Current Electricity GRADE 10 PHYSICAL SCIENCE Robyn Basson CAPS
What is current electricity? The flow of moving charge, usually carried by moving electrons in a wire.
Circuits A path in which charges continually move through a complete loop, returning to their original position and cycling through again. The following conditions are necessary for an electric current to flow: A source of energy Conductors in a circuit A closed circuit A closed circuit allows an electric current to flow, as there are no gaps/spaces in the circuit If the switch is not closed or if there are any spaces/breakages, the current will not flow. This is called an open circuit. TB pg. 113
Components of a circuit Conductor TB pg. 113
Components of a circuit Resistor R TB pg. 113
Components of a circuit Light Bulb TB pg. 113
Components of a circuit Battery / Cell + - Direction of conventional current? From positive to negative TB pg. 113
Components of a circuit Variable resistor/ rheostat TB pg. 113
Components of a circuit Open switch TB pg. 113
Components of a circuit Closed switch TB pg. 113
Components of a circuit Voltmeter TB pg. 113
Components of a circuit Ammeter TB pg. 113
Components of a circuit Multimeter Can be used as an ammeter and a voltmeter TB pg. 113
Connection of components Components can be connected in two ways: Series Only one path for the current to flow Parallel More than one path for the current to flow TB pg. 114
Circuit Diagrams Use the symbols of components to draw representing diagrams Description? TB pg. 113
Activity: Drawing Electrical Circuits PG. 115-116
Potential Differenc In a an gravitational electric field, field, the you have electrons the potential have the to fall, thus potential you posses to move, thus gravitational they posses electrical potential energy. potential energy. TB pg. 116
Potential Differenc Potential difference means that there is a difference in potential energy between two points. Charge moves from where it has a high potential energy to where it has a lower potential energy. TB pg. 117
This difference in potential energy between the 2 points is used to do work. The work can be in the form of light, heat or other types of energy. If the resistance between the two points increases, the potential difference increases. TB pg. 117
Potential Differenc Definition: The difference in the electric potential energy per unit charge between any two points in a circuit. Also called voltage. Electrical potential difference across the poles of a battery when no current is flowing is known as emf when a current is flowing is called the potential difference or voltage. TB pg. 117
Potential Difference There are chemicals inside a battery. The chemical reaction provides chemical energy, which is converted into electrical energy. This allows the charges to move. The battery gets hot? The battery also offers some resistance to the flow of charge and so it converts some potential energy to heat energy inside the battery. Unit for EMF and voltage (V). TB pg. 117
Write this down: The difference between EMF and potential difference The voltage measured across the terminals of a battery when NO CURRENT is flowing through the battery is called EMF. The voltage measured across the terminals of a battery when CURRENT IS FLOWING is called terminal potential difference. EMF and potential difference are both measured in volts. (V)
Formula sheet
Potential Difference Calculations V W Q V = voltage V W= work/energy J Q = charge C TB pg. 117
Example 1 Calculations 120J of work is done in lighting a bulb when a charge of 20C flows through it. What will the potential difference be across the bulb? W V Q TB pg. 117
Example 2 Calculations How much energy is transferred when a 60V source allows a 6C charge to flow through a conductor? W V Q TB pg. 118
Potential Difference in series and parallel Potential difference is measured with a voltmeter. Voltmeters are always connected in parallel so that it can measure the difference in potential energy between 2 points in a circuit. A voltmeter has a high resistance so that almost no current flows through it. TB pg. 118
The positive terminal of the voltmeter is always connected to the side of the circuit that is closest to the positive terminal of the battery. - + - + The negative terminal of the voltmeter is connected to the side of the circuit that is closest to the negative terminal of the battery. TB pg. 118
Diagrams page 119-120
Homework Exercise 11 pg. 121-123
Current Strength A source of electrical energy is needed for electric current to flow. The charges collect energy from the source and deliver it to various components of the circuit The rate at which electric charges flow in a circuit. Electric current cannot be used up. It is not the amount of charge but rather how fast or slow the charges move around the circuit. TB pg. 123
Current is measured with an ammeter. The positive terminal of the ammeter is connected so that it is closest to the positive terminal of the battery. Ammeter is always connected in series. Low resistance TB pg. 118
Formula sheet
Current Strength Calculations I Q t I = current A Q = charge C t = time s
Example 1 Calculations How much current is flowing through when 10C of charge moves past a point in 5s? I Q t
Diagrams page 124-125 In a series circuit, the current is the same at all points. In a parallel circuit, the current is divided. The current is equal to the sum of the ammeter readings in parallel.
