ELECTRIC CURRENT AND POTENTIAL DIFFERENCE LEARNING GOALS Students will: Define what is meant by electric current. Solve problems involving current, charge and time. Know the difference between electron flow and conventional current. Know the definition of electric potential difference. Solve problems relating potential difference to energy and charge. ELECTRIC CURRENT Electric Current is the movement of electric charges from one place to another. As objects develop a charge, these charges can move from one object to another and be distributed over a conductor. In metals, the moving charges are electrons. Considering a cylindrical wire with a known cross-sectional area, the total charge, Q, flowing through that area, A, in a time,, constitutes the electric current. Q is the electric charge in coulombs (C) t is the time in seconds (s) I is the electric current measured in amperes (A), 1 A = 1 C/s, which represents 1 C of charge moving past a point in a conductor in 1 second. The charge (Q=Ne) is the magnitude of the net charge passing through the cross-sectional area. N is the number of electrons e is the charge on a single electron or on a single proton and is equal to 1.6 x 10-19 C. Q1. Calculate the current in an electric toaster if it takes 900 C of charge to toast two slices of bread in 1.5 minutes. THE DIRECTION OF ELECTRIC CURRENT By the definition of electric current, it is assumed that the charge moves from an area of excess (positive charge) to an area where there is a deficit (negative charge). Therefore, the direction of the current is defined as moving from the positive side to the negative side of an electric potential. This assumption about the direction of the electric current is called conventional current or electric current. It was after this definition was proposed that the electron was discovered. The electrons were found to flow from the negative side of a potential source to the positive side. This is referred to as electron flow. Conventional or electric current is the most widely accepted view and it is the one we will use in this course. However, the reality is that it is the electrons that flow not the positive charges. Batteries supply direct current (DC) and wall sockets supply alternating current (AC). 1
ELECTRIC POTENTIAL DIFFERENCE In the illustration to the right, the source (the battery) does work on the positive charge by separating it from the negative. The energy used by the source is transferred to the charges. This energy is called the electric potential energy. Consider the analogy of a skier (the + charge) at the bottom of a hill. When the skier takes the ski lift to the top of the hill, the lift transforms mechanical energy from the lift into gravitational potential energy stored in the skier. Different skiers will gain different amounts of gravitational potential energy, but all will gain the same amount per kilogram (E p /m). Gravitational Potential Energy depends only on the height of the hill and the acceleration due to gravity. Similarly, a skier on an "electric hill" is analogous to positive charges in a battery. The chemical reactions inside the voltaic cell take the positive charges from the cathode (the bottom of the electric hill) to the anode (the top of the electric hill) giving them electric potential energy. The charge will flow through a conductor (the wire). When it passes through the light bulb (or load), it will experience an opposition to flow resulting in a loss of potential energy as light and heat. Since the charges lose energy they also lose electric potential, resulting in an electric potential difference. Electric Potential Difference is the change in electric potential energy per Coulomb caused by the load. E is the change in energy in joules (J) Q is the charge in coulombs (C) V is the potential difference in volts (V) 2
PRACTICE PROBLEMS 1. A battery uses 45J in order to place 15 C of electrons at the negative terminal. What is the potential difference across the battery? 2. If 10 C of charge passes through a light bulb and a total of 500 J of electric potential energy was used up, what is the potential difference across the light bulb? 3. If 5.0 x 10 2 C of charge passes through a buzzer that operates at 6.0 V, how much electrical energy is converted into sound energy? 4. In the circuit below the potential difference between the two terminals of the battery is 6V. The light bulb uses 1/3 of the electrical energy from the charges and the buzzer uses the remaining 2/3. A B E D C What is the electric potential difference between each of the following pairs of points? a) A and B b) B and C c) A and C d) C and D e) D and E f) C and E g) A and D h) B and E i) A and E j) B and D CLASS WORK Read 11.3, 11.5 Do P513 Q1-5, P518 Q1-5 3
Current electricity: Electrical circuit: Open circuit: Closed circuit: Control: Source: Load: Electrical conductor: Short circuit: Electric Current: Ampere (amp): Direct current (DC): Standard Electrical Circuits Definitions and Symbols the flow of electric charges an electrical energy source is connected by conductors to other components such as a load an electrical circuit that contains a gap; no current can flow an electrical circuit that does not contain any gaps; the current can flow a switch that opens or closes a circuit a device that provides energy that can be transformed into electrical energy; e.g., a battery is a source - it transforms chemical potential energy into electrical energy a device that transforms electrical energy into another form of energy; e.g., a light bulb is a load - it transforms electrical energy into light energy and thermal energy a substance through which electrons can move easily; e.g., copper wires are good electrical conductors a circuit in which the load is bypassed, resulting in dangerous overheating; in a short circuit, the current is allowed to travel directly across the source or another component the 'flow rate' of electrons; measured as the total number of electrons that pass a point in the circuit each second the unit used for electric current; the symbol for ampere is A, e.g., current = 2.5 ampere or 2.5 amp or 2.5 A an electric current in which the electrons move in the same forward direction; batteries produce direct current Alternating current (AC): an electric current in which the electrons move in a back and forth pattern in an overall forward direction; electrical generating stations produce alternating current Ammeter: Electric Potential: an instrument used to measure current; since it counts the electron flow rate, it must be connected in the flow rate - in series (aka Voltage) the difference in electrical potential energy per unit charge before and after a source or a load Electric Potential Energy: The total potential energy possessed by electric charges due to their interaction with each other. Electric potential rise: the amount of energy that an electron gains after passing through a source Electric potential drop: the amount of energy that an electron loses after passing through a load Volt: the unit used for electrical potential difference; the symbol for volt is V, e.g.., electrical potential difference = 1.5 volt or 1.5 V Voltmeter: Series Circuit: Parallel Circuit: an instrument used to measure electrical potential difference; since it compares the electron energy before and after a device, it must be connected at the two ends of the device, in parallel A circuit where the current flows in one continuous path from the sources to the load (or loads) A circuit where current can take two or more different paths moving from the source to the load (or loads)
Standard Electrical Symbols