ExamLearn.ie. Current Electricity

Transcription

1 ExamLearn.ie Current Electricity

2 Current Electricity An electric current is a flow of electric charge. If a battery is connected to each end of a conductor, the positive terminal will attract the free electrons of the conductor. At the other end, the negative terminal will replace the missing electrons. The result will be a constant flow of charges (electrons) through the conductor. A flow of charge like this is called an electric current. Mandatory Experiment *To test if a substance is a conductor or an insulator Apparatus required: 6 V battery, 6 V bulb, conducting wires, two crocodile clips, items to be tested (copper wire, candle, key, coins, plastic bottle, nail, comb, ruler, compass, paper clip, fruit, cotton wool etc.) Method 1. Make list of all materials you are going to test. Divide them into two columns, predicting whether they are conductors or insulators. 2. Set up circuit. At beginning there is gap between crocodile clips. The bulb does not light. The circuit is not complete yet. 3. Now, bridge the gap with each item you are testing, by inserting them between the clips. Note the result each time. Result The materials that cause the bulb to light brightly, like copper, steel and most metals, are good conductors. Other materials, like plastic, paper etc., are insulators. Charge cannot travel through these materials. Thus, the light does not come on.

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4 Simple electric circuits When charges move around, as in current electricity, they become really useful. These charges flow around a path (electric circuit). Points to remember about electric circuits: 1. Circuits must be complete if there is a gap anywhere, the charges cannot flow. Current flows when a switch in the circuit is closed (or on). The current cannot flow when the switch is open (or off). 2. Potential difference A circuit must have a source of potential difference. This is also referred to as voltage or e.m.f (electron-moving-force). This provides the energy to push the electrons around the circuit. Electrons cannot move around a circuit without this energy. A voltmeter is used to measure the difference in potential/ voltage between any two points in a circuit. Volt (V) is the unit of voltage. 3. Current The potential difference in a circuit causes electrons to flow, i.e. a current. To measure electric current, we use an ammeter. Ampere/Amp (A) is the unit of current. The symbol for current is I. 4. Resistance Resistors are often used in circuits to produce heat or light or to reduce the current flowing in a circuit. Resistors, whose resistance can be changed, also exist. They are called variable resistors or rheostats. If you increase the resistance, less current will flow. An ohmmeter is the instrument used to measure the resistance of a resistor. Ohms is the unit of resistance. The following is the symbol for resistance:

5 Symbols for electric currents

6 Resistance between voltage, current and resistance The following experiment will find a link between voltage, current and resistance across a coil of wire. An electric current will be used. 1. The resistance of the coil will be measured. 2. Then the voltage across coil and current going through coil will be measured. 3. We then try to establish a link between the resistance, voltage and current. Mandatory Experiment (a) To measure the resistance of a coil (or resistor)

7 Apparatus required: ohmmeter, coil (resistor), test leads and connecting wires Method 1. Set the multimeter to act as an ohmmeter by changing the dial to lowest resistance setting. Switch it on. 2. Connect test leads to each end of resistor. Note reading on ohmmeter. Result The reading on ohmmeter is resistance of coil. (b) To measure potential difference (voltage) and current and establish a relationship between potential difference, current and resistance Apparatus required: low voltage power supply, ammeter, voltmeter, switch, coil in beaker of water ( same coil as used above), connecting wires Method 1. Use connecting wires to connect up circuit. 2. Switch on power supply at lowest setting. Close switch. Current will flow. 3. Note reading on voltmeter and ammeter. 4. Change to next setting on power supply. Take readings of voltage and current again. Repeat process for several other settings. 5. Plot graph of voltage (vertical axis) against current (horizontal axis). 6. Divide each voltage by its corresponding current, 7. Find average of these results. Compare this average result with resistance of same coil. Results and conclusion The voltage across resistor is given by voltmeter reading. The reading on ammeter is the current in circuit. A straight-line graph going through origin shows that current is proportional to voltage. This means that the current increases directly with the voltage. The resistance of a resistor can be found by dividing potential difference across it by current passing through it. The resistance of coil can be found by finding slope of graph of potential difference plotted against current.

8 Calculations based on relationship between potential difference, current and resistance: Voltage(v) Current(I) = resistance (R) I = V R V = I R Series Circuits and Parallel Circuits Two bulbs in a circuit can be connected in two different ways: 1. They can be connected in series. Here, they are connected one after the other. There is only one path through which the current can flow. 2. They can be connected in parallel. Here, they are connected side by side. There is a branch in the circuit and the current will have a choice of paths.

9 Mandatory Experiment *To demonstrate simple series and parallel circuits, containing a switch and two bulbs Apparatus required: battery, two similar 6 V bulbs, switch, connecting wires Method 1. Set up series circuit. Close switch and observe bulb brightness. 2. Then set up in circuit. Here, an extra similar bulb is added in series with first bulb. Close switch. Observe bulb brightness. 3. Now remove one of these two bulbs from its holder. 4. Set up parallel circuit. This time, second similar bulb is added in parallel with first bulb. Close switch. Observe bulb brightness. 5. Remove one of these bulbs from its holder. Results Two bulbs in series shine less brightly than if one of these bulbs was on its own. If one bulb is removed/blows, the circuit is no longer complete. Two bulbs in parallel shine as brightly as one of these bulbs on its own. If one bulb is removed/blows, the circuit is still complete.

10 Effects of an electric current 1. Heating effect A demonstration of this is that electric current causes light bulb to heat up and emit light. An everyday application is electric kettle. 2. Magnetic effect A demonstration of this is that electric current will deflect a magnetic current. An everyday application of this is electro magnet. 3. Chemical effect Electrolysis occurs when electric current splits water into hydrogen and oxygen. An everyday application is electro plating (used in making cutlery). Direct and alternating current Direct current (d.c.) is current that flows in one direction only. An example is a battery. Alternating current (a.c) is current that changes direction constantly (100 times a second). An example of this is the mains electricity.