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Boardworks GCSE Additional Science: Physics Resistance and Resistors

Boardworks GCSE Additional Science: Physics Resistance and Resistors

What obstacles affect the flow of current?
Boardworks GCSE Additional Science: Physics Resistance and Resistors How do different components affect the flow of current? Teacher notes This illustration contains several discussion points relating to resistance and resistors including: electrons (represented by blue circular characters): the electrons are completing a circuit. George Ohm (represented by man in tracksuit): this scientist completed the early research on resistance. treacle pit and pegged down sheet: these obstacles in the circuit represent components that resist the flow of electrons and reduce the current. trampoline: this provides the electrons with more energy and allows them to reach a higher level. slide: this represents a component in which electrons lose energy as they go down it.

What affects the size of the current?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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.

Resistance simulation
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This animated activity illustrates why resistance occurs in conductors and why some resistors have a higher degree of resistance than others. While using this activity it should be highlighted that energy is neither created or destroyed, but transferred from electrical energy to heat and light energy.

Boardworks GCSE Additional Science: Physics Resistance and Resistors
What is a resistor? Boardworks GCSE Additional Science: Physics Resistance and Resistors 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. Teacher notes To extend this slide, students could be asked to identify other household items that use resistance in this way.

Boardworks GCSE Additional Science: Physics Resistance and Resistors

How can resistance be investigated?
Boardworks GCSE Additional Science: Physics Resistance and Resistors Set up this circuit with a resistor and a variable resistor. voltage (V) 0.0 0.6 1.1 1.8 2.5 3.0 3.5 4.2 0.5 1.0 1.5 2.0 current (A) 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.

Investigating current and voltage
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This virtual experiment can be used to investigate how resistance changes in a variety of components and also how voltage and current change. It should be highlighted to students that the voltmeter is wired in parallel and the ammeter is wired in series. Please note, this virtual experiment is designed to demonstrate the principle of Ohm's Law and so has some simplified features: A single cell has been used as the power source but more sophisticated apparatus would be needed if setting up this experiment in the laboratory. The resistance values have been chosen to show the general behaviour of each component so that comparisons can be made with other components. This is why an ordinary piece of copper wire has a measurable resistance.

Current voltage graphs
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This animated graph could be used to introduce the topic of current-voltage graphs and Ohm’s law. Suitable prompts include: Stage 1: What is the shape of the line? Stage 2: What will happen to the size of the current if voltage is multiplied by a factor of four? Stage 3: This stage introduces Ohm’s law, which states that voltage = current x resistance (V = I x R). Students could be asked to consider which factors control the size of the current.

Is resistance the same in all wires?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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 steeper the gradient of a current-voltage graph, the lower the resistance of the wire.

Current-voltage graph for a bulb
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This animated graph could be used to introduce work on the current-voltage graphs for a bulb. While using this activity, it should be highlighted to students that a filament light bulb is a non-ohmic conductor. Stage 3: If the voltage becomes too high, the filament will get too hot and melt. This will break the circuit, and the light will go out.

Current voltage graph for a diode
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This animated graph could be used to introduce work on the current-voltage graph for a diode. More information about diodes is available in the “Using resistance” section of this presentation.

Current – voltage graphs
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This identifying activity could be used as a plenary or revision exercise on the current-voltage graphs.

Boardworks GCSE Additional Science: Physics Resistance and Resistors

The irresistible Georg Ohm
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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. Teacher Notes The illustration shows Georg Ohm carrying out an electrical experiment with a simple circuit based on examples of early 19th century electrical equipment (cell and galvanometers). There are wires of different lengths and thicknesses on the right-hand side of the image.

What is the formula/equation for Ohm’s law?
Boardworks GCSE Additional Science: Physics Resistance and Resistors Ohm’s law is usually written as: voltage = current x resistance V = I x R This formula can also be written as: resistance = R = voltage current V I 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 ().

What does Ohm’s Law show?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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. V = I x R The voltage and current are proportional, so the resistance of a material is constant, as long as the temperature does not change. R = V / I Teacher notes This slide could be used with higher ability students to help improve their understanding of the subject matter. 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. I = V / R

Resistance formula triangle
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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… I = VR x

Calculating the resistance of a bulb
Boardworks GCSE Additional Science: Physics Resistance and Resistors A filament bulb has a current of 0.2 A running through it, with a potential difference of 5 V across it. What is the resistance of the filament in the bulb? V = IR R = V I = 5 V 0.2 A = 25 

Resistance calculations
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This calculations activity could be used as a plenary exercise to check students’ ability to use Ohm’s law.

Boardworks GCSE Additional Science: Physics Resistance and Resistors

What affects resistance?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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?

Investigating resistance and length
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This simulation activity could be used to investigate the effect of the length of a coil on resistance. It could be used as a precursor to running the practical in the lab, or as a revision exercise. To extend the activity, students could be asked to calculate the resistance in ohms in their books before revealing the resistance on the IWB.

