Download
1 / 46
490 likes | 687 Views
What obstacles affect the flow of current?. How do different components affect the flow of current?. What affects the size of the current?. Current is a measure of the rate of flow of electric charge through a circuit. A large current means that the rate of flow is more rapid.
E N D
What obstacles affect the flow of current? How do different components affect the flow of current?
What affects the size of the current? Current is a measure of the rate of flow of electric charge through a circuit. A large current means that the rate of flow is more rapid. Current can be changed by increasing or decreasing the voltage of the circuit. Components in a circuit may reduce the size of the current. This is called resistance. All wires and components resist current. Resistance is a measure of how hard it is for electrons to move in an electrical circuit.
What causes resistance? Resistance is a measure of how much a material tries to stop electricity passing through it. Electricity is the flow of electrons along a wire. As the electrons move along the wire they collide with the metal ions in the wire. These collisions make the atoms vibrate more, which makes the metal hotter. All wires and components have some resistance, so electrical appliances always waste some energy as heat.
What is a resistor? A resistor is a component designed to reduce the current. • A variable resistor has a resistance that can be changed. • A fixed resistor has a resistance that remains the same. Many domestic appliances use resistance to transfer electrical energy to heat and light energy. The heating element in this kettle is a coil of metal that resists electricity. As current is passed through the metal, resistance causes the metal to get hot and so boils the water.
voltage (V) current (A) 0.0 0.0 0.5 0.6 1.0 1.1 1.5 1.8 2.0 2.5 2.5 3.0 3.0 3.5 3.5 4.2 How can resistance be investigated? Set up this circuit with a resistor and a variable resistor. A V Slowly move the variable resistor so that the voltage increases by 0.5 V and record the current for each setting. Plot a current-voltage graph of the results.
Is resistance the same in all wires? The steeper the gradient of a current-voltage graph, the lower the resistance of the wire. Plotting current–voltage results for nichrome and copper wires on the same graph gives straight lines with different gradients. copper current (A) At the same voltage, a copper wire allows a larger current than a nichrome wire of the same size. nichrome voltage (V) This means that the copper wire has a lower resistance than the nichrome wire. What does the gradient tell you about resistance?
The irresistible Georg Ohm Resistance is a measure of how hard it is for electrons to move in an electrical circuit. The connection between current, voltage and resistance was discovered in 1827 by Georg Ohm, a German physics and maths teacher. The formula R = V/I is known as Ohm’s Law. It was such an important discovery in electricity that the unit of resistance is called the ohm. This unit is represented by the symbol W.
voltage current V I voltage = current x resistance V = I x R resistance = R = What is the formula/equation for Ohm’s law? Ohm’s law is usually written as: This formula can also be written as: What are the units of voltage, current and resistance? • Voltage is measured in volts (V). • Current is measured in amps (A). • Resistance is measured in ohms ().
V = I x R R = V / I I = V / R What does Ohm’s Law show? What do the different arrangements of Ohm’s law show about the links between current, voltage and resistance? This version of Ohm’s Law shows that as the voltage increases, the current increases. The voltage and current are proportional, while the resistance remains constant. The voltage and current are proportional, so the resistance of a material is constant, as long as the temperature does not change. For a low resistance material, more current is allowed to flow for a given voltage. For a high resistance material, less current will flow at the same voltage.
VR I = Resistance formula triangle A formula triangle helps you to rearrange a formula. The formula triangle for V = IR is shown below. Cover up the quantity that you have to work out and this gives the formula needed. So to find current (I), cover up I… …which gives the formula… x
= 5V 0.2A Calculating the resistance of a bulb A filament bulb has a current of 0.2A running through it, with a potential difference of 5V across it. What is the resistance of the filament in the bulb? V = IR R = V I = 25
What affects resistance? The resistance of a wire depends on several factors: • material • thickness • temperature • length For example, a copper wire has a lower resistance than a nichrome wire of the same size. Copper’s excellent ability to conduct electricity means it is often used in wiring. What experiments could be used to test how length and thickness affect resistance?
length (cm) voltage (V) current (A) resistance (W) 5 2.6 5.0 0.52 10 2.9 4.3 0.67 20 3.0 2.1 1.48 How does length affect resistance? The table shows the resistance of different lengths of wire. What general pattern do these results show? When the length of the wire increases, resistance increases. Why would increasing the length of the wire result in a increase in resistance?
Why does length affect resistance? The effect of length of a wire on resistance can be understood by looking at the atomic structure. Resistance is caused by electrons colliding with metal ions. When the length of the wire is increased, the electrons have to travel further. So the chance of collisions will increase, causing the resistance to increase.
thickness (mm) voltage (V) current (A) resistance (W) 1 3.0 2.1 1.48 2 2.9 4.3 0.67 4 2.6 5.0 0.52 How does thickness affect resistance? The table shows the resistance of different thicknesses of wire. What general pattern do these results show? When the thickness of the wire increases, resistance decreases. Why would increasing the thickness of the wire result in a decrease in resistance?
Why does thickness affect resistance? Increasing the thickness of a wire increases the surface area that the electrons can flow through. This decreases the chance of collisions with metal ions. In thick materials the charge carrying particles are able to move through the conductor more easily, reducing resistance.
How does temperature affect resistance? If a component obeys Ohm’s law, then decreasing the temperature decreases the resistance of the material. This is because the positively charged ions in the metal do not vibrate as fast, and the electrons can flow more easily. A superconductor is a material that conducts electricity with very little resistance at very low temperatures. Superconductors can be used to make very fast circuits and to make magnets levitate.
2 4 total resistance in series = R1 + R2 How is resistance affected in a series circuit? When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors. There is only one path for the current to travel, which means that it flows through the resistors one after the other. Total resistance = R1 + R2 This has the same effect as using a longer piece of wire. = 4 + 2 = 6
Calculating resistance in series What is the total resistance for each of these circuits? Total resistance = R1 + R2 = 6 + 34 34 6 = 40 Total resistance = R1 + R2 = 15 + 5 = 20 5 15
= R1 x R2 R1 + R2 4 = 4 x 2 4 + 2 1 1 1 = + 2 total resistance in parallel R1 R2 How is resistance affected in a parallel circuit? When two (or more) resistors are connected in parallel, the current splits at the branches and does not go through each resistor. This means the total combined resistance is less than any of the individual resistors. Total resistance This has the same effect as using a thicker piece of wire. = 1.33
Total resistance = R1 x R2 R1 + R2 = 8 x6 8 + 6 Total resistance = R1 x R2 R1 + R2 = 5 x5 5 + 5 Calculating resistance in parallel What is the total resistance for each of these circuits? 8 = 3.4 6 5 = 2.5 5
How can resistance be used? When a material resists current, some of the electrical energy is transferred to heat or light. This property of resistance is used in many electrical appliances. Hairdryers and electric heaters contain coils of nichrome wire. Nichrome, an alloy of nickel and chromium, resists current and gets very hot. Filament light bulbs contain tungsten. Tungsten resists current and gets so hot that it glows. This is why bulbs waste energy as heat.
Glossary • diode – A component that allows current to flow in one direction only. • light dependent resistor – A component whose resistance changes in response to light intensity. • ohm – The unit of electrical resistance, named afterGeorg Ohm. • Ohm’s law – Formula used to relate current, voltage and resistance, if the temperature remains constant. R = V/I. • resistance – The opposition to the flow of charge. • resistor – A component that opposes the flow of charge. • thermistor – A component whose resistance changes in response to temperature. • variable resistor – A component whose resistance can be adjusted to vary the amount of current.
