| You cannot assume that it is providing you with exact voltage across its terminals as suggested by the value of e.m.f. Give 3 examples why? | 1. Supply may not be made to high degree of precision
2. Batteries may have become flat
3. All sources of e.m.f have an internal resistance |
| The charges moving round a circuit have to pass through what 2 things | 1. External components
2. Internal resistance of power supply |
| Power supplies and batteries get warm when they are in use, why? | Some of the electrical potential energy of the charge is transformed to internal energy as they do work against the internal resistance of the cell |
| Potential difference across the internal resistance indicates what? | Indicates energy transferred to the internal resistance of the supply |
| What is the formula used to calculate Potential difference across a power source: | V = E - Ir |
| What does the terminal p.d. of the battery depend on? | the resistance of an external resistor |
| What arrangement of resistors do we use when you only want to use part of the e.m.f. of a supply | Potential divider circuit |
| What is the potential divider equation | V'out' = (R₂ ➗(R₁ + R₂)) ✖️ V 'in' |
| How is a light-dependent resistor (LDR) used as a sensor or transducer | Voltage is needed to drive the output devices, such as a voltmeter, yet the LDR only produces a change in resistance
Sensor must use this change in resistance to generate the change in voltage
Solution is to place the LDR in series with a fixed resistors |
| Negative temperature coefficient means that, | when temperature rises, resistance of the theristor falls |
| In a negative temperature coefficient, when the temperature rises, resistance of the theristor falls why? | Thermistor is made from a semiconductor material, so when temperature rises the number of free electrons increases, thus resistance falls |
| What is a potentiometer | device used for comparing potential differences |
| What can a potentiometer be used for | measure e.m.f. of a cell, provided you already have a source whose e.m.f. is known accurately |
| A potentiometer consists of what | a piece of resistance wire, usually 1m in length, stretched horizontally between 2 points |
| What is a jokey | Simple connecting device with a very sharp edge that allows very precise positioning on the wire |
| What is a null method | finding a point where the reading is zero |