How does length affect resistance?
Boardworks GCSE Additional Science: Physics Resistance and Resistors The table shows the resistance of different lengths of wire. What general pattern do these results show? length (cm) 5 10 20 voltage (V) current (A) resistance (W) 2.6 2.9 3.0 5.0 4.3 2.1 0.52 0.67 1.48 Teacher notes The table provides a summary of the results from the virtual experiment on length and resistance. Increasing the length of the wire will increase the resistance because the electrons will have farther to travel. 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?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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.

Investigating resistance and thickness
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This simulation activity could be used to investigate the affect of the thickness of a coil on resistance. It could be used a precursor to running the practical in the lab, or as a revision exercise. To extend the activity, students could be asked to calculate the resistance in ohms in their books before revealing the resistance on the IWB.

How does thickness affect resistance?
Boardworks GCSE Additional Science: Physics Resistance and Resistors The table shows the resistance of different thicknesses of wire. What general pattern do these results show? thickness (mm) 1 2 4 voltage (V) current (A) resistance (W) 3.0 2.9 2.6 2.1 4.3 5.0 1.48 0.67 0.52 Teacher notes The table provides a summary of the results from the virtual experiment on thickness and resistance. Increasing the thickness of the coil will decrease resistance because the electrons will have a larger surface area to travel through. 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?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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. Photo Credit: David Parker / IMI / Univ. of Birmingham High TC Consortium / Science Photo Library Image shows a demonstration of magnetic levitation of one of the new high-temperature superconductors - yttrium- barium- copper oxide (Y-Ba2-Cu3-O7-x). Discovered in 1986, the new superconducting ceramic materials are expected to lead to a technological revolution & are the subject of intensive worldwide research. The photograph shows a small, cylindrical magnet floating freely above a nitrogen-cooled, cylindrical specimen of a superconducting ceramic (made by IMI Ltd). The glowing vapour is from liquid nitrogen, which maintains the ceramic within its superconducting temperature range. Photographed at the University of Birmingham. Superconductors can be used to make very fast circuits and to make magnets levitate.

Resistance – true or false?
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This true-or-false quiz could be used as a starter exercise to work on resistance. Students could be given coloured traffic light cards (red = false, green = true) to vote on the statements shown. To stretch students, they could be asked to explain their voting.

How is resistance affected in a series circuit?
Boardworks GCSE Additional Science: Physics Resistance and Resistors When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors. 4  2  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  total resistance in series = R1 + R2 = 6 

Calculating resistance in series
Boardworks GCSE Additional Science: Physics Resistance and Resistors What is the total resistance for each of these circuits? Total resistance = R1 + R2 = 6  + 34  6  34  = 40  Total resistance = R1 + R2 = 15  + 5  = 20  15  5 

How is resistance affected in a parallel circuit?
Boardworks GCSE Additional Science: Physics Resistance and Resistors When two (or more) resistors are connected in parallel, the current splits at the branches and does not go through each resistor. 2  4  This means the total combined resistance is less than any of the individual resistors. Total resistance = R1 x R2 R1 + R2 This has the same effect as using a thicker piece of wire. 1 total resistance in parallel R1 R2 = + = 4 x 2 4 + 2 = 1.33

Calculating resistance in parallel
Boardworks GCSE Additional Science: Physics Resistance and Resistors What is the total resistance for each of these circuits? Total resistance = R1 x R2 R1 + R2 = 8  x 6 8  + 6  8  = 3.4  6  Total resistance = R1 x R2 R1 + R2 = 5  x 5  5  + 5  5  = 2.5  5 

Boardworks GCSE Additional Science: Physics Resistance and Resistors
Resistors in a circuit Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This calculations activity could be used as a plenary exercise to check students’ ability to calculate the total resistance when resistors are placed in series or in parallel.

Boardworks GCSE Additional Science: Physics Resistance and Resistors

How can resistance be used?
Boardworks GCSE Additional Science: Physics Resistance and Resistors 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.

Boardworks GCSE Additional Science: Physics Resistance and Resistors
Uses of resistors Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This activity provides illustrated information about components that are specifically designed to resist current. It could be used as an introduction to the topic or for revision purposes.

Resistor circuit diagram symbols
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This matching activity could be used as a plenary exercise on resistors’ circuit symbols. Students could be asked to complete the questions in their books and the activity could be concluded by the completion on the IWB.

Resistor circuit diagram symbols
Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This true-or-false quiz could be used as a plenary exercise to work on resistors. Students could be given coloured traffic light cards (red = false, green = true) to vote on the statements shown. To stretch students, they could be asked to explain their voting.

Boardworks GCSE Additional Science: Physics Resistance and Resistors

Boardworks GCSE Additional Science: Physics Resistance and Resistors
Glossary Boardworks GCSE Additional Science: Physics Resistance and Resistors 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 after Georg 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.

Boardworks GCSE Additional Science: Physics Resistance and Resistors
Anagrams Boardworks GCSE Additional Science: Physics Resistance and Resistors

Boardworks GCSE Additional Science: Physics Resistance and Resistors
Multiple-choice quiz Boardworks GCSE Additional Science: Physics Resistance and Resistors Teacher notes This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of resistance and resistors. The questions can be skipped through without answering be clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by the completion on the IWB.

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