Ch 23-- Electric current, Ohms law

What is an ampere?

1amp= 1 coulomb/sec

In a circuit of two lamps in parallel, if there is a voltage of 6 volts across one lamp, what is the voltage across the other lamp?

6 volts. All electrical devices in parallel will have the same voltage.

Does a battery produce ac or dc? Does the generator at a power station produce ac or dc?

A battery produces direct current, dc. The power station produces alternating current, ac.

Would you expect to find ac or dc in the filament of a light bulb in your home? How about in the headlight of an automobile?

AC in your home, DC in your car

Distinguish between dc and ac

Electricity flows in two ways: either in an alternating current (AC) or in a direct current (DC). Electricity or "current" is nothing but the movement of electrons through a conductor, like a wire. The difference between AC and DC lies in the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forward" and then going "backward." Alternating current is the best way to transmit electricity over large distances.

Solve the formula P=IV for I, then find the current drawn by a 1200 watt hair dryer connected to a 120V source. Then, using Ohms law, find the resistance of the hair dryer.

I= P/V I= 1200watt/120V= 10 amps I= V/R R=V/I= 120V/10amps= 12Ω

If the resistance in a circuit is doubled while the voltage is kept constant, what happens to the current?

I= V/R We can see from Ohms law that current (I) is inversely proportional to Resistance (R), therefore if the resistance is doubled, while the voltage is kept constant, the current will be half of its original value.

The total charge that an automobile battery can supply without being recharged is given in terms of ampere-hours. A typical 12-V battery has a rating of 60 ampere-hours. Suppose you forget to turn off the headlights in your parked car. If each of the two headlights draws 3amps, how long will it be before your battery is "dead"?

I= q/t therefore t= q/I t= 60 amp hrs/6 amps= 10 hours

If the resistance in a circuit remains constant while the voltage across the circuit is changed to 1/2 its original value, what happens to the current?

I=V/R Current is directly proportional to voltage. If you cut the voltage in half, you cut the current in half.

Will the current in a light bulb connected to a 220-V source be greater or less than when the same bulb is connected to a 110-V source?

I=V/R Voltage is directly proportional to Current. If the voltage is cut in half, the current is cut in half. The bulb connected to the 220V source will have a greater current.

Only a small percentage of the electric energy fed into a common light bulb is transformed into visible light. What happens to the rest?

It comes off as invisible light in the infra-red part of the spectrum.

Your tutor tells you that an ampere and a volt are the same thing. Are they?

No. An amp is a measure of the charge per unit time that flows through a device, Voltage is the amount of energy per unit charge. They are related but very different things.

Are your car headlights wired in parallel or in series?

Parallel, when one bulb burns out, the other keeps on shining.

Rearrange the equation I= V/R to express resistance in terms of current and voltage. Then solve the following: A certain device in a 120-V circuit has a current rating of 20amps. What is the resistance of the device?

R= V/I R= 120V/20amps= 6Ω

What role do resistors normally play in an electrical circuit?

Resistors are used to regulate the amount of current and therefore the power in various parts of an electrical circuit. When you turn the volume up on your radio, you are actually decreasing the resistance and increasing the current in the portion of the circuit that produces the sound.

To connect a pair of resistors so that their combined resistance will be greater than the resistance of either one, should they be connected in series or in parallel?

Series

In a circuit of two lamps in series, if the current through one lamp is 1amp, what is the current through the other lamp?

Since resistors in series have exactly the same current through them, the current in the second lamp must also be 1amp

How much energy is supplied to each coulomb of charge that flows through a 12V battery?

Since voltage is the amount of energy in joules per coulomb of charge, 12V will yield 12 joules of energy per coulomb of charge. V=E/q

What is the relationship between electrical power, current and voltage?

The relationship is given by P=IV We can see that power is directly proportional to the current I, as well as the voltage V. If the current is measured in amps and the voltage is in volts, the power will be in watts.

If a voltage of 6volts is impressed across the circuit in the preceding question and the voltage across the first lamp is 2volts, what is the voltage across the second lamp?

The second lamp will have 4volts (conservation of energy)

What is the effect on current in a wire if both the voltage across it and the resistance are doubled? both halved?

They current remains unchanged. I=V/R

How does the sum of the currents through the branches of a simple parallel circuit compare with the current that flows through the voltage source?

They will be equal, charge is conserved.

For a given voltage, what happens to the amount of current that flows in your skin when you perspire?

When you perspire, your skin's resistance decreases. If voltage remains constant, when resistance decreases, current will increase. I= V/R

When you pay a household electric bill, which of the following are you paying for-- voltage, current, power or energy?

You are billed for a certain number of kilowatt hrs. Kilowatts are units of power. Power x time= energy. (you're being billed for energy)

How does water moisture on your skin affect your skin's electrical resistance?

Your skin's resistance decreases as it becomes more moist. (your skin becomes a good conductor)

What unit is represented by: a) joule/coulomb b) coulomb/sec c) wattᐧsec

a) joule/coulomb= volt b) coulomb/sec=amp c) wattᐧsec= joule

What does it mean to say that a certain current is 60Hz?

it means that the current is changing direction at a rate of 60 complete cycles per second. This is an example of alternating current (AC).

What happens to the brightness of light emitted by a light bulb when the current that flows in it increases?

since the power in a device is proportional to current, P=IV, the larger the current, the brighter the bulb

Which of these is a unit of power and which is a unit of energy: a watt, a kilowatt, a kilowatt hour?

watts and kilowatts are power, kilowatt hours are energy