What happens when: The length of the wire is doubled? The length of the wire is tripled? The length of the wire is halved? The cross-sectional area is doubled? The cross-sectional area is halved? R is doubled R is tripled R is halved R is halved R is doubled The length is tripled and the area is doubled at the same time? = 1.5 R R is tripled x R is halved = R is 3 2

20 Key words Ohmic conductor components have a constant resistance. They follow Ohm s law Ohm s Law -The current that flows through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant Filament lamp is an electric light with a wire filament heated to a high temperature, by passing an electric current through it, until it glows with visible light. It is an example for a non ohmic conductor Diode - typically allowing the flow of current in one direction only. It is a non ohmic conductor