Physics Subject Area Test ELECTRICITY & MAGNETISM
Electric Charge and Electrical Forces: • Electronshave a negative electrical charge. • Protonshave a positive electrical charge. • These charges interact to create an electrical force. • Like charges produce repulsive forces –they repel each other • Unlike charges produce attractive forces –they attract each other
A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus. The nucleus has positively charged protons and neutral neutrons. Negatively charged electrons move around the nucleus at much greater distance. Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons.
Electrostatic Charge: • Electrons move from atom to atom to create ions. • positively charge ions result from the loss of electrons and are called cations. • Negatively charge ions result from the gain of electrons and are called anions.
A neutral atom has no net charge because the numbers of electrons and protons are balanced. (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion (cation). (C) The addition of an electron produces a net negative charge and a negative ion (anion).
The charge on an ion is called an electrostatic charge. • An object becomes electrostatically charged by • Friction, which transfers electrons between two objects in contact, • Contactwith a charged body which results in the transfer of electrons, • Inductionwhich produces a charge redistribution of electrons in a material.
ElectricalConductors and Insulators Electrical conductorsare materials that can move electrons easily. • Good conductors include metals. Copper is the best electrical conductor. Electrical nonconductors (insulators)are materials that do not move electrons easily. • Examples are wood, rubber etc. • Semiconductors are materials that sometimes behave as conductors and sometimes behave as insulators. Examples are silicon, arsenic, germanium.
fundamental charge- the electrical charge on an electron -has a magnitude of 1.6021892 X 10-19C (measured in coulombs). coulomb - the charge resulting from the transfer of 6.24 x 1018 of the charge carried by an electron. magnitudeof an electrical charge (Q)is dependent upon how many electrons (n) have been moved to it or away from it.Mathematically, Q = n e- where e-is the fundamental charge.
Coulomb’s law: Electrical force is proportional to the product of the electrical charge and inversely proportional to the square of the distance. This is known as Coulomb’s law. where, • F is the force, • k is a constant and has the value of 9.00 x 109 Newtonmeters2/coulomb2 (9.00 x 10 9 Nm2/C2), • q1 represents the electrical charge of object 1 and q2represents the electrical charge of object 2, and • dis the distance between the two objects.
Force Fields: The electrical charge produces a force field, that is called an electrical fieldsince it is produced by electrical charge.
Voltage is a measure of the potential difference between two places in a circuit. • Voltage is measured in joules/coloumb. • The rate at which an electrical current (I) flows is the charge (Q) that moves through a cross section of a conductor in a give unit of time (t), I = Q/t. • the units of current are coulombs/second. • A coulomb/second is an ampere (amp).
The CELL The cell stores chemical energyand transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.
What is an electric current? An electric current is a flow of microscopic particles called electronsflowing through wires and components. - + In which direction does the current flow? from the Negativeterminal to the Positiveterminal of a cell.
A simple circuit is a connection of batteries and resistors that meets 2 criteria • All batteries are in series • The equivalent resistance of the entire circuit can be obtained by repeated use of just the series and parallel equivalent resistance formulas Simple Circuits
simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. wires cell lamp switch To make the circuit, these components are connected together with metal connecting wires.
simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric currentto flow around. If there were any breaks in the circuit, the current could not flow.
circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires
circuit diagrams In circuit diagrams components are represented by the following symbols; cell battery switch lamp buzzer ammeter voltmeter motor resistor variable resistor
types of circuit There are two types of electrical circuits; SERIES CIRCUITS PARALLEL CIRCUITS
SERIES CIRCUITS The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.
PARALLEL CIRCUITS The components are connected side by side. The current has a choice of routes. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.
measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A
measuring current This is how we draw an ammeter in a circuit. A A PARALLEL CIRCUIT SERIES CIRCUIT
measuring current SERIES CIRCUIT 2A • current is the same • at all points in the • circuit. 2A 2A PARALLEL CIRCUIT 2A 2A • current is shared • between the • components 1A 1A
copy the following circuits and fill in the missing ammeter readings. ? 3A 3A ? 4A ? 1A ? 4A 4A 1A ? 1A
measuring voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V
measuring voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter a voltmeter is connected across the components Scientist usually use the term Potential Difference(pd) when they talk about voltage.
