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KS4 Physics. Resistance and Power. Contents. Resistance and Power. Resistance. Calculating resistance. Resistance components. Electrical power. Summary activities. Electron flow in a wire. Electron flow and resistance.
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KS4 Physics Resistance and Power
Contents Resistance and Power Resistance Calculating resistance Resistance components Electrical power Summary activities
Electron flow and resistance Electricity is the flow of electrons along a wire. As the electrons move along the wire they collide with the metal atoms. These collisions make the atoms vibrate more… …which makes the metal hotter. Resistance is a measure of how much a material tries to stop electricity passing through it. All wires and devices have some resistance, which is why electrical appliances always waste some energy as heat.
Factors that affect resistance The resistance of a wire depends on several factors: • Material – Different materials have different resistances because some materials are better conductors. Nichrome wire has a higher resistance than a copper wire of the same size. • Length – The longer a wire is the higher its resistance. When electrons travel down a long wire they can collide with more metal ions than in a short wire. • Thickness – A thin wire has a higher resistance than a thick wire. • Temperature – The higher the temperature of a wire the higher its resistance. Metal ions vibrate more at higher temperatures and so collisions with electrons are more likely to happen.
How are current and voltage related for a resistor? 0 0.6 1.1 1.8 2.5 3.0 3.5 4.2 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.
Current-voltage graph for a resistor Current-voltage graph for a resistor Ohm’s Law The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant. Plot the current and voltage readings for a resistor on a graph and draw a line of best fit. x x x The graph is a straight line so the two quantities current and voltage are proportional. x Current / A x x x So if voltage doubles then the current doubles and so on. x Potential difference / V This is called Ohm’s Law after the scientist Georg Ohm.
Current-voltage graphs for different wires Plot the current and voltage readings for nichrome and copper wires of the same size. copper x x x x x The points produce straight lines with different gradients. x x nichrome Current / A x x x The copper wire has a steeper gradient and so has a lower resistance than the nichrome. x x x Potential difference / V At the same potential difference, a copper wire lets a larger current flow than a nichrome wire of the same length and thickness. The steeper the gradient of a current-voltage graph, the lower the resistance of the wire.
Current-voltage graphs for a bulb Plot the current and voltage readings for a filament bulb. x x x x x x The graph produced is not a straight line but a curved line. Increasing the voltage across the filament in the bulb causes this wire to get very hot and give out light. x Current / A x Potential difference / V As the wire gets hotter, its resistance gets higher, which means the current flow is less. So as the temperature rises the current is not proportional to the voltage. The higher the temperature of a wire, the higher its resistance.
Current-voltage graphs and Ohm’s Law Which of the components obeys Ohm’s Law? 1 2 3 I I I V V V 3. Wires of different materials 2. A filament lamp 1. A wire or resistor
Contents Resistance and Power Resistance Calculating resistance Resistance components Electrical power Summary activities
The irresistable 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 equation 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, which is represented by the symbol W.
Resistance formula voltage current V I resistance = R = voltage = current x resistance V = I x R The resistance of a conductor can be calculated using: This equation can also be written as: What are the units of voltage, current and resistance? • Potential difference is measured in volts (V). • Current is measured in amps (A). • Resistance is measured in ohms ().
Resistance formula triangle VR I = A formula triangle helps you to rearrange a formula. The formula triangle for V = IR is shown below. Whatever quantity you are trying to find cover it up and it will leave you with the calculation required. So if you are trying to find current (I)... …which gives the formula… V …cover up I… R I x
Calculating the resistance of a bulb A filament bulb has a current of 20A running through it, with a potential difference of 100V across it. What is the resistance of the filament in the bulb? V = IR R = V I = 100V 20A = 5
Resistors in series 2 4 When resistors are connected in series, the total resistance can be calculated using: Total resistance = R1 + R2 What is the total resistance for this circuit? Total resistance = R1 + R2 = 4 + 2 = 6
Resistors in series What is the total resistance for this circuit? 34 6 Total resistance = R1 + R2 = 6 + 34 = 40
Resistors in parallel 4 2 When resistors are connected in parallel, the total resistance can be calculated using: Total resistance = R1 x R2 R1 + R2 What is the total resistance for this circuit? Total resistance = R1 x R2 R1 + R2 = 4 x 2 4 + 2 = 1.33
Resistors in parallel What is the total resistance for this circuit? 8 6 Total resistance = R1 x R2 R1 + R2 = 8 x 6 8 + 6 = 3.4
Contents Resistance and Power Resistance Calculating resistance Resistance components Electrical power Summary activities
Thermistors A thermistor is a component that has a high resistance when cold but a low resistance when hot. The graph shows how the resistance of a thermistor decreases as its temperature increases. This is the reverse of the normal trend for resistance. This property makes thermistors useful in circuits that control and monitor temperatures. x x x x x Resistance / x x x x Temperature / C
Light dependent resistors The resistance of a light dependent resistor (LDR) is not fixed and depends on the light intensity. A LDR has a high resistance in the dark but a low resistance in the light. The graph shows how the resistance of an LDR decreases as the light intensity increases. This makes LDRs useful in circuits that are controlled by light intensity. x x x x x Resistance / k x x x x Light intensity
Diodes No current flows A diode is a component that allows a current to flow in one direction only. It has a low resistance in one direction and a very high resistance in the other. Current flows in the direction with low resistance but is not proportional to the voltage. If the voltage is reversed or the diode is connected the other way around, the high resistance of the diode ‘blocks’ the flow of current. x x Current / A x x x x x x x Voltage / V
Calculating the current through a diode A diode has a current of 5A running through it and a resistance of 5. What is the potential difference across the diode? V = IR = 5A x 5 = 25V
Contents Resistance and Power Resistance Calculating resistance Resistance components Electrical power Summary activities
Electrical power power = current x voltage P =I x V The relationship between power, current and voltage is shown by the equation: What are the units of power, current and voltage? • Power is measured in watts (W). • Current is measured in amps (A). • Voltage is measured in volts (V).
Power formula triangle P V I = A formula triangle helps you to rearrange a formula. The formula triangle for P = IV is shown below. Whatever quantity you are trying to find cover it up and it will leave you with the calculation required. So if you are trying to find current (I)... …which gives the formula… P …cover up I… V I x
Calculating power A filament bulb has a potential difference of 200V across it and a current of 0.2A running through it. What power is the filament bulb operating at? P = IV = 0.2A x 200V = 40W
Calculating current A filament bulb operates at a power of 60W and has a potential difference of 240V across it. What is the current running through the bulb? P = IV I = P V = 60W 240V = 0.25 A
Converting units 1 kV = 1000 V 1 kJ = 1000 J 1 kW = 1000 W 6 000 How many volts in 6kV? _________ V 12 300 How many joules in 12.3kJ? _________ J 600 How many watts in 0.6kW? _________ W
Converting units 1 kV = 1000 V 1 kJ = 1000 J 1 kW = 1000 W 9.0 How many kilovolts in 9 000V? _________ kV 23.5 How many kilojoules in 23 500J? _________ kJ 0.325 How many kilowatts in 325W? _________ kW
Contents Resistance and Power Resistance Calculating resistance Resistance components Electrical power Summary activities
Glossary • diode – A device that allows current to flow in one direction only. • ohm – The unit of electrical resistance, named after Georg Ohm. • power – The rate at which energy is transferred. • resistance – The reduction in the flow of electrons through a piece of wire or a device. • resistor – A component that opposes the flow of electrons. • thermistor – The resistance of this device decreases as its temperature increases. • variable resistor – The resistance of this device can be changed. • watt – The unit of power. 1 watt = 1 joule of energy transferred every second.