What influences current strength? What annoys you the most when you go shopping on a Saturday? Queue of people Till = current = resistors If the queue is the current, and the tills are the resistors then the people in the queue (current) would be able to move forward faster if another till opens up. (another path for the current to flow)
What has an effect on the current strength? RESISITORS When more resistors are connected in series, there is a greater obstruction to the flow of charge. Therefore the total resistance increases. As a result of this increase, the current flowing through the battery decreases. TB pg. 125
What has an effect on the current strength? When more resistors are added in parallel, there are some more paths opening for the current to flow through, causing greater ease of current flow. Therefore, the total resistance in the circuit increases. As a result of this, the total current through the battery increases. TB pg. 125
The difference between conventional current and electron flow Conventional Current = The direction in which positive charges would flow if they could. It is from the positive to the negative pole of a battery. Electron flow = The direction in which electrons move in a circuit. This is from the negative to the positive pole of a battery. In circuit diagrams, we indicate the direction of CONVENTIONAL CURRENT. WRITE DOWN
Homework Exercise 12 PG. 128
RESISTANCE TB pg. 129
A resistor converts electrical energy into other types of energy e.g. heat or light. TB pg. 129 Metals are used in electric circuits as the are good conductors of electricity. (Delocalised electrons) When these electrons flow through the resistor, the electrons bump into the particles of the conductor. Kinetic energy is transferred and the particles of the conductor start to vibrate more quickly. The conductor becomes hot and the flow of current decreases, as the electrons find it harder to move through the vibrating particles. Resistors are components in a circuit that resists the flow of charge in the circuit. A resistor is a conductor of electricity but a bad one.
Resistors in series There is only one route for the flow of current. Total resistance of circuit Calculated by taking the sum of individual resistors. R s = R 1 + R 2 + R 3 Increases as more resistors are added. The greater the number of resistors in series, the smaller the total current. Potential dividers (V). Total potential difference of the battery is divided between the resistors. Resistors in Parallel There is alternative routes for the flow of current. Total resistance of circuit Calculated using the inverse of the resistor strength. 1 R p = 1 R 1 + 1 R 2 + 1 R 3 Decreases as more resistors are added. The greater the number of resistors connected in parallel, the greater the total current. Current dividers (A). Total current is divided between the resistors. TB pg. 129
This describes the relationship between current and resistance. TB pg. 130-131
The potential difference over a resistor is directly proportional to the current flowing through the resistor, provided the temperature of the resistor remains constant. V = IR Conductors that obey Ohms law are known as Ohmic conductors. Here, the ratio of V to I remains constant. Non-Ohmic conductors are those that do not obey Ohms Law. They tend to get hot and their resistance increases so much that the ratio of V to I does not remain constant. TB pg. 131 & 133
Formula sheet
Ohms Law Calculations R V I R = resistance Ω V = voltage V I = current A
Example 1 The following series circuit consists of a 12V battery, two resistors R 1 = 4Ω and R 2 = 2Ω and an ammeter. Voltmeters V T, V 1 and V 2 are connected across the battery and the resistors. 1.1 Calculate the total resistance of the circuit. 1.2 Calculate the current through the ammeter. 1.3 Calculate the reading on V 1. 1.4 Calculate the reading on V 2 TB pg. 132
Example 2 The following circuit consists of a source of 6V and three resistors R 1 = 6Ω, R 2 = 2Ω and R 3 = 3Ω connected in parallel with each other. Voltmeter V p is connected over the parallel resistors and V 1 over R 1. 2.1 Calculate the resistance of the parallel arrangement. 2.2 Calculate the current through the ammeter. 2.3 What is the reading on V p? 2.4 What is the reading on V 1? 2.5 Calculate the current through the 6Ω resistor. 2.6 Calculate the current through R 2. TB pg. 132-133 2.7 Calculate the current through R 3.
Homework Exercise 13 PG. 134-138
Question 2 Diagrams from Exercise 13
Question 3 Diagrams from Exercise 13
Question 4 Diagrams from Exercise 13
Question 5 Diagrams from Exercise 13
ELECTROMOTIVE FORCE