measuring voltage This is how we draw a voltmeter in a circuit. V V SERIES CIRCUIT PARALLEL CIRCUIT
measuring voltage V V V V
series circuit • voltage is shared between the components 3V 1.5V 1.5V
parallel circuit • voltage is the same in all parts of the circuit. 3V 3V 3V
measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.
measuring current & voltage a) 6V 4A A V V A
measuring current & voltage b) 6V 4A A V A V A
answers a) b) 6V 4A 6V 4A 6V 4A 4A 2A 3V 3V 4A 6V 2A
In electricity, the concept of voltage will be like pressure. Water flows from high pressure to low pressure(this is consistent with our previous analogy that Voltage is like height since DP = rgh for fluids) ; electricity flows from high voltage to low voltage. But what flows in electricity? Charges! How do we measure this flow? By Current: current = I = Dq / Dt UNITS: Amp(ere) = Coulomb / second Electric Circuits
The rate at which electrons move along field lines is called drift speed, typically about 10-4 m/s Electric current defined in terms of the flow of positive charge opposite the electrons is called conventional current Current will always be in the same direction as the local electric field
A battery or power supply supplies voltage. This is analogous to what a pump does in a water system. Question: Does a water pump supply water? If you bought a water pump, and then plugged it in (without any other connections), would water come out of the pump? Question: Does the battery or power supply actually supply the charges that will flow through the circuit? Voltage Sources:batteries and power supplies
Charges move from higher to lower potential • For the process to continue, charges that have moved from a higher to lower potential must be raised back to a higher potential again • A battery is able to add charges and raise the charges to higher electric potential Symbol for a battery
Just like a water pump only pushes water (gives energy to the water by raising the pressure of the water), so the voltage source only pushes the charges (gives energy to the charges by raising the voltage of the charges). Just like a pump needs water coming into it in order to pump water out, so the voltage source needs charges coming into it (into the negative terminal) in order to “pump” them out (of the positive terminal). Voltage Sources:batteries and power supplies
Because of the “pumping” nature of voltage sources, we need to have a complete circuit before we have a current. Voltage Sources:batteries and power supplies
two of the common circuit elements: capacitor resistor The capacitor is an element that stores charge for use later (like a water tower). The resistor is an element that “resists” the flow of electricity. Circuit Elements
The current established is directly proportional to the voltage difference Ohm’s Law: ΔV ∝ I In a plot of ΔV vs I, the slope is called theelectrical resistance Electrical Resistance & Ohms’ Law
Current is somewhat like fluid flow. Recall that it took a pressure difference to make the fluid flow due to the viscosity of the fluid and the size (area and length) of the pipe. So to in electricity, it takes a voltage difference to make electric current flow due to the resistance in the circuit. Resistance
By experiment we find that if we increase the voltage, we increase the current: Vis proportional to I. The constant of proportionality we call the resistance, R: V = I*R Ohm’s Law UNITS:R = V/I soOhm = Volt / Amp. Resistance The symbol for resistance is
Just as with fluid flow, the amount of resistance does not depend on the voltage (pressure) or the current (volume flow). The formula V=IR relates voltage to current. If you double the voltage, you will double the current, not change the resistance. The same applied to capacitance: the capacitance did not depend on the charge and voltage - the capacitance related the two. As was the case in fluid flow and capacitance, the amount of resistance depends on the materials and shapes of the wires. Resistance
The resistance depends on material and geometry (shape). For a wire, we have: R = r L / A where r is called the resistivity (in Ohm-m) and measures how hard it is for current to flow through the material,L is the length of the wire, and Ais the cross-sectional area of the wire. The second lab experiment deals with Ohm’s Law and the above equation. Resistance
The electrical potential energy of a charge is: U = q*V . Power is the change in energy with respect to time: Power = DU / Dt . Putting these two concepts together we have: Power = D(qV) / Dt = V(Dq) / Dt = I*V. Electrical Power
Besides this basic equation for power: P = I*V remember we also have Ohm’s Law: V = I*R . Thus we can write the following equations for power: P = I2*R = V2/R = I*V . To see which one gives the most insight, we need to understand what is being held constant. Electrical Power