Physics Past Paper PDF

The nervous system Explain saltatory coduction. What Are myelin sheaths and the Nodes of Ranvier? Neurons communicate by sending...

The nervous system Explain saltatory coduction. What Are myelin sheaths and the Nodes of Ranvier? Neurons communicate by sending electrical signals along an axon. Myelin insulates the axon, similar to how copper electrical wires are insulated with rubber or plastic. Insulation prevents the loss of electrical current and optimizes function. When neurons lose insulation, they likewise lose function. Loss of myelin is called demyelination. Electrical signals that move along the axon are called an action potential and need to be incredibly fast. Action potentials can move as fast as 150 meters a second in a myelinated axon. But in an unmyelinated axon, action potentials can only move up to 10 meters a second. When the axon is demyelinated, insulation is reduced, and the electrical current leaks out into the extracellular space outside the neuron as it moves along the axon. The leaking current causes poor action potential conduction, leading to poor signal and neurological disease. Multiple sclerosis is an example of a demyelinating neurological disease. Schwann cells are wrapped around the axon of the neurone. These cells form the myelin sheath A myelinated nerve cell has short unmyelinated segments which are known as Nodes of Ranvier. The action potential is generated in these areas. The action potential jumps from one node to another node. This type of conduction is called saltatory conduction. Explain the process of depolarization by an action potential. Diagram the path of the electrical system of the heart. Potential di@erence At resting state there is a diKerence in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside. This is because there are more positive ions outside the cell than inside. The diKerence in charge is called a potential diKerence. Hyperpolarization is when the membrane potential becomes more negative at a particular spot on the neuron’s membrane, while depolarization is when the membrane potential becomes less negative (more positive). Depolarization and hyperpolarization occur when ion channels in the membrane open or close, altering the ability of particular types of ions to enter or exit the cell ELECTRICITY Apply knowledge and understanding of: The coulomb as the unit of charge The elementary charge e equals 1.6 × 10 ̶ 19 C Net charge on a particle or an object is quantised and a multiple of e Q = n. e n is a positive or negative integer Conventional current and electron flow Potential difference (p.d.); Energy transferred per unit charge the unit volt ! ! Coulomb’s law; F = k !" # # for the force between two point charges Define an ohmic conductor Interpret IV graphs of their characteristic electrical behaviour Explain how to measure the resistance of a component by drawing an appropriate circuit diagram using correct circuit symbols Explain that components in series have the same current passing through them Explain that components connected in parallel have the same potential diKerence across each of them Calculate the total resistance of two components in series as the sum of the resistance of each component using the equation: [ R total = R1 + R2 ] Explain qualitatively why adding resistors in series increases the total resistance whilst adding resistors in parallel decreases the total resistance Solve problems for circuits which include resistors in series and parallel using the concept of equivalent resistance Calculate the total resistance of two components in parallel using the equation: 1 1 1 = + 𝑅#$#%& 𝑅( 𝑅) Charging by Friction Rubbing two different materials together, a process known as charging by friction (charging by rubbing), is the simplest way to give something a charge. This is what you do every time you drag your feet along a carpet so you can reach out and zap someone's ear. Your feet in socks and the carpet are doing charging by friction. Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths. As they pass over each other the electrons with weaker bonds are “ripped” off one material and collect on the other material. Rub a glass rod with a piece of silk. Explain what happens. This is the same sort of situation as the one above. In this case the silk holds onto the electrons more strongly than the glass. Electrons are ripped off of the glass and go on to the silk. The glass is now positive and the silk is negative. Charging by Conduction Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called “charging by contact”). Overall the total negative charge remains constant. We started with six negative charges, and we ended up with a total of six negative. Notice that this also means a negative object causes a negative charge on the other object. What would happen if you used a positive rod at the start to touch the metal sphere? The same sort of thing, except that the neutral object will transfer electrons to the positively charged object. In any of these cases, the results depend on the materials being conductors or insulators.  In the examples I've given so far, all the materials have been conductors, so the charges move easily from one to the other and spread out. If the materials used were insulators, only the specific areas that actually touched would show any change in charge. That's because the charges can't move through the insulators easily. Charging by Induction It is possible to charge a conductor without touching it. You do have to follow some special procedures. Most important is the use of a grounding wire. A grounding wire is simply a conductor that connects the object to the ground. Think of the earth as a huge reservoir of charge... it can both gain or donate electrons as needed. Depending on what the situation is, either electrons will travel up the grounding wire to the object being charged, or travel down to the ground. Charging by induction is a more complex process than conduction, as the example below shows